退化草地恢复过程中土壤氮素状况以及与植被地上绿色生物量形成关系的研究

 研究了在不同放牧率下形成的不同退化阶段的草地各形态氮素（全氮、硝态氮、铵态氮、无机氮和微生物氮）的变化情况，同时也研究了植被地上绿色生物量与各形态氮素季节变化的同步性关系。土壤全氮含量相对稳定，随草地植被状况和植物生长时期变化不大，说明土壤总氮库有相当的弹性。土壤硝态氮（NO－3-N）、铵态氮（NH+4-N）、无机氮（IN）和微生物氮（Micro-N）季节变化明显。土壤Micro-N和NO-3-N含量随植物生长逐渐降低，到植物枯黄期含量又回复到较高的水平；土壤NH+4-N含量随植物生长有逐渐升高的趋势；IN则随着植物的生长出现低-高-低-高的特点，且与植被地上绿色生物量呈显著负相关（R=-0.247, p<0.01）。在放牧条件下草原植物优先利用NO－3-N，NO－3-N与植被地上绿色生物量有显著的负相关性，是形成草原植被地上绿色生物量的有效性氮素。Micro-N能解释土壤IN 22.3%的变异（R2=0.223, p<0.01），Micro-N是土壤无机氮的重要来源。土壤NH+4-N与Micro-N呈显著负相关（R=-0.222, p<0.01），说明土壤微生物对土壤NH+4-N有偏好吸收。总体上，不同形态的氮素在各土壤层次间差异显著，随土壤层次的加深含量逐步降低。连续放牧11年恢复两年后，各氮素组分对放牧压力消除的响应并不一致。土壤全氮含量与停止放牧前相比变化差异不显著；而Micro-N对放牧压力消失的响应在不同处理下整个生长季的结果比较一致，即以前过度和中度放牧处理的Micro-N含量较高，无牧和轻牧含量较低；IN、NH+4-N和NO－3-N变化比较复杂，在不同放牧恢复处理上结果并不一致。总的来看，以前中度和过度放牧的IN、NH+4-N和NO－3-N含量较高，存在潜在损失的可能。经过两年的恢复，植被地上绿色生物量（8月）过牧处理与无牧处理差异不显著。

SOIL NITROGEN REGIME AND THE RELATIONSHIP BETWEEN ABOVEGROUND GREEN PHYTOBIOMASS AND SOIL NITROGEN FRACTIONS AT DIFFERENT STOCKING RATES IN THE XILIN RIVER BASIN,INNER MONGOLIA

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.