氮素添加对内蒙古羊草草原净氮矿化的影响

EFFECTS OF NITROGEN ADDITION ON NET NITROGEN MINERALIZATION IN LEYMUS CHINENSIS GRASSLAND, INNER MONGOLIA, CHINA

为了更好地了解天然草原氮素矿化对全球氮沉降背景和草原施肥管理模式的响应,从2000年起对内蒙古典型草原羊草(Leymu schinensis)群落开展了长期的氮素添加实验,分别设置对照(N0),添加5gNH4NO3·m–2(N1.75)、30gNH4NO3·m–2(N10.5)和80gNH4NO3·m–2(N28)4个氮素添加梯度。2002年,从相邻的同时进行施肥的两个生态系统类型,即1979年围封的样地A和1999年围封的样地B进行土壤取样,在最佳温度(25℃)和最适土壤湿度(即60%田间持水量)下进行5周的室内培养,并用阶段性淋溶方法研究了氮素添加对土壤氮矿化动态的影响。在A和B两个样地内,氮素添加都显著改变了土壤的累积氮矿化量。最高氮素处理N28对应于最低的累积氮矿化量,而低氮素处理N1.75使得累积氮矿化量达到最高。在N0和N1.75处理中,硝态氮的含量高于铵态氮;在N28处理中,却表现出相反的趋势。氮素添加显著降低了土壤的pH值,但累积氮矿化量与土壤pH值、有机碳和全氮均没有显著的相关性。大多数氮素添加处理水平在样地A具有比样地B更高的土壤累积氮矿化量。

Aims Our objective was to examine the response of soil nitrogen mineralization in natural grassland to N addition, which is pertinent to global N deposition and grassland fertilization management. Methods We began a long-term N addition experiment in 2000 in typical steppe, Leymus chinensis community, in Inner Mongolia of China, at four N addition levels： control （N0）, 5 g NH4NO3·m^–2 （N1.75）, 30 g NH4NO3·m^–2 （N10.5） and 80 g NH4NO3·m^–2 （N28）. In 2002, soils were collected from two adjacent sites ― A and B （fenced since 1979 and 1999, respectively）, with simultaneous fertilization. The soils were incubated under optimal temperature （25 °C） and moisture （60% of water holding capacity （WHC）） in the laboratory to examine effects of nitrogen addition on the potential net N mineralization in 5 weeks by using a periodically leaching method. Important findings Nitrogen addition significantly influenced accumulated mineralized nitrogen （Nm） in all plots of the two grassland sites. The highest N addition level （N28） was associated with the lowest Nm, whereas the N1.75 had the highest Nm among the four nitrogen addition levels. Accumulated nitrate nitrogen in N0 and N1.75 was higher than ammonium nitrogen, but that was not the case at the higher N addition levels （N10.5, N28）. N addition significantly decreased soil pH value, but no significant correlations were found between Nm and soil pH value, total organic C or total N. N addition causes higher Nm at site A （fenced from 1979） than at site B （fenced from 1999）.