Effects Of Grazing On Soil Physical Chemistry Characteristics And Microbes At Loess Plateau, China

Grazing is one of the main uses of grassland. Livestock interacts with grassland by its activities of feeding, trampling and excreting. Effects of grazing on soil physical and chemical properties, and soil microbes in semi-arid grassland were studied during the period of 2004 to 2006 at Huanxian County, eastern Gansu Province, which belongs to the Loess Plateau. The stocking rates of sheep used in the experiments were 0 sheep unit/ha, 2.7 sheep unit/ha, 5.3 sheep/ha and 8.7 sheep unit/ha. Soil samples were taken for seven times at various growing stages of grassland plants at the depth of 0-30cm from paddocks with different stocking rates. The soil samples were studied for the contents of soil particles at various size, nutrient components, and soil microbial components. The main results are summarized as the follows:1,The contents of clay, silt, fine sand and coarse sand were significantly different among the soils taken from paddocks with various stocking rates. The contents of clay and silt at paddocks with 0 sheep unit/ha were significantly higher than the paddocks with other stocking rates. Whereas the contents of coarse sand were the highest at paddock grazed at 8.7 sheep unit/ha , and were the lowest at the paddocks grazed at both 0 sheep unit/ha and 2.7 sheep unit/ha. The highest contents of clay, silt and coarse sand were all obtained on September during the year. In the 0-30 cm soil layers, content of coarse sand was the highest at the 0-10cm depth, while the contents of clay, silt and fine sand were increasing with soil depth.2,The contents of soil organic matter and total N were no significant at different treatments. However, the contents of total P and NH₄⁺-N at the paddocks with 0 sheep unit/ha were significantly higher than those at the paddocks with 2.7 sheep unit /ha. NO₃^-- N content at paddocks with 8.7 sheep unit /ha was significantly higher than that obtained at the paddocks grazed at 0 sheep unit/ha. Contents of available P at paddocks with at 0 sheep unit /ha and 2.7 sheep unit /ha were significantly higher than that at paddocks with 8.7 sheep unit/ha. Contents of soil organic matter, total N, NOV - N and pH were highest on September during the year regardless the stocking rates and soil depth, while contents of total P and available P were highest on November. NH₄⁺- N was highest on May or June. In terms of spatial distribution contents of soil organic matter, total N, total P and NO₃^-- N were all increasing with soil depth. However, contents of NH₄⁺- N, available P and pH were decreasing with soil depth. 3,Among the soil organisms isolated from various paddocks the number of soil fungi was the lowest, which was followed by the number of actinomycete, and the number of bacteria was the highest. They account for 0.10-1.29 %, 17.37-59.63 % and 40.27- 82.23 %, of the total number of microorganism isolated, respectively. The total numbers of soil microorganisms and the number of bacteria were both the highest at paddocks with 8.7 sheep unit/ha and were the lowest at the paddocks with 5.3 sheep unit/ha. Whereas the number of soil fungi was the highest at the paddocks with 2.7 sheep unit/ha, followed by the paddocks with 5.3 sheep unit/ha and 8.7 sheep unit/ha, and the lowest number obtained from the paddocks with 0 sheep unit/ha. The number of actinomycete was highest at the paddocks with 2.7 sheep unit/ha, followed by the stocking rates of 5.3 and 8.7 sheep unit/ha and lowest number was obtained at the paddocks with 0 sheep unit/ha. Both the numbers of fungi and bacteria were the highest on September and the lowest on April and November, while the highest number of actinomycete was on November. The numbers of soil fungi, bacteria and actinomycete were all significantly decreasing with soil depth. The significant positive correlations were found between the numbers of fungi and bacteria, of fungi and actinomycete, of bacteria and actinomycete, the correlation coefficience were 0.3362-0.4626, 0.2087-0.4535 and 0.6434-0.6456, respectively. Soil fungi, bacteria and soil NH₄⁺- N, NO₃^-- N was also correlated significantly and positively (p <0.05). Actinomycete number and total P was also significantly and positively correlated (p<0.05). Another significant correlation was found between soil pH and the number of bacteria, the number of actinomycetes, respectively (p<0.05).4,Soil microbial biomass C the was lowest at the paddocks with 8.7 sheep unit/ha. However, microbial N was the highest under the same stocking rate. Microbial biomass C and N was not differed significantly among other stocking rates. Soil microbial biomass P was the lowest at paddocks with 0 sheep unit/ha, it was significantly higher at paddocks with 2.7 sheep unit/ha or 5.3 sheep unit/ha than the 8.7 sheep unit/ha treatment. At different sampling dates, soil microbial C and P were the highest on September, microbial N was the highest on November. Two years results showed that the cumulative effect of grazing was very obviously. Microbial biomass C and N in 2005 was significantly lower than those obtained at the same period of 2004. Soil microbial biomass C, N and P were significantly decreasing with soil depth and a significant difference was also found among the soil depths of 0 - 10 cm, 10 - 20 cm and 20 - 30 cm. (p<0.05). A significant and positive correlation was existed between soil microbial biomass C and N, N and P, C and P (p<0.05), the correlation coefficient was 0.3649, 0.2195 and 0.3390, respectively. The corresponding regression equation was y = 0.036x +7.2752, y = 0.0562x + 5.5049 and y = 0.0064x + 5.1656, respectively. Soil microbial biomass C and the content of clay and silt was correlated with each other positively (p<0.05). Microbial C and fine sand content was correlated negatively.Based on the results mentioned above it may be conclude that the stocking rates of 2.7 sheep unit /ha and 5.3 sheep unit /ha were more appropriated for the grassland studied.