Water-holding capacity and plant growth in compost-based substrates modified with polyacrylamide, guar gum or bentonite

1 Water-holding capacity and plant growth in compost-based substrates 1 modified with polyacrylamide, guar gum or bentonite 2 R. Paradelo*, R. Basanta, M.T. Barral 3 Departamento de Edafoloxía e Química Agrícola, Universidade de Santiago de 4 Compostela. Facultade de Farmacia, Praza Seminario de Estudos Galegos s/n, Campus 5 Sur, 15782 Santiago de Compostela (Spain) 6 7 *Corresponding author. Remigio Paradelo Núñez. Phone: +34 881 815 042. E-mail: 8 [email protected] 9 10 11 Abstract 12 Humectant agents are used to improve seed germination and survival in hard 13 environments through their effect on water-holding capacity. In this work, we have 14 evaluated the combined effect of substrate composition and humectant agents on the 15 water-holding capacity and growth of Lolium multiflorum in different mixtures based on 16 municipal solid waste composts. A commercial polyacrylamide, bentonite or guar gum 17 have been added to mixtures of two municipal solid waste composts with either manure 18 vermicompost or pine bark compost. Water-holding capacity of the substrates and plant 19 production after five weeks in the greenhouse were evaluated. Only bentonite increased 20 water retention of the substrates, likely because the compost-based substrates already 21 presented high water retention. None of the humectant agents had significant effects on 22 the growth of Lolium multiflorum. The use of bentonite can be recommended for 23 increasing water-holding capacity of organic substrates in this case, but given the 24 2 overall small effects of the humectant agents, an adequate choice of the organic 1 materials used could suffice for the formulation of adequate plant substrates in this case. 2 Keywords: Water-holding capacity; Municipal solid waste; Compost; Hydrogel; 3 Polyacrylamide; Bentonite; Guar gum. 4 5 1. Introduction 6 The use of soilless substrates has been a constant in commercial horticulture during the 7 last decades and recently, a new field of application for these substrates has been found 8 in urban agriculture, in particular in green roofs or vegetal gardens (Farrell et al., 2013; 9 Young et al., 2014; Grard et al., 2015; Matlock and Rowe, 2017). Due to the 10 environmental issues associated to the extraction of peat, which was traditionally the 11 main component in most substrates, the search for alternative components has been a 12 subject of abundant research during the last decades (Abad et al., 2002). Special 13 attention has been paid in this sense to the use of composts, urban wastes and other 14 organic materials of residual origin (Barral et al., 2007; Paradelo et al., 2009, 2012b, 15 2016). In the new scenario of increasing (or at least sustained) demand for soilless 16 substrates due to the requirements of urban agriculture, research on the use of residual 17 materials and the main properties of the substrates produced is still necessary. 18 For almost all applications of soilless substrates, water retention is a decisive factor 19 due to its influence in seed germination and plant growth. This is specially true in some 20 applications where severe conditions that limit water availability exist. For example, in 21 green roofs, the substrate layer is often restricted to a depth of 10 cm or less. Other 22 problems are for example due to irrigation limitations, high rate of desiccation due to 23 exposure and free draining, so water stress is one of the most common limitations for 24 plant growth on green roofs (Farrell et al., 2013; Young et al., 2014). Plant production 25 3 under these conditions is challenging in what concerns water supply to seeds or plant 1 during implantation, as it is in other contexts where water retention is limited, such as in 2 post-fire environments or minesoils. 3 The extreme relevance of water supply in these cases makes the study of water- 4 holding capacity and potential increase in substrate formulations an essential issue. In 5 this sense, the use of humectant agents for increasing water retention in soilless 6 substrates is an interesting subject of research, even for materials with relatively high 7 water-holding capacity. Water-retention additives have the potential to increase 8 substrate water availability leading to greater plant growth and survival without the 9 need for large amounts of extra growing media (Martin and Szort, 2001; Farrell et al., 10 2013). These include substances of diverse nature and origin, either organic or inorganic, 11 natural or artificial. Natural materials employed include clays, which have been 12 traditionally used for the amendment of sandy soils in many arid and semiarid areas of 13 the world (Ajayi and Horn, 2016), and natural polymers such as guar gum (Patil et al., 14 2011). Among the artificial additives, hydrogels are among the most widely employed 15 for agricultural, horticultural or forestry applications (Hüttermann et al., 2009; Agaba et 16 al., 2010; Shi et al., 2010; Kabiri et al., 2011; Paradelo et al., 2012a; Crous, 2017); they 17 are cross-linked crystalline forms of insoluble polyacrylamide gel that absorb and store 18 water up to 500 times their own weight when saturated (Johnson, 1984). However, 19 information regarding combinations of humectants and compost for use as substrates is 20 still scarce. 21 In this work, we have studied the possibility of recycling municipal solid waste 22 (MSW) composts as components of substrates, focusing on water retention. However, 23 previous studies have shown that substrates based solely on MSW compost may be 24 inadequate for plant production due to high salinity and phytotoxicity (Moldes et al., 25 4 2006; Barral et al., 2007), so we have blended MSW composts with composted pine 1 bark or manure vermicompost, that are easily available in the region and do not present 2 plant toxicity. We have assessed the water-holding capacity of these mixtures with or 3 without three substances with high water retention as additives (polyacrylamide, 4 bentonite and guar gum). Given that for substrate applications it must be assured that no 5 plant harm exist, potential phytotoxicity of the substrates produced has been assessed by 6 a growth trial using Lolium multiflorum. 7 8 2. Materials and Methods 9 2.1 Composts 10 Four composts were used for the experiment. MSWC1 is an compost obtained from 11 aerobic treatment of source-separated biodegradable fraction of municipal solid waste 12 (MSW); MSWC2 is a compost obtained by anaerobic fermentation of the biodegradable 13 fraction of MSW, separated before collection, followed by an aerobic composting step, 14 to stabilize the incompletely digested residue. Both MSW composts were provided by 15 industrial composting facilities located in A Coruña (Spain). MV is a mixed manure 16 vermicompost supplied by a local producer in Galicia (Spain). CPB is composted pine 17 bark produced by aerobic composting in windrows, supplied by Costiña (O Pino, A 18 Coruña, Spain). Their main properties are summarized in Table 1. 19 20 2.2 Humectant agents 21 Three humectant agents were employed in the study: a commercial polyacrylamide 22 (Alcosorb® Pearl) supplied by Ciba Especialidades Químicas S.L. (Barcelona, Spain); 23 bentonite, a commercial mixture of smectitic clays supplied by Minas de Gador S.L. 24 (Almería, Spain); and guar gum, supplied by Sigma-Aldrich (Madrid, Spain);. 25 5 1 2.3 Substrates 2 A total of 28 different mixtures with three replicates per mixture were prepared 3 following the scheme summarized in Figure 1. Each MSW compost was blended either 4 with manure vermicompost or with composted pine bark, producing four mixtures 1:1 5 (on a volume basis). With each of these four blends, three series were prepared with 6 different doses of each of the three humectant agents: dose 0 (no humectant agent 7 added); dose 1 (1 g L-1 of guar gum or Alcosorb or 100 g L-1 of bentonite); and dose 2 8 (10 g L-1 of guar gum or 3 g L-1 of Alcosorb or 300 g L-1 of bentonite). These doses 9 were selected to match those used in similar studies in the literature. 10 11 2.4 Analyses of the substrates and greenhouse experiment 12 Water retention at pF 2 (0.1 bar) was determined using a Richards membrane extractor 13 following the method described by Guitián and Carballas (1976). For the growth 14 experiment, pots, 12 cm diameter and 7 cm high, were filled with 400 cm3 of each 15 mixture in three replicates, and 50 seeds of Italian ryegrass (Lolium multiflorum) were 16 sown in each pot and transferred to the greenhouse. A multinutrient solution (Welgro 17 Standard Plus commercial; Química Masso S.A., Barcelona, Spain) containing 17% N, 18 15% K2O, 30% P2O5, 0.13% Fe, 0.052% Mn, 0.06% Zn, 0.02% B and 0.005% Mo, was 19 added twice to the pots in order to obtain final concentrations of 220 mg N L-1, 160 mg 20 K L-1 and 170 mg P L-1. In order to avoid heat and water stress, pots were watered daily 21 during the experiment. At the end of the growth period (five weeks) the plants were 22 harvested by cutting them off between the root and stalk, and the fresh and dry (65ºC) 23 weight of the shoots were recorded. 24 25 6 2.5 Statistics 1 Four factors were included in the statistics for the study of the effect of substrate 2 composition and humectant agents on water-holding capacity and plant production: type 3 of MSW compost (factor 1, two levels), use of either manure or pine bark (factor 2, two 4 levels), humectant agent (factor 3, four levels) and dose of humectant agent (factor 4, 5 three levels). Linear regression model analysis and ANOVA for each individual factor 6 plus multi-way analysis of variance for the interactions between factors were performed 7 using the R statistical package for MacOSX (R Core Team 2015). 8 9 3. Results and discussion 10 All the substrates presented high water holding capacity values irrespective of the 11 materials employed in their formulation (Figure 2), as expected given their 12 predominantly organic composition, with values ranging from 83 to 151 g 100 g-1. 13 There were no significant differences between the two MSW composts, but blending 14 them with pine bark increased water holding capacity to a greater extent than manure 15 vermicompost (Table 2). A significant effect of the humectant agent employed as well 16 as the dose employed was also found: bentonite was the most effective to increase water 17 holding capacity, followed by the polyacrylamide, whereas the highest rate (dose 2) was 18 more effective than the lowest one. In addition to the significant effect found for these 19 individual factors, significant interactions were also found between them: it is 20 noteworthy that, while the choice of MSW compost (factor 1) did not influence water 21 holding capacity by itself, it did influence the effect of the rest of factors on this 22 property. Also, the significant interaction between factors 3 and 4 show that the positive 23 effect of humectant agents in water holding capacity depends both on their nature and 24 their dose. 25 Comentado [RPN1]: Reviewer #1: Table 2 was updated to show that there are statistically significant interactions. However, the authors have not addressed these interactions or their meaning in their discussion. COMMENTS ON INTERACTION FACTORS ADDED 7 In any case, the increments in water holding capacity by the humectant agents were 1 small and it would have been expected to observe larger effects, as per previous works 2 where increases have been mainly observed using similar additive doses. This is the 3 case for guar gum, that has been shown to increase water-holding capacity in soils and 4 soil-less media (Patil et al., 2011; Chandrika et al., 2014; Thombare et al., 2018), clays 5 and specially bentonite (Ajayi and Horn 2016) and polyacrylamides and similar 6 hydrogels in soils (Akhter et al., 2004; Geesing and Schmidhalter, 2004; Agaba et al., 7 2010; Farrell et al., 2013) and substrates (Young et al., 2014). Among the probable 8 causes of this lower-than-expected performance, the nature of the substrate/soil to 9 which the humectant agent is added could decisively affect their performance in many 10 cases. In this sense, it has been observed that the effects of water retention additives are 11 more important in soils than in soilless substrates, and within soils, in those coarse- 12 textured soils with respect to heavy soils. For example, Agaba et al. (2010) found that 13 0.4% hydrogel amendment significantly increased the available water by a factor of 14 about three in sand, but only one fold in sandy loam, loam and clay soils, whereas 15 Chandrika et al. (2014) observed that the increase of water retention due to hydrogel 16 was less pronounced in an organic soil-less medium than in a soil. These observations 17 suggest that the magnitude of the effect of additives is lower when the substrate has 18 previous high water-holding capacity, and imply that the major components of the 19 substrate may be as important for water holding capacity as humectant agent used. 20 In the case of plant production in the greenhouse experiment (Figures 3 and 4), 21 significant differences were also observed due to the composition of the substrates. 22 Regarding the MSC composts, MSWC1 produced significantly lower yields than 23 MSWC2. This fact is likely due to a negative effect of the high salinity of the first 24 compost, as shown by its high electrical conductivity (Table 1). Blending the composts 25 Comentado [RPN2]: Reviewer #1: Table 2 was updated to show that there are statistically significant interactions. However, the authors have not addressed these interactions or their meaning in their discussion. COMMENTS ON INTERACTION FACTORS ADDED 8 with pine bark produced better results for plant growth than manure; in this case, this 1 difference is attributed to the observed higher water holding capacity of the pine bark 2 substrates (Figure 2). In turn, the addition of the humectant agents had no significant 3 effect on plant productivity, at either dose employed. Significant interactions of factors 4 1 and 2 between them and with the dose of humectant were found, what reinforces the 5 idea that the choice of components for the substrate was by far the most important factor 6 for their productivity. Overall, even if a positive effect of the additives was not evident, 7 the lack of negative effect is convenient because, at least at the doses employed here, 8 indicates that water-holding capacity can be increased without negatively affecting 9 plants. 10 In contrast to water retention, where increases due to additives are the rule, the effect 11 of humectant agents in plant productivity is much less obvious in the literature. Farrell 12 et al. (2013) have stated that the effects of hydrogel on plant growth and yield are often 13 contrasting, even within the same species and substrate, and Geesing and Schmidhalter 14 (2004) observed ether increases or decreases in wheat yield in hydrogel-amended soils, 15 depending on the level of water stress. Yet, beneficial effects of these additives are not 16 unusual. Akhter et al. (2004) found that the addition of 0.1-0.3% hydrogel increased 17 seedling growth of wheat and barley, Patil et al. (2011) observed positive effects of guar 18 gum, hydrogels and other polymers in germination and seedling growth, whereas Agaba 19 et al. (2010) found that the addition of either 0.2 or 0.4% hydrogel to five soil types 20 resulted in prolonged tree survival. Those authors who have observed positive effects of 21 plant growth and/or development have generally related them to the increase in water 22 holding capacity (Young et al., 2014), and in this sense, significant correlations have 23 been found between both effects (Agaba et al., 2010). In any case, given the complexity 24 9 of the mechanisms involved in seed germination and plant growth, it is not surprising 1 that mixed results have been observed in the literature. 2 What is clear, even more than in the case of water-holding capacity, is that that the 3 difference in performance between the substrates owes more to the nature of their 4 components than to the presence of additives. This is in agreement with the opinion of 5 other researchers, based on similar experiments. Farrell et al. (2013), for example, 6 highlighted that the effectiveness of water-retention additives for increasing plant 7 growth and plant available water is dependent on both substrate and additive properties 8 and Matlock and Rowe (2017) also observed that compost selection have a strong effect 9 on water-holding capacity and initial plant establishment. Young et al. (2014) noted that, 10 despite the relevant and positive effects of water retention agents on water holding 11 capacity, the composition of the substrate is often even more decisive and therefore a 12 careful choice of the components can reduce or even eliminate the need for additional 13 expensive agents. Consequently, substrate composition should be considered carefully 14 as a previous step in order to take the decision of using or not humectant agents, which 15 can increase the cost of substrate production. 16 17 4. Conclusions 18 In compost-based substrates prepared with or without three water retention additives 19 (polyacrylamide, bentonite or guar gum), the presence and dose of the humectant agents 20 and the type of compost used had significant effects on water-holding capacity, but only 21 the type of compost produced positive effects in the growth of Lolium multiflorum. The 22 high salinity of some composts produced negative effects on plant growth and must be 23 taken into account when preparing substrates. Blending municipal solid waste compost 24 with pine bark was more effective in increasing water retention than manure, and also 25 16 Table 2. Results of ANOVA for the effect of each substrate component, humectant 1 agent and dose on water holding capacity and productivity of Lolium multiflorum. 2 MSW: municipal solid waste. Significance of the effect of factors is indicated as 3 follows: * significant at a P-value of 0.05; ** significant at a P-value of 0.01; *** 4 significant at a P-value of 0.001. 5 Water holding capacity Dry productivity Fresh productivity Factor F-value Pr(>F) F-value Pr(>F) F-value Pr(>F) F1 (MSW compost) 0.21 0.65 53.7 <0.001*** 57.2 <0.001*** F2 (Manure/Pine bark) 144.8 <0.001*** 21.6 <0.001*** 21.2 <0.001*** F3 (Humectant agent) 2.87 0.042* 0.16 0.85 1.92 0.13 F4 (Dose of humectant agent) 7.44 0.001** 1.72 0.16 0.08 0.92 Interactions F1*F2 18.5 <0.001*** 46.9 <0.001*** 60.5 <0.001*** F1*F3 3.25 0.046* 7.40 0.001** 10.7 <0.001*** F1*F4 5.64 0.006** 5.62 0.006** 7.74 0.001** F2*F3 6.69 0.002** 1.66 0.20 2.24 0.12 F2*F4 2.84 0.07 4.22 0.02* 5.00 0.01* F3*F4 17.1 <0.001*** 2.78 0.07 1.38 0.26 6 7