Sensitivity of organic matter mineralisation to water availability: role of solute diffusivity and the 'Birch effect'

Context Several functions are used to describe the effects of soil water content on organic matter mineralisation. A meta-analysis of published studies identified relative water content (RWC; available water relative to the soil's available water holding capacity) as the best water descriptor for N mineralisation. Aims To evaluate RWC as a predictor of C and N mineralisation in New Zealand soils; and to investigate how solute diffusivity and the 'Birch effect' may help to explain this relationship. Methods Three agricultural soils (0-15 cm), differing in water holding capacity were incubated (8-week; 20 degrees C) under a range of RWCs to measure carbon (respiration) and net N mineralisation. After 4 weeks, a subset of samples from each treatment were re-wetted to field capacity for a further 4-weeks to quantify the respiration response to re-wetting. Key results For all three soils, there was a linear relationship between respiration and RWC where the C respired at the wilting point (RWC = 0) was similar to 25-30% of that at field capacity (RWC = 1.0). Results from a solute diffusivity model suggested that a decrease in microbial substrate supply, owing to restricted diffusion of dissolved organic compounds, contributed to moisture-induced decline in respiration. A respiration flush was not observed when RWC was >0 at re-wetting. Nitrogen mineralisation was non-linearly related to RWC, with small decreases in RWC below 1.0 (optimum) having a greater effect on N, than C, mineralisation. Conclusions RWC may be a reliable 'water modifier' to describe the influence of soil moisture on respiration. Further work is recommended to verify the RWC vs net N mineralisation relationship observed in this study.