Effects of annual plant functional group on biomass and soil respiration in a grazing community of a typical steppe grassland

As a critical synusia in steppe grasslands, annual plant functional groups can compensate for changes in community biomass and maintain its stability in wet seasons and across years. This compensation should be evident because they are more sensitive to precipitation than perennial plant functional groups, especially in grazing communities with low biomass of perennial plant functional groups. However, it is not clear how the compensation effects of the annual plant functional groups improve productivity and affect soil respiration (Rs) in grazing areas in the steppe. We conducted a 2-year grazing experiment, including light-grazing (L) and moderate-grazing (M) treatments, in a typical steppe in Inner Mongolia in 2012 and 2013. Above-ground biomass (AGB), under-ground biomass (UGB), Rs, and environmental factors were monitored during the growing season. We found that: 1) either compensation of the annual plant functional groups occurred in the AGB and UGB, or Rs was higher in the wet year (2012) than in the dry year (2013); 2) both compensation effects in the annual plant functional groups in the AGB and UGB and Rs were significantly greater in moderate-grazing than light-grazing; 3) Rs was positively correlated with both the AGB and UGB of the annual plant functional groups, as well as with soil temperature and moisture (P<0.001). This study showed that grazing utilization decreased biomass of the perennial plant functional groups, and the annual plant functional groups dramatically developed in wet seasons and years to compensate for loss in community biomass to improve productivity and maintain ecosystem stability, which as a principal biotic mechanism that affects the ecosystem carbon cycle in grasslands.