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Reduced compensatory effects explain the nitrogen-mediated reduction in stability of an alpine meadow on the Tibetan Plateau Ming-Hua Song1 and Fei-Hai Yu2 1

Key Laboratory of Ecosystem Network Observation and Modelling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China;

2

School of Nature Conservation, Beijing Forestry University, Beijing 100083, China

Summary Author for correspondence: Fei-Hai Yu Tel: +86 10 62336173 Email: [email protected] Received: 3 November 2014 Accepted: 12 January 2015

New Phytologist (2015) doi: 10.1111/nph.13329

Key words: compensatory effect, dominance effect, long-term experiment, mean-variance scaling, N fertilization, nitrogen (N) chemical form, richness, temporal stability.


Many ecosystems are facing strong perturbations such as nitrogen (N) fertilization, which can greatly alter ecosystem stability via different mechanisms. Understanding such mechanisms is critical for predicting how ecosystems will function in the face of global changes.
We examined how 8 yr of N fertilization with different N rates (no N addition or N addition at a low, medium or high rate) and different forms of N (ammonium, nitrate or ammonium nitrate) affected the temporal stability of the aboveground biomass of an alpine meadow on the Tibetan Plateau, and tested four mechanisms (diversity effect, mean-variance scaling, compensatory dynamics and dominance effect) that may alter stability.
Compared with the control (no N addition), a high N rate did not affect the diversity effect, the mean-variance scaling or the dominance effect, but significantly decreased compensatory dynamics among species and functional groups, which contributed to the reduction in community stability of the alpine meadow. The form of N did not affect any of the four mechanisms and thus did not affect community stability.
A high N rate can change community stability by altering compensatory dynamics, whereas the form of N may not have an effect.

Introduction Ecosystems have the ability to maintain their functional stability under a certain level of environmental or anthropogenic perturbation (Grime, 1998; Ernest & Brown, 2001; Hooper et al., 2005; Grman et al., 2010). However, many ecosystems are facing frequent and intense perturbations, and such perturbations can often change the stability of ecosystems (Naeem et al., 1994; Polley et al., 2007; Fowler et al., 2012). Understanding the mechanisms driving changes in ecosystem stability is a challenge in ecological research and also a critical step to predicting how ecosystems will function in the face of global changes (Naeem et al., 1994; Grman et al., 2010; Loreau & de Mazamcourt, 2013; Hautier et al., 2014). Several mechanisms have been proposed to explain how ecosystem stability can be altered by perturbations (Grman et al., 2010; Sasaki & Lauenroth, 2011; Yang et al., 2012; Loreau & de Mazamcourt, 2013; Hautier et al., 2014). First, perturbations may greatly change species diversity (Naeem et al., 1994; Lehman & Tilman, 2000; Reich et al., 2003; Grman et al., 2010). Both theoretical and empirical studies have shown that communities with more species can be more resistant to perturbations and thus more temporally stable (Yachi & Loreau, 1999; McGrady-Steed & Morin, 2000; Proulx et al., 2010; Loreau & de Mazamcourt, 2013). Therefore, if perturbations can reduce species diversity, then they may decrease ecosystem stability (Naeem et al., 1994; Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust

Tilman, 1996; Yang et al., 2012). Second, compensatory effects, arising from negatively correlated responses of different plants or functional groups to perturbations, are an important mechanism for sustaining ecosystem stability (Houlahan et al., 2007; Valone & Barber, 2008; Gonzalez & Loreau, 2009; Grman et al., 2010; Hautier et al., 2014). If perturbations can reduce the strength of compensatory effects, then they may reduce ecosystem stability. Third, statistical averaging (described by mean-variance scaling), albeit not a genuine biological mechanism (Kalyuzhny et al., 2014), may also contribute to ecosystem stability because more abundant species are normally more variable (Taylor, 1961; Grman et al., 2010). Perturbations may change the mean-variance scaling relationships and thus alter ecosystem stability (Smith & Knapp, 2003; Lep
s, 2004; Polley et al., 2007). Fourth, perturbation-mediated changes in species dominance may also alter ecosystem stability (Smith & Knapp, 2003; Polley et al., 2007). We tested these four mechanisms using data from an 8-yr nitrogen (N) fertilization experiment. N enrichment by atmospheric deposition or agricultural fertilization is a common anthropogenic perturbation of ecosystems (Hooper & Vitousek, 1997; Hooper et al., 2005). Nitrogen fertilization can reduce plant richness (Suding et al., 2005; Tilman et al., 2006; Xia & Wan, 2008), shift plant dominance (Suding et al., 2005; Hillebrand et al., 2008), and alter plant interactions (Grman et al., 2010; Yang et al., 2012). However, effects of N enrichment New Phytologist (2015) 1 www.newphytologist.com

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on ecosystem stability are controversial (Tilman, 1996; Grman et al., 2010; Yang et al., 2012). For instance, N enrichment decreased the stability of grasslands in Minnesota and Inner Mongolia by decreasing species richness (Tilman, 1996; Romanuk et al., 2006; Yang et al., 2012), but did not affect the stability of grassland in Michigan (Grman et al., 2010). In contrast to the studies that examined effects of N supply rates on community stability (Tilman et al., 2006; Grman et al., 2010; Yang et al., 2012), no published data are available on the effects of different chemical forms of N (i.e. ammonium, nitrate and free amino acids), although atmospheric N deposition and agricultural fertilization commonly introduce N into soil with different proportions of N chemical forms (Stevenson, 1982; McKane et al., 2002). It is well known that species differ in the capacity to take up different chemical forms of N from soil (Kielland, 1994; N€asholm et al., 1998; McKane et al., 2002; Miller & Bowman, 2002; Kahmen et al., 2006). In N-limited communities, abundant species can show a higher capacity to use the most abundant N form in soil than rare and uncommon species, whereas rare and uncommon species prefer to use the less abundant N form (Kielland, 1994; N€asholm et al., 1998; McKane et al., 2002; Miller & Bowman, 2002; Kahmen et al., 2006). If N forms affect species dominance and interactions (compensatory effects), then they may affect ecosystem stability. To examine how N form and N rate affect ecosystem stability and to investigate the underlying mechanisms, we analyzed data collected over 8 yr (2005–2012) from a long-term N-fertilization experiment conducted in an alpine meadow on the Tibetan Plateau. In this experiment, we tested the effects of both N rate and N form. Results for the first 6 yr of data from this fertilization experiment showed that N rate affected aboveground biomass and species richness at both community and functional group levels, but N chemical form affected them only at functional group level (Song et al., 2012). In this study, we specifically addressed the following questions. Does N rate affect the stability of the alpine meadow? Does the chemical form of N affect the stability of the alpine meadow? What are the underlying mechanisms?

Materials and Methods Site description We analyzed data from an 8-yr N-fertilization experiment established in an alpine meadow at the Haibei Alpine Meadow Ecosystem Research Station (37°370 N, 101°120 E, 3240 m above sea level), located on the northeastern Tibetan Plateau in Qinghai Province, China (Song et al., 2012). The mean annual temperature is 1.7°C, and mean annual precipitation is 560 mm, 85% of which occurs in the growing season (from May to September). The soil is classified as Mat Cry-gelic Cambisols (Chinese Soil Taxonomy Research Group, 1995), corresponding to Gelic Cambisol (WRB, 1998). The soil pH is c. 8.0. Atmospheric wet N deposition through precipitation was 0.46  0.03 g m2 yr1 measured from May 2008 to May 2009 in the alpine meadow New Phytologist (2015) www.newphytologist.com

(Jiang, 2010). Data on atmospheric dry N deposition are not available in this region, but it is expected that both atmospheric wet and dry N deposition will increase as a consequence of the increased impacts of anthropogenic activities (Jiang, 2010). In 2005, an area of 80 m 9 60 m was enclosed to prevent grazing. The area has uniform vegetation and is within the permanent research area of the station. The permanent research area has not been fertilized since the foundation of the station in 1976. Since then, this area had been lightly grazed by domestic yaks and sheep only in winter (1 yak ha1). The lightly grazed meadow maintained c. 25–30 plant species per m2 quadrat. The meadow was dominated by the perennial tussock sedge Kobresia humilis (C.A.Mey. ex Trautv.) Serg. and the perennial grasses Elymus nutans Griseb., Stipa aliena Keng. and Festuca ovina L. The abundant species include the grasses Poa pratensis L. and Koeleria cristata (L.) Pers., the legumes Oxytropis ochrocephala Bunge., Oxytropis coerulea (pall.) DC. and Astragalus adsurgens Pall., the sedges Kobresia pygmaea C.B. Clarke in Hook and Carex spp. and the nonlegume forbs Saussurea superb Anth., Gentiana lawrencei var. farreri T. N. Ho, Gentiana straminea Maxim., Potentilla nivea L., Potentilla anserina L. and Scirpusdis tigmaticus Tang et Wang. This meadow is N limited and the growth of plants peaks in August (Zhou, 2001). Vegetation coverage is over 95%. Experimental design Within the enclosed area, an inner area was fenced off at the start of the fertilization experiment and thereby released from grazing. Within this ungrazed area, we established 30 plots of 2 m 9 2 m, arranged in three rows (blocks) with ten plots each. Plots were 1 m apart. Iron sheets 35 cm high were inserted into the ground along the four edges of the plots to a depth of 30 cm to separate the plots from the surrounding vegetation, with the remaining 5 cm above the soil surface. The fertilization experiment had 10 N treatments with three replicate plots for each treatment. In the control treatment (CK) no N was added. The other nine treatments were the combinations of three N forms and three N rates. The three N forms were (1) ammonium (Am), (2) nitrate (Ni), and (3) ammonium nitrate (AN). (NH4)2SO4 was used for the Am treatment, NaNO3 for the Ni treatment and NH4NO3 for the AN treatment. The three N rates were 0.375, 1.5 and 7.5 g N m2 yr1, coded as low N (LN), medium N (MN) and high N (HN), respectively. The N was supplied twice a year during the growing season (on 10–15 July and 10–15 August) in 2006–2012, with half of the total amount of N supplied on each occasion. In 2005, N was added only once, on 10 July, with the total amount. The N was applied in aqueous solution and 5 l of solution was evenly sprayed into each plot. For the control, 5 l of water was supplied (Song et al., 2012). Sampling and measurements A 1 m 9 1 m quadrat was established at the center of each plot. The occurrence of each vascular species in each quadrat was Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust

New Phytologist recorded in the middle of August each year (2005–2012), and species richness was then calculated. The coverage of each species was measured in August of 2005, 2008, 2010 and 2012. For cover measurements, a 1 m 9 1 m frame with 100 grid cells (10 cm 9 10 cm) was placed above the canopy in each quadrat, and the cover of each species was visually estimated in each grid cell. For biomass measurements, we clipped aboveground shoots within a 0.25 m 9 0.25 m quadrat outside the central 1 m 9 1 m quadrat but within the plot in the middle of August every year, when biomass peaked. The quadrat for clipping shifted each year within the plot to avoid harvesting the same area in successive years. Shoots were clipped at ground level and sorted into four plant functional groups, that is, grasses, sedges, legumes and forbs. All shoots were oven-dried at 60°C for 48 h, and then weighed. Data analyses We determined ecosystem stability using aboveground biomass data collected within each plot in the 8 yr (2005–2012). Ecosystem stability was measured by calculating the temporal mean in each plot over the 8 yr divided by the temporal standard deviation (Lehman & Tilman, 2000; Tilman et al., 2002). There was little temporal autocorrelation in aboveground biomass (see year to year variation of biomass in Supporting Information Fig. S1) and detrending (Tilman et al., 2006) was not necessary (see detrending results in Figs S2,S3). We calculated mean species richness in each plot over the 8 yr. Simpson’s dominance index (Smith & Wilson, 1996) was calculated based on species relative cover in each plot, and mean dominance was the average over the 4 yr (2005, 2008, 2010 and 2012). Evenness in each plot was calculated as the reciprocal of the dominance index divided by species richness, and mean evenness was the average over the 4 yr. Two-way ANOVAs were used to examine the effects of N form and N rate on community stability, mean aboveground biomass, variance of aboveground biomass, mean richness, mean dominance and mean evenness. In these analyses, the control treatment (no N addition) was not included because it did not belong to any N form. Because a significant interaction effect between N form and N rate was absent (see ANOVA results in Supporting Information Table S1), we pooled the data across the three N form treatments for each N rate treatment and also pooled those across the three N rate treatments for each N form treatment. We then used one-way ANOVA followed by Tukey’s test to compare the differences in community stability, mean aboveground biomass, variance, mean richness, mean dominance and mean evenness among the three N form treatments, and in these analyses the control was not included. We also used oneway ANOVA followed by Tukey’s test to compare the differences among the four N rate treatments, and in these analyses the control was included because it represented the lowest amount of N addition. To measure compensatory effects among species, we calculated community-wide synchrony of species (Loreau & de Mazancourt, 2008; Loreau, 2010) as follows: Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust

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r2 uc ¼ P cT 2 S i¼1 rci (φc, community-wide synchrony of species based on species cover; r2cT , the variance in community cover (i.e. the sum of all species in a community); rci, the standard deviation in the cover of species i in the community; S, species number (richness) in the community (Yang et al., 2012).) Similarly, to measure compensatory effects among functional groups, we calculated the community-wide synchrony of functional groups based on the aboveground biomass of the community and of the four plant functional groups. The value of the synchrony measure equals 1 if species or functional groups fluctuate synchronously, which indicates that there is no significant compensatory effect, and is smaller than 1 if they do not, which indicates that there is a significant compensatory effect (Loreau & de Mazancourt, 2008; Hautier et al., 2014). We used a one-sample, one-tailed t-test to examine whether the synchrony measure was significantly smaller than 1. We used one-way ANOVA followed by Tukey’s test to compare the synchrony measure among the three N form and four N rate treatments, respectively. We calculated temporal mean and temporal variance of each species’ cover within each replicate plot, and conducted linear regressions to examine the relationships between log(species variance of cover) and log(species mean cover) for each of the three N form treatments and four N rate treatments. We used ANCOVA to test whether the slopes of the linear regressions differed significantly among the three N form and the four N rate treatments. A significant difference in the slopes suggests that N fertilization affects the mean-variance scaling relationship (Grman et al., 2010). We used residuals from the linear regressions to measure the relative stability of each species. Generally, the more negative the residual was, the more stable was the species (Grman et al., 2010). We conducted linear regressions to examine the relationships between species relative stability and species relative cover (abundance), and used ANCOVA to examine whether the slopes of the regression differed significantly among the N treatments.

Results Effects of N fertilization on stability Nitrogen form did not affect the stability, mean or variance of the aboveground biomass of the alpine meadow (Fig. 1a,c,e). Compared with the control, community stability in HN was significantly lower, but the mean and variance of aboveground biomass were significantly higher (Fig. 1b,d,f). None of the three variables differed significantly among the control, LN and MN or among LN, MN and HN (Fig. 1b,d,f). Effects of N fertilization on compensatory dynamics For all three N form and four N rate treatments, community-wide synchrony of both species and functional groups was significantly smaller than 1 (Fig. 2), suggesting that there existed compensatory New Phytologist (2015) www.newphytologist.com

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Fig. 1 Effects of nitrogen (N) form and rate on the stability (a, b), mean (c, d) and variance of aboveground biomass (e, f) of the alpine meadow community. For the ammonium-N (Am), nitrate-N (Ni) and both ammonium- and nitrate-N (AN) treatments, (NH4)2SO4, NaNO3 and NH4NO3 were added, respectively. For the control (CK), low N (LN), medium N (MN) and high N (HN) treatments, 0, 0.375, 1.5 and 7.5 g N m2 yr1 were added, respectively. Mean + SE are given. Bars sharing the same letters are not statistically different.

effects among species and among functional groups. N form did not significantly affect synchrony of either the species or the functional groups (Fig. 2a,c), indicating that N form did not affect compensatory dynamics. However, synchrony of both species and functional groups was higher in HN than in the control (Fig. 2b,d), suggesting that HN significantly decreased compensatory dynamics among species and among functional groups. Effects of N fertilization on mean-variance scaling Log-transformed values of variance in species abundances were positively correlated to log-transformed values of mean abundances in all the three N form treatments (Fig. 3a; slopes = 1.569–1.626; all P < 0.001) and four N rate treatments (Fig. 3b; slopes = 1.527– 1.629; all P < 0.001). However, neither N form (ANCOVA log (mean) 9 N form interaction: F2,1042 = 0.90; P = 0.41) nor N rate (ANCOVA log (mean) 9 N rate interaction: F3,1151 = 1.08; P = 0.36) significantly affected the slopes (i.e. scaling coefficients) of the mean-variance relationships (Fig. 3). These results suggest that the reduced community stability in HN was not attributable to changes in the mean-variance scaling. New Phytologist (2015) www.newphytologist.com

Nitrogen form did not significantly affect the species richness or dominance of the alpine community (Fig. 4a,c). Compared with the control, HN significantly reduced the species richness and increased species dominance of the community (Fig. 4b,d). Neither N form nor N rate affected community evenness (P > 0.05; data not shown). Considering all the treatments together, community stability was not significantly correlated with species richness or dominance (Fig. 4e–h; all P > 0.05). Community stability was also not significantly related to species diversity in Am or AN, and was significantly but negatively related to richness and dominance in Ni (Fig. 4e,f). Community stability was significantly related to species diversity in none of the four N rate treatments (Fig. 4g,h). These results suggest that changing species diversity was not a mechanism for N-mediated changes in community stability. Effects of N fertilization on dominance shift Species relative abundance was significantly (P < 0.05) or marginally significantly (P < 0.1) negatively related to species relative stability (as measured by the residuals from the mean-variance relationships) in all four N rate treatments and in two of the three N form treatments, the exception being Ni (Fig. 5). These results suggest that species dominance, at least to some extent, contributed to ecosystem stability. However, neither N form (ANCOVA species relative abundance 9 N form interaction: F2,1042 = 0.359; P = 0.698) nor N rate (ANCOVA species relative abundance 9 N rate interaction: F3,1151 = 0.624; P = 0.598) significantly affected the slopes of the linear regressions, suggesting that neither N rate nor N form altered the dominance effect on community stability.

Discussion Our 8-yr N fertilization experiment shows that the chemical form of N had little effect on the community stability of this speciesrich alpine meadow ecosystem, but a high N rate reduced stability by reducing compensatory dynamics among species and among functional groups. Many studies have shown that perturbations affect species richness and community stability in the same direction and to a comparable extent, so that there is a positive relationship between species richness and community stability (Dodd et al., 1994; Tilman, 1996; McGrady-Steed & Morin, 2000; Steiner et al., 2005; Jiang & Pu, 2009; Yang et al., 2012). These studies suggest that changing species richness is a mechanism to impact community stability (Romanuk et al., 2006; Tilman et al., 2006; Loreau & de Mazamcourt, 2013). Interestingly, although we found that a high N rate also significantly decreased the species richness of the alpine meadow, we failed to find a positive relationship between species richness and community stability. Therefore, our results suggest that the mechanism underlying the effect of a high N rate on community stability was not ascribed to its effect on species richness. A diverse community often consists of a few dominant species and many common and rare species (Lep
s, 2004). Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust

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fertilized grassland in Michigan in the USA (Grman et al., 2010). One of the reasons may be that N-mediated species fluctuations are asynchronous in the alpine meadow. An increase in the abundance of dominant species was found to help maintain or even increase the stability of ecosystem functions (Bai et al., 2004; Lep
s, 2004; Steiner et al., 2005; Polley et al., 2007; Grman et al., 2010; Wilsey et al., 2014). In our study, the high N rate significantly increased the dominance of the community. For instance, the high N rate increased the aboveground biomass of grasses by 54.1%, mainly through an increase in the abundance of the dominant grasses Stipa aliena and Elymus nutans. Despite this, we found no significant change in the relationship between community dominance and community stability with N addition. Although species relative abundance was significantly or marginally significantly related to species relative stability, a high N rate did not alter the slope of this relationship. Therefore, our results provide little evidence that a change in the dominance of the species or community is the mechanism

Dominant species often contribute greatly to ecosystem functions such as biomass, whereas rare and less dominant species contribute very little (Smith & Knapp, 2003; Polley et al., 2007; Grman et al., 2010). Thus, loss of dominant species can greatly impact the functional stability of communities, whereas loss of a few rare species in most cases cannot (Smith & Knapp, 2003; Polley et al., 2007). In the present study, a high N rate decreased species number, on average, only by 10% (two species). Such a small loss of species did not greatly affect the temporal stability of aboveground biomass. The mean-variance relationship is also considered as a mechanism to maintain community stability, and factors altering the slope of such a relationship can change ecosystem stability. Lep
s (2004) found that fertilization could increase the slope of the mean-variance relationship, and attributed the driving factor to the effect of environmental productivity. In our study, N rate had little effect on the slope of the mean-variance relationship. The same phenomenon was also observed in a long-term 0.3

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Fig. 2 Effects of nitrogen (N) form and rate on compensatory dynamics, measured by community-wide synchrony, at both species (a, b) and functional group (c, d) level. For the ammonium-N (Am), nitrate-N (Ni) and both ammonium- and nitrate-N (AN) treatments, (NH4)2SO4, NaNO3 and NH4NO3 were added, respectively. For the control (CK), low N (LN), medium N (MN) and high N (HN) treatments, 0, 0.375, 1.5 and 7.5 g N m2 yr1 were added, respectively. Mean + SE are given. All values are significantly < 1. Bars sharing the same letters are not statistically different.

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Fig. 3 Effects of nitrogen (N) form and rate on mean-variance scaling relationships. For the ammonium-N (Am), nitrate-N (Ni) and both ammonium- and nitrate-N (AN) treatments, (NH4)2SO4, NaNO3 and NH4NO3 were added, respectively. For the control (CK), low N (LN), medium N (MN) and high N (HN) treatments, 0, 0.375, 1.5 and 7.5 g N m2 yr1 were added, respectively. Significant relationships were found in all the three N form and four N rate treatments, but neither N form nor N rate significantly affected the linear regression slopes (tested by ANCOVA). Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust

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Fig. 5 Effects of nitrogen (N) form (a–c) and rate (d–g) on relationships between species relative abundance (cover) and relative species stability (residuals of the linear regression in Fig. 3). For the ammonium-N (Am), nitrate-N (Ni) and both ammoniumand nitrate-N (AN) treatments, (NH4)2SO4, NaNO3 and NH4NO3 were added, respectively. For the control (CK), low N (LN), medium N (MN) and high N (HN) treatments, 0, 0.375, 1.5 and 7.5 g N m2 yr1 were added, respectively. Neither N form nor N rate significantly affected the linear regression slopes (tested by ANCOVA). Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust

New Phytologist underlying the effect of a high N rate on community stability in the alpine meadow ecosystem (Smith & Knapp, 2003; Polley et al., 2007; Grman et al., 2010; Wilsey et al., 2014). Compensatory dynamics existed both among species and among functional groups in all the four N rate treatments and thus contributed to the stability of this alpine meadow. Compensatory effects were also found in both annual and perennial communities and both fertilized and unfertilized communities (Grman et al., 2010). We found that a high N rate markedly decreased the strength of compensatory dynamics, very likely because it significantly decreased the intensity of competitive interactions or negatively correlated responses to environmental perturbations. For example, the high N rate increased the abundance of the two dominant grasses S. aliena and E. nutans and synchronously decreased the abundance of the two dominant legumes Trigonellaruth enica and O. ochrocephala, showing negatively correlated responses to the high N rate in dominant species of the two functional groups. However, compared with the control, the high N rate significantly increased the synchrony measure of these four dominant species (0.35  0.07 in the high N rate versus 0.19  0.05 in the control; t = 3.67; P = 0.036), implying that the high N rate reduced compensatory effects among these four dominant species. Moreover, the synchrony measure of less dominant species was also significantly higher (t = 3.14; P = 0.043) in the high N rate (0.26  0.01) than in the control (0.11  0.02). All these results indicate that reduced community stability in the high N rate was attributable to the reduction in the strength of compensatory effects. By contrast, increasing nutrient availability increased compensatory dynamics in annual and perennial grassland communities (Grman et al., 2010) and in algal communities in aquatic microcosms (Zhang & Zhang, 2006). Over the 8-yr study period, fertilization with different N forms did affect the abundance of some species, including the grass P. pratensis, the legume Astragalus strictus, and the forbs S. superb, Ranunculus membranaceuse and Lancea tibetica (see species cover in Fig. S4), suggesting that species in this meadow showed a preference for different N forms (Xu et al., 2003; Song et al., 2012). Moreover, during the 8-yr period of fertilization with different N forms, some rare forb species such as Ajaniatenui folia, Viola yedoensis and Gentiana arenaria occurred randomly in certain plots fertilized with a specific N form, but were absent in other plots fertilized with other N forms. At the functional group level, fertilization with nitrate also caused nonrandom grass species loss compared with fertilization with ammonium. However, fertilization with different N forms significantly affected neither the abundance of the dominant species (see species cover in Fig. S4) nor species richness at the community level. Although the form of N caused nonrandom species loss at functional group level, loss of those rare species did not affect stabilizing mechanisms, and consequently did not affect community stability. Conclusions A high N rate can decrease the stability of the alpine meadow community, but the chemical form of N may not. A diminution Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust

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in compensatory dynamics is the mechanism underlying the long-term effects of a high N rate on the stability of the alpine meadow. Our results also suggest that with increasing N deposition in future we should pay more attention to the risk of changing ecosystem stability mediated by an increased N rate. Our results provide a thorough elucidation of the effects of long-term N fertilization on community stability, and improve understanding of the mechanisms of the effects of N deposition on ecosystem processes in this alpine meadow.

Acknowledgements We thank Guangmin Cao and Shun Wang for help with the field work, and Prof. Michel Loreau and two anonymous reviewers for their valuable comments. The study was supported by NSFC (31270503), Excellent Scientist grant from the Institute of Geographic Sciences and Natural Resources Research, CAS (2011RC101) and the Fundamental Research Funds for the Central Universities (TD-JC-2013-1).

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Supporting Information Additional supporting information may be found in the online version of this article. Fig. S1 Aboveground biomass during the 8 yr in the three N form and four N rate treatments. Fig. S2 Results from repeated-measures ANOVA for effects of N form, time and the interaction on community stability. Fig. S3 Results from repeated-measures ANOVA for effects of N rate, time and the interaction on community stability. Fig. S4 Total cover (%) summed over years of species in each replicate plot of the four N rate treatments. Table S1 Effects of N form and N rate on stability, mean and variance of aboveground biomass, mean species richness, dominance and evenness of the alpine meadow communities over the 8 yr Please note: Wiley Blackwell are not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing material) should be directed to the New Phytologist Central Office.

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