Topographic differences in nitrogen cycling mediate nitrogen retention in a subtropical, N-saturated forest catchment
Topographic differences in nitrogen cycling mediate nitrogen retention in a subtropical, N-saturated forest catchment
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DOI：
10.1016/j.soilbio.2021.108303
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发表时间：
2021-08
影响因子：
9.7
通讯作者：
Jing Zhu;A. Jansen-Willems;C. Müller;P. Dörsch
中科院分区：
文献类型：
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作者：
Jing Zhu;A. Jansen-Willems;C. Müller;P. Dörsch
森林集水区的氮素淋溶和气态氮排放是由土壤控制的，土壤的氮素状况和周转率因景观位置的不同而不同。为了解地形对受大气高氮沉降影响的森林集水区氮素存留和耗散的影响，采用AneX-Situite15N示踪研究了中国南部一片氮饱和的亚热带森林的坡面土壤和地下水排泄区土壤。尽管土壤处于严重的N饱和状态，但HS的土壤立即将大量添加的15N-NH4+结合到顽固的有机N中。剩余的NH4+通过快速的N固定和再矿化的微生物循环，缓慢地释放NH4+用于自养硝化。异养硝化作用只有在示踪剂应用后才能观察到。添加的15N-NO3-在土壤微生物量和溶解有机氮之间循环，没有储存在顽固的有机氮库中，解释了HS土壤对NO3-−淋失的强烈倾向。相反，GDZ的土壤通过将15N-NH4+结合到顽固性有机N中，并将15N-NO3-反硝化为气态N，从而充当了增加N的汇。在这里，N的固定超过了N的矿化，表明N的限制。异养硝化作用是GDZ土壤NH3氧化的主要途径，而N2O-N对氮素去除有较大贡献。非生物过程对N3DNRA向有机N的吸收起作用，但对N2O的产生没有影响，而−在两种土壤中都可以忽略不计。总体而言，我们的发现表明地形对氮素循环的控制很强，这可能解释了亚热带中国地区氮素饱和森林中氮的意外高保留和迁移。
N leaching and gaseous N emissions from forested catchments are controlled by soils differing in nitrogen (N) status and turnover depending on landscape position. To understand the impact of topography on N retention and dissipation in forested catchments suffering from high atmospheric N deposition, we carried out anex-situ15N-tracing study with soils from a hillslope (HS) and a hydrologically connected groundwater discharge zone (GDZ) of an N-saturated subtropical forest in South China. Despite being severely N-saturated, soil from HS incorporated a substantial amount of added15N–NH4+instantly into recalcitrant organic N. The remaining NH4+was cycled via a microbial loop of fast N immobilization and re-mineralization, slowly releasing NH4+for autotrophic nitrification. Heterotrophic nitrification was only observed right after tracer application. Added15N–NO3-cycled between soil microbial biomass and dissolved organic N without being stored in the recalcitrant organic N pool, explaining the strong propensity of HS soils for NO3−leaching. By contrast, the soil from GDZ acted as a sink for added N by incorporating15N–NH4+into recalcitrant organic N and denitrifying15N–NO3-to gaseous N. Here, N immobilization exceeded N mineralization, suggesting N limitation. Heterotrophic nitrification was the main pathway of NH3oxidation in the GDZ soil, and N2O–N contributed substantially to N removal. Abiotic processes played a role in NO3−incorporation into organic N but not in N2O production, while DNRA was negligible in either soil. Overall, our findings suggest strong topographic control on N cycling, which might explain the unexpectedly high N retention and removal from N-saturated forests in subtropical China.