混交对亚热带针叶树根际土壤氮矿化和微生物特性的影响

Effects of tree mixture on rhizosphere soil nitrogen mineralization and microbial characteristics of coniferous trees in subtropical plantations

混交阔叶树是退化红壤区针叶林改造的重要措施之一,土壤养分供应和转化是评价混交效应的重要参数,但混交后针叶树根际土壤氮矿化特征及其影响因素还不清楚。选取退化红壤区马尾松(Pinus massoniana Lamb.)纯林、湿地松(Pinus elliottii Engelmann)纯林及其补植木荷(Schima superba Gardn. et Champ.)形成的马-木混交林和湿-木混交林为研究对象,采集4种林分下针叶树根际土壤,测定速效养分含量、氮矿化速率、氮水解酶活性和微生物磷脂脂肪酸含量,探究混交对针叶树根际土壤氮供应及微生物特性的影响,分析土壤氮矿化和微生物特性间的相关性。结果表明:混交显著增加了针叶树根际土壤铵态氮,矿质氮和有效磷含量,而对硝态氮影响不显著。根际土壤氮矿化以硝化作用为主,混交后针叶树根际土壤氨化速率降低了27.0%,硝化速率增加了55.4%,而最终净氮矿化速率增加了24.1%。两个树种间,马尾松根际土壤矿质氮含量和净氮矿化速率显著高于湿地松。针叶树根际土壤真菌、丛枝菌根真菌,总微生物生物量及真菌/细菌比在混交后显著增加,且马尾松根际土壤总微生物和真菌生物量分别比湿地松高18.9%和27.0%。同时,针叶树根际土壤β-N-乙酰氨基葡萄糖苷酶活性在混交后显著增强,且根际土壤硝化速率和净氮矿化速率与微生物指标和酶活性正相关。总的来说,混交阔叶树显著提高了针叶树根际土壤氮供应,以此应对阔叶树混交后带来的养分竞争压力,而马尾松倾向于积极性的应对策略,通过增加土壤氮矿化以适应外界环境改变。

Mixing broad-leaved trees into coniferous plantations is one of the important measures for plantation reconstruction in the degraded red soil regions. Soil nutrient supply and transformation are important parameters for evaluating mixed effects. However, the characteristics and influencing factors of nitrogen(N) mineralization in the coniferous rhizosphere soil after mixing are not clear. In this study, we chose the Masson pine(Pinus massoniana Lamb.) plantation, slash pine(Pinus elliottii Engelmann) plantation, Pinus massoniana-Schima superba mixed plantation and Pinus elliottii-Schima superba mixed plantation as research objects. The rhizosphere soil of coniferous trees in the four plantations were collected to measure available nutrient concentrations and N mineralization rate. Enzyme activities associated with N hydrolysis were determined with microplate fluorometer and soil microbial biomass and community composition were also analyzed with phospholipid fatty acid method. The aims of this research were to study the effects of mixing on N supply and microbial characteristics in the rhizosphere soil of conifers and to analyze the correlation between soil N mineralization and microbial characteristics. The results showed that mixing with broad-leaved trees significantly increased the concentrations of ammonium N, mineral N and available phosphorus in the coniferous rhizosphere soil. However, no significant difference was found for nitrate N concentration among the four plantations. The N mineralization in rhizosphere soil of conifers was dominated by nitrification. After mixing, the ammoniation rate of coniferous rhizosphere soil significantly decreased by 27.0%, while the rhizosphere soil nitrification rate of conifers increased by 55.4%, which induced an increase in the soil net N mineralization rate by 24.1% of coniferous trees. Among the two tree species, the mineral N concentration and net N mineralization rate in rhizosphere soil of Masson pine were significantly higher than those of slash pine. Mixing significantly increased the biomass of fungi, arbuscular mycorrhizal fungi and total microbes, and the fungi/bacteria ratio in coniferous rhizosphere soil. The total microbial and fungal biomass in the rhizosphere soil of Masson pine were 18.9% and 27.0% higher than that of slash pine, respectively. The activity of rhizosphere soil β-N-acetylglucosaminidase increased significantly after mixing, and the rates of nitrification and net N mineralization positively correlated with soil microbial parameters and enzyme activity. Generally speaking, mixing significantly enhances rhizosphere soil nitrogen supply, which helps coniferous trees to cope with the pressure of nutrient competition caused by the mixture of broad-leaved trees. Masson pine tends to adopt positive strategies by increasing soil N mineralization to cope with external environment changes.