Soil physical and chemical properties affected by long-term grazing on the desert steppe of Inner Mongolia, China

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It remains unclear how long-term and different intensities of grazing affect the soil infiltration process, total nitrogen (TN), organic carbon (SOC), and grain size of desert steppes of Inner Mongolia, China. We quantified the effects of consecutive years of grazing from 2004 to 2018 on the soil particle size distribution (PSD, using the American soil classification system), mean soil infiltration rate (r), and cumulative infiltration volume (VC), in addition to the contents of SOC and TN, and their ratio (C/N). We defined grazing intensity at four levels, as non-grazed enclosure (NG, 0-), light (LG, 0.15-), moderate (MG, 0.30-), and heavy grazing (HG, 0.45 sheep per month per hectare steppe). The annual grazing period lasted from May to November. We established twelve adjacent experimental plots containing three replications for each grazing intensity. In July 2018, we collected 600 soil samples (0–40 cm soil depth: 4 depths with a 10–cm interval) in the plots (50 per plot) in a sigmoid configuration, to determine the PSD, SOC, TN, and C/N. Grazing evidently changed the PSD in diameter ranges of 20–150 (fine particles) and 200–500 μm (coarse particles). Grazing significantly decreased the uniformity and distribution range of PSD but increased its dispersion, and these changes were most pronounced in the topsoil (0–10 cm depth). LG promoted r and VC, whose augmentation was evident within the initial 10 min of the infiltration process. The Kostiakov infiltration model was better at dynamically and quantitatively fitting the changing process of r than either the Horton or Philip model. Compared with NG, LG and MG significantly increased both SOC and TN (p < 0.01) whereas these were significantly decreased under HG (p < 0.01). Across differing grazing intensities, topsoil SOC contents can be well predicted by the range and heterogeneity of PSD, as well as the fraction of fine soil particles. These results are helpful for better understanding how continuous grazing impacts soil characteristics and for specifically enacting reasonable grazing scenarios to mitigate desertification on vulnerable desert steppes.