Abstract: The grassland in the ecotone of agriculture and animal husbandry in northern China plays an important role in supplying for animal husbandry,however,long-term grazing activities may have lead to changes in the supply capacity of soil nitrogen (N),and thereby impact the ecological and productive function of the grassland. In order to explore the effects of different grazing intensities on soil nitrogen mineralization,we set up four grazing intensities in 2016,including no grazing (NG),light grazing (2.35 Sheet Unit (SU) hm^-2·growing season;LG),medium grazing (4.80 SU hm^-2·growing season;MG) and heavy grazing (7.35 SU hm^-2·growing season;HG). Soil net N mineralization rates were determined using the situ closed-top tube incubation technique in 2017 and 2018. Our results showed that different grazing intensities had significant effects on soil inorganic N content (P<0.05),and sampling time had a significant effect on soil inorganic nitrogen content (P<0.01). Heavy grazing improved the rate of soil N mineralization compared to other treatments,and the lowest net N mineralization rate was found under light grazing treatment. Significant relationship was found between soil net N mineralization rate and soil temperature or moisture,which had significant effects on soil net N mineralization rates. Therefore,in a short period of time,heavy grazing accumulated soil inorganic nitrogen and increased available nitrogen for plants. Heavy grazing significantly increased soil net nitrogen mineralization rate,but decreased the biomass and caused great surface exposure,and made grassland at the risk of degradation. The long-term experiments are required to determine the appropriate grazing intensity to maintain the stability of grassland ecosystem in this area.

Key words: Nitrogen mineralization, Grazing intensities, Inorganic nitrogen, Net N mineralization rate

摘要： 北方农牧交错带的草地是我国重要的畜牧业基地，但长期不合理的放牧使其土壤氮素的供应能力发生变化，草地生态和生产功能受到影响。因此，为探究不同放牧强度对土壤氮矿化的影响，本试验于2016年设置4个放牧强度，分别为不放牧（No Grazing，NG）、轻度放牧（2.35 Sheep Unit （SU） hm^-2·生长季；Light Grazing，LG）、中度放牧（4.80 SU hm^-2·生长季；Medium Grazing，MG）和重度放牧（7.35 SU hm^-2·生长季；Heavy Grazing，HG），分别在2017年和2018年采用顶盖埋管培养法测定土壤净氮矿化速率。结果表明：不同放牧强度对土壤无机氮含量有显著影响（P<0.05），生长季不同月份土壤无机氮含量存在极显著差异（P<0.01）；重度放牧的净氮矿化速率最大，轻度放牧最低；放牧主要通过影响土壤温度和水分的变化进而影响土壤净氮矿化速率。可见，短期内重度放牧会使土壤无机氮累积，提高植被可利用氮含量，显著提高土壤的净氮矿化速率，但造成地上生物量减少，地表裸露，草地有退化的风险，因此，为更好地维持北方农牧交错带草地生态系统的稳定，还需进行长期的试验监测来确定适宜的放牧强度。

关键词: 氮矿化, 放牧强度, 无机氮, 净氮矿化速率