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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 (α) 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°C ≤ T a < 25°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 α 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 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 from July 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. © 2011 The Author. Published by Oxford University Press on behalf of the Institute of Botany, Chinese Academy of Sciences and the Botanical Society of China. All rights reserved.

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