降水和氮沉降增加对草地土壤酶活性的影响

为探究降水和氮沉降增加对草地生态系统土壤酶活性的影响,于2014年生长季在内蒙古温带典型羊草草原开展了野外原位控制实验。试验共设置降水(对照,W0,自然降水;W15,增加15%的年均降水量)、施氮(对照,CK,0 kg N hm~(-2)a~(-1);低氮,LN,25 kg N hm~(-2)a~(-1);中氮,MN,50 kg N hm~(-2)a~(-1);高氮,HN,100 kg N hm~(-2)a~(-1))及其交互作用等8个不同的处理水平来模拟降水和氮沉降增加的全球变化情景,分别定量探讨了不同水、氮添加条件下草地表层土壤中与氮循环相关的蛋白酶,脲酶,硝酸还原酶,亚硝酸还原酶活性的月动态变化及其与土壤理化性质之间的相关性。研究结果表明:在自然降水条件下,不同施氮水平蛋白酶、脲酶和硝酸还原酶活性无显著差异,亚硝酸还原酶活性相比于对照显著降低;在增加降水条件下,不同施氮水平对蛋白酶和硝酸还原酶活性未产生显著性影响,高氮水平显著降低脲酶和亚硝酸还原酶活性。不同施氮水平是否添加降水对亚硝酸还原酶活性无影响,而增添降水使低氮处理的蛋白酶活性和中、高氮处理水平的硝酸还原酶活性增加、高氮处理的脲酶活性降低。降水在影响蛋白酶和硝酸还原酶活性方面具有主效应,氮沉降在影响亚硝酸还原酶活性方面具有主效应,而降水和施氮处理未表现出明显地交互作用。土壤亚硝酸还原酶活性与土壤碳氮比和NH~+_4-N含量极显著正相关,与NO-3-N含量显著正相关。

Effects of increased precipitation and nitrogen deposition on soil enzyme activities

An in-situ field experiment was established to examine the effects of increased precipitation and nitrogen deposition on the activities of soil enzymes in the semi-arid Leymus chinensis grassland of inner Mongolia during the 2014 growing season. Water was added at two levels (W0, no additional water; and W15, a 15% surplus on average rainfall) and N was added at four levels (CK, 0 kg N hm^-2a^-1; LN, 25 kg N hm^-2a^-1; MN, 50 kg N hm^-2a^-1; and HN, 100 kg N hm^-2a^-1) to simulate increasing precipitation and N deposition in a future global change scenario. Effects of different water and nitrogen additions on the monthly dynamics of N cycle-related protease, urease, nitrate reductase, and nitrite reductase activities in grass topsoil were quantified. We also analyzed the correlations between all variables and soil physical and chemical properties. Results showed that, under natural rainfall conditions, there were no significant differences in protease, urease, and nitrate reductase activity between elevated nitrogen deposition levels, although nitrite reductase activity decreased significantly compared to the control. Under increased precipitation conditions, different nitrogen levels did not significantly affect protease or nitrate reductase activity, although high levels of nitrogen loading (HN) significantly reduced nitrite reductase activity. Additional precipitation increased protease activity in the low nitrogen (LN) treatment and nitrate reductase activity in the medium and high nitrogen (MN, HN) treatments; it also reduced urease activity in the high nitrogen treatment (HN). Individually, precipitation had major effects on protease and nitrate reductase activity, while nitrogen deposition had a major effect on nitrite reductase activity, however, precipitation and nitrogen treatments did not show any significant interactive effects on all the enzyme activity. Finally, soil nitrate reductase activity was significantly, positively correlated with soil C/N ratio and NH₄⁺-N content, and positively correlated with NO₃^--N content.