降雨强度和时间频次对内蒙古典型草原土壤氮矿化的影响

Effects of precipitation intensity and temporal pattern on soil nitrogen mineralization in a typical steppe of Nei Mongol grassland

为了深入地理解和认识全球变化背景下草原生态系统土壤氮矿化的变化动态及其对气候变化的响应机制,以内蒙古典型草原不同围封年限样地(1999年围封和2013年围封)为研究对象,通过改变降雨量(增加降雨50%和减少降雨50%)和降雨时间频次(连续3年降雨处理;连续2年降雨处理,然后自然恢复1年;降雨1年处理,然后自然恢复1年),共设置7个降雨处理,研究不同降雨强度和时间频次对内蒙古典型草原土壤氮矿化的影响及其调控因子。研究结果表明:1)随着减雨或增雨时间频次的增加(由1年减雨或增雨至连续3年减雨或增雨处理),土壤净氮矿化速率和净硝化速率会降低,且土壤净氮矿化速率和净硝化速率的最大值发生在增雨或减雨1年恢复1年处理中,表明较高的降雨强度和时间频次对土壤净氮矿化速率和净硝化速率会产生抑制作用,而适宜的土壤水分和温度条件更有利于土壤氮矿化作用。2)与2013年围封样地相比,1999年围封样地的土壤净氮矿化速率和净硝化速率、土壤累积氮矿化量和硝化量更高,表明长期的自然封育有利于养分储存和土壤质量恢复。3)长期的连续性增雨或减雨处理会显著影响土壤含水量和土壤温度(短期的间断性增雨或减雨处理则无显著影响),但二者对土壤氮矿化的影响在两块样地的表现不同,如在2013年围封样地,土壤无机氮和净氮矿化速率主要受土壤水分的影响,而在1999年围封样地,土壤无机氮和净氮矿化速率主要受土壤温度的影响,土壤水分甚至对净氮矿化速率产生了明显的负效应。研究表明,降雨强度和时间频次对内蒙古典型草原土壤氮矿化具有重要影响,但影响大小因样地而异,与土壤质地、群落组成和干扰程度等因素有关。

Aims Our objective is to： 1） explore the dynamics of soil nitrogen（N） mineralization in a grassland ecosystem in response to the changes in precipitation intensity and temporal distribution,and 2） identify the controlling factors.Methods The two study sites located in a typical steppe of the Nei Mongol grassland were fenced in 2013 and 1999,respectively.Our field experiment includes manipulations of three levels of precipitation intensity（increased 50%,decreased 50%,control） in three temporal patterns（increased or decreased precipitation for three years; increased or decreased precipitation for two years and no manipulation for one year; increased or decreased precipitation for one year and no manipulation for one year）.Important findings 1） The soil net N mineralization and net nitrification rates decreased with changes in the temporal distributions of precipitation from one year to three years,with the maximum values of soil net N mineralization and nitrification rates observed in the treatments of increased or decreased precipitation for one year and no manipulation for one year（＋PY1 or –PY1）.This indicates that the high precipitation intensity and longer precipitation may have negative effects on soil net N mineralization and nitrification rates,while the moderate soilmoisture and temperature may stimulate soil mineralization.2） The soil net N mineralization and nitrification rates,soil cumulative N mineralization,and nitrification in the fenced site in 1999 were higher than those in the site fenced in 2013,implying that a long-term enclosure may have promoted nutrient storage and soil quality restoration.3） The long-term treatments of increased or decreased precipitation had significant effects on soil water content and temperature,whereas the short-term,discontinuous precipitation produced minor effects on soil moisture and temperature.Moreover,the controlling factors for soil N mineralization were different between the two fields.Soil moisture had a major effect on soil inorganic N content and net N mineralization rate in the site fenced in 2013,while soil temperature played a dominant role in the site fenced in 1999,with the net N mineralization rate depressed by higher soil moisture.Our findings suggest that the precipitation intensity and temporal distribution had important impacts on soil N mineralization in the Inner Mongolia grassland; these effects was site-dependent and particularly related to soil texture,community composition,and disturbance,and other factors.