神农架地区次生林主要树种林冠雨的氮磷营养特征

Nitrogen and phosphorous characteristics of canopy rainfall of eight dominant tree species in secondary forests of Shennongjia Mountain

林冠水文过程对大气降雨氮磷营养特征的影响,对于森林生态系统营养物质循环、森林演替和生物多样性维持具有重要意义。在神农架地区次生林中选择了8个优势树种,定期收集、测定了林外降雨及各树种穿透雨、树干茎流的氮磷浓度和pH值,分析了该地区的湿氮、磷沉降强度以及种特异性的茎流、穿透雨氮、磷浓度和pH值的影响。结果表明:该地区生长季节的湿氮、湿磷沉降强度分别为11.89 kg N·hm-2·a-1和1.98 kg P·hm-2·a-1,其中,硝酸盐氮占全氮的40.75%,亚硝酸盐氮占15.19%,铵态氮占44.08%;降雨偏酸性,全年pH平均值为5.84;8个树种穿透雨和树干茎流的氮、磷年均浓度显著高于大气降雨,表现出较强的淋溶作用,同时,各树种林内雨氮、磷特征存在显著的月度差异和种间差异,尤其是原生树种巴山冷杉的硝酸盐氮、铵态氮、总氮及P浓度均较高,而引进树种华山松和阔叶树种的浓度则较低;8个树种的茎流和穿透雨的pH值分别介于5.42~6.82和6.23~6.83,均略高于大气降雨;在种组水平,引进树种的pH值显著低于原生树种,但氮、磷浓度在两组间不显著。因此,该地区次生林演替过程中林分变化对林内雨氮磷营养特征的影响值得进一步关注。

Variation of N and P characteristics induced by hydrologic process of forest canopy rainfall plays an important role in nutrient cycling, community succession and biodiversity conservation in forests. In 2015, eight dominant tree species in secondary forests of Shennongjia Mountain were selected. Rainfall in open area, throughfall and stemflow of each species were collected monthly. The N and P characteristics of water samples, including nitrate, nitrite, ammonium,total nitrogen, total P and pH, were measured. Intensities of N and P deposition and the impacts of speciesspecific canopy hydrology on N and P concentrations and pH were analyzed. Ourresults showed that N and P depositions in Shennongjia Mountain were 11.89 kg N·hm-2·a-1 and 1.98 kg P·hm-2·a-1, respectively. Nitrate, nitrite and ammonium accounted for 40.75%, 15.19%, and 44.08% of the total N in rainfall, respectively. Annual mean concentrations of N and P in throughfall and stemflow of eight species were significantly higher than those of the atmospheric rainfall, indicating that N and P leaching occurred when rainfall dropped through tree canopy and stem. Meanwhile, we observed significant interspecific differences of N and P in both stemflow and throughfall. Interestingly, nitrate, ammonium, total N and total P of stemflow and throughfall of native speciesAbies fargesii were the highest among the eight species. pH of stemflow and throughfall were slightly higher than that of atmospheric rainfall. Specifically, pH of stemflow of eight species ranged from 5.42 to 6.82, and those of throughfall varied from 6.23 to6.83. Furthermore, pH of stemflow and throughfall of introduced species was significantly lower than that of the native ones. In contrast, there were no significant differences of N and P between introduced and native tree groups. Therefore, more attention should be paid on the impacts of secondary forest succession on forest N and P cycling in Shennongjia Mountain.