Abstract:

Limited information exists on synchronized nutrient release, especially of nitrogen, to meet the nutrient demand of plants in the Xilin River Basin, Inner Mongolia. We conducted a field experiment to examine soil nitrogen fractions during the first two years of grassland restoration following 11 years of consecutive grazing under four different stocking rates. The seasonal variation of different soil nitrogen fractions in different soil layers was investigated. Soil and plant samples were randomly taken from each block on May 12, June 23, August 3, and September 13, 2002 at depths of 0-10 cm, 10-20 cm and 20-40 cm. The concentrations of NO－3-N, NH+4-N, inorganic-N (the sum of NO-3N and NH+4-N) and total Kjeldahl-N in the different soil layers were determined. At the same time, we determined the microbial biomass N of the top 10 cm soil using the chloroform fumigation-extraction method. Our results showed that the total nitrogen in different soil layers had no significant seasonal change under all treatments. This indicated that grazing rates had no significant effects on the pool size of total soil N. However, there were significant seasonal change patterns of the soil NO－3-N, NH+4-N, inorganic-N and microbial biomass N. During the growing season, the NO－3-N decreased and was negatively correlated with aboveground green phytobiomass, suggesting that soil NO－3-N concentrations were controlled primarily by synchronized N uptake by plants although other N transformation processes such as microbial immobilization, denitrification and leaching also can exert some control over the NO－3-N pools. Microbial biomass N could explain 22.3% of the variation in inorganic nitrogen concentrations whereas NH+4-N was negatively correlated with microbial biomass N (p<0.01) indicating that soil microbial organisms can have an important impact on soil N transformation processes. Total soil inorganic nitrogen showed an inconsistent pattern during the growing season, but was significantly negatively correlated with aboveground green phytobiomass. We also found that NH+4-N pools were relatively constant in the top 10 cm of soil from June to September, but NO－3-N fluctuated throughout the year and was almost undetectable by the end of plant growing season. After plant senescence in September, inorganic nitrogen concentrations increased again. In general, the amounts of different soil nitrogen fractions decreased with soil depths.Following a two-year exclosure period after 11 consecutive years of grazing at four different stocking rates, the different soil N fractions showed a differential response. Total nitrogen concentrations were not affected by previous stocking rates while the soil microbial biomass nitrogen differed significantly among treatments during the growing season. Soil microbial biomass N was the highest in the previously overgrazed and moderately grazed treatments followed by ungrazed and lightly grazed treatments. The responses of soil inorganic nitrogen, NH+4-N and NO－3-N to two years of no grazing were complicated. In general, soil inorganic N, NH+4-N and NO－3-N concentrations were higher in the moderately grazed and overgrazed treatments. After two years of restoration, there were no significant differences in total aboveground green phytobiomass (maximum biomass in August) between the previously ungrazed and overgrazed treatments.