Response of soil CO2 efflux to water addition in temperate semiarid grassland in northern China: the importance of water availability and species composition

Knowledge of the effects of water addition and the composition of plant species on soil CO2 efflux (SCE) and its temperature sensitivity (Q10) in temperate semiarid grassland is essential for understanding carbon (C) cycles and for evaluating future C balance. Our objective was to examine the seasonal variability of SCE and its sensitivity to temperature in response to water addition in four grasslands on the northern Loess Plateau. SCE was measured during the growing seasons (April–October) in 2010 and 2011 in the four grasslands dominated by different species: purple alfalfa (ALF), bunge needlegrass (BNE), degraded alfalfa (DALF), and fallow land for the recovery of natural vegetation (NAF), in two treatments: water addition (50 mm year−1) and a control. Water addition consistently increased mean seasonal SCE by 8.4–33.8 % across the four communities, attributable to enhanced plant growth, root and microbial activities, and respiration. The magnitudes of the positive effects, however, varied with plant community and measuring period. Soil temperature was primarily responsible for the seasonal changes in SCE in both the control (37–63 %) and the watered (40–75 %) plots. The addition of water increased the Q10 of SCE by 11.0 %, 14.4 %, and 13.4 % for ALF, DALF, and NAF, respectively, but had no significant effect on the Q10 for BNE. We therefore conclude that water addition can significantly increase SCE and may stimulate the sensitivity to temperature in semiarid grassland. The composition of plant species may regulate the dependence of SCE on water and temperature.