Soil organic matter and texture affect responses to dry/wet cycles: Effects on carbon dioxide and nitrous oxide emissions

Soil organic matter (SOM) content and texture are important factors affecting carbon (C) and nitrogen (N) mineralisation under constant soil moisture but their effects on organic matter mineralisation and associated biogenic gas (carbon dioxide (CO2) and nitrous oxide (N2O)) production during dry/wet cycles is poorly understood. A laboratory incubation study was conducted to quantify CO2 and N2O production during sequential dry/wet cycles and under constant soil moisture conditions along a gradient of SOM contents in two soil types representing different texture classes (silt loam vs. clay loam). Three soil moisture treatments were established: wet (WW; field capacity), moderately dry (MD; 120% of soil moisture content (SMC) at wilting point (WP)) and very dry (VD; 80% of SMC at WP). To each of the two ‘dry’ treatments two different dry/wet treatments were applied where the soils were either maintained continuously dry (MD & VD) or subjected to three sequential 20-day long dry/wet cycles (MDW & VDW) during the treatment phase of the experiment. At field capacity soil moisture content, the rate of C mineralisation increased with increases in SOC content and the increase per unit of C was twice as high in silt loam (0.30mgCO2-Cg−1SOCd−1) as in clay loam (0.13mgCO2-Cg−1SOCd−1) soils. N2O-N emissions also increased with increasing in SOC content. However, in contrast to C mineralisation, the effect was four-fold greater for clay loam (1.38μgN2O-Ng−1SOCd−1) than silt loam (0.32μgN2O-Ng−1SOCd−1) soils. Following rewetting, the VDW and MDW soils produced a short-term C mineralisation flush that was, on average, 30% and 15% greater, respectively, than in WW soils. However, the flush of C mineralisation was not sufficient to compensate for the reduction in mineralisation during the drying phase of each cycle, resulting in a lower total C mineralisation from MDW and VDW soils, on average, compared with WW soils over the three sequential dry/wet cycles. The C mineralisation flush also remained a relatively constant proportion of the total C mineralised from both silt loam (23%) and clay loam soils (22%), irrespective of their SOC content. In contrast, the short-term flush of N2O that followed rewetting of dry soil accounted for 62% and 68% of the total N2O emissions from silt loam and clay loam soils, respectively. On average, the total N2O emissions from dry/wet treatments imposed on silt loam and clay loam soils were 33% and 270% greater, respectively, than from the WW treatments, though the effect varied greatly and depended on SOC content. Overall, N2O emissions were highest where we had a combination of fine texture, an adequate supply of available C (i.e. high SOM content), and a water-filled pore space (WFPS) > 0.60 cm cm−3 at field capacity. Prediction of C mineralisation over dry/wet cycles using mineralisation data from soils at constant moisture content is possible, but knowledge of the stress history for the soil would be required to improve accuracy. The prediction of N2O-N emissions during dry/wet cycles using emission data from soils at constant moisture was very inaccurate, due to the inherent spatial variability of N2O emissions.