NOx and N2O fluxes in an upland agroecosystem of the north China plain: field measurements, biogeochemical simulation, and climatic sensitivity

Abstract

Chinese agriculture represents one of the most intensively managed and biogeochemically important ecosystems in the world. High fertilizer application rates and poor nutrient use efficiency by crops result in high nitrogen losses to the surrounding environment, with consequences to atmospheric composition, groundwater quality, public health, and ironically, agriculture itself. One loss mechanism, the gaseous production of NOx and N2O is examined here. Flux measurements revealed seasonal emission factors of 1.24% and 0.22% of added nitrogen for NOx and N2O, respectively. An unequivocal relationship between the amount of added nitrogen and the magnitude of gaseous efflux was evident. A relationship to organic matter amendment, whose proportions have been steadily declining in China, was significant only for N2O. Relative seasonal fluxes were lower than those determined by other authors, possibly the result of particularly high ammonia volatilization under Chinese management regimes. Mathematical simulations with the biogeochemical model DAYCENT adequately represented the temporal dynamic and peak size of measured fluxes. Ammonia volatilization levels were important considerations in comparing measured versus simulated trace gas fluxes. DAYCENT tended to underestimate the number of very small fluxes, however overall seasonal fluxes were similar for measured versus simulated fluxes. In response to sensitivity to the climatic variables of temperature and precipitation, standard biogeochemical diagnostics including soil carbon, organic soil nitrogen, grain yield, net primary productivity, and actual evapotranspiration behave according to expectations. Emissions of NO and N2O, however, show a complex and nonlinear response, reflecting interactions and fluctuations within the many driving parameters of trace gas production.