Effects of drying-rewetting, previous and current soil water content on soil respiration, microbial biomass and nutrient availability in soils without or with plant residues differing in C/N ratio

Abstract

Soil water content is a major factor influencing organic matter turnover and nutrient cycling through its effect on microbial activity, either directly or by modifying substrate availability. Organic soil amendments e.g. plant residue can improve soil fertility. The release of nutrients from plant residue is a complex process. Residue decomposition requires water for microbial growth and for diffusion of nutrients and by-products during decomposition. Recent studies showed that nutrient availability and microbial biomass after the addition of a second residue are influenced by the C/N ratio of the first residue amendment, which is referred to as a legacy effect. However, little is known about the effect of drying-rewetting (DRW) and variable soil water content on nutrient availability in soil amended with residues differing in C/N ratio and on the legacy effect of the previous residue addition. A better understanding of residue decomposition is important to help managing soil fertility using plant residues. The aims of the study were to determine how soil respiration, microbial biomass and nutrient availability and the legacy effect are influenced by i) Drying and rewetting (DRW) cycle frequency between the first and second residue addition and residue addition upon rewetting, ii) current and previous water content after rewetting with three rewetting events iii) soil water content between the first and second residue addition and number of days between rewetting, and iv) previous and current soil water content in soil amended with residue differing in C/N ratio. In the first experiment, soil was amended twice (days 0 and 32) with plant residues with either high (H) or low C/N ratio (L) giving treatments LH or HL. Soil was incubated for 64 days. Between the first and the second residue addition (day 0-32) the soil was maintained at 50% WHC or exposed to one, two or four DRW cycles. All treatments were kept at 50% WHC (optimal for soil respiration) during the second period (day 33-64). During the first period, N, P availability and microbial biomass C were higher in LH than HL in all moisture treatments. Cumulative respiration was higher in LH than HL only in the constantly moist control. After the second residue addition compared with other water regimes, four DRW cycles stimulated decomposition of the low C/N ratio residue added on day 32, but the effect was transient as moisture treatment did not influence available N and MBN from day 48 onwards (8 days after the second residue amendment). The study showed that drying can temporarily increase N and P availability and reduce soil respiration, but after rewetting there was little difference to the constantly moist soil. Further, DRW between residue additions had little effect on the legacy effect of the first residue addition. In the second experiment, soil was exposed to two wet-dry cycles with 5 days moist and 5 days dry each. Residues with high (H) or low C/N ratio (L) were added in eight residue treatments at different rates (10 or 20 g kg-1 soil) and timing (day 0 or day 10, before rewetting). Available N and P on day 11 were similar as on day 1 suggesting that if residues are added upon rewetting, nutrient release is not greater than if residues are added to moist soil. However, in the treatment where L had been added only on day 0, rewetting of dry soil induced N release from partially decomposed L residue left in the soil from the first period. When H was added to moist soil on day 0, MBN on days 1 and 5 was higher than in unamended soil. But when H was added on day 10, MBN increased only on day 11 indicating that with H microbial utilisation of residue N may be restricted if addition of residue was combined with rewetting. From day 11 to 20, MBN, available N and P were lower in LH than in HL, suggesting that the second residue had a strong effect and thus the legacy effect was weaker than if the soil was moist throughout the experiment. Experiments 1 and 2 showed that DRW had little or no influence on the legacy effect of the first residue addition. In the third experiment, we investigated how current and previous water content after rewetting influences soil respiration, microbial biomass and nutrient availability in unamended soils. Soil was exposed to two wet-dry cycles (5 days moist, 3 days dry) with soil being rewetted to 50%, 30% or 10% WHC on days 0 and 8. All treatments were rewet to 50% WHC on day 16 and maintained at this water content for 7 days (day 23). The flush of respiration after the first two rewetting events was more than two-fold higher with 50% than 10% WHC and the second flush was about five times lower than the first. After rewetting of all treatments to 50% WHC on day 16, the flush was three-fold greater in soil previously rewet to 10% WHC than soil rewet to 50% previously. In soil previously rewet to 10% WHC compared to that rewet to 50% WHC, MBN and available P on days 17 and 23 were about two-fold higher whereas available N did not differ between treatments. The greater respiration and microbial biomass after the third rewetting event in soil previously rewet to 10% WHC compared to that rewet to 50% WHC can be explained by the greater amount of available substrate remaining after the first two rewetting events. The study showed that rewetting of dry soil to low water content induces only a small flush of respiration and thus little decomposition of organic matter. In the fourth experiment, soil was amended with high (H) or low C/N ratio (L) residue and then maintained at 10% or 50% WHC for 10 days after which the soil at 10% WHC was rapidly rewetted to 50% WHC. A second residue with a different C/N ratio than the first was added one, two or five days after rewetting. Rewetting of soil that was at 10% WHC in the first 10 days increased MBC and respiration after day 10 in soil amended with L. After day 10, MBN increased with number of days between rewetting and the second residue addition. After the second residue addition, respiration rate in the first three to four days and available N two days after residue addition were higher when residue was added five days after rewetting than if added after one day. But MBN was higher in treatments with residues added one day after rewetting compared to amendment after five days. It can be concluded that soil water content between the first and the second residue addition influenced soil respiration whereas the time between rewetting and the second residue addition affected N availability. However, neither previous soil water content nor time between rewetting and the second residue addition influenced the legacy effect. In the following two experiments, we studied how previous and current soil water content influence soil respiration, microbial biomass and nutrient availability in soils amended with residues differing in C/N ratio. In the fifth experiment from day 1 to day 10, soil was incubated at 10%, 30% or 50% WHC and there were three residue treatments (unamended (C), amended with high (H) or low C/N ratio (L) residue). After sampling on day 10, soil water content was adjusted to 50% WHC and either H or L was added to soil. Therefore, the water content treatments were 10-50, 30-50 and 50-50. Cumulative respiration from day 1 to day 10, MBC and MBN on day 1 and available N and P on both day 1 and day 10 were lower at 10% than at 50% WHC. When L was added on day 10, cumulative respiration from day 11 to day 20, microbial biomass C and N on day 11 and available N on day 20 were higher in soil kept at 10% WHC in the first 10 days than in that maintained at 50% WHC. The previous water content had little effect on respiration and nutrient availability when H was added on day 10. Differences in MBC, MBN, MBP and available N on day 11 between HL and LL and between LH and HH were greater with 10% WHC in the first period than with 50% WHC. The results showed that the water content between the first and second residue amendment affects the extent of the legacy effect after the second residue addition. In the last experiment, soil was amended with low (L) or high C/N ratio (H) residues on days 0 and 10 and the soil was incubated at 10 or 50% WHC from day 1 to day 10, but unlike previous experiment the water content from day 10 to 20 was either 10 or 50% WHC, not only 50%. Therefore the water content treatments were 10-10, 10-50, 50-10 and 50-50. In LH at 50% WHC from day 11 to day 20, previous low water content (10% WHC) enhanced N immobilisation. In HL with 10% WHC from day 11 to day 20, MBN, available N and P on day 20 were higher in soil that was at 50% WHC in the first 10 days compared to that at 10% WHC. The study showed that the influence of the previous water content on respiration and microbial biomass was stronger when the first amendment was low C/N residue because its decomposition was more affected by water content than that of high C/N residue. It can be concluded that soil water content influences organic matter decomposition and therefore nutrient availability, particularly in soil amended with low C/N residue whereas drying and rewetting has only small and transient effects.