Centennial Fertilization-Induced Soil Processes Control Trace Metal Dynamics. Lessons from a Long-Term Bare Fallow Experiment

Article Centennial Fertilization-Induced Soil Processes Control Trace Metal Dynamics. Lessons from a Long-Term Bare Fallow Experiment Folkert van Oort 1,*, Remigio Paradelo 2ID , Nicolas Proix 3, Ghislaine Delarue 1, Denis Baize 4 and Fabrice Monna 5 1INRA-AgroParisTech, UMR 1402 EcoSys, Soil Ecotoxicology, RD10, F-78026 Versailles CEDEX, France; [email protected] 2Facultade de Farmacia, Departamento de Edafoloxía e Química Agrícola, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain; [email protected] 3INRA, US 0010 Laboratoire d’Analyse des Sols, 273 Route de Cambrai, F-62000 Arras, France; [email protected] 4INRA, UR 0272 Science du Sol, F-45075 Orléans CEDEX 2, France; [email protected] 5 Universitéde Bourgogne—Franche Comté, UMR 6298, ArTéHis, CNRS-Culture, 6 bd Gabriel, Bât. Gabriel, F-21000 Dijon, France; [email protected] *Correspondence: [email protected]; Tel.: +33-130-833-251 Received: 15 March 2018; Accepted: 17 April 2018; Published: 19 April 2018   Abstract: Long-term bare fallow (LTBF) experiments with historical sample archives offer unique opportunities to study long-term impacts of anthropogenic activities on mineral soil fractions. In natural agro- and ecosystems, such impacts are often masked by organic matter due to its buffering action and rapid turnover. The 42-plot LTBF trial of INRA (Institut National de la Recherche Agronomique) started in Versailles (France) in 1928 to assess the impacts of prolonged application of fertilizers and amendments on the composition and properties of loamy soils. Here, we established geochemical budgets of major and trace elements on surface samples from 1929 and 2014 for four groups of treatments relevant for developed soil processes. We considered accompanying effects of soil compaction or decompaction due to changing physicochemical conditions over 85 years. Element losses from the surface horizon were quantified via fertilization-induced or -amplified soil processes: clay leaching favored by Na- or K-based fertilization, and lixiviation of major and trace elements in acidic or alkaline soil conditions. Enhanced mineral weathering was shown for acidified and nonamended plots. Conclusions on trace metal migration were confirmed by selected analyses on subsurface horizons. Additional information was provided on specific element inputs via fertilizers and/or diffuse inputs via atmospheric deposition. Keywords: fertilization; bare fallow soils; long-term agronomic experiments; loess Luvisol; acidification; lixiviation; clay leaching; geochemical budgets; major elements; trace metals 1. Introduction Soil degradation or, better, degradation of soil properties may result from both anthropogenic activities and natural processes, i.e., land misuse, soil mismanagement, or climatic change and related factors [ 1 ]. Soil acidification was an up-to-date topic in the 1970s to 1990s with a view to acid rain and related risks for increased leaching of soil nutrients. Currently, soil organic carbon (SOC) pools occupy a central place in problematic global change, with respect to reduced biodiversity, decreased aggregate stability and soil compaction, and nutrient loss [ 1 , 2 ]. Such items are widely documented. The SOC pool was reported as the defining constituent of soil [3] and reputed to be the Soil Syst. 2018,2, 23; doi:10.3390/2020023 www.mdpi.com/journal/soilsystems Soil Syst. 2018,2, 23 2 of 20 most reliable indicator for monitoring soil degradation [ 4 ], particularly by soil erosion. This reputation is due, in large part, to short-term response on climatic or anthropogenic constraints and rapid turnover dynamics in soils [5,6] . However, in most ecosystems, the decomposition of organic matter is compensated by restitution of plant debris or organic fertilization and amendments in agricultural soils. Therefore, soil organic matter (SOM) is not necessarily the most suitable indicator of long-term constraints affecting soils, or in any case not the sole indicator. Changing land use and management or agricultural practices may also affect the soil’s mineral fraction, in particular the finest reactive phases. Phyllosilicate clay minerals register such impacts by changes in their composition (substitution), behavior (hydration), surface properties (electric charge), or clay-particle thickness [ 7 – 9 ]. The chemical and mineralogical evolution of clay minerals is generally less rapid than that of SOC pools. Impacts of long-term anthropogenic activities on mineral fractions are less well documented, because study in real field conditions is often hampered by soil heterogeneity, changing land use, management practices, or duration of field monitoring. Long-term agronomic experiments are particularly valuable for studying time impacts of agricultural management on soils and cultivars, under more or less known and/or controlled conditions. Among them, long-term bare fallow (LTBF) experiments, in the absence of vegetation and organic matter restitution, offer unique opportunities to unravel the complex impacts of anthropogenic activities on mineral soil fractions. In 1928, such a long-term agronomic experiment, simply designated as the “42-plots trial,” was created at the research center of INRA (Institut National de la Recherche Agronomique) in Versailles, located in the gardens of the Chateau de Versailles. The main initial objective was to “examine the impacts of prolonged applications of main N, P and K fertilizers, as well as organic and basic amendments on the composition and properties of loam soils” [ 10 ]. The experiment was conducted on bare soils, in the absence of vegetation. Fertilizer doses were about twice those applied in the 1930s [ 10 ]. Such conditions are more extreme than those observed in real field situations, but were intentionally selected to exacerbate the impacts of the different treatments on mineral soil fractions. The restricted surface of the experiment in a fairly flat geographical position ensured the similarity of soils and soil properties at the start of the experiment. Moreover, the historical soil archive of annually collected surface samples makes it possible to study time effects, such as SOM dynamics [ 11 ] or atmospheric deposition of metal pollutants [ 12 , 13 ]. Organic C and N dynamics in the 42 plots have been extensively discussed elsewhere [11,14,15]. Recently, soil damage effects of chemical fertilization of intensively cultivated soils in modern industrialized agriculture in the quest for ever-increasing productivity were questioned in an article titled “Why It’s Time to Stop Punishing Our Soils with Fertilizers” [ 16 ]. Such questioning implies disposing of good indicators for time effects of fertilization practices on soils. Study of long-term bare fallow trials provides valuable insight into fertilizer-induced impacts on soils. The aim of the present work was to interpret, from a pedological point of view, recently obtained analytical data on soils of the LTBF 42-plots trial of INRA Versailles. We established geochemical budgets for major and trace elements covering a time span of 85 years and assessed the impacts of fertilization treatments on the composition and properties of loess Luvisols. For that, we selected surface soil samples collected in 1929 and 2014 in plots under monovalent (Na, K), ammonium-based, dried blood, or basic amendment treatments, and in nonamended plots, thus offering a wide panel of physicochemical soil conditions. Geochemical budgets were examined in terms of inputs by fertilizers, amendments, and atmospheric deposition, or losses via fertilization-induced soil processes. Selected exploratory data on soil characteristics in subsurface horizon are briefly discussed to support the major findings of our work with respect to migration pathways of major and trace elements toward soil depth. Soil Syst. 2018,2, 23 3 of 20 2. Materials and Methods 2.1. Long-Term Bare Fallow Experiment of INRA Versailles The experiment was conducted on silt-loam textured Haplic Luvisols [ 17 ] developed in loess, representative of large areas used for conventional cereal production in northern France and northwestern Europe. The experiment was and still is managed according to a strict plan, unchanged since 1928: plots are 2 m × 2.5 m wide; annual inputs of N-fertilizers are in the spring, whereas K and P fertilizers, as well as organic and basic amendments, are added in autumn; the upper 25 cm soil of all plots is dug by spade twice a year, following fertilization; 16 fertilization treatments are tested in duplicate and 10 plots remain without applications as a reference. Weeds are removed manually and occasionally by herbicide treatment. The first applications were performed on 12 November 1928. Samples of the upper 25 cm surface layer have been collected throughout the experiment, generating today a 90-year-old unique bare-soil sample archive. Initial soil data used in our work were taken from [ 10 ]: pH 6.3, and bulk density estimated at 1300 kg/m3 based on soil porosity data. Initial chemical data were determined on the first reference-plot samples collected in March 1929: clay content: 189 g/kg and concentrations of major and trace elements [ 18 , 19 ]. The fertilizers used were commercial products. The fertilizer stocks were renewed partly in 1954 and completely in 1991. The applied doses of the target elements N, P, K, and Ca were adapted according their concentration in the fertilizers, but little or no information on the contents of trace elements was available. The experimental design of the 42 plots is presented in Figure 1. For the present study, 26 plots were selected and divided into 4 groups with comparable analytical characteristics: (1) ammonium-based fertilizers (sulfate, phosphate, chloride, nitrate) and dried blood; results indicated by red bars (or spots) and black borders; (2) monovalent Na- and/or K-based fertilizers added as sulfate, chloride, or nitrate; for the sodic treatments, the results are indicated by yellow bars with red borders; (3) reference plots, 4 from the left-hand nitrogen block (reference L) and 4 from the right-hand K, P, and basic amendments block (reference R); results indicated by gray bars with black borders; and (4) basic amendments, indicated by green bars with black borders. The samples from plots along a small road in sight of the INRA’s research center (19–21,40–42) were excluded from our study due to suspected localized dust contamination [19]. Soil Syst. 2018, 2, x FOR PEER REVIEW 3 of 20 2. Materials and Methods 2.1. Long-Term Bare Fallow Experiment of INRA Versailles The experiment was conducted on silt-loam textured Haplic Luvisols [17] developed in loess, representative of large areas used for conventional cereal production in northern France and northwestern Europe. The experiment was and still is managed according to a strict plan, unchanged since 1928: plots are 2 m × 2.5 m wide; annual inputs of N-fertilizers are in the spring, whereas K and P fertilizers, as well as organic and basic amendments, are added in autumn; the upper 25 cm soil of all plots is dug by spade twice a year, following fertilization; 16 fertilization treatments are tested in duplicate and 10 plots remain without applications as a reference. Weeds are removed manually and occasionally by herbicide treatment. The first applications were performed on 12 November 1928. Samples of the upper 25 cm surface layer have been collected throughout the experiment, generating today a 90-year-old unique bare-soil sample archive. Initial soil data used in our work were taken from [10]: pH 6.3, and bulk density estimated at 1300 kg/m 3 based on soil porosity data. Initial chemical data were determined on the first reference-plot samples collected in March 1929: clay content: 189 g/kg and concentrations of major and trace elements [18,19]. The fertilizers used were commercial products. The fertilizer stocks were renewed partly in 1954 and completely in 1991. The applied doses of the target elements N, P, K, and Ca were adapted according their concentration in the fertilizers, but little or no information on the contents of trace elements was available. The experimental design of the 42 plots is presented in Figure 1. For the present study, 26 plots were selected and divided into 4 groups with comparable analytical characteristics: (1) ammonium-based fertilizers (sulfate, phosphate, chloride, nitrate) and dried blood; results indicated by red bars (or spots) and black borders; (2) monovalent Na- and/or K-based fertilizers added as sulfate, chloride, or nitrate; for the sodic treatments, the results are indicated by yellow bars with red borders; (3) reference plots, 4 from the left-hand nitrogen block (reference L) and 4 from the right-hand K, P, and basic amendments block (reference R); results indicated by gray bars with black borders; and (4) basic amendments, indicated by green bars with black borders. The samples from plots along a small road in sight of the INRA’s research center (19–21,40–42) were excluded from our study due to suspected localized dust contamination [19]. Figure 1. Experimental design of the 42-plots trial, groups of selected plots with their color codes, and equivalent annual fertilizer input rates per hectare. White plots were not considered in this work. In addition to global sampling in the surface horizon, bulk and undisturbed samples were also collected in the underlying E, B and C horizons of a dozen plots, using a root auger device (diameter 8 cm, height 15 cm), until the carbonated parent material was reached, between 90 and >120 cm. The core samples were separated according soil horizon limits. Selected undisturbed parts were resin-impregnated before thin-section preparation; the rest were ground to 2 mm and stored for Figure 1. Experimental design of the 42-plots trial, groups of selected plots with their color codes, and equivalent annual fertilizer input rates per hectare. White plots were not considered in this work. In addition to global sampling in the surface horizon, bulk and undisturbed samples were also collected in the underlying E, B and C horizons of a dozen plots, using a root auger device (diameter 8 cm, height 15 cm), until the carbonated parent material was reached, between 90 and >120 cm. The core samples were separated according soil horizon limits. Selected undisturbed parts were Soil Syst. 2018,2, 23 4 of 20 resin-impregnated before thin-section preparation; the rest were ground to 2 mm and stored for chemical analysis. Relevant chemical and microscopic data are presented, providing a first short overview of the impacts of 85 years of experimentation on soil characteristics in subsurface horizons, which supports our interpretation of geochemical budgets established in the 0–25 cm surface horizons. 2.2. Analysis of Soil Characteristics and Total Element Concentrations On representatively quartered soil samples from the selected plots, the following analyses were performed according to international standard conditions [ 20 ]: grain size distribution (NF X 31-107), pH in water (ISO 10390), organic carbon and nitrogen (ISO 10694, ISO 13878), and cation exchange capacity (CEC) and exchangeable cations by the cobalt hexammine method (ISO 23470). For analysis of total element concentrations, subsamples were ground to 250 µ m before complete dissolution by hydrofluoric and perchloric acid (ISO NF X31-147). Major agronomic elements (K, P, Ca, Mg, and Na), pedological key indicator elements (Al, Fe, and Mn), and 12 trace elements—micro-oligo-elements (Cu, Zn, Co, Cr, Ni, Mo, and As) and nonessential elements (Cd, Pb, U, Tl, and Sc)—were determined by either Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) or Mass Spectrometry (ICP-MS) (NF ISO 22036 and NF ISO 17294-2, respectively). Total element concentrations were analyzed simultaneously on samples from 1929 and 2014. The studied elements were chosen based on a preliminary analytical screening of 25 elements in soils of the 42 plots. Our final selection coincides to a large extent with the panel of major and trace elements considered in a large French national research program on trace metal contents in agricultural soils [21]. 2.3. Computation of Element Stocks and Budgets Following decomposing organic matter and diverging physicochemical ambience induced by fertilization, the physical properties of soils in the surface horizon were significantly modified, notably with regard to soil structure, aggregate stability, and bulk density [ 2 , 22 , 23 ]. Among the plots selected for our work, the bulk densities in 2014 varied from about 1200 to 1600 kg/m 3 , i.e., ≈ 30% variation with respect to the estimated 1300 kg/m 3 at the start of the experiment. Consequently, the weight of 1 m 2 of the 25 cm thick Ap horizon ranged from ≈ 300 to 400 kg. In view of such differences, the use of element concentrations expressed as mass per kg of soil does not appropriately account for their amounts in the surface horizon. Therefore, we calculated stocks of elements in the 0–25 cm Ap horizons, expressed as mass per m 2 . The use of element stocks was reported as an ecologically relevant unit for soil quality studies [ 24 , 25 ], preventing biased comparisons of element amounts in soils with variable bulk density, organic matter content, or horizon thickness [ 26 ]. Stocks of elements in a soil horizon were calculated by multiplying the element concentration, the bulk density, and the horizon thickness. Thus, in 1929 the initial stock of elements (S1929) in the Ap horizon was: S1929 = [M]1929 ×ρ1929 ×tAp (1) where [M] 1929 = element concentration (mass/kg), determined on March 1929 reference-plot samples; ρ1929 = bulk density (kg/m3) of the Ap horizon in 1929; and tAp = thickness of Ap, set to 0.25 m. For 2014, the computation of element stock was more complex. First, due to the combining effects of soil compaction and successive soil collection, and a constant 25 cm depth of annual digging and sampling, the upper centimeters of the underlying E horizon was progressively incorporated into the surface Ap horizon (Figure 2). Second, a small but nonnegligible part of the surface horizon elements is currently stored in the soil archive collection. Therefore, in 2014 the element stock (S 2014 ) in the Ap horizon included 3 terms: S2014 = S’2014 −SE+ Sarch (2) S’2014: overall element stock (kg/m3) in the 25 cm surface layer = [M]2014 ×ρ2014 ×0.25 Soil Syst. 2018,2, 23 5 of 20 where [M]2014 = element concentration (mass/kg) and ρ2014 = bulk density; and SE: element stock from the E horizon incorporated into the Ap horizon = [M]E1929 ×ρE×(tE1 +tE2)/100 where [M] = element concentration (mass/kg) in the upper few cm of the E horizon in 1929, set to the initial surface horizon concentration; ρE = bulk density (kg/m 3 ) of the E horizon, set to 1580 kg/m3 according to several local soil survey studies [ 27 , 28 ]; and t E = thickness (cm) of the E horizon incorporated into the surface Ap horizon, with: tE1: thickness linked to densification in the Ap horizon = [(ρ2014 −ρ1929)×0.25]/ρE/100 tE2: thickness linked to soil sampling, estimated at 75 kg for 5 m2= (75/5)/ρE/100 Soil Syst. 2018, 2, x FOR PEER REVIEW 5 of 20 SE: element stock from the E horizon incorporated into the Ap horizon = [M]E1929 × ρE × (tE1 + tE2)/100 where [M] = element concentration (mass/kg) in the upper few cm of the E horizon in 1929, set to the initial surface horizon concentration; ρE = bulk density (kg/m3) of the E horizon, set to 1580 kg/m3 according to several local soil survey studies [27,28]; and tE = thickness (cm) of the E horizon incorporated into the surface Ap horizon, with: tE1: thickness linked to densification in the Ap horizon = [(ρ2014 − ρ1929) × 0.25]/ρE/100 tE2: thickness linked to soil sampling, estimated at 75 kg for 5 m2 = (75/5)/ρE/100 The estimation of 75 kg of soil collected for each plot is based on the actual amounts stored in the historical sample archive, and probably slightly underestimates the amount of soil used with time for laboratory analyses. Therefore, tE2 was set to 1 cm. Sarch: element stock of the Ap in the historical sample collection = (75/5) × ([M]2014 + [M]1929)/2 The final geochemical budget for each element is calculated as: B2014−1929 = S2014 − S1929 (3) Figure 2. Effects of soil deepening or heightening (Δept) of the 0–25 cm layer due to changing bulk density caused by fertilizing treatments (Δep) and repetitive soil sampling (Δ’ep) since 1929. The manure treatment is added here to demonstrate the amplitude of soil height (modified after [19]). NB: For treatments with decreasing bulk density during the experiment (dried blood, lime), the soil surface rose, producing a new layer between the Ap and E horizons, Ap/E. This effect was particularly visible for the manure treatment (Figure 2), where the surface Ap bulk density decreased to 960 kg/m3 [2,19]. The corresponding element stock SA/E was calculated using average values for element concentrations and bulk densities between 1929 and 2014: SA/E: = ([M]1929 + [M]2014)/2 × (ρ2014 + ρ1929)/2 × (tA/E1 + tA/E2)/100 where tA/E1 = [(ρ2014 − ρ1929) × 0.25]/[(ρ2014 + ρ1929)/2]/100 and tA/E2 is set to 1 cm. NB: tAE1 and tAE2 have opposite signs, thus expressing a soil rising or deepening effect, respectively. Figure 2. Effects of soil deepening or heightening ( ∆ep t) of the 0–25 cm layer due to changing bulk density caused by fertilizing treatments ( ∆ep ) and repetitive soil sampling ( ∆ ’ ep ) since 1929. The manure treatment is added here to demonstrate the amplitude of soil height (modified after [19]). The estimation of 75 kg of soil collected for each plot is based on the actual amounts stored in the historical sample archive, and probably slightly underestimates the amount of soil used with time for laboratory analyses. Therefore, tE2 was set to 1 cm. Sarch: element stock of the Ap in the historical sample collection = (75/5) ×([M]2014 + [M]1929)/2 The final geochemical budget for each element is calculated as: B2014−1929 = S2014 −S1929 (3) NB: For treatments with decreasing bulk density during the experiment (dried blood, lime), the soil surface rose, producing a new layer between the Ap and E horizons, Ap/E. This effect was particularly visible for the manure treatment (Figure 2), where the surface Ap bulk density decreased Soil Syst. 2018,2, 23 6 of 20 to 960 kg/m 3 [ 2 , 19 ]. The corresponding element stock S A/E was calculated using average values for element concentrations and bulk densities between 1929 and 2014: SA/E: = ([M]1929 + [M]2014)/2 ×(ρ2014 +ρ1929)/2 ×(tA/E1 +tA/E2)/100 where t A/E1 = [( ρ2014 −ρ1929 ) × 0.25]/[( ρ2014 + ρ1929 )/2]/100 and t A/E2 is set to 1 cm. NB:t AE1 and tAE2 have opposite signs, thus expressing a soil rising or deepening effect, respectively. 2.4. Validity of Calculations The validity of our computation approach was checked by comparing budgets of one soil component, a key indicator of dominant soil processes (<2 µ m clay fraction), and one trace element (scandium, Sc), assuming their concomitant migration in loess Luvisols. This hypothesis was based first on the geochemistry literature from the 1960s and 1970s: (i) in ferromagnesian minerals, Sc 3+ can substitute for Fe 3+ and Al 3+ [ 29 ]; (ii) in sedimentary soils, scandium is primarily located in easily weatherable minerals, such as hornblende, biotite, and chlorite [ 30 ]; and (iii) in Luvisols of North America, Sc translocation was shown to be closely related to clay and iron via pedogenesis processes [ 29 ]. Second, Sc is disseminated a little or not at all by anthropogenic activity in the environment [ 31 ] and has very low mobility in soils under oxidizing and acidic environmental conditions [ 32 ]. Third, our previous work [ 19 ] revealed remarkably high correlation between Sc and clay for the entire soil set of the 42-plots experiment (Figure 3a, excluding manure plots), supporting the literature findings. The Sc content in clay fractions, analyzed on samples from plots with highly contrasting physicochemical conditions, showed an average of 18.4 ± 0.37 mg/kg. Such low variability of the Sc content of clay makes it useful as an indicator of clay loss/accumulation. The stocks of <2 µ m clay fractions and scandium were established following Equations (1) and (2) for the selected groups of fertilization treatments (cf. Figure 1) and their geochemical budgets were established following Equation (3). Comparing the correlation of concentrations and budgets of scandium and clay (Figure 3a,b) reveals an appreciable increase of R 2 from 0.83 to 0.92. In both graphs, the slopes of linear regression were very close to measured clay Sc contents (18.4 mg/kg), 17.6 and 17.3 , respectively. For plots displaying clearly lower clay content in 2014, i.e., ≥ 20% since 1929 , the budget ratio B clay /B Sc ranged from 16.2 to 15.6, diverging by 12.5–15.2% with respect to the measured 18.4 mg/kg Sc content in the clay fraction. In view of the estimations made for unknown parameters at the start of and during the experiment, this difference was found to be satisfactory for validating our computation approach. Soil Syst. 2018, 2, x FOR PEER REVIEW 6 of 20 2.4. Validity of Calculations The validity of our computation approach was checked by comparing budgets of one soil component, a key indicator of dominant soil processes (<2 µm clay fraction), and one trace element (scandium, Sc), assuming their concomitant migration in loess Luvisols. This hypothesis was based first on the geochemistry literature from the 1960s and 1970s: (i) in ferromagnesian minerals, Sc 3+ can substitute for Fe 3+ and Al 3+ [29]; (ii) in sedimentary soils, scandium is primarily located in easily weatherable minerals, such as hornblende, biotite, and chlorite [30]; and (iii) in Luvisols of North America, Sc translocation was shown to be closely related to clay and iron via pedogenesis processes [29]. Second, Sc is disseminated a little or not at all by anthropogenic activity in the environment [31] and has very low mobility in soils under oxidizing and acidic environmental conditions [32]. Third, our previous work [19] revealed remarkably high correlation between Sc and clay for the entire soil set of the 42-plots experiment (Figure 3a, excluding manure plots), supporting the literature findings. The Sc content in clay fractions, analyzed on samples from plots with highly contrasting physicochemical conditions, showed an average of 18.4 ± 0.37 mg/kg. Such low variability of the Sc content of clay makes it useful as an indicator of clay loss/accumulation. The stocks of <2 µm clay fractions and scandium were established following Equations (1) and (2) for the selected groups of fertilization treatments (cf. Figure 1) and their geochemical budgets were established following Equation (3). Comparing the correlation of concentrations and budgets of scandium and clay (Figure 3a,b) reveals an appreciable increase of R 2 from 0.83 to 0.92. In both graphs, the slopes of linear regression were very close to measured clay Sc contents (18.4 mg/kg), 17.6 and 17.3, respectively. For plots displaying clearly lower clay content in 2014, i.e., ≥20% since 1929, the budget ratio B clay /B Sc ranged from 16.2 to 15.6, diverging by 12.5–15.2% with respect to the measured 18.4 mg/kg Sc content in the clay fraction. In view of the estimations made for unknown parameters at the start of and during the experiment, this difference was found to be satisfactory for validating our computation approach. Figure 3. Linear correlation between clay (<2 µm fractions) and scandium in soils of the 42-plots experiment. (a) Soil concentrations of all plots (except manure plots); (b) scandium and clay budgets for the selected fertilizer groups. 2.5. Data Presentation and Statistical Treatment Due to the initial setup of the experiment, including 2 replicates per treatment, no statistics could be operated on data from fertilized plots; therefore max and min bars are presented on histograms to indicate intragroup variability. For the 8 reference plots, the significance of differences in geochemical budgets in the soil surface horizon between 1929 and 2014 was statistically examined using the paired Wilcoxon test. A principal component analysis (PCA) projection of the 2014–1929 element and clay budgets and the 2014 soil pH was made to help assess the impacts of soil processes on element dynamics. Figure 3. Linear correlation between clay (<2 µ m fractions) and scandium in soils of the 42-plots experiment. ( a ) Soil concentrations of all plots (except manure plots); ( b ) scandium and clay budgets for the selected fertilizer groups. 2.5. Data Presentation and Statistical Treatment Due to the initial setup of the experiment, including 2 replicates per treatment, no statistics could be operated on data from fertilized plots; therefore max and min bars are presented on histograms to Soil Syst. 2018,2, 23 7 of 20 indicate intragroup variability. For the 8 reference plots, the significance of differences in geochemical budgets in the soil surface horizon between 1929 and 2014 was statistically examined using the paired Wilcoxon test. A principal component analysis (PCA) projection of the 2014–1929 element and clay budgets and the 2014 soil pH was made to help assess the impacts of soil processes on element dynamics. 3. Results 3.1. Present Physicochemical Soil Characteristics Notable modifications of chemical and physical soil properties in the surface horizons were reported early, as soon as 10 years after the start of experimentation [ 10 ], and progressively amplified over the years. The most striking effects of fertilizers are presented in Figure 4. The range of pH values in 2014 was outstandingly wide, more than 5 units from 8.8 in CaCO 3 amended plots to 3.5 in the ammonium sulfate plot (Figure 4a). The pH decrease was higher for K-based than for Na-based fertilizers. Although variation of pH was remarkably fast during the first decades (4 units in 1939, [ 10 ]), the magnitude of ∆pH is still increasing: 4.6 units in 1999 [33] and 5.3 units in 2014 [18]. Soil Syst. 2018, 2, x FOR PEER REVIEW 7 of 20 3. Results 3.1. Present Physicochemical Soil Characteristics Notable modifications of chemical and physical soil properties in the surface horizons were reported early, as soon as 10 years after the start of experimentation [10], and progressively amplified over the years. The most striking effects of fertilizers are presented in Figure 4. The range of pH values in 2014 was outstandingly wide, more than 5 units from 8.8 in CaCO3 amended plots to 3.5 in the ammonium sulfate plot (Figure 4a). The pH decrease was higher for K-based than for Na-based fertilizers. Although variation of pH was remarkably fast during the first decades (4 units in 1939, [10]), the magnitude of ΔpH is still increasing: 4.6 units in 1999 [33] and 5.3 units in 2014 [18]. Figure 4. Remarkable physicochemical data in selected plots of the long-term bare fallow (LTBF) experiment: (a) soil pH; (b) proportion of exchangeable cations on cation exchange capacity (CEC); (c) clay content; (d) bulk density. The acid plots include ammonium sulfate, chloride, and nitrate; P– NH4: ammonium phosphate fertilization; reference L and reference R: mean data of four reference plots for the left and right blocks, respectively; sodic plots include NaNO3 and sylvinite treatments, potassic plots include KCl and K2SO4 treatments, basic plots include CaO and CaCO3 amendments. Data modified after [18]. Concomitantly, the composition of the exchange complex markedly changed: the proportion of bivalent cations decreased from >98% in 1928 to ≈80% in the reference plots, to 75 and 55% in the sodic and potassic plots, respectively, and to <5% in strongly acidified plots (Figure 4b). In the latter, exchangeable Al occupied up to 96% of the CEC in 2014, strongly contrasting with soils under basic amendments, where exchangeable Ca + Mg occupied almost 99% of the CEC. In the monovalent plots, the proportion of exchangeable Na + K reached 15–35% in 2014 and was highest under K fertilization. Clay content considerably varied (Figure 4c) from 134 g/kg for the NaNO3 treatment to 197 g/kg for the (NH4)2HPO4 treatment. With respect to 1929, the decrease of clay content was more marked for Nathan for K-fertilized plots and indicates a process of enhanced clay leaching under monovalent dispersive conditions. In the reference plots, an average clay content in 2014 of 174 g/kg also pointed to a loss of clay by leaching toward depth. Such widely diverging physicochemical soil conditions impacted the physical properties of the surface horizon: effects of surface sealing are easily visible at the surface of the plots after major rain events [22], as well as effects of soil compaction due to increasing bulk density with respect to 1928, notably in the reference and monovalent plots, reaching 1590 kg/m3 for sylvinite (Figure 4d). By contrast, bulk density remained Figure 4. Remarkable physicochemical data in selected plots of the long-term bare fallow (LTBF) experiment: ( a ) soil pH; ( b ) proportion of exchangeable cations on cation exchange capacity (CEC); ( c ) clay content; ( d ) bulk density. The acid plots include ammonium sulfate, chloride, and nitrate; P–NH 4 : ammonium phosphate fertilization; reference L and reference R: mean data of four reference plots for the left and right blocks, respectively; sodic plots include NaNO 3 and sylvinite treatments, potassic plots include KCl and K 2 SO 4 treatments, basic plots include CaO and CaCO 3 amendments. Data modified after [18]. Concomitantly, the composition of the exchange complex markedly changed: the proportion of bivalent cations decreased from >98% in 1928 to ≈ 80% in the reference plots, to 75 and 55% in the sodic and potassic plots, respectively, and to <5% in strongly acidified plots (Figure 4b). In the latter, exchangeable Al occupied up to 96% of the CEC in 2014, strongly contrasting with soils under basic amendments, where exchangeable Ca + Mg occupied almost 99% of the CEC. In the monovalent plots, the proportion of exchangeable Na + K reached 15–35% in 2014 and was highest under K fertilization. Clay content considerably varied (Figure 4c) from 134 g/kg for the NaNO 3 treatment to 197 g/kg for the (NH 4 ) 2 HPO 4 treatment. With respect to 1929, the decrease of clay content was more marked for Nathan for K-fertilized plots and indicates a process of enhanced clay leaching under monovalent dispersive conditions. In the reference plots, an average clay content in 2014 of 174 g/kg also pointed Soil Syst. 2018,2, 23 8 of 20 to a loss of clay by leaching toward depth. Such widely diverging physicochemical soil conditions impacted the physical properties of the surface horizon: effects of surface sealing are easily visible at the surface of the plots after major rain events [ 22 ], as well as effects of soil compaction due to increasing bulk density with respect to 1928, notably in the reference and monovalent plots, reaching 1590 kg/m 3 for sylvinite (Figure 4d). By contrast, bulk density remained unchanged or even decreased under basic amendments and dried blood, and the lowest value reached 1210 kg/m3. 3.2. Geochemical Budgets of Major and Trace Elements between 1929 and 2014 3.2.1. Element Budgets in Nonamended Reference Plots Geochemical budgets in soils of the reference plots provide information about cumulative gains and losses of elements under local climatic conditions over a period of 85 years. Tables 1and 2present the median values of element stocks in 1929 and 2014 in the eight selected reference plots, as well as p-values calculated at a 95% significance level by paired Wilcoxon test. Significant differences are expressed as both the element mass/m 2 and a rough estimation of mass/ha. Among the major elements, significant changes (p<0.01) were observed for Ca, Mg, and Na (Table 1). Losses reached 1.2 kg/m2for Ca and ≈50g/m2for Mg, and there was a gain of ≈100 g/m2for Na. Table 1. Median values of major element stocks calculated for reference plots in 1929 and 2014; significance of differences estimated by paired Wilcoxon test at p<0.05; ** significant at 99%. Element P K Ca Mg Na Al Fe Mn S1929 (g/m2)156 4250 2069 918 1766 11,823 5913 114 S2014 (g/m2)158 4248 876 862 1877 12,006 6091 129 p-value 0.64 0.15 <0.01 ** <0.01 ** <0.01 ** 0.25 0.055 0.31 B2014–1929 (kg/m2)– – −1.2 −0.05 +0.1 – – Table 2. Median values of trace element stocks calculated for reference plots in 1929 and 2014; significance of differences estimated by paired Wilcoxon test at p<0.05; ** significant at 99%. Element Cu Zn Co Cr Ni Mo As Cd Pb U Tl Sc S1929 (mg/m2)6530 15,380 2062 13,312 5328 161 2583 39.5 16,753 615 129 1781 S2014 (mg/m2)6895 20,437 2492 18,374 5285 178 2539 52.0 25,856 593 128 1743 p-value 0.31 <0.01 ** <0.01 ** <0.01 ** 0.55 <0.01 ** 0.31 <0.01 ** <0.01 ** 0.31 0.55 0.11 B2014−1929 (mg/m2)– +5057 +430 +5026 – +17 – +12.5 +9103 – – – For the 12 trace elements (Table 2), six of them showed significant gains: Cd, Co, Cr, Mo, Pb, and Zn, ascribed to atmospheric deposition between 1929 and 2014. The gains of Cd and Mo ranged between 10 and 20 mg/m 2 , but for Co, Cr, Pb, and Zn, the gains were in the order of 0.5–10 g/m2, and their sum totaled about 20 g/m2, representing an average yearly deposition rate of more than 200 mg/m2. 3.2.2. Element Budgets in Fertilized Plots For plots receiving chemical fertilizers and amendments, budgets are presented in Figures 5and 6. The order of presentation is based on similar patterns of gain/loss distribution in the fertilization treatment groups. In the graphs, the gray right-angle represents the 95% confidence interval of element budgets calculated for the reference plots with maximum (T + ) and minimum (T − ) values. In case of significant inputs of elements via atmospheric deposition (Tables 1and 2), the net element gains and losses in fertilized plots were estimated with respect to the T+or T−value, respectively. A first group of elements (Mn, Co, and Cd) demonstrated distinct losses for ammonium-based fertilization and dried blood (Figure 5a–c). Budgets of monovalent and basic treatments were found within or close to the 95% confidence interval of the reference plots. In the acidified plots, Mn loss from the surface horizon reached − 55 to − 65 g/m 2 in 85 years (Table 3). Cobalt budgets in these Soil Syst. 2018,2, 23 9 of 20 treatments were between about − 250 and − 600 mg/m 2 (Figure 5b). When considering the Co input by atmospheric deposition, the net Co budgets represent a loss of − 600 to − 800 g/m 2 (Table 3). Cadmium budgets showed a loss of − 30 to − 40 mg/m 2 for the dried blood and acid plots, respectively. However, a small gain was observed under P–NH 4 fertilization (Figure 5c), highlighting the frequently reported presence of Cd as an impurity in phosphate fertilizers [ 32 ]. When considering atmospheric deposition of Cd revealed in the reference plots (Table 3), the net budgets reached − 45, +3.5, and − 35 mg/m 2 for the acid, N–PH 4 , and dried blood plots, respectively. It must be noted that a loss of − 39 mg/m 2 represents the total estimated Cd stock present in the 0–25 cm surface layer in 1929. Soil Syst. 2018, 2, x FOR PEER REVIEW 9 of 20 deposition of Cd revealed in the reference plots (Table 3), the net budgets reached −45, +3.5, and −35 mg/m 2 for the acid, N–PH 4 , and dried blood plots, respectively. It must be noted that a loss of −39 mg/m 2 represents the total estimated Cd stock present in the 0–25 cm surface layer in 1929. Figure 5. Element budgets between 2014 and 1929 in the 0–25 cm layer of fertilized plots. The color code is according to Figure 1. The gray-tone box represents the maximum (T + ) and minimum (T − ) values of the 95 % confidence interval of the element budget calculated in the reference plots: (a) manganese; (b) cobalt; (c) cadmium; (d) calcium; (e) magnesium; (f) nickel; (g) aluminum; (h) iron; (i) scandium; (j) arsenic; (k) thallium; (l) clay. A second group of elements (Ca, Mg, and Ni) displayed losses for both the ammonium-based fertilizers and the monovalent plots (Figure 5d–f). Calcium loss was outstandingly high, between −1.4 and −1.6 kg/m 2 (i.e., −14 to −16 t/ha), exceeding Ca loss in the reference plots by 200 to 400 g/m 2 . In the monovalent treatments, the loss of Ca also slightly exceeded that observed for the reference plots. In the basic amendments, the gain of Ca reached about +2.5 kg/m 2 (i.e., +25 t/ha). The distribution of budgets for Mg and Ni showed comparable tendencies. Mg loss in soils under Figure 5. Element budgets between 2014 and 1929 in the 0–25 cm layer of fertilized plots. The color code is according to Figure 1. The gray-tone box represents the maximum (T + ) and minimum (T − ) values of the 95% confidence interval of the element budget calculated in the reference plots: ( a ) manganese; ( b ) cobalt; ( c ) cadmium; ( d ) calcium; ( e ) magnesium; ( f ) nickel; ( g ) aluminum; ( h ) iron; ( i ) scandium; (j) arsenic; (k) thallium; (l) clay. A second group of elements (Ca, Mg, and Ni) displayed losses for both the ammonium-based fertilizers and the monovalent plots (Figure 5d–f). Calcium loss was outstandingly high, between − 1.4 and − 1.6 kg/m 2 (i.e., − 14 to − 16 t/ha), exceeding Ca loss in the reference plots by 200 to 400 g/m 2 . In the monovalent treatments, the loss of Ca also slightly exceeded that observed for the reference plots. Soil Syst. 2018,2, 23 16 of 20 deposition (Table 2). High losses were observed for Ca (Figure 5d) in the reference plots ( − 1.2 kg/m 2 ) and for acid treatments ( − 1.6, − 1.5, − 1.4 kg/m 2 , respectively). Such magnitudes bring into question the origin of Ca washed out of the surface horizon. Data of exchangeable Ca in the reference plots decreased roughly from 15 to 5 cmol/kg between 1929 and 2014 [ 18 ], representing a loss of about 0.73 kg/m2 of Ca. In the strongly acidified plots, where virtually no more exchangeable Ca is detected at present, such a Ca loss accounted for about 0.98 kg/m 2 . These calculations indicate that, in view of total Ca loss, about 0.4–0.6 kg/m 2 must be attributed to mineral dissolution, likely of the more weatherable plagioclase feldspars. Magnesium also washed out of the surface horizon in the reference soils (Table 1) and more in the acid and P–NH 4 plots (Figure 5e). Loss of Mg was less evident for dried blood, suggesting consistent input via this organic nitrogen source. A second indication of mineral weathering was given by positive budget data observed for Al, Fe, Sc, Tl, As, and clay in the acid, P–NH 4 , and dried blood plots (Figure 5g–l, Table 4). High inputs of H + would expectedly lead to clay destruction and decreasing clay content (cf. Section 4.1). However, soils of the strongly acidified plots demonstrated the highest clay content (Figure 4c). Such apparently contradictory findings may be explained by subsidiary hydrolysis weathering of primary phyllosilicate minerals in loess soils (mica and chlorite) affecting the fine silt fractions. Their breakdown would produce clay-sized particles with low electric charge and thus compensate for the acid dissolution of clay . A mechanism of clay mineralogical development of silt fraction in acid loess soils was reported by [ 52 ]. Such a dissolution process of clay minerals in acid soils would explain the omnipresence of Al on the exchange complex (Figure 4b), and the decreasing CEC values inversely proportional with the produced proton loads by ammonium fertilizers reported by [ 18 ] is consistent with a mineralogical mechanism of breakdown of silt into clay-sized mica or chlorite particles. Positive element budgets for Al, Fe, Sc, Tl, and Sc suggest their immobilization by Al oxyhydroxides, or phosphate, in the surface horizon. The incorporation of a part of the underlying E horizon by constant 25 cm depth digging in a compacting A horizon would lead to a relative accumulation of Al, Fe, Sc, Tl, and Sc during the 85 years of the experiment. For Fe, its relative accumulation in strongly acidic soil conditions may explain the difference between the observed positive Fe budget in the dried blood plots (+6.6 t/ha) and the theoretical inputs (+1.8 to +2.7 t/ha) (cf. Section 4.1.1). Remarkably, some elements, such as Mg, Ni, and Ca, were affected by both clay leaching and acidification (Table 4), explaining their intermediate position between clay and pH in the PCA budget projection (Figure 8). Furthermore, all three elements showed positive budgets for the basic amendment plots (Figure 5d–f). Alkalinization In Figure 8, molybdenum was opposed to the soil pH, illustrating a clear loss observed for the basic plots ( ≈ 400 g/ha), despite a diffuse input of Mo of about 170 g/ha by atmospheric deposition (Figure 6a). Actually, in neutral aerated soils, Mo is present in an anionic form (MoO 4 ) 2− whose mobility increases in alkaline soil conditions [ 45 ]. This finding demonstrates a distinct behavior of Mo with respect to the other trace elements studied here. Considering the input of Mo by atmospheric deposition (Table 2), its net loss reached − 30 mg/m 2 (Table 3). This finding is noteworthy, as it means that about 20% of the initial Mo stock has been lost by 85 years of liming practices. 4.2. Significance of Agronomic Experiments to Long-Term Soil Evolution Studies Agronomic long-term experiments are of great interest for studies aiming at unraveling the physicochemical mechanisms of soil degradation and their consequences for declining sustainability of ecosystem quality and services. In the 42-plots trial, outstanding variable physicochemical and physical properties are observed today in the surface 0–25 cm horizon that relate the fascinating story of almost 90 years of application of fertilizers and amendments, or their absence. The changes of soil properties that have occurred since 1928 are due, in large part, to the cation composition of the soil exchange complex [ 53 ], i.e., the action of compensating cations in N, P, and K fertilizers (Na + or H + ) or Ca 2+ in Soil Syst. 2018,2, 23 17 of 20 the basic amendments. Bivalent Ca 2+ cations in soils favor the flocculation of electronegative colloids, enhancing aggregate stability [ 15 ], low bulk density (Figure 2d), and well-aerated hydrodynamic conditions. Exchangeable Na + (and K + ) is known to promote swelling of expendable clay minerals and clay dispersion [ 8 ]. When it exceeds 10 to 15% of the soil CEC (Figure 2b), Na + may become toxic for plants and favors degradation of aggregate stability and soil compaction (Figure 2d), leading to temporary water stagnation and hydromorphic conditions [ 51 ]. High input of H + (NH 4+ -based fertilizers) leads to strong soil acidification and the formation of hydrogen clay [ 54 ]. Such H + clay is highly unstable and evolves rapidly into Al-saturated clay [ 55 ], explaining the presence of consistent amounts of exchangeable Al. In the presence of strong complexing organic agents that immobilize exchangeable Al, acidification leads to clay-mineral dissolution, as reported in podzol-B horizons [ 56 , 57 ]. Considering the low SOC content in soils of the 42-plots trial, strong complexing anions of ammonium fertilizers, such as (PO 4 ) 3− or (SO 4 ) 2− , may possibly play an Al-immobilizing role. Previous work on the impacts of 85 years of fertilization practices on pedological characteristics showed a large series of active soil processes: acidification, mineral dissolution, aluminization, neoformation of secondary phases, retrogression of phosphates or K via illitization of smectite, clay leaching, lixiviation, or temporary water logging [ 18 ]. In real field situations, study sites that offer such a wide panel of processes and physicochemical conditions united in a restricted area with initially identical soils are exceptional, if not nonexistent. Actually, due to continuous experimental constraints, the 42-plots experiment acts as a pedotron in real climatic conditions. Together with its historical soil archive, such a long-term bare fallow experiment enables access to synchronic and diachronic aspects of soil evolution on a centennial time scale. 5. Conclusions In this work, the soils of the 42-plots LTBF tell fascinating stories about historical dynamics of major and trace elements and constraint by continuous application of fertilizers and amendments, or by their absence. Taking account of modified physical properties in the 0–25 cm surface horizon during the experiment, stocks of elements were calculated for 1929 and 2014 and budgets were established. The relevance of our computational approach was checked by the equivalent losses of scandium and clay in Na-based fertilized plots, under the hypothesis of a predominant location of Sc in phyllosilicate minerals in loess soil. The budgets demonstrated clear gains of elements by fertilizer inputs (notably P, K, Ca, Na, Cd, and U) or atmospheric deposition in the reference plots (Na, Cd, Co, Cr, Mo, Pb, and Zn ). Additionally, the budgets gave insight into a series of diverging element losses controlled by different soil processes that were induced by changing physicochemical soil conditions over 85 years, which can be considered as a long-term time span in agronomic experimentation studies, but rather a short-term time span in soil development. Clay leaching was strongly favored in Na- and K-based fertilizers and coincided with consistent losses of Al, Fe, Mg, Sc, Ni, Cr, Tl, and As. In the acidified soils under ammonium-based fertilization, large losses were observed for Ca, Mg, Mn, Co, and Ni, whereas under basic amendments with high soil pH, a clear loss of Mo was demonstrated. However, elements were sometimes affected by both inputs and losses, i.e., Cd inputs by phosphate fertilizers and atmospheric deposition and its lixiviation in acid soil conditions, but often the geochemical budgets provided insight into the main processes. Calculation of decreasing contents of total and exchangeable Ca gave evidence for enhanced mineral weathering in the acid and reference plots, probably of feldspars, whereas relative accumulations of Al, Fe, Sc, and Tl (and As) in the acid plots pointed to mineral weathering of phyllosilicate. Presently, the 42-plots LTBF experiment shows a remarkable wide diversity of element dynamics in reaction to fertilization treatments. However, despite a strict and unchanged experimental plan, clear reactions were not found for all elements. For some elements, inputs may counterbalance losses. For elements such as Zn and Pb, the magnitude of inputs via atmospheric deposition masks their fate under the constraint of diverging physicochemical conditions. More comprehensive insight on Soil Syst. 2018,2, 23 18 of 20 element dynamics may be obtained from detailed study of samples from the historical soil archive, giving access to the chronology of geochemical changes of the soil surface. Acknowledgments: The authors greatly acknowledge all people who were, since 1928, or are still today in charge of the maintenance and technical exploitation of the 42-plots experiment: application of fertilizers, soil digging, sample collection and preparation, removal of weeds, etc. Their conscientious efforts have made it possible to develop our geochemical budget approach and test its validity. Financial support came from INRA and from the Agence de l’Environnement et de la Maîtrise de l’Energie ADEME. R. Paradelo thanks the Spanish Ministry of Economy and Competitiveness (MINECO) for a Ramón y Cajal fellowship (RYC-2016-19286). We thank Sébastien Breuil for sample preparation and Christian Lelay for thin section preparation. We are grateful for two anonymous reviewers for their positive reactions to our work and helpful suggestions. Author Contributions: Folkert van Oort conceived and designed the study; Folkert van Oort, Remigio Paradelo, Ghislaine Delarue, Nicolas Proix, Denis Baize and Fabrice Monna analyzed the experimental data; Nicolas Proix supervised the chemical analyses; Folkert van Oort wrote the paper with the help of Remigio Paradelo, Ghislaine Delarue, Nicolas Proix, Denis Baize and Fabrice Monna. Conflicts of Interest: The authors declare no conflict of interest. References 1. Lal, R. Restoring Soil Quality to Mitigate Soil Degradation. Sustainability 2015,7, 5875–5895. [CrossRef] 2. Paradelo, R.; van Oort, F.; Chenu, C. 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