Effects of grazing on soil respiration in typical steppe during growing season in Inner Mongolia

Objectives Grazing changes the productivity and soil nutrient cycling of typical steppe, affecting the growth of vegetation and soil microbes, and thus changes the carbon emissions of grassland soils. This study analyzed the difference of soil respiration (Rs) rate in the growing season of typical steppe under different grazing measures, to understand the relationship between grassland carbon exchange and carbon balance for grazing. It has great theoretical significance for exploring and clarifying the rational utilization of grassland ecological resources and the trend of carbon emission.

Methods On three types of grazing plots (yearlong grazing, rest rotation grazing and grazing exclusion) in Inner Mongolia typical grassland, Rs was measured in situ in July and September of 2014 and 2015. The vegetation, soil nutrient contents and microbiological indicators under three grazing system were also analyzed.

Results 1) The Rs in rest rotation grazing, grazing exclusion and yearlong grazing systems were CO₂ 1.94, 2.00 and 1.56 μmol/(m²·s), respectively, and showed an order of grazing exclusion > rest rotation grazing > yearlong grazing system. Grazing impacted Rs differently during the growing season, inhibiting Rs in July and increasing Rs in September. 2) Compared with grazing exclusion plots, the aboveground biomass of yearlong grazing and rest rotation grazing were reduced by 70.6% and 47.3%, the root biomass increased by 18.6% and 37.2%, the total soil carbon decreased by 34.5% and 32.0%, and the total soil nitrogen decreased by 37.0% and 34.5%. And compared with grazing exclusion, the root biomass of rest rotation grazing increased by 37.2% (P<0.05). Grazing had different effects on soil dissolved organic carbon (DOC) and soil phospholipid fatty acid (PLFAs). Compared with grazing exclusion control, yearlong grazing increased DOC but decreased PLFAs. 3) Soil respiration was positively correlated with soil moisture and aboveground biomass in July, and positively correlated with soil temperature and soil PLFAs in September. The Structure Equation Model showed that soil temperature (0.905) and moisture (0.188) dominated the variations of Rs through effecting the environment of soil microbial and root development during the growing season; Grazing inhibited Rs (–0.137) by decreasing soil moisture and aboveground biomass.

Conclusions The main path affecting soil respiration is different with grazing modes. Yearlong grazing inhibits soil respiration by reducing microbial biomass in the growing season, while rest rotation grazing increases soil respiration by enhancing the underground biomass.