Grazing exclusion alters soil microbial respiration, root respiration and the soil carbon balance in grasslands of the Loess Plateau, northern China

Abstract

Grassland ecosystems are a significant component of the global carbon cycle. To better understand how grazing affects the carbon cycle of grasslands, soil microbial respiration (Rm) and root respiration (Rr), which are the main soil respiration components, we investigated with a trenching method in grazed grasslands (GG) and fenced (FG) grasslands on the Loess Plateau, northern China in 2008. The annual carbon balance in the two grasslands were also assessed and compared. After exclusion of grazing for about 3 years, soil organic carbon (SOC) and microbial biomass carbon (MBC) in the surface soil increased significantly (P < 0.05), resulting in the increase of Rm in most seasons. Exclusion of grazing did not change the diurnal variations of Rm, Rr and total soil respiration (Rt). Grazing decreased the temperature dependence of Rm. The annual accumulations of Rm were 165.9 g C m−2 in FG and 116.1 g C m−2 in GG. On most dates, Rr in FG was higher than in GG, but significant differences were only found in some seasons. The seasonal average value of Rr was 0.374 µmol carbon dioxide (CO2) m−2 s−1 in FG, 21.0% higher than that in GG (0.309 µmol CO2 m−2 s−1). Net primary production (NPP) in FG and GG were 243.6 and 205.8 g C m−2, respectively. The annual C balance resulted in net C sequestrations of 77.7 and 89.7 g C m−2 in FG and GG, respectively, suggesting that the grassland in this region may act as a C sink both under grazing and fencing.