A review on the effects of nitrogen and phosphorus addition on tree growth and productivity in forest ecosystems

Nitrogen (N) and phosphorus (P) inputs induced by anthropogenic activities and atmospheric N and P deposition have largely increased the availability of soil N and P in terrestrial ecosystems, which have considerably affected terrestrial carbon cycling processes. Tree growth and productivity in forest ecosystems play an important role in global carbon cycling, and determine the magnitude and direction of terrestrial carbon sequestration. Currently, a large number of field manipulation experiments have been conducted to investigate the effects of N and/or P addition on tree growth and forest productivity, but the results from these studies were inconsistent. Such inconsistent results might be affected by multiple factors, including biological, environmental and experimental variables. Here, we reviewed the present research status of the effects of N and P addition on tree growth and forest productivity in forest ecosystems based on three aspects, including the number of publications and experiments with field N and P addition, and the global distributions of these experiments. Then, we summarized the methods for assessing tree growth and forest productivity at ecosystem level in forest ecosystems, including relative growth rate and absolute increment. According to the related results, we reviewed the regulating factors that affect tree growth and productivity, and the potential mechanisms for such factors, including climate, tree size and stand age, plant functional traits (including type of tree-associated mycorrhizal fungi, N-fixation property of trees, and conservative and acquisitive functional traits), plant-microbe interaction, ambient nutrient (i.e., N and P) deposition rate, and experimental variables. Finally, we summarized the current studies, and pointed out five aspects that are urgently needed to provide further insights in future studies, including the physiological mechanism of how tree growth responds to N and P addition, the tradeoff and allocation among growth of various parts of tree under N and P addition, the role of plant functional traits in regulating and predicting the responses of tree growth to N and P addition, how the competition among trees regulates the responses of tree growth to N and P addition, and conducting long-term and coordinated distributed field experiments investigating the effects of N and P addition on tree growth and forest productivity at the global scale.     

氮磷添加对树木生长和森林生产力影响的研究进展

人为活动所导致的氮、磷输入和大气氮、磷沉降使生态系统中的氮、磷可利用性大幅提高, 对陆地生态系统的碳循环过程产生了显著影响。树木生长和森林生产力在全球碳循环中发挥着重要作用, 它决定着陆地碳固存的大小和方向。目前, 在全球范围内开展了很多氮、磷添加调控树木生长和森林生产力的野外控制实验, 但是研究结果并不一致, 受到多种生物、环境和实验处理条件等因素的影响。该文从野外氮添加和磷添加实验的文献数量、实验数量及其全球空间分布三个方面概述了氮、磷添加对树木生长和森林生产力影响的研究现状, 并总结了氮、磷添加实验中树木生长和森林生产力的评估方法, 包括相对生长速率和绝对增长量。基于相关的研究结果, 阐述了氮、磷添加影响树木生长和森林生产力的调控因素及其潜在影响机制, 包括气候、树木径级与林龄、植物功能性状(共生菌根类型、树木固氮属性和保守性与获得性性状)、植物和微生物相互作用关系、区域养分沉降速率和实验处理条件等。最后, 基于当前的研究进行了系统总结, 并指出今后需要加强的几个方面的研究, 以期为后续研究提供参考: 树木生长响应氮、磷添加的生理学机制, 树木各部分生长对氮、磷添加响应的权衡与分配, 植物功能性状在调节与预测树木生长响应氮、磷添加中的作用, 树木之间的竞争关系如何调控氮、磷添加对树木生长的影响, 以及开展长期的和联网的氮、磷添加对树木生长和森林生产力影响的野外控制实验。     

Long‐term nitrogen loading alleviates phosphorus limitation in terrestrial ecosystems

Increased human‐derived nitrogen (N) deposition to terrestrial ecosystems has resulted in widespread phosphorus (P) limitation of net primary productivity. However, it remains unclear if and how N‐induced P limitation varies over time. Soil extracellular phosphatases catalyze the hydrolysis of P from soil organic matter, an important adaptive mechanism for ecosystems to cope with N‐induced P limitation. Here we show, using a meta‐analysis of 140 studies and 668 observations worldwide, that N stimulation of soil phosphatase activity diminishes over time. Whereas short‐term N loading (≤5 years) significantly increased soil phosphatase activity by 28%, long‐term N loading had no significant effect. Nitrogen loading did not affect soil available P and total P content in either short‐ or long‐term studies. Together, these results suggest that N‐induced P limitation in ecosystems is alleviated in the long‐term through the initial stimulation of soil phosphatase activity, thereby securing P supply to support plant growth. Our results suggest that increases in terrestrial carbon uptake due to ongoing anthropogenic N loading may be greater than previously thought.     

陆地生态系统植物多样性对矿质元素输入的响应

人类活动的加剧改变了陆地生态系统矿质元素(如氮、磷、钾等)循环的速度和方向,并且对生态系统的结构和功能也产生重要影响。如今,矿质元素输入量的改变及其产生的后续效应对陆地生态系统生物多样性的影响备受学者们的关注。从4个方面综述了全球氮沉降背景下主要矿质元素输入的改变对陆地植物多样性的影响及其机理:1)矿质营养元素限制的概念、确定方法以及与植物多样性的耦合关系;2)概述了氮、磷、钾等主要矿质元素输入对陆地植物多样性的影响:主要表现为负面效应;3)探讨了矿质元素输入影响植物多样性的可能机制,包括生态系统水平上的机制(如竞争排斥、酸化铝毒、物种入侵、同质性假说,间接诱导机制等)和植物个体水平上的机制(如元素失衡和环境敏感性增加等);4)根据目前研究现状,指出了已有研究的局限性,分析了未来可能的研究方向和重点。     

氮添加对森林植物磷含量的影响及其机制

氮沉降对森林生态系统磷循环产生了不可忽视的影响, 尤其是加剧了植物生长的磷限制, 从而使得氮沉降背景下植物磷含量变化备受关注。该文综述了氮添加对森林植物磷含量的影响, 认为氮添加通过促进土壤磷酸酶活性进而提高土壤有效磷含量, 有利于植物的磷吸收并增加植物磷含量。同时, 森林植物磷含量对氮添加的响应还受物种、生活型以及施氮时间长短等因素的影响。基于森林植物磷含量对氮添加响应的差异性, 该文进一步探讨氮富集背景下森林植物磷含量变化的可能机制: 1)外源氮输入通过改变土壤中有效磷含量从而对植物磷的来源产生影响; 2)通过影响植物的根系分泌物、菌根共生和根系形态结构等进而影响植物的磷吸收能力; 3)通过影响植物的磷养分再分配、磷养分重吸收对植物磷利用效率产生影响。综上所述, 外源氮输入使植物磷含量发生改变, 首要原因是土壤有效磷含量的改变, 其次是植物磷吸收能力和磷利用效率的改变起调控作用。     

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m^–2 yr^–1 P and 694 mg m^–2 yr^–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m^–2 yr^–1 P), while sediment burial of N (1890–4027 mg m^–2 yr^–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.     

Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems

The cycles of the key nutrient elements nitrogen (N) and phosphorus (P) have been massively altered by anthropogenic activities. Thus, it is essential to understand how photosynthetic production across diverse ecosystems is, or is not, limited by N and P. Via a large-scale meta-analysis of experimental enrichments, we show that P limitation is equally strong across these major habitats and that N and P limitation are equivalent within both terrestrial and freshwater systems. Furthermore, simultaneous N and P enrichment produces strongly positive synergistic responses in all three environments. Thus, contrary to some prevailing paradigms, freshwater, marine and terrestrial ecosystems are surprisingly similar in terms of N and P limitation.     

氮添加对中国陆地植被地上-地下生物量分配的影响

大气氮沉降水平持续升高导致的外源氮输入增加,强烈影响了陆地生态系统的碳循环。目前,已有大量报道证实了氮沉降升高对全球陆地植被固碳的积极影响。虽然之前大部分研究将这一结果归因于光合作用增强导致的地上生物量增加,但最近的研究发现长期氮添加对植物地下根系的影响也同样重要。归纳整理了181篇公开发表的我国野外模拟氮沉降试验结果,采用整合分析(Meta-analysis)方法,定量评估了氮添加对我国陆地植被地上-地下生物量分配的影响特征和不同生态系统类型及施氮方式之间的影响差异。通过分析地上-地下生物量分配对氮添加的响应差异来探究植被碳增益对长期大气氮沉降增加的潜在响应机制。结果表明,氮添加显著增强了我国陆地植被的光合作用及碳固存,且植物碳增益在不同生态系统类型及施氮制度间有所差异。植物叶片的氮含量显著增加,使得叶片碳氮比及凋落物碳氮比显著降低,但并未显著影响细根的碳氮比。氮添加总体上显著提高了植物的净光合速率,但降低了光合利用效率。地上生物量,凋落物产量和根生物量平均分别显著增加了38%,17%和18%,总体上植物地上部分对氮添加的响应程度比地下部分更高。然而,不同生态系统类型的地上-地下生物...     

Sources,fates, and impacts of nitrogen inputs to terrestrial ecosystems: review and synthesis

The relative importance of nitrogen inputs from atmospheric deposition and biological fixation is reviewed in a number of diverse, non-agricultural terrestrial ecosystems. Bulk precipitation inputs of N (l–l2 kg N ha^–1 yr^–1) are the same order of magnitude as, or frequently larger than, the usual range of inputs from nonsymbiotic fixation (< 1=" –=" 5=" kg=" n=">^–1 yr^–1), especially in areas influenced by industrial activity. Bulk precipitation measurements may underestimate total atmospheric deposition by 30–40% because they generally do not include all forms of wet and dry deposition. Symbiotic fixation generally ranges from 10–160 kg N ha^–1 yr^–1) in ecosystems where N-fixing species are present during early successional stages, and may exceed the range under unusual conditions.Rates of both symbiotic and nonsymbiotic fixation appear to be greater during early successional stages of forest development, where they have major impacts on nitrogen dynamics and ecosystem productivity. Fates and impacts of these nitrogen inputs are important considerations that are inadequately understood. These input processes are highly variable in space and time, and few sites have adequate comparative information on both nitrogen deposition and fixation.

The unseen majority: soil microbes as drivers of plant diversity and productivity in terrestrial ecosystems

Microbes are the unseen majority in soil and comprise a large portion of life's genetic diversity. Despite their abundance, the impact of soil microbes on ecosystem processes is still poorly understood. Here we explore the various roles that soil microbes play in terrestrial ecosystems with special emphasis on their contribution to plant productivity and diversity. Soil microbes are important regulators of plant productivity, especially in nutrient poor ecosystems where plant symbionts are responsible for the acquisition of limiting nutrients. Mycorrhizal fungi and nitrogen-fixing bacteria are responsible for c. 5–20% (grassland and savannah) to 80% (temperate and boreal forests) of all nitrogen, and up to 75% of phosphorus, that is acquired by plants annually. Free-living microbes also strongly regulate plant productivity, through the mineralization of, and competition for, nutrients that sustain plant productivity. Soil microbes, including microbial pathogens, are also important regulators of plant community dynamics and plant diversity, determining plant abundance and, in some cases, facilitating invasion by exotic plants. Conservative estimates suggest that c. 20 000 plant species are completely dependent on microbial symbionts for growth and survival pointing to the importance of soil microbes as regulators of plant species richness on Earth. Overall, this review shows that soil microbes must be considered as important drivers of plant diversity and productivity in terrestrial ecosystems.     

The effects of ultraviolet-B radiation on plant competition in terrestrial ecosystems

Evidence regarding the interaction of ultraviolet-B (UV-B, 280-320 nm) radiation and plant competition in terrestrial ecosystems is examined. The competitive interactions of some species pairs were affected even by ambient solar UV-B radiation (as exists without ozone depletion), when compared to control pairs grown without UV-B. Also, the total shoot biomass of these species pairs was depressed under ambient UV-B. Relatively large increases in UV-B radiation (approximating a 40% ozone layer reduction when weighted with the generalized plant action spectrum) altered the competitive interactions of some species pairs grown in pots under field conditions, but did not affect the total shoot biomass production of those pairs. Recent field experiments have examined the competitive interactions of wheat ( Triticum aestivum L. cv. Bannock) and wild oat ( Avena fatua L.) under a simulated increased UV-B regime resulting from a 16% ozone layer reduction when weighted with the generalized plant action spectrum. This increase in UV-B altered the competitive interactions of these two species without affecting the total shoot biomass production of the species pair. The manner in which increased UV-B affected the relative competitive abilities of the two species was highly dependent upon the environmental conditions during the early life stages of the plants. The implications of these results for both agricultural and natural plant communities are discussed.     

Missing effects of anthropogenic nutrient deposition on sentinel alpine ecosystems

Anthropogenic nitrogen (N) deposition affects unproductive remote alpine and circumpolar ecosystems, which are often considered sentinels of global change. Human activities and forest fires can also elevate phosphorus (P) deposition, possibly compounding the ecological effects of increased N deposition given the ubiquity of nutrient co‐limitation of primary producers. Low N : P ratios coupled with evidence of NP‐limitation from bioassays led us to hypothesize that P indirectly stimulates phytoplankton by amplifying the direct positive effect of N (i.e. serial N‐limitation) in alpine ponds. We tested the hypothesis using the first replicated N × P enrichment experiment conducted at the whole‐ecosystem level, which involved 12 alpine ponds located in the low N deposition backcountry of the eastern Front Range of the Canadian Rockies. Although applications of N and P elevated ambient N and P concentrations by 2–5×, seston and plankton remained relatively unaffected in the amended ponds. However, additions of ammonium nitrate elevated the δ¹⁵N signals of both primary producers and herbivores (fairy shrimp; Anostraca), attesting to trophic transfer of N deposition to consumers. Further, in situ bioassays revealed that grazing by high ambient densities of fairy shrimp together with potential competition from algae lining the pond bottoms suppressed the otherwise serially N‐limited response by phytoplankton. Our findings highlight how indirect effects of biotic interactions rather the often implicit direct effects of chemical changes can regulate the sensitivities of extreme ecosystems to nutrient deposition.     

Lake‐type‐specific seasonal patterns of nutrient limitation in German lakes,with target nitrogen and phosphorus concentrations for good ecological status

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