Resource Ecology Group, Wageningen University, Wageningen, The Netherlands,
Oecologia. 2014 Apr;174(4):1107-16. doi: 10.1007/s00442-013-2848-8. Epub 2013 Nov 28.
A key question in savanna ecology is how trees and grasses coexist under N limitation. We used N stable isotopes and N content to study N source partitioning across seasons from trees and associated grasses in a semi-arid savanna. We also used (15)N tracer additions to investigate possible redistribution of N by trees to grasses. Foliar stable N isotope ratio (δ(15)N) values were consistent with trees and grasses using mycorrhiza-supplied N in all seasons except in the wet season when they switched to microbially fixed N. The dependence of trees and grasses on mineralized soil N seemed highly unlikely based on seasonal variation in mineralization rates in the Kruger Park region. Remarkably, foliar δ(15)N values were similar for all three tree species differing in the potential for N fixation through nodulation. The tracer experiment showed that N was redistributed by trees to understory grasses in all seasons. Our results suggest that the redistribution of N from trees to grasses and uptake of N was independent of water redistribution. Although there is overlap of N sources between trees and grasses, dependence on biological sources of N coupled with redistribution of subsoil N by trees may contribute to the coexistence of trees and grasses in semi-arid savannas.
在热带稀树草原生态学中，一个关键问题是树木和草本植物如何在氮限制下共存。我们使用氮稳定同位素和氮含量，研究了半干旱热带稀树草原中树木和相关草本植物在整个季节的氮源分配情况。我们还使用了（15）N 示踪剂添加来研究树木可能向草本植物重新分配氮的情况。除了在雨季它们转而利用微生物固定的氮之外，所有季节的叶片稳定氮同位素比值（δ（15）N）都与树木和草本植物利用菌根供应的氮一致。基于克鲁格国家公园地区氮矿化率的季节性变化，树木和草本植物对矿化土壤氮的依赖似乎不太可能。值得注意的是，尽管通过根瘤固氮的潜力不同，但三种树木的叶片 δ（15）N 值在所有季节都相似。示踪剂实验表明，氮在所有季节都由树木重新分配给林下草本植物。我们的研究结果表明，氮从树木到草本植物的再分配和氮的吸收与水分的再分配无关。尽管树木和草本植物的氮源有重叠，但对生物源氮的依赖以及树木对亚土壤氮的再分配可能有助于半干旱热带稀树草原中树木和草本植物的共存。
