Soil organic matter and texture affect responses to dry/wet cycles: Changes in soil organic matter fractions and relationships with C and N mineralisation

Abstract

The extent to which SOM content and texture affect C and N dynamics during dry/wet cycles is poorly understood. A laboratory incubation study was conducted to quantify short-term changes in SOM (C & N) fractions and their relationship to C and N mineralisation in response to dry/wet cycles along a SOM gradient in two soil types of differing texture. The experiment consisted of three phases: pre-incubation, treatment and recovery. Three soil water content (SWC) treatments were established: continuously wet (WW; field capacity (FC)), moderately dry (MD; 120% of SWC at wilting point (WP)) and very dry (VD; 80% of SWC at WP). Each of the two ‘dry’ treatments were either maintained continuously dry (MD & VD) or subjected to three sequential 20 d-long dry/wet cycles (MDW & VDW) during the experiments treatment phase. All soils were returned to FC at the start of the recovery phase and analyses were carried out at the end of each phase. Over all, the results of this study showed that SOC content and texture are important factors affecting the size of several commonly measured fractions of soil organic matter, but the stability and resilience of these fractions depended on the duration and amplitude of dry/wet cycles. Whereas most of the measured C and N fractions (cold water extractable C [CWEC] and hot water extractable C [HWEC], microbial biomass C [MBC] and N [MBN], inorganic N) were affected by both the duration (e.g. MDW vs. MD) and amplitude (e.g. MDW vs. VDW) of dry/wet cycles, the response differed between fractions and the effects tended to be much stronger in silt loam than in clay loam soils. The duration and amplitude of dry/wet cycles also suppressed the rate of both C and N mineralisation in both soils compared to continuously wet conditions. There was also strong evidence that the C mineralised from both soils during the recovery phase (i.e. following rewetting of dry soils) compensated for the reduction in C mineralised during the treatment phase. For N mineralisation, the amplitude of dry/wet cycles was at least as important as the duration of the cycles in affecting N mineralisation during the recovery phase. Much higher rates of N mineralisation were observed in soils that had previously been exposed to very dry conditions, particularly in the silt loam soil. There was some evidence that HWEC was the primary source of the C made available during the rewetting of dry soil and that it contributed to the increased availability of CWEC and supported an increase in MBC and C mineralisation in both soils during the recovery phase. In contrast, there was no evidence that differences in availability of C and N fractions affected the rate of N mineralisation following the return of dry soils to continuously wet conditions. Further research is need to resolve the primary factors that regulate N mineralisation response to the recovery from dry soil conditions.