Temporal stability of net primary productivity (NPP) is important for predicting the reliable provisioning of ecosystem services under global changes. Although nitrogen (N) addition is known to ...affect the temporal stability of aboveground net primary productivity (ANPP), it is unclear how it impacts that of belowground net primary productivity (BNPP) and NPP, and whether such effects are scale dependent. Here, using experimental N addition in a grassland, we found different responses of ANPP and BNPP stability to N addition at the local scale and that these responses propagated to the larger spatial scale. That is, N addition significantly decreased the stability of ANPP but did not affect the stability of BNPP and NPP at the two scales investigated. Additionally, spatial asynchrony of both ANPP and BNPP among communities provided greater stability at the larger scale and was not affected by N addition. Our findings challenge the traditional view that N addition would reduce ecosystem stability based on results from aboveground dynamics, thus highlighting the importance of viewing ecosystem stability from a whole system perspective.
It remains unknown how would nitrogen (N) enrichment affect the stability of belowground productivity. Using a field experiment with six N addition rates in a temperate steppe, we found that N addition decreased the stability of ANPP, but did not affect the stability of BNPP and total NPP at both local and larger scales. Spatial asynchrony of both ANPP and BNPP provided greater stability at larger scale and was not affected by N addition, highlighting the importance of understanding stability from a whole system perspective.
Summary
The continuing nitrogen (N) deposition observed worldwide alters ecosystem nutrient cycling and ecosystem functioning. Litter decomposition is a key process contributing to these changes, but ...the numerous mechanisms for altered decomposition remain poorly identified.
We assessed these different mechanisms with a decomposition experiment using litter from four abundant species (Achnatherum sibiricum, Agropyron cristatum, Leymus chinensis and Stipa grandis) and litter mixtures representing treatment‐specific community composition in a semi‐arid grassland under long‐term simulation of six different rates of N deposition.
Decomposition increased consistently with increasing rates of N addition in all litter types. Higher soil manganese (Mn) availability, which apparently was a consequence of N addition‐induced lower soil pH, was the most important factor for faster decomposition. Soil C : N ratios were lower with N addition that subsequently led to markedly higher bacterial to fungal ratios, which also stimulated litter decomposition.
Several factors contributed jointly to higher rates of litter decomposition in response to N deposition. Shifts in plant species composition and litter quality played a minor role compared to N‐driven reductions in soil pH and C : N, which increased soil Mn availability and altered microbial community structure. The soil‐driven effect on decomposition reported here may have long‐lasting impacts on nutrient cycling, soil organic matter dynamics and ecosystem functioning.
Human activities have significantly altered nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. Although studies of how ...changes in N availability affect biodiversity and community composition are relatively common, much less remains known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data exist regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver of plant nutrient economies and of the quality of litter plants produce. Accordingly, resorption patterns have marked ecological implications for plant population and community fitness, as well as for ecosystem nutrient cycling. In a semiarid grassland in northern China, we studied the effects of a wide range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis. After 4 years of treatments, N and P availability in soil and N and P concentrations in green and senesced grass leaves increased with increasing rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption‐mediated, positive plant–soil feedback induced by N inputs. Furthermore, N : P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of plant N and P stoichiometry to N inputs. Taken together, the results demonstrate that N additions accelerate ecosystem uptake and turnover of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasizes the importance of nutrient resorption as a pathway by which plants and ecosystems adjust in the face of increasing N availability.
Identifying the thresholds for the positive responses of total net primary productivity (NPP) to nitrogen (N) enrichment is an essential prerequisite for predicting the benefits of N deposition on ...ecosystem carbon sequestration. However, the responses of below‐ground NPP (BNPP) to N enrichment are unknown in many ecosystems, which limits our ability to understand the carbon cycling under the scenario of increasing N availability. We examined the changes in above‐ground NPP (ANPP), BNPP, and NPP of a temperate meadow steppe across a wide‐ranging N addition gradient (0, 2, 5, 10, 20, and 50 g N m−2 year−1) during 5 years. Both ANPP and NPP increased nonlinearly with N addition rates. The N saturation threshold for ANPP (TA) and NPP (TN) was at the rate of 13.11 and 6.70 g N m−2 year−1, respectively. BNPP decreased with increasing N addition when N addition rates ˃5 g N m−2 year−1, resulting in much lower TN than TA. Soil N enrichment played a key role in driving the negative impacts of high N addition rates on BNPP, and consequently on the earlier occurrence of N saturation threshold for NPP. Our results highlight the negative effects of soil N enrichment on NPP in natural grasslands super‐saturated with N. Furthermore, by considering ANPP and BNPP simultaneously, our results indicate that previous findings from above‐ground might have over‐estimated the positive effects of N deposition on primary productivity.
Based on a field experiment with six nitrogen (N) addition rates in a temperate steppe, we found that the N saturation threshold for ANPP (TA) and NPP (TN) was 13.11 and 6.70 g N m−2 year−1, respectively. Soil N enrichment played a key role in driving the negative impacts of high N addition rates on BNPP, resulting in much lower TN than TA. Our results indicate that previous findings from above‐ground might have over‐estimated the positive effects of N deposition on primary productivity.
Between‐species variation in nutrient resorption is one of the mechanisms explaining the positive relationship between biodiversity and primary productivity. Yet, the role of within‐species ...variations in nutrient resorption in mediating the relationship between biodiversity and productivity remains unclear.
We examined how within‐species nutrient resorption, and ultimately productivity, respond to changes in species richness by using four traits related to nitrogen and phosphorus use in four dominant species from different plant functional groups in a biodiversity removal experiment in the temperate steppe.
Nitrogen and phosphorus concentrations in both green and senesced leaves in all species significantly decreased with increasing plant species richness, suggesting that plants used those limiting nutrients more efficiently with increasing biodiversity. Plants in higher diversity communities resorbed more nutrients during senescence, which may facilitate reproduction and vegetative regrowth in the next year.
Synthesis. Our results highlight the importance of considering within‐species variation in nutrient resorption as an important underlying mechanism explaining the positive effects of biodiversity on primary productivity and ecosystem carbon accumulation.
Plants in higher‐diversity communities resorbed more nutrients during senescence, which may facilitate reproduction and vegetative regrowth in the next year. Our results highlight the importance of considering within‐species variation in nutrient resorption as an important underlying mechanism explaining the positive effects of biodiversity on primary productivity and ecosystem carbon accumulation.
摘要
生物多样性与生产力之间存在正向相关关系， 对这一关系内在驱动机制的探讨是国际生态学研究的热点问题， 主要的科学假说包括取样效应 (Sampling effect） 和互补效应 （Complementarity effect）。以往的研究认为植物群落内不同物种对土壤资源的互补性利用 （种间作用） 是导致生物多样性和生产力存在正向关系的重要原因之一， 却忽视了单种植物的养分利用特征 （种内变化） 随着群落中生物多样性的变化会发生怎样的改变， 这种改变进而会如何作用于生产力。本研究利用位于中国内蒙古典型草原的物种去除实验平台， 分析了羊草、大针茅、细叶葱、猪毛菜等四种来自四个不同的植物功能群的代表性植物在枯萎过程中养分回收效率随群落中植物物种多样性的变化情况。研究结果表明上述四种植物夏季绿色成熟叶片和秋季枯萎叶片中的氮磷养分含量均随着群落中物种多样性的增加而降低， 这说明在高多样性的群落中植物会提高自身的养分利用效率。在枯萎过程中， 多数植物的养分回收效率随着群落中植物多样性的增加而升高， 回收的这部分养分会进入植物的繁殖器官和储藏器官从而有利于植物的繁殖生长和下一个生长季的营养生长。本研究从植物养分利用特征种内变化这一新的角度解释了为什么生物多样性与生产力之间存在正向相关关系。
Aim: Trees associating with ectomycorrhizal (ECM) fungi typically occur in infertile soils and use nutrients more conservatively than arbuscular mycorrhizal (AM) trees. We hypothesized that ECM trees ...would have greater nutrient resorption (i.e., proportion of nutrients resorbed during leaf senescence) than AM trees. Location: Global. Methods: We synthesized nitrogen (N) and phosphorus (P) resorption data from 378 species from sub/tropical, temperate and boreal forests, including 43 studies where ECM and AM trees co-occurred, and conducted a meta-analysis. Additionally, we quantified N resorption in 45 plots varying in ECM-AM tree abundances in the temperate deciduous forests of southern Indiana, USA. Results: Overall, resorption patterns were driven primarily by mycorrhizal type, climate zone, and to a lesser degree, leaf habit. In the boreal forest, P resorption was 76% greater for ECM than AM trees (p < .05). In the sub/tropics, AM trees resorbed 30% more N than ECM trees. At the sites where AM and ECM trees co-occurred, ECM trees resorbed more N in temperate forests (15% greater; p < .001) whereas AM trees tended to resorb more N in sub/tropical forests (by 29%; p = .08). Besides, deciduous ECM trees resorbed more N (10%) and P (15%) than deciduous AM trees, while evergreen ECM and AM trees did not differ. In the deciduous forests of Indiana, where ECM and AM trees co-occurred, the relative abundance of ECM trees in a plot was positively correlated to plot-scale N resorption (R2 = .25, p = .001), indicating greater nutrient conservatism with increasing ECM-dominance. Main conclusions: Our results indicate that mycorrhizal association – in addition to other factors – is correlated with the degree to which trees recycle nutrients, with the strongest effects occurring for N resorption by temperate deciduous trees.
Upland forests are traditionally thought to be net sinks for atmospheric methane (CH4). In such forests, in situ CH4 fluxes on tree trunks have been neglected relative to soil and canopy fluxes.
We ...measured in situ CH4 fluxes from the trunks of living trees and other surfaces, such as twigs and soils, using a static closed-chamber method, and estimated the CH4 budget in a temperate upland forest in Beijing.
We found that the trunks of Populus davidiana emitted large quantities of CH4 during July 2014–July 2015, amounting to mean annual emissions of 85.3 and 103.1 μg m−2 h−1 on a trunk surface area basis on two replicate plots. The emission rates were similar in magnitude to those from tree trunks in wetland forests. The emitted CH4 was derived from the heartwood of trunks. On a plot or ecosystem scale, trunk CH4 emissions were equivalent to c. 30–90% of the amount of CH4 consumed by soils throughout the year, with an annual average of 63%.
Our findings suggest that wet heartwoods, regardless of rot or not, occur widely in living trees on various habitats, where CH4 can be produced.
Increasing availability of reactive nitrogen (N) threatens plant diversity in diverse ecosystems. While there is mounting evidence for the negative impacts of N deposition on one component of ...diversity, species richness, we know little about its effects on another one, species evenness. It is suspected that ecosystem management practice that removes nitrogen from the ecosystem, such as hay-harvesting by mowing in grasslands, would mitigate the negative impacts of N deposition on plant diversity. However, empirical evidence is scarce. Here, we reported the main and interactive effects of N deposition and mowing on plant diversity in a temperate meadow steppe with 4-year data from a field experiment within which multi-level N addition rates and multiple N compounds are considered. Across all the types of N compounds, species richness and evenness significantly decreased with the increases of N addition rate, which was mainly caused by the growth of a tall rhizomatous grass, Leymus chinensis. Such negative impacts of N addition were accumulating with time. Mowing significantly reduced the dominance of L. chinensis, and mitigated the negative impacts of N deposition on species evenness. We present robust evidence that N deposition threatened biodiversity by reducing both species richness and evenness, a process which could be alleviated by mowing. Our results highlight the changes of species evenness in driving the negative impacts of N deposition on plant diversity and the role of mowing in mediating such negative impacts of N deposition.
Summary
Differences in litter quality and in soil microbial community composition can influence the litter decomposition and ‘home‐field advantage’ (HFA). However, our knowledge about the relative ...role of litter and soil characteristics on litter decomposition and HFA effects is still limited, especially under long‐term N deposition.
We collected soil and two types of litter (monospecific and mixed species litter) from five replicate plots from a long‐term N deposition field experiment with seven N addition treatments (0, 2, 5, 10, 15, 20, 50 g N m−2 year−1). We examined the effects of N addition on litter quality and soil characteristics. We then carried out a three‐pronged microcosm decomposition experiment with (i) litter from different N addition treatments decomposed on a standard field soil; (ii) standard litter decomposed on soils from the different N addition treatments; and (iii) litter decomposed on soil from the same N addition treatment plot.
Decomposition of litter on standard soil was influenced strongly by the N addition treatment, but did not consistently decrease or increase with increasing N addition rates. Instead, decomposition of standard litter on soils collected from different N addition treatments decreased with increasing rates of N addition. Decomposition of litter on soil collected from the same plot increased with increasing N addition rates. Soil characteristics explained more of the variation in litter decomposition than litter characteristics.
There was a clear HFA effect for litter decomposition, both from a litter and from a soil perspective. HFA effects increased when the dissimilarity in litter quality (N content and C : N ratio) increase among the different N addition treatments and the soil effect was strongest at high N addition rates.
N addition influenced litter decomposition by changing both litter and soil characteristics. Importantly, N addition decreased the capability of soils to decompose litter and it increased the HFA effect indicating that soils decomposed local litter better than other litter, due to specialization in soil communities. Nitrogen deposition is an important threat to ecosystems worldwide and our study emphasizes that ecosystem functions such as decomposition can be greatly influenced by these global changes.
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