Data from: Stream nitrogen concentration, but not plant N-fixing capacity, modulates litter diversity effects on decomposition

1. We are facing major biodiversity loss and there is evidence that such
loss can alter ecosystem functioning. However, the effects of plant
diversity on decomposition – a key component of the global carbon cycle –
are still unclear. A recent study suggested that a plant trait – their
nitrogen (N)-fixing capacity – could mediate effects of litter diversity
on decomposition by means of a microbial transfer of N from N-fixers to
non-fixers. 2. We explored this possibility in a microcosm experiment in
which we manipulated litter species richness (1, 2 or 4 species), N-fixing
capacity (N-fixer or non-fixer species), the presence of detritivores
(Sericostoma pyrenaicum larvae present or absent), and water N
concentration [natural stream water (0.366 mgL-1 of NO3-N) or elevated N
concentration (5 times the natural concentration: 1.835 mgL-1)]. 3. We
show that litter diversity accelerated decomposition by microorganisms and
detritivores (by 7 and 15%, respectively), mostly through complementarity
effects. However, enhanced decomposition did not result in higher
detritivore growth, possibly because all litter combinations provided
sufficient resources for their maximum growth. 4. The plant N-fixing
capacity had no effect on decomposition, which varied among species most
likely because of differences in a combination of litter traits.
Detritivores maximized the consumption of their preferred resource in
litter mixtures, but also exploited less preferred resources, and their
C:N ratios increased during the experiment regardless of litter type or
water N concentration. 5. Microbial decomposition of litter with low N
content was enhanced at elevated water N concentration, suggesting that
microorganisms used nutrients from the water when those nutrients were
limiting in leaf litter. In contrast, detritivore growth was impaired at
elevated water N concentration, possibly because a stoichiometric
imbalance entails metabolic costs. 6. Our findings suggest that loss of
plant diversity in riparian forests would mostly affect decomposition in
streams of high nutrient status, where effects on microbial decomposition
would be more evident and detritivore populations may be reduced.

1. 当前全球正面临严重的生物多样性丧失问题，且有证据表明此类丧失会改变生态系统功能。然而，植物多样性对分解作用——全球碳循环的关键组成部分——的影响仍不明确。近期一项研究提出，某一植物功能性状——即其固氮能力——可通过固氮植物向非固氮植物的微生物氮转移，介导凋落物多样性对分解作用的调控效应。
2. 本研究通过微宇宙实验探究了这一可能性，实验中我们设置了4个调控变量：凋落物物种丰富度（1、2或4种）、固氮能力（固氮植物或非固氮植物）、食碎屑动物（Sericostoma pyrenaicum幼虫存在或缺失）以及水体氮浓度[天然溪流水体（NO3-N浓度0.366 mg·L⁻¹）或高氮浓度（为天然浓度的5倍，即1.835 mg·L⁻¹）]。
3. 研究结果显示，凋落物多样性可通过互补效应分别使微生物和食碎屑动物介导的分解速率提升7%和15%。但分解作用的增强并未伴随食碎屑动物生长量的提升，这可能是因为所有凋落物组合均能为食碎屑动物的最大生长提供充足的资源。
4. 植物固氮能力对分解作用无显著影响；不同物种间的分解速率存在差异，这大概率是由凋落物性状组合的差异所致。食碎屑动物会优先取食凋落物混合物中的偏好性资源，同时也会利用非偏好性资源；且实验期间，无论凋落物类型或水体氮浓度如何，食碎屑动物的碳氮比均有所升高。
5. 当水体氮浓度升高时，低氮含量凋落物的微生物分解速率会被提升，这表明当叶片凋落物中氮元素匮乏时，微生物可利用水体中的氮营养。与之相反，高氮水体环境会抑制食碎屑动物的生长，这可能是因为化学计量失衡会带来代谢成本。
6. 本研究结果表明，河岸森林的植物多样性丧失，主要会对高营养状态溪流中的分解过程产生影响：在此类溪流中，微生物分解受到的影响会更为显著，且食碎屑动物种群可能会出现缩减。