Responses of carbon, nitrogen and phosphorus to two consecutive drying-rewetting cycles in soils

Drying and rewetting cycles are known to be important for the dynamics of carbon (C), phosphorus (P) and nitrogen (N) in soils. This study reports the short-term responses of these nutrients to consecutive drying and rewetting cycles and how varying soil moisture content affects the presence of different forms of C, P and N, microbial biomass C and P (MBC and MBP), as well as associated carbon dioxide (CO2) and nitrous oxide (N2O) emissions. Soils were incubated for 14 days during which two successive drying-rewetting episodes were imposed on the soils. Soils subjected to drying (DRW) were rewetted on the seventh day of each drying period to return them to 60% water holding capacity, whilst continually moist samples (M), with soil maintained at 60% water holding capacity, were used as control samples. During the first 7 days, the DRW samples showed significant increases in extractable ammonium, total oxidised nitrogen, and bicarbonate extractable P concentrations. Rewetting after the first drying event produced significant increases only in CO2 flux. The MBC and MBP concentrations fluctuated throughout the incubation in both treatments and only the second drying-rewetting event resulted in a significantly MBC decrease. The two drying-rewetting events impacted the microbial biomass, but distinguishing the different impacts of microbial versus physical impacts of the perturbation is difficult. Moreover, this study has shown that the interactions between the macronutrient cycles in soils under drying-rewetting events is complex and different for each nutrient. However, it might be important to understand how soils will react to changing patterns of longer period of drying-rewetting to forecast the impacts of future climate change.