Biophysical regulations of NEE light response in a steppe and a cropland in Inner Mongolia

Aims Ecosystem carbon models often require accurate net ecosystem exchange of CO_(2)(NEE)light-response parameters,which can be derived from the Michaelis–Menten equation.These parameters include maximum net ecosystem exchange(NEE_(max)),apparent quantum use efficiency(a)and daytime ecosystem respiration rate(R_(e)).However,little is known about the effects of land conversion between steppe and cropland on these parameters,especially in semi-arid regions.To understand how these parameters vary in responses to biotic and abiotic factors under land conversions,seasonal variation of light-response parameters were evaluated for a steppe and a cropland of Inner Mongolia,China,during three consecutive years(2006–08)with different precipitation amounts.Methods NEE was measured over a steppe and a cropland in Duolun,Inner Mongolia,China,using the eddy covariance technique,and NEE light-response parameters(NEE_(max),α and R_(e))were derived using the Michaelis–Menten model.Biophysical regulations of these parameters were evaluated using a stepwise regression analysis.Important Findings The maximum absolute values of NEE_(max) occurred in the meteorological regimes of 15℃<T_(a)<25℃,vapor pressure deficit(VPD)<1 KPa and 0.21 m^(3) m^(-3)<volumetric soil water content at 10 cm(SWC)<0.28 m^(3) m^(-3) for both the steppe and the cropland ecosystems.The variations of α and R_(e) showed no regular variation pattern in different T_(air),VPD and SWC regimes.Under the same regime of T_(air),VPDand SWC,the cropland had higher absolute values of NEE_(max) than the steppe.Canopy conductance and leaf area index(LAI)were dominant drivers for variations in NEE light-response parameters of the steppe and the cropland.The seasonal variation of NEE light-response parameters followed the variation of LAI for two ecosystems.The peak values of all light-response parameters for the steppe and the cropland occurred fromJuly to August.The values of NEE light-response parameters(NEE_(max),α and R_(e))were lower in the driest year(2007).Seasonally averaged NEE light-response parameters for the cropland surpassed those for the steppe.Land conversion from steppe to cropland enhanced NEE light-response parameters during the plant growing period.These results will have significant implications for improving the models on regional NEE variation under climate change and land-use change scenarios.