杨-桤混交林及其凋落物对土壤氮矿化的影响

【目的】研究含氮量中等的立地条件下杨-桤混交林相比于杨树纯林能否增加土壤养分,分析桤木是否为合适的固氮树种,探讨桤木与杨树营造混交林以缓解人工林连栽带来的土地肥力衰退问题。【方法】以苏北地区杨树人工纯林以及杨-桤(1∶1)混交林为研究对象,采用尼龙网袋法观察两种林分下凋落叶质量分解以及养分释放的过程,比较两种凋落物处理(去除凋落物与保留凋落物)对土壤全氮以及碱解氮含量的影响。采用树脂芯田间原位培养法,研究杨-桤混交林与杨树纯林两种林分在两种凋落物处理下,林地土壤氮年净矿化量的变化。【结果】营造杨-桤混交林显著提高了林地土壤全氮和碱解氮含量。杨树纯林凋落物分解时需从周围土壤中吸收更多氮素,且分解速率较慢。杨-桤混交林凋落物分解更快进入氮释放状态,因此去除凋落物后混交林土壤碱解氮显著减少。保留凋落物处理下,杨-桤混交林中每公顷土壤氮年净氨化量显著高于杨树纯林中的,去除凋落物后两者差异不显著。【结论】研究发现,桤木是杨树较合适的伴生树种,营造杨-桤混交林可以起到增加土壤全氮和碱解氮含量的作用,有助于缓解杨树纯林连栽带来的地力下降问题。混交林凋落物分解速率与氮素释放速率更快,有利于人工林生态系统氮循环。保留凋落物处理下,混交林中每公顷土壤氮年净氨化量显著高于杨树纯林的,而每公顷土壤氮年净硝化量没有显著差异,表明混交林凋落物分解释放的氮素主要促进土壤氨化过程,对土壤硝化过程影响较弱。

The influence of poplar-alder mixed forest and litter on soil nitrogen mineralization

【Objective】To investigate whether soil nutrient concentrations would increase in a poplar(Alus trabeculosa)-alder(Populus×euramericana‘Nanlin 95’)mixed forest relative to the nutrient concentrations in a pure poplar forest under moderate nitrogen(N)concentrations,and analyze the N-fixing potential of alder,to explore the establishment of poplar-alder mixed forests as a potential strategy for mitigating a decline in soil fertility caused by successive plantation rotations.【Method】Poplar plantations and poplar-alder mixed forest sites were set up in experimental plots in the north of Jiangsu Province.A nylon mesh belt was used to investigate the mass decomposition and nutrient release from poplar and/or alder litter and the effect of litter removal and non-removal on the soil total N and available N concentrations.The in-situ resin core method was used to investigate the annual changes in net N mineralization rates in the two forests soil with and without litter.【Result】The establishment of a poplar-alder mixed forest increased soil total N and available N concentrations significantly.Litter in the pure poplar forest decomposed slowly and the N in the surrounding soil was adsorbed by the litter during decomposition.Conversely,the litter in the poplar-alder mixed forest decomposed and entered the N release state rapidly.Therefore,the soil available N decreased significantly in the poplar-alder mixed forest when the litter was removed.In the litter non-removal treatment,the annual net amount of N from ammonification per hectare of soil in the mixed forest was significantly higher than that in the pure poplar forest.However,in the litter removal treatment,there was no significant difference in N amount from ammonification between the two treatments with or without litter in the mixed forest.【Conclusion】Alder is a suitable tree species for integration with poplar.Poplar-alder mixed forest could increase soil total N and available N concentrations and facilitate the mitigation of soil fertility decline following successive rotations of pure poplar plantations.The rates of litter decomposition and N release in the mixed forests were more rapid,which would be beneficial to the N cycle in plantation ecosystems.In the litter non-removal treatment,although the nitrification N was not considerably different between the pure poplar plantation and the poplar-alder mixed forest,the ammonification N amount was higher in the mixed forest soil than that in the pure poplar soil,which indicated that the N released during litter decomposition in the mixed forest largely enhanced the soil ammonification process and weakly influenced the soil nitrification process.