Clay amended sandy soil – influence of clay concentration and particle size on nutrient availability and organic carbon content after plant residue addition

Abstract

Sandy soils have low water and nutrient holding capacity which limit crop growth. In the short-term, these constraints can be overcome by increased fertiliser application or irrigation. However, long-term solutions are needed to improve farm productivity and sustainability. Clay added to sandy soils may be such a solution. Compared to sandy soils, clay soils have smaller pores and higher cation exchange capacity and therefore greater water and nutrient holding capacity. Clay can also bind organic matter via cation bridges and thereby reduce its accessibility to decomposing microbes. In sandy soils with clay subsoil, the clay can be mixed into the sandy top soil by delving or spading. However, the clay subsoil is not uniformly distributed in the sandy top soil. It forms clay peds of varying size which creates a highly non-uniform soil environment with patches of sandy soil with clay peds next to sandy soil with little or no clay. The clay-rich patches can hold more water and nutrients compared to the surrounding sandy soil that could influence nutrient availability and organic C binding. Little is known about the influence of ped size and rate of clay added to sandy soil on nutrient availability and organic C binding after residue addition. The aims of the study were i) to determine the effect of clay addition rate and ped size in residue amended sandy soil on soil respiration, nutrient availability and organic C retention ii) to assess the effect of clay soil particle size and clay soil properties on nutrient availability and organic C binding after addition of residues with low or high C/N ratio iii) to determine the effect of clay addition rate and ped size on nutrient leaching after mineral fertilizer addition. A series of incubation experiments were carried out to assess the effect of clay addition rate and ped size on nutrient availability and organic C retention on < 53 μm fraction after mixing with low and high C/N ratio residue. In the first study, clay peds of 1, 2 or 3 mm size derived from a clay-rich Vertosol (73% clay) were added to a sandy soil (3% clay) at clay addition rates of 10% and 20% w/w. After addition of ground mature faba bean residue (C/N 37) at 10 g kg-1, the soils were incubated for 45 days at 80% of water holding capacity. Clay addition to sandy soil influenced nutrient availability after plant residue addition, particularly when small peds are added at higher rates. Sandy soil with clay peds had a greater maximum NH4 and P sorption capacity than sandy soil alone, sorption capacity was higher at 20% compared to 10% clay addition and greater with 1 mm than 3 mm peds. Retrieval of clay peds at the end of the experiment showed ped breakdown during the experiment but also formation of larger peds. Compared to the < 53 μm fraction added at the start of the experiment, total organic carbon (TOC) content of the < 53 μm fraction was up to two-fold higher, particularly in the smaller peds (1 and 2 mm). The study confirmed that claying can increase organic C sequestration, but also showed that organic C sequestration is likely to be greatest when the added clay peds are small. The capacity to bind organic C and nutrients may depend on clay soil properties such as mineralogy, clay concentration and exchangeable Fe and Al. A 45-day experiment was carried out to investigate the effect of clay type on nutrient availability and organic carbon retention with residues differing in C/N ratio (20 or 47). Two clay soils with smectite as a dominant mineral were used. They differed in smectite percentage [high (40%) or low (5-10%)], clay content (73 or 42%) and exchangeable Fe and Al concentration (low or high). The clay soils were added to sandy soil at rate of 20% w/w either finely ground or as 2 mm peds. Over 45 days, available N and P, microbial biomass N and P concentrations and cumulative respiration were greater with low C/N than high C/N residue. With low C/N residue, compared to sandy soil alone clay addition increased available N concentration and initial microbial biomass C and N, but decreased cumulative respiration and P availability. This study showed that addition of clay soil to sandy soil influences nutrient availability, but there were no clear differences between clay soils or sizes. The lack of differences between high and low smectite clay soil suggests that a high concentration of Fe and Al oxides can compensate for a lower clay concentration and proportion of smectite with respect to binding of organic matter and nutrients. In the previous studies, we found that clay addition had no consistent effect on cumulative respiration and ped size effect was variable. Secondly, low C/N ratio residue had stronger effect on nutrient availability due to its high decomposition rate and nutrient release compared to high C/N ratio residue. The third experiment was conducted to investigate that if clay addition has a different effect on respiration and nutrient availability when added as peds with a greater range of sizes (1, 3 and 5 mm) in presence of plant residue with lower C/N ratio. The aims of this experiment were to (i) determine the effect of clay addition rate and ped size in residue amended sandy soil on nutrient availability, and (ii) assess breakdown of peds during the experiment and organic C retention by the < 53 μm fraction of the peds. Clay soil addition to sandy soil amended with plant residue reduced respiration rate and available P concentration. Ped size had little effect on respiration and nutrient availability. Clay soil addition increased soil organic carbon retention compared to sandy soil alone. With respect to ped size, the experiment showed substantial ped breakdown and but also formation of larger peds over 45 days. The first three experiments were conducted over 45 days. But longer term studies are needed to better evaluate the effect of claying in the field. To investigate the effect of repeated addition of residue (finely ground wheat mature shoots added every 2 months) in clay amended sandy soil, a longer term (8 months) study was conducted with clay soil added as finely ground soil, 1 and 3 mm peds. The organic C content of the whole soil increased during the experiment with a greater increase in clay amended soils. The organic C content of the > 53 μm fraction was very low and changed little over time. With finely ground clay soil and 1 mm peds, the organic C content of the < 53 μm fraction increased mainly in the first 2 months while in 3 mm peds it increased over 6 months to reach similar concentrations as with finely ground clay soil and 1 mm peds. Excessive use of fertilizer in sandy soils can cause leaching of nutrient elements N and P into water ways and cause eutrophication. In the fifth experiment, clay soil was added in sandy soil at 10% or 20% clay soil w/w finely ground or as 2 and 5 mm peds with and without N and P fertiliser (27 mg N kg-1 and 7 mg P kg-1). The clay sand mixture (30 g) was placed in cores with nylon mesh at the bottom. The soils were incubated at 80% water holding capacity and leachate was collected weekly for 50 days. Clay addition significantly reduced leaching of N and P as compared to sandy soil alone. In sandy soil alone, the highest N (68%) leaching occurred after the first week whereas the highest amount (41%) of P was leached after two weeks. It can be concluded that clay addition to sandy soil can reduce the risk of nutrient leaching and enhance carbon sequestration in sandy soils by decreasing C loss via respiration and leaching. This effect will be greatest with finely ground clay soil or small peds.