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