林牧系统的树木中豆科植物的比例减少并不影响其内部的氮吸收效率

Trees in silvopastoral systems reduce legume proportion with no consequence for internal N resorption efficiency

在以豆科植物为主的草地中,豆科植物通过生物固氮为草地提供氮(N)。以往对林牧系统的研究发现,树木附近的豆科植物比例下降,将导致N供应以及树间草坪收获的牧草中N浓度的空间变化。在自然生态系统中,氮吸收效率(NRE)随着养分限制而增加,所以在树木附近豆科植物比较稀缺。本研究验证了以下假设,即树木附近的NRE增加,补偿了豆科植物相对于牧草中N浓度的损失。为此,本研究分析了5年前建立的巷道种植系统中,分布在林木间的两种由豆科草本植物为主的植被组成。研究结果表明,豆科植物的比例向树线方向下降了45%,因此,活的和枯死的牧草中的N浓度也下降了,但平均仅下降了15%。由于N浓度降低以及靠近树木的牧草质量下降,N产量最多下降了50%。尽管树木附近的豆科植物减少,但无论植被组成如何,NRE都不受树线的影响。此外,与未管理的生态系统相比,收获之间的落叶间隔较短的林牧草地中管理草坪的内部N循环的相关性较低。豆科植物的比例控制着活的和枯死的草本植物的N浓度,而与林牧系统中的树木遮阴无关。

In legume-based grasslands,legumes supply the sward with nitrogen(N)from biological N2-fixation.Previous studies in silvopastoral systems have emphasized that legume proportions decline near trees which would cause spatial variation in the N supply and the concentration of N in the harvested herbage of grass swards between trees.In natural ecosystems,it was shown that the N resorption efficiency(NRE)was increased by nutrient limitation,hence,near trees where legumes are scarce.Therefore,we tested the hypothesis that the NRE is increased near trees and compensates for the loss of legumes with respect to N concentration in live herbage.Two vegetation compositions consisting of legume-based grass swards were analysed across positions between tree lines in an alley cropping system established 5 years prior to this study.Legume proportion declined up to 45%towards the tree line,and also N concentration in live and dead herbage but on average only declined by 15%.As a consequence of a reduced N concentration and also herbage mass close to trees,the N yield decreased by up to 50%.Despite the loss of legumes near trees,the NRE was unaffected by the tree line irrespective of vegetation composition.Further,the results indicate that internal N cycling of managed grass swards in silvopastoral grassland with short defoliation intervals between harvests is of lesser relevance than in unmanaged ecosystems.Legume proportions control the N concentrations of live and dead herbage irrespective of tree shading in silvopastoral systems.