凋落物和氮添加对亚热带森林土壤浸提氮组分的影响

Effects of Litter and Added Nitrogen on Soil Extractable Nitrogen Fractions in Mid-subtropical Forests

土壤不同氮组分对氮素循环和转化过程具有不同程度的调节作用。为了准确评价和理解氮添加和凋落物对土壤氮动态的影响,以亚热带罗浮栲(Castanopsis fabri)天然林和杉木(Cunninghamia lanceolata)人工林土壤为研究对象,在保留凋落物(留凋)和去除凋落物(去凋)情况下,模拟氮沉降试验,采用不同浸提剂(水、K_2SO_4、2.5 mol·L^(-1)和13 mol·L^(-1) H2SO4)逐步浸提土壤氮组分,研究氮添加[对照(CK,0 kg·hm^(-2)·a^(-1))、低氮(LN,75 kg·hm^(-2)·a-1)和高氮(HN,150 kg·hm^(-2)·a-1)]对亚热带红壤浸提组分氮含量的影响。结果表明:硫酸钾浸提的NH_4^+-N和可溶性有机氮(SON)高于水浸提的,而水浸提的NO_3^--N含量高于硫酸钾浸提的,酸水解性氮高于水溶性氮和盐溶性氮。林分显著影响不同组分氮含量,对水溶性氮含量的影响最显著,而阔叶天然林土壤氮组分对氮添加的响应更明显。留凋处理增加水溶性氮,且有利于惰性氮的分解,而去凋处理有利于惰性氮积累。氮添加对酸解性氮组分的影响不显著,氮添加显著降低2种林分土壤的惰性氮指数,降幅为64.7%～82.2%,去凋处理的降幅更大。土壤水溶性、交换性和弱酸浸提的SON与微生物生物量氮呈显著正相关,土壤水溶性和交换性SON可能是参与土壤氮矿化的重要组分。不同浸提组分氮之间呈显著正相关,说明组分之间存在相互影响。可见,从不同氮组分角度研究氮动态,更能反映其内在机理。

Soil different extracted nitrogen components have different degrees of regulation on nitrogen cycle and transformation process.A simulated nitrogen deposition and litter layer manipulation experiments were conducted in a subtropical broad-leaved forest dominated by Castanopsis fabri(CAF)and a conifer plantation forest of Cunninghamia lanceolata(CUL)with three levels of nitrogen amended as NH4NO3:control(0 kg·hm^-2·a^-1),low nitrogen(75 kg·hm^-2·a^-1),high nitrogen(150 kg·hm^-2·a^-1)and two litter layer treatments of removed litter layer and intact litter layer in order to understand the effects of nitrogen deposition and litter layer on soil extractable nitrogen pools.Soil samples collected from the topsoil(0~15 cm)were extracted by using in a sequence of water,0.5 mol·L^-1 K2SO4,2.5 mol·L^-1 H2SO4,13 mol·L^-1 H2SO4.The results showed that the ammonium nitrogen and soluble organic nitrogen(SON)of potassium sulfate extract were higher than that of water extract,while the nitrate content of water extract was higher than that of potassium sulfate,and acid-hydrolyzed nitrogen was higher than that of soluble nitrogen and salt-soluble nitrogen.The forest types,especially the broadleaf forest,had significant effect on those extractable nitrogen fractions,especially the content of water-soluble nitrogen.The treatment of intact litter layer increased the water-soluble nitrogen,which favored the decomposition of inert nitrogen,and the removed litter layer was beneficial to the accumulation of inert nitrogen.The nitrogen addition had no significant effect on the acid-hydrolysis nitrogen,and the nitrogen addition significantly reduced the inert nitrogen index of two forest soils by 64.7%~82.2%,and the decline was greater in the removed litter layer treatment.There was a significant positive correlation of the soil water-soluble,exchangeable and weak acid extracts of SON with microbial biomass nitrogen,and soil water-soluble and exchangeable SON might be an important component in soil nitrogen mineralization.There was a significant positive correlation between the different species of extracted nitrogen,indicating the interaction among the nitrogen components.It can be seen that the study of different nitrogen components could reflect the intrinsic mechanism of soil nitrogen dynamics.