华北盐渍化草地土壤净氮矿化速率对不同水平氮添加的响应

Response of soil net nitrogen mineralization to different levels of nitrogen addition in a saline-alkaline grassland of northern China

对于养分贫瘠的盐渍化草地生态系统,大气氮沉降如何影响土壤氮循环过程是一个目前尚未解决的问题。该研究在位于华北地区山西省右玉县境内的盐渍化草地建立了一个模拟氮沉降的试验平台,设置8个氮添加水平,分别为0、1、2、4、8、16、24、32 g·m^(-2)·a^(-1) (N0、N1、N2、N4、N8、N16、N24、N32),生长季5–9月,每月月初以喷施的方式等量添加NH_(4)NO_(3)。从2017年5月到2019年10月,运用顶盖PVC管法每月一次进行净氮矿化速率的测定同时计算了净氮矿化速率对不同水平氮添加的敏感性。主要结果表明:(1)高水平氮添加(N16、N24、N32)显著增加土壤无机氮库;(2)该盐渍化草地土壤氮矿化以硝化作用为主,经过3年氮添加以后,高氮添加(N24、N32)显著促进了土壤净硝化速率,并且不同氮添加水平在不同的月份和年份中表现出差异性响应;(3)不同氮添加水平对土壤净氮矿化敏感性的影响在不同降水年份差异显著,短期低水平氮添加提高了土壤净氮矿化的敏感性,而高水平氮添加降低土壤净氮矿化敏感性;(4)盐渍化草地土壤净氮矿化速率与土壤温度和水分呈正相关关系,与土壤pH呈负相关关系。因此,在当前氮沉降增加的背景下,北方盐渍化草地土壤氮矿化速率对低氮添加的敏感性较高,结合氮沉降的特点,未来模型预测应该同时考虑氮沉降对盐渍化草地的可能影响。

Aims The salinized grassland ecosystem has low level of nutrients, and how increasing nitrogen(N) deposition affects N cycling has not been solved yet. Methods An experimental platform was set up to simulate increasing N deposition in the saline alkaline grassland ecosystem in Youyu County, Shanxi Province, eight levels of N addition were designed, expressed by 0, 1, 2, 4, 8, 16, 24, 32 g·m^(-2)·a^(-1)(N0, N1, N2, N4, N8, N16, N24, N32). The same amount of ammonium nitrate(NH_(4)NO_(3)) was sprayed at the beginning of each month during the growing seasons from May to September every year. From May of 2017, net N mineralization rate was measured once a month using top-cover PVC cylinders. The experiment has been conducted for three years from 2017 to 2019. Important findings The results showed that,(1) High N addition levels(N16, N24, N32) significantly increased soil inorganic N pool.(2) Soil N mineralization in the salinized grassland ecosystem was dominated by nitrification. After three years of N addition, high levels of N addition(N24, N32) significantly promoted the rate of soil net nitrification, and different N levels showed different effects in different months and years.(3) The sensitivity of soil net N mineralization to different levels of N addition showed significant difference among three years with different precipitation. Lower N addition improved the sensitivity of soil net N mineralization, while higher N addition reduced the sensitivity of soil net N mineralization.(4) There was a positive correlation between soil net N mineralization rate and soil temperature, water content, and a negative correlation between soil net N mineralization rate and soil pH value. The sensitivity of soil N cycle to N addition was higher at low level of N addition than at high level of N addition in the saline-alkaline grassland of north China. The influence of increasing N deposition on soil N cycle should be considered in process-based models in the future.