Organic carbon, water repellency and soil stability to slaking under different crops and managements: a case study at aggregate and intra-aggregate scales

SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | SOIL Discuss., 1, 295–325, 2014 www.soil-discuss.net/1/295/2014/ doi:10.5194/soild-1-295-2014 © Author(s) 2014. CC Attribution 3.0 License. This discussion paper is/has been under review for the journal SOIL. Please refer to the corresponding final paper in SOIL if available. Organic carbon, water repellency and soil stability to slaking under different crops and managements: a case study at aggregate and intra-aggregate scales A. Jordán1, J. García-Moreno1, Á. J. Gordillo-Rivero1, L. M. Zavala1, and A. Cerdà2 1MED_Soil Research Group, Department of Crystallography, Mineralogy and Agricultural Chemistry, University of Seville, 1, 41012, Sevilla, Spain 2SEDER Research Group, Department of Geography, University of Valencia, Blasco Ibáñez, 10, 46010, Valencia, Spain Received: 22 July 2014 – Accepted: 11 August 2014 – Published: 29 August 2014 Correspondence to: A. Jordán ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 295 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract This research studies the distribution of organic C and intensity of water repellency in soil aggregates with different size and in the interior of aggregates from Mediterranean soils under different crops (apricot, citrus and wheat) and management (conventional tilling and no tilling/mulching). For this, undisturbed aggregates were sampled5 and carefully divided in size fractions (0.25–0.5, 0.5–1, 1–2, 2–5, 5–10 and 10–15mm) or peeled to obtain separated aggregate layers (exterior, transitional and interior). Organic C content in the fine earth fraction of soils under different crops did not show important variations, although it increased significantly from conventionally tilled to mulched soils. The distribution of organic C content in aggregates with different size10 varied among soils under different crops, generally increasing with decreasing size. At the intra-aggregate level, organic C concentrated preferably in the exterior layer of aggregates from conventionally tilled soils, probably because of recent organic inputs or leachates. In the case of mulched soils, higher concentrations were observed, but no significant differences among aggregate regions were found. The intensity of water15 repellency, determined by the ethanol method, did not show great variations among crops, but increased significantly from conventionally tilled to mulched soils. Coarser aggregates were generally wettable, while finer aggregates showed slight water repellency. Regardless of variations in the distribution of organic C in aggregate layers from conventionally tilled soils, great or significant differences in the distribution of water re-20 pellency at the intra-aggregate level were not found in any case. Finally, the intensity of water repellency was much more important than the concentration of organic C in the stability to slaking of aggregates. 1 Introduction Water repellency (WR) is a soil property that inhibits or delays water infiltration dur-25 ing periods of time varying between a few seconds and days or weeks. Inhibited or 296 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | delayed infiltration rates contribute to enhanced runoffflow, often increasing soil erosion risk (Doerr et al., 2000; Shakesby et al., 2000). Other important consequences are irregular soil wetting patterns, the development of preferential flow paths and accelerated leaching of nutrients (Blackwell, 2000; Leighton-Boyce et al., 2005; Ritsema and Dekker, 1994).5 Although low inputs of hydrophobic organic substances and high mineralization rates lead to low degrees of WR in cropped soils, it has been reported that conservative agricultural practices (reduced or no tilling, mulching treatments, etc.) may induce the development of soil WR (Blanco-Canqui and Lal, 2009; Buczko et al., 2006; García- Moreno et al., 2013; González-Peñaloza et al., 2012). New evidences show that sub-10 critical or slight WR are common states for many soils (Bodí et al., 2013; Buczko et al., 2006; Goebel et al., 2005; Hallett et al., 2001a; Lozano et al., 2013; Urbanek et al., 2007; Zavala et al.,2014). Many authors have studied the impact of WR at catchment, slope or plot scales (De- Bano, 2000; Doerr et al., 2000; Jordán et al., 2013). Nevertheless, comparatively few15 studies have been carried out at particle or aggregate scale. Some researchers have reported variations in persistence or intensity of WR among aggregates with different size. Intra-aggregate heterogeneity of physical, biological and chemical properties have been reported by Dexter (1988), Horn (1990), Horn et al. (1994) and, more recently, by Fan et al. (2013) and Urbanek et al. (2007). This heterogeneity conditions the trans-20 port of substances, microbial activity and biochemical processes, including changes in the amount, distribution and chemical properties of organic matter (Urbanek et al., 2007). Soil WR does not only have negative consequences. Some authors have reported positive relationships with, for example, aggregate stability (Mataix-Solera and Doerr, 2004) or carbon sequestration rates (Piccolo and Mbagwu, 1999).25 Studies focused on the intra-aggregate distribution of OC and WR are necessary to shed light on the soil processes at a detailed scale. The objectives of this research are to study (i) the OC content and the intensity of WR in aggregates of different sizes. (ii) the intra-aggregate distribution of OC and the intensity of WR and (iii) the structural 297 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | stability of soil aggregates relative to the OC content and the intensity of WR in soils under different crops (apricot, citrus and wheat) and different treatments (conventional tilling and mulching). 2 Methods 2.1 Experimental design and soil sampling5 Soil samples were collected from an experimental area in the province of Sevilla (Southern Spain). Climate is Mediterranean type, with warm dry summers and moderately wet cool winters. According to data from the nearby weather station Las Cabezas (25m a.s.l.; 37◦10N, 5◦530W), mean temperature is 17.6◦C, with monthly mean temperature ranging between 9.8 (January) and 26.0◦C (August). Annual mean rainfall10 is 449.4mm, with mean monthly rainfall ranging between 0.9 (July and August) and 79.3mm (November). Soils in the studied area are developed from calcareous sandstone, and are classified as Luvic Calcisols and Calcic Luvisols (WRB, 2006). For this study, soil plots under different crops were selected (apricot, citrus and wheat). At each case, two management types were considered: conventional tillage with moldboard15 plow and mulching (no-tilling and addition of wheat residues at rates varying between 5 and 8Mg ha−1year−1). At each sampling site, soil blocks (50cm long ×50cm wide ×10cm deep) were carefully collected to avoid disturbance of aggregates as much as possible and transported to the laboratory. At field moist condition, undisturbed soil aggregates were separated.20 Individual aggregates were arranged in paper trays and air-dried during 7 days under laboratory standard conditions. After air-drying, part of each sample was separated for different analyses: (i) part of the original samples was reserved for soil chemical and physical characterization; (ii) part of the aggregates were carefully measured with a caliper and separated in different size classes (0.25–0.5, 0.5–1, 1–2, 2–5, 5–10 and25 10–15mm) for determination of WR and OC content; (iii) aggregates 10–15mm in size 298 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | were selected for obtaining aggregate layers and determination of WR and OC content; finally, (iv) aggregates about 10mm in size were selected for assessing stability to slaking, WR and OC content. 2.2 Separation of aggregate layers Part of coarser aggregates were selected for obtaining aggregate layers. Aggregate5 layers were separated using the soil aggregate erosion abrasion chamber described by Park and Smucker (2005), shown in Fig. 1a. For this purpose, single air-dried aggregates (10–15mm) were placed in the abrasion chamber and rotated in a rotary shaker at 400rpm. The eroded material fell through a 340µm sieve and was collected in a retainer base chamber. During each experiment, the eroded material was weighted pe-10 riodically to obtain the exterior and transitional layers. Eroded material corresponding to these layers (Fig. 1b) was collected when the percentage of eroded mass reached 33.3±2 and 66.7 ±2%, respectively (Park and Smucker, 2005). 2.3 Soil analyses Part of air-dried soil samples were sieved (2 mm) to eliminate coarse soil particles15 and homogenized. Soil OC (OC) content was determined by the modified Walkley- Black method (USDA, 2004). Soil acidity (pH) was measured in aqueous soil extracted in de-ionised water (1 : 2.5 soil: water). Total nitrogen was measured by the Regular Macro-Kjeldahl method and C/N ratio was calculated. For texture analysis, air-dried soil subsamples were pre-treated with H2O2(6%) to20 remove organic matter and soluble salts, dried in the oven to obtain the initial weight, dispersed with a sodium hexametaphosphate solution, and mechanically shaken. The sand fraction (0.05–2mm) was removed from the suspension by wet sieving and then fractionated by dry sieving; the fine silt (0.002–0.02mm) and clay (<0.002mm) fractions were determined by the pipet method (USDA, 2004). Coarse silt (0.02–0.05 mm)25 299 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | was calculated as the difference between 100% and the sum of the sand, clay, and fine silt percentages. The intensity of WR was assessed using the ethanol percentage test (EPT). Drops (0.5µL) of decreasing ethanol concentrations (increasing surface tensions) were applied onto the soil surface with a micro-pipet until one of the drops balled out in the5 first 5 seconds after application. This allows the classification of the soil into a surface tension category between two ethanol concentrations. EPT classes were classified as in Doerr (1998): (1) very wettable (0.0 % ethanol), (2) wettable (3.0%), (3) slightly water repellent (5.0%), (4) moderately water repellent (8.5%), (5) strongly water repellent (13.0%), (6) very strongly water repellent (24.0%) and (7) extremely water repellent10 (36.0%). In order to study the relation between stability to slaking, WR and OC, 90 air-dried aggregates (about 10mm in size) selected per treatment (mulched or conventional tillage) and crop (apricot, citrus and wheat). Every set of aggregates was randomly divided in three groups (n=30) for assessing stability to slaking, WR and OC, respectively. For15 analysing stability to slaking, selected aggregates were placed on a 1.5 mm sieve and immersed in distilled water (20mm depth) during 5 min, and the time for 50 % loss of structural integrity was recorded. If structural integrity of aggregates is maintained after 5min, immersion was repeated 5 times and the soil material remaining on the sieve was dried and weighted. Stability to slaking was determined according to Herrick et20 al. (2001) (Table 1). 2.4 Data analysis The normal distribution of data was assessed using the Shapiro–Wilk test. When data fitted the normal distribution, data analysis included basic data descriptions (means and standard deviations, ANOVA). When data did not fit the normal distribution, non-25 parametric tests were applied (Wilcoxon test for comparison of median values, Spearman rank correlation coefficient). Differences between the intensity of WR in aggregate layers were assessed using the Kruskal-Wallis and the median Mood’s test, and it was 300 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | considered that significant differences existed when confirmed at least by one of these tests. All computations and graphical displays were performed using SPSS (IBM Corp., 2013). 3 Results 3.1 Characterization of studied soils5 The results of soil characterization (0–10cm) is shown in Table 2. Studied soils are neutral (pH 7.1±0.1, on average), with OC content varying between 1.35 and 1.55% (conventional tillage) and 4.60 and 5.25 (mulched soils). Soil texture varied between silt loam and silty clay loam, with average sand and clay contents 9.7 ±1.6 and 24.6±2.6 %, respectively.10 3.2 Organic C content and water repellency in aggregate size fractions Soil OC content from aggregate size fractions showed significant differences according to crop, treatment and size (Table 3). On average, OC content varied between 1.50±0.88 (wheat) and 2.00 ±0.93% (apricot). Mulching increased OC content from 1.00±0.35 (conventional tillage) to 2.49±0.57 % (mulched soils). OC content varied15 with size, with maximum value between 1.91±0.90 and 2.20±1.105 (size fractions 0.5–1 and 0.25–0.5mm, respectively). Figure 2 shows the distribution of OC content from soils under different crop and treatment per size fractions. The distribution of OC content from different crops under conventional tillage did not show any particular behaviour, with values ranging between20 0.62±0.25 (wheat, 5–10 mm) and 1.41±0.23 % (apricot, 0.25–0.5mm), on average. In contrast, OC content decreased whit increasing size in mulched soils under all crops. In this case, OC content varied between 2.44±0.11 (5–10 mm) and 3.46±0.22 % (0.25– 0.5mm) under apricot, 2.11±0.2 (10–15 mm) and 3.28±0.61 % (0.25–0.5mm) under citrus and 1.62 ±0.44 (10–15 mm) and 2.85±0.29 % (0.25–0.5mm) under wheat.25 301 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The intensity of WR did not show significant differences among crops, but varied significantly per treatment and size fraction (Table 4). Figure 3 shows the distribution of EPT values from soils under different crop and treatment per size fractions. The intensity of soil WR decreased with increasing size under all crops and treatments. Median EPT values generally varied between 2 (fractions between 2 and 15mm) and5 3 (fractions between 0.25 and 2mm), shifting from slightly water repellent to wettable between size fractions 1–2 and 2–5mm. In contrast, median EPT values from mulched soils under wheat were 2 (10–15mm), 4 (1–2, 2–5 and 5–12) and 5 (0.25–0.5 and 0.5–1mm). 3.3 Intra-aggregate distribution of organic C10 The distribution of OC content from aggregate layers varied with soil treatment. Table 5 shows the results of the ANOVA for OC content of soil samples from each crop and treatment for different aggregate layers. On average, OC content in aggregate layers from conventionally tilled soils varied between 0.34±0.13 (interior layer of aggregates from conventionally tilled soils under citrus) and 2.97±0.52 % (transitional15 layer of aggregates from mulched soils under apricot). In aggregates from soils under conventional tillage, the distribution of OC content decreased strongly between the exterior and interior layers. In citrus cropped conventionally tilled soils, for example, OC content decreased by 30.10%. In contrast, mulched soils did not show intra-aggregate variations, with average OC contents of 2.93±0.50 (apricot), 2.75 ±0.69 (citrus) and20 2.27±0.61 % (wheat). 3.4 Intra-aggregate distribution of water repellency In general, the intensity of soil WR from aggregate layers of soil samples under different crops and treatments varied between EPT=1 (very wettable) and 4 (moderately water repellent). The range of EPT values was 1–3 (median 2–2.5) in conven-25 tionally tilled soils) and 1–4 (median 2–3) in mulched soils. Table 6 shows the results 302 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | of the Kruskal–Wallis and Mood’s median tests. Results show that significant differences among EPT median values from different layers were found only in aggregates from conventionally tilled soils under wheat and mulched soils under apricot. In the first case, median EPT varied between 2 (interior and transitional layers) and 2.5 (exterior layer). Although this difference is considered significant, it only implies a jump between5 wettable and wettable to slightly water-repellent classes and has not any hydrological meaning. 3.5 Slaking stability and relation with water repellency and organic C content Median values of stability to slaking determined in aggregates (∼10mm) from soil samples under different crops and treatments are shown in Table 7. Stability to slaking10 varied between crops and treatments. Median slaking values varied between 3 (apricot and citrus) and 4 (wheat) in conventionally tilled soils and between 4 (apricot and citrus) and 5 (wheat) in mulched soils. In all cases, stability to slaking in mulched soils was 1 unit greater than in conventionally tilled soils. Table 8 shows the R-Spearman coefficients for slaking/EPT, slaking/OC content and15 EPT/OC content. When all cases are considered together, stability to slaking was significantly correlated with EPT (R-Spearman=0.8699). Significant positive correlations were found between stability to slaking and EPT in all cases, except for aggregates under wheat and conventional tillage. No significant correlations were found between stability to slaking and OC content or WR and OC content in aggregates under dif-20 ferent crops and treatments, except when all cases were considered together (0.5245 and 0.4317, respectively). 303 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | of the redistribution of hydrophobic substances between and within micro-and macroaggregates, as well as physical, chemical and biological processes involved. Acknowledgements. Alfonso Mazuelos (Dept. of Chemical Engineering, University of Seville) allowed us to use his laboratory and kindly offered his generous assistance in some of the experiments. The authors are also thankful to Antonio Jordán senior, who helped with the5 design and making of micro-sieves for the slaking test and Luis F. Pérez (IFAPA Las Torres- Tomejil). This study has been funded by Spanish Ministry of Economy and Competitiveness through the research projects “HYDFIRE: Water repellency in Mediterranean fire-affected soils. Involved factors, temporal changes and implications for hydrology and soil system function-10 ing” (CGL2010-21670-C02-01) and “GEOFIRE: Geochemical alterations in fire affected soils” (CGL2012-38655-C04-01) and the Andalusian Regional Ministry for Science (2011/RNM-364). References Amelung, W. and Zech, W.: Organic species in ped surface and core fractions along a climosequence in the prairie, North America. Geoderma, 74, 193–206, 1996.15 Benito, E., Santiago, J. L., de Blas, E., and Varela, M. E.: Deforestation of water-repellent soils in Galicia (NW Spain): effects on surface runoffand erosion under simulated rainfall, Earth Surf. 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Simon, T., Javurek, M., Mikanova, O., and Vach, M.: The influence of tillage systems on soil30 organic matter and soil hydrophobicity, Soil Till. Res., 105, 44–48, 2009. 313 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Urbanek, E., Hallett, P., Feeney, D., and Horn, R.: Water repellency and distribution of hydrophilic and hydrophobic compounds in soil aggregates from different tillage systems, Geoderma, 140, 147–155, 2007. USDA: Soil survey laboratory methods manual. Soil survey investigation report No. 42. Version 4.0. US Department of Agriculture-NCRS: Lincoln, NE, 2004.5 WRB: World reference base for soil resources 2006, 2nd Edn., World Soil Resources Reports No 103, Food and Agriculture Organization of the United Nations, Rome, 2006. Zavala, L. M., Granged, A. J. P., Jordán, A., and Bárcenas-Moreno, G.: Effect of burning temperature on water repellency and aggregate stability in forest soils under laboratory conditions, Geoderma, 158, 36–374, 2010.10 Zavala, L. M., García-Moreno, J., Gordillo-Rivero, A. J., Jordán, A., and Mataix-Solera, J.: Natural soil water repellency in different types of Mediterranean woodlands, Geoderma, 227–227, 170–178, 2014. 314 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 1. Criteria for classification of stability to slaking (Herrick et al., 2001). Slaking Criteria for each slaking class class 0 50% of structural integrity is lost immediately after immersion. 1 50% of structural integrity is lost 5s after immersion. 2 50% of structural integrity is lost 5–30s after immersion. 3 50% of structural integrity is lost 30–300s after immersion or <10% of soil material remains on the sieve after 5 immersion cycles. 4 10–25% of soil material remains on the sieve after 5 immersion cycles. 5 25–75% of soil material remains on the sieve after 5 immersion cycles. 6>75% of soil material remains on the sieve after 5 immersion cycles. 315 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 2. Characterization of studied soils in the 0–10 cm layer. SD: standard deviation. Crop Treatment pH Organic C (%) Sand (%) Silt (%) Clay (%) Apricot Conventional tillage 7.0 1.55 7.4 69.4 23.2 Apricot Mulch 7.1 4.60 10.2 66.7 23.1 Citrus Conventional tillage 6.9 1.40 11.2 60.6 28.2 Citrus Mulch 7.1 5.25 11.7 65.4 22.9 Wheat Conventional tillage 7.2 1.35 8.7 63.7 27.6 Wheat Mulch 7.2 4.85 8.9 68.6 22.5 Mean±SD 7.1±0.1 3.2±1.9 9.7±1.6 65.7±3.3 24.6±2.6 316 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 3. Results of the ANOVA for organic C content by factors crop, treatment and size fraction. At each group, mean values followed by the same letter did not show significant differences. Factor Group NMean±standard deviation ANOVA, pvalue Crop Apricot 60 2.00±0.93 b 0.0062 Citrus 60 1.74±0.89 ab Wheat 60 1.50±0.88 a Treatment Conventional tillage 90 1.00 ±0.35 0.0000 Mulch 90 2.49 ±0.57 Size fraction 0.25–0.5mm 30 2.20±1.10 b 0.0159 0.5–1mm 30 1.91±0.90 ab 1–2mm 30 1.66±0.77 a 10–15mm 30 1.42±0.72 a 2–5mm 30 1.69±0.86 a 5–10mm 30 1.49±0.76 a 317 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 4. Results of the Kruskal–Wallis analysis of EPT data by factors crop, treatment and size fraction. Factor Group NMedian Minimum Maximum Kruskal–Wallis pvalue Crop Apricot 60 3 2 4 >0.05 Citrus 60 3 2 4 Wheat 60 3 2 5 Treatment Conventional tillage 90 2 1 3 0.0000 Mulch 90 4 2 5 Size fraction 0.25-0.5mm 30 3.5 3 5 0.0000 0.5–1mm 30 3.5 3 5 1–2mm 30 3 2 5 10–15mm 30 2 1 3 2–5mm 30 2.5 2 4 5–10mm 30 2.5 2 4 318 SOILD 1, 295–325, 2014 Organic C and water repellency distribution in soil aggregates A. Jordán et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 5. Results of the ANOVA for organic C content (OC %, mean±standard deviation) of soil samples from each crop and treatment for different aggregate layers. Mean values followed by different letters showed significant differences for the same use and treatment. N=30 for each case. Crop Treatment Layer OC% ANOVA, pvalue Apricot Conventional tillage Exterior 1.25±0.38 c <0.0001 Transitional 0.94±0.29 b Interior 0.59 ±0.20 a Mulch Exterior 2.93±0.49 a >0.05 Transitional 2.97±0.52 a Interior 2.88 ±0.50 a Citrus Conventional tillage Exterior 1.03 ±0.35 c <0.0001 Transitional 0.79±0.29 b Interior 0.34 ±0.13 a Mulch Exterior 2.77±0.66 a >0.05 Transitional 2.73±0.72 a Interior 2.76 ±0.73 a Wheat Conventional tillage Exterior 0.96 ±0.38 c <0.0001 Transitional 0.71±0.28 b Interior 0.43 ±0.19 a Mulch Exterior 2.28±0.54 a >0.05 Transitional 2.24±0.60 a Interior 2.28 ±0.71 a 319