干旱区生态环境知识资源中心

Ecosystem carbon models often require accurate net ecosystem exchange of CO2 (NEE) light-response parameters, which can be derived from the Michaelis-Menten equation. These parameters include maximum net ecosystem exchange (NEEmax), apparent quantum use efficiency (alpha) 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.

NEE was measured over a steppe and a cropland in Duolun, Inner Mongolia, China, using the eddy covariance technique, and NEE light-response parameters (NEEmax, alpha and R-e) were derived using the Michaelis-Menten model. Biophysical regulations of these parameters were evaluated using a stepwise regression analysis.

The maximum absolute values of NEEmax occurred in the meteorological regimes of 15 degrees C < T-a < 25 degrees C, 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 alpha and R-e showed no regular variation pattern in different T-air, VPD and SWC regimes. Under the same regime of T-air, VPD and SWC, the cropland had higher absolute values of NEEmax 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 from July to August. The values of NEE light-response parameters (NEEmax, alpha 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.