Tradeoffs and thresholds in the effects of nitrogen addition on biodiversity and ecosystem functioning: evidence from inner Mongolia Grasslands
TL;DR: It is suggested that the critical threshold for N-induced species loss to mature Eurasian grasslands is below 1.75gNm � 2 yr � 1, and that changes in aboveground biomass, species richness, and plant functional group composition to both mature and degraded ecosystems saturate at N addition rates of approximately 10.5 gNm� 2 yr� 1.
Abstract: Nitrogen (N) deposition is widely considered an environmental problem that leads to biodiversity loss and reduced ecosystem resilience; but, N fertilization has also been used as a management tool for enhancing primary production and ground cover, thereby promoting the restoration of degraded lands. However, empirical evaluation of these contrasting impacts is lacking. We tested the dual effects of N enrichment on biodiversity and ecosystem functioning at different organizational levels (i.e., plant species, functional groups, and community) by adding N at 0, 1.75, 5.25, 10.5, 17.5, and 28.0gNm � 2 yr � 1 for four years in two contrasting field sites in Inner Mongolia: an undisturbed mature grassland and a nearby degraded grassland of the same type. N addition had both quantitatively and qualitatively different effects on the two communities. In the mature community, N addition led to a large reduction in species richness, accompanied by increased dominance of early successional annuals and loss of perennial grasses and forbs at all N input rates. In the degraded community, however, N addition increased the productivity and dominance of perennial rhizomatous grasses, with only a slight reduction in species richness and no significant change in annual abundance. The mature grassland was much more sensitive to N-induced changes in community structure, likely as a result of higher soil moisture accentuating limitation by N alone. Our findings suggest that the critical threshold for N-induced species loss to mature Eurasian grasslands is below 1.75gNm � 2 yr � 1 , and that changes in aboveground biomass, species richness, and plant functional group composition to both mature and degraded ecosystems saturate at N addition rates of approximately 10.5gNm � 2 yr � 1 . This work highlights the tradeoffs that exist in assessing the total impact of N deposition on ecosystem function.
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TL;DR: The data is synthesized to assess the status of the anthropogenic N(r) emissions and N deposition as well as their impacts on different ecosystems, including empirical critical loads for different ecosystems.
648 citations
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...richness accompanied by a shift in plant functional group composition (Huang et al., 2009; Bai et al., 2010)....
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...As found by Bai et al. (2010) in a mature community of Inner Mongolia grasslands, N addition led to a large reduction in species richness accompanied by increased dominance of early successional annuals and loss of perennial grasses and forbs at all N input rates....
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...Nitrogen addition may lessen the N limitation by increasing soil N availability and thus stimulate plant growth (Pan et al., 2004; Bai et al., 2010)....
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...Plant diversity commonly responds to N addition as a loss of richness accompanied by a shift in plant functional group composition (Huang et al., 2009; Bai et al., 2010)....
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...lessen the N limitation by increasing soil N availability and thus stimulate plant growth (Pan et al., 2004; Bai et al., 2010)....
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TL;DR: In this paper, the authors conducted a meta-analysis of 106 studies to reveal global patterns of soil acidification in responses to N addition and found that N addition significantly reduced soil pH by 0.26 on average globally.
Abstract: Nitrogen (N) deposition-induced soil acidification has become a global problem. However, the response patterns of soil acidification to N addition and the underlying mechanisms remain far from clear. Here, we conducted a meta-analysis of 106 studies to reveal global patterns of soil acidification in responses to N addition. We found that N addition significantly reduced soil pH by 0.26 on average globally. However, the responses of soil pH varied with ecosystem types, N addition rate, N fertilization forms, and experimental durations. Soil pH decreased most in grassland, whereas boreal forest was not observed a decrease to N addition in soil acidification. Soil pH decreased linearly with N addition rates. Addition of urea and NH4NO3 contributed more to soil acidification than NH4-form fertilizer. When experimental duration was longer than 20 years, N addition effects on soil acidification diminished. Environmental factors such as initial soil pH, soil carbon and nitrogen content, precipitation, and temperature all influenced the responses of soil pH. Base cations of Ca2+, Mg2+ and K+ were critical important in buffering against N-induced soil acidification at the early stage. However, N addition has shifted global soils into the Al3+ buffering phase. Overall, this study indicates that acidification in global soils is very sensitive to N deposition, which is greatly modified by biotic and abiotic factors. Global soils are now at a buffering transition from base cations (Ca2+, Mg2+ and K+) to non-base cations (Mn2+ and Al3+). This calls our attention to care about the limitation of base cations and the toxic impact of non-base cations for terrestrial ecosystems with N deposition.
640 citations
TL;DR: In this article, the authors introduce a special issue on biodiversity of Palaearctic grasslands and provide a synthesis of the current knowledge on this topic and provide some promising conservation and management approaches and call for a strong and comprehensive Convention on Grassland Conservation.
411 citations
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...Long-term atmospheric nitrogen deposition may be the major source of excess nitrogen at grassland sites, which has been identified as a reason for decreasing species richness in grasslands (Bai et al., 2010; Bobbink et al., 2010; Dupré et al., 2010; Stevens et al., 2004)....
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TL;DR: In this article, the authors analyzed the grazing-induced steppe degradation process and identified an efficient and sustainable grazing management system for the widely degraded Inner Mongolian typical steppe ecosystem.
Abstract: The major aims of this study were, firstly, to analyse the grazing-induced steppe degradation process and, secondly, to identify an efficient and sustainable grazing management system for the widely degraded Inner Mongolian typical steppe ecosystem. From 2005–2008 a grazing experiment was conducted to compare two grazing management systems, the Mixed System (MS) and the Traditional System (TS), along a gradient of seven grazing intensities, i.e. ungrazed (GI0), very-light (GI1), light (GI2), light-moderate (GI3), moderate (GI4), heavy (GI5), and very-heavy (GI6). Each grazing intensity treatment was considered a production unit comprising two adjacent plots, one for hay-making (single-cut system) and one for grazing. Hay-making and grazing alternated annually in the MS, while in the TS the same plots were used either for hay-making or for grazing. Effects of management system, grazing intensity, and year on end-of-season standing biomass (ESSB), aboveground net primary production (ANPP), relative difference in ANPP between 2005 and 2008 (ANPPDiff), relative growth rate (RGR), and sward characteristics (litter accumulation, soil coverage) were analysed. Litter accumulation of production units was affected by grazing intensity (P < 0.001) and decreased from GI0 to GI6 by 83%. Correspondingly, soil coverage decreased (P < 0.001) from GI0 to GI6 by 43%, indicating an increased vulnerability to soil erosion. We found varying compensatory growth responses to grazing intensity among years, probably because of temporal variability in precipitation. The ability of plants to partially compensate for grazing damage was enhanced in years of greater seasonal precipitation. The ANPP of production units was negatively affected by grazing intensity and decreased from GI0 to GI6 by 37, 30, and 55% in 2006 (P < 0.01), 2007 (P < 0.05), and 2008 (P < 0.001), respectively. The effect of management system × grazing intensity interaction on ANPP (P < 0.05) and ANPPDiff (P < 0.05) suggested greater grazing resilience of the MS as compared to the TS at GI3 to GI6.
277 citations
Cites background from "Tradeoffs and thresholds in the eff..."
...Due to the positive effect of nitrogen addition on ANPP (Bai et al. 2010) the wider distribution of excremental nutrients to both grazing and hay-making plots was probably also beneficial to ANPP in the MS. Relative growth rate (RGR) The RGR representing the production per unit of producing tissue…...
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...Due to the positive effect of nitrogen addition on ANPP (Bai et al. 2010) the wider distribution of excremental nutrients to both grazing and hay-making plots was probably also beneficial to ANPP in the MS....
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TL;DR: In this paper, the authors apply an approach to 30-60 years of data on environmental drivers, biological responses, and associated evidence from pelagic ocean, coastal benthic, polar marine, and semi-arid grassland ecosystems.
Abstract: The occurrence and causes of abrupt transitions, thresholds, or regime shifts between ecosystem states are of great concern and the likelihood of such transitions is increasing for many ecological systems. General understanding of abrupt transitions has been advanced by theory, but hindered by the lack of a common, accessible, and data-driven approach to characterizing them. We apply such an approach to 30–60 years of data on environmental drivers, biological responses, and associated evidence from pelagic ocean, coastal benthic, polar marine, and semi-arid grassland ecosystems. Our analyses revealed one case in which the response (krill abundance) linearly tracked abrupt changes in the driver (Pacific Decadal Oscillation), but abrupt transitions detected in the three other cases (sea cucumber abundance, penguin abundance, and black grama grass production) exhibited hysteretic relationships with drivers (wave intensity, sea-ice duration, and amounts of monsoonal rainfall, respectively) through a variety of response mechanisms. The use of a common approach across these case studies illustrates that: the utility of leading indicators is often limited and can depend on the abruptness of a transition relative to the lifespan of responsive organisms and observation intervals; information on spatiotemporal context is useful for comparing transitions; and ancillary information from associated experiments and observations aids interpretation of response-driver relationships. The understanding of abrupt transitions offered by this approach provides information that can be used to manage state changes and underscores the utility of long-term observations in multiple sentinel sites across a variety of ecosystems.
270 citations
Cites result from "Tradeoffs and thresholds in the eff..."
...For example, a 304 different slope and intercept for a linear regression fitting response-driver relationships or a non-305 linear versus linear fit for data and after the identified breakpoint would support the hysteresis 306 model (Scheffer and Carpenter 2003, Bai et al. 2010)....
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References
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01 Jan 2005
8,490 citations
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...This situation is likely to worsen if grazing pressure continues to increase as projected (Reynolds & Smith, 2002; Asner et al., 2004; Millennium Ecosystem Assessment, 2005)....
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TL;DR: In this article, a review of available scientific evidence shows that human alterations of the nitrogen cycle have approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle, with these rates still increasing; increased concentrations of the potent greenhouse gas N 2O globally, and increased concentration of other oxides of nitrogen that drive the formation of photochemical smog over large regions of Earth.
Abstract: Nitrogen is a key element controlling the species composition, diversity, dynamics, and functioning of many terrestrial, freshwater, and marine ecosystems. Many of the original plant species living in these ecosystems are adapted to, and function optimally in, soils and solutions with low levels of available nitrogen. The growth and dynamics of herbivore populations, and ultimately those of their predators, also are affected by N. Agriculture, combustion of fossil fuels, and other human activities have altered the global cycle of N substantially, generally increasing both the availability and the mobility of N over large regions of Earth. The mobility of N means that while most deliberate applications of N occur locally, their influence spreads regionally and even globally. Moreover, many of the mobile forms of N themselves have environmental consequences. Although most nitrogen inputs serve human needs such as agricultural production, their environmental conse- quences are serious and long term. Based on our review of available scientific evidence, we are certain that human alterations of the nitrogen cycle have: 1) approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle, with these rates still increasing; 2) increased concentrations of the potent greenhouse gas N 2O globally, and increased concentrations of other oxides of nitrogen that drive the formation of photochemical smog over large regions of Earth; 3) caused losses of soil nutrients, such as calcium and potassium, that are essential for the long-term maintenance of soil fertility; 4) contributed substantially to the acidification of soils, streams, and lakes in several regions; and 5) greatly increased the transfer of nitrogen through rivers to estuaries and coastal oceans. In addition, based on our review of available scientific evidence we are confident that human alterations of the nitrogen cycle have: 6) increased the quantity of organic carbon stored within terrestrial ecosystems; 7) accelerated losses of biological diversity, especially losses of plants adapted to efficient use of nitrogen, and losses of the animals and microorganisms that depend on them; and 8) caused changes in the composition and functioning of estuarine and nearshore ecosystems, and contributed to long-term declines in coastal marine fisheries.
5,729 citations
"Tradeoffs and thresholds in the eff..." refers background in this paper
...Widespread nitrogen enrichment through industrial and agricultural processes, however, lessens N limitation and, in so doing, dramatically impacts ecosystem functioning (Galloway et al., 1995; Jordan & Weller, 1996; Vitousek et al., 1997; Phoenix et al., 2003)....
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...Widespread nitrogen enrichment through industrial and agricultural processes, however, lessens N limitation and, in so doing, dramatically impacts ecosystem functioning (Galloway et al., 1995; Jordan & Weller, 1996; Vitousek et al., 1997; Phoenix et al., 2003)....
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TL;DR: In this paper, the authors examine both how the biogeochemistry of the nitrogen cycle could cause limitation to develop, and how nitrogen limitation could persist as a consequence of processes that prevent or reduce nitrogen fixation.
Abstract: The widespread occurrence of nitrogen limitation to net primary production in terrestrial and marine ecosystems is something of a puzzle; it would seem that nitrogen fixers should have a substantial competitive advantage wherever nitrogen is limiting, and that their activity in turn should reverse limitation. Nevertheless, there is substantial evidence that nitrogen limits net primary production much of the time in most terrestrial biomes and many marine ecosystems. We examine both how the biogeochemistry of the nitrogen cycle could cause limitation to develop, and how nitrogen limitation could persist as a consequence of processes that prevent or reduce nitrogen fixation. Biogeochemical mechansism that favor nitrogen limitation include: A number of mechanisms could keep nitrogen fixation from reversing nitrogen limitation. These include: The possible importance of these and other processes is discussed for a wide range of terrestrial, freshwater, and marine ecosystems.
3,332 citations
01 Jan 1996
TL;DR: In this article, Bremner et al. defined the nonexchangeable NHt as the NHt in soil that cannot be replaced by a neutral potassium salt solution (SSSA, 1987), in contrast to NHt which is extractable at room temperature with such a solution.
Abstract: Most soils contain inorganic nitrogen (N) in the form of ammonium (NHt) and nitrate (NO)"). Nitrite (NOz) also may be present, but the amount is usually too small to warrant its determination, except in cases where NHt or NHt-forming fertilizers are applied to neutral or alkaline soils. Several other forms of inorganic N have been proposed as intermediates during microbial transformations of N in soils, including hydroxylamine (NH20H), hyponitrous acid (H2N20 2), and nitramide (NH2N02), but these compounds are thermodynamically unstable and have not been detected in soil. Until the 1950s, inorganic N was believed to account for <2% of total soil N, on the assumption that NHt and NO)" are completely recovered by extracting soil with a neutral salt solution. The validity of this assumption was challenged by the finding that some soils contain NHt in a form that is not extracted by exchange with other cations (e.g., Rodrigues, 1954; Dhariwal & Stevenson, 1958; Stevenson & Dhariwal, 1959; Bremner & Harada, 1959; Bremner, 1959; Schachtschabel, 1960, 1961; Young, 1962), and by estimates that the proportion of soil N in this form can exceed 50% for some subsurface soils (Stevenson & Dhariwal, 1959; Young, 1962). In such cases, NHt is said to be fixed, and fixed NHt has subsequently been defined as the NHt in soil that cannot be replaced by a neutral potassium salt solution (SSSA, 1987), such as 1 or 2 M KCI or 0.5 M K2S04, in contrast to exchangeable NHt, which is extractable at room temperature with such a solution. Existing information indicates that fixed NHt occurs largely, if not entirely, between the layers of 2: I-type clay minerals, particularly vermiculite and illite (hydrous mica), and that fixation results from entrapment of NHt in ditrigonal voids in the exposed surfaces upon contraction of the clay lattice (Nommik & Vahtras, 1982). The term, nonexchangeable NHt, has been used by Bremner (1965) and Keeney and Nelson (1982) in previous editions of this publication as a more precise alternative to fixed NHt. The same term is used in the present treatment, with specific reference to NHt determined by the method described in "Determination of Nonexchangeable Ammonium," which involves digestion with an HF-HCI solution following treatment of the soil with alkaline KOBr to remove exchangeable NHt and labile organic-N compounds.
2,810 citations