氮沉降对杉木和枫香土壤氮磷转化及碳矿化的影响

氮沉降是全球变化的重大环境问题,根际是地下生态过程研究的前沿,但目前氮沉降对亚热带地区不同树种土壤氮、磷供应和碳矿化根际过程的影响及其机制尚不清楚。选取典型红壤区15a针叶树杉木(Cunninghamia lanceolata)和阔叶树枫香(Liquidamba formosana)为对象,野外原位开展10 g N m~(-2)a~(-1)氮沉降试验3a,于2014年8月收集杉木和枫香根际土壤和非根际土壤,测定其p H值、有效氮、速效磷、水溶性有机碳及其34 d有机碳矿化动态,并计算根际效应。结果表明:氮沉降显著降低两个树种土壤p H值和杉木根际土壤速效磷(P0.05);提高枫香非根际土壤NO~-_3-N和杉木非根际土壤水溶性有机碳含量。同时,氮沉降显著提高杉木土壤有机碳矿化速率,根际和非根际的增幅分别为71.2%和41.2%,降低枫香土壤有机碳矿化速率,根际和非根际的降幅分别为10.6%和44.1%。此外,氮沉降显著降低枫香土壤NO~-_3-N和有机碳前期矿化速率的根际效应,增强后期矿化速率的根际效应,而杉木对氮沉降响应不显著。可见,氮沉降可显著改变树木土壤养分供应和有机碳稳定性,且丘陵红壤区针叶树和阔叶树根际过程对氮沉降的响应模式有别。率先报道了亚热带不同树种根际碳、氮、磷耦合过程对氮沉降的响应格局,并较好地揭示了针叶树和阔叶树对氮沉降响应的分异机制。

Influence of nitrogen deposition on soil nutrient supply and organic carbon mineralization in Cunninghamia lanceolata and Liquidambar formosana plantations

Nitrogen (N) deposition is a major environmental issue that affects global climate change. Study of the rhizosphere has become a research frontier in underground ecology. However, the effects of N deposition on N and phosphorus (P) supplies, and organic carbon (OC) mineralization in the rhizosphere of trees in subtropical China remains unclear, and whether the underlying mechanisms differ between tree species need to be determined. In the present study, a coniferous tree (Cunninghamia lanceolata) and a broadleaf tree (Liquidambar formosana) each 15 years of age were selected at the Qianyanzhou Experimental Station of the Chinese Academy of Sciences to evaluate in situ N deposition. Trees were treated with 10 g N m^-2 a^-1 and compared to the control. After 3 years, rhizosphere and bulk soils for both tree species were collected to assess pH value, mineral N, available P, water-soluble organic carbon (WSOC), and the dynamic of OC mineralization following 34 days incubation. Rhizosphere effects of all measured variables were assessed using ratios of rhizosphere to bulk soil, and OC mineralization parameters at early (0-9 d), middle (9-19 d), and late stages (19-34 d) were obtained using the traditional dynamics model. N deposition significantly decreased rhizosphere and bulk soil pH, available P in rhizosphere soil of C. lanceolata, and OC mineralization intensity in bulk soil of L. formosana. N deposition also increased NO₃^--N in bulk soil of L. formosana and WSOC in bulk soil of C. lanceolata (P < 0.05). In contrast, NH₄⁺-N, mineral N, and the ratio of mineral N to available P in rhizosphere and bulk soils did not differ significantly between both species. N deposition significantly increased OC mineralization rates in C. lanceolata rhizosphere and bulk soil by 71.2% and 41.2%, respectively; but decreased OC mineralization rates in L. formosana rhizosphere and bulk soil by 10.6% and 44.1%, respectively. N deposition significantly decreased both the rhizosphere effects of NO₃^--N and the early OC mineralization rate in L. formosana, but increased its late OC mineralization rate; whereas the rhizosphere effects of all measured variables in C. lanceolata showed no significant differences between control (CK) and N treatments. N deposition significantly altered soil nutrient supply and OC stability, and the synchronicity of responses to N deposition between rhizosphere and bulk soil was stronger for C. lanceolata than for L. formosana. We concluded that response of rhizosphere processes to N deposition differs between conifer and broadleaf tree species. The response of rhizosphere OC mineralization to N deposition showed a positive effect for C. lanceolata, and a negative effect for L. formosana, as their rates preferentially increased and decreased, respectively with N deposition. This difference could be attributed to their nutrient requirements and root traits, because C. lanceolata prefers fertile soil and are not associated with mycorrhizal fungi, whereas L. formosana can tolerate infertile soil with roots that are associated with mycorrhizal fungi. To our knowledge, the present study is the first to report on the effects of N deposition on the interactions between supplies of N and P in soil, and the stability of OC in different tree species of subtropical China, and reports divergent mechanisms in coniferous versus broadleaf tree species.