Seasonal dynamics of carbon and nitrogen pools and fluxes under continuous arable and ley-arable rotations in a temperate environment

Improved understanding of the seasonal dynamics of C and N cycling in soils, and the main controls on these fluctuations, is needed to improve management strategies and to better match soil N supply to crop N demand. Although the C and N cycles in soil are usually considered to be closely linked, few data exist where both C and N pools and gross N fluxes have been measured seasonally. Here we present measurements of inorganic N, extracted soluble organic N, microbial biomass C and N, gross N fluxes and CO2 production from soil collected under wheat in a ley-arable and continuous arable rotation within a long-term experiment. The amounts of inorganic N and extracted soluble organic N were similar (range 5-35 kg N ha(-1); 0-23 cm) but had different seasonal patterns: whilst inorganic N declined during wheat growth, extracted soluble organic N peaked after cultivation and also during maximal stem elongation. The microbial biomass was significantly larger in the ley-arable (964 kg C ha(-1); 0-23 cm) than the continuous arable rotation (518 kg C ha(-1); 0-23 cm) but with no clear seasonal pattern. In contrast, CO2 produced from soil and gross N mineralization showed strong seasonality linked to soil temperature and moisture content. Normalization of soil CO2 production and gross N mineralization with respect to these environmental regulators enabled us to study the underlying influence of the incorporation of fresh plant material into soil on these processes. The average normalized gross rates of N mineralized during the growing season were 1.74 and 2.55 kg N ha(-1) nday(-1) in continuous arable and ley-arable rotations respectively. Production rates (gross N mineralization, gross nitrification) were similar in both land uses and matched rates of NH4+ and NO3- consumption, resulting in periods of net N mineralization and immobilization. There was no simple relationship between soil CO2 production and gross N mineralization, which we attributed to changes in the C : N ratio of the mineralizing pool(s).