Tradeoffs and thresholds in the effects of nitrogen addition on biodiversity and ecosystem functioning: evidence from inner Mongolia Grasslands

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 28.0 g N m À2 yr À1 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 1.75 g N m À2 yr À1 , 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 g N m À2 yr À1 . This work highlights the tradeoffs that exist in assessing the total impact of N deposition on ecosystem function.

Nitrogen loading levels affect abundance and composition of soil ammonia oxidizing prokaryotes in semiarid temperate grassland

“…Significant decrease in the Shannon and Evenness index values of AOB in higher nitrogen loading treatments (more than 16 g N m −2 year −1 ) was observed in this study. The results implied the potential risk of diversity loss in microorganisms caused by nitrogen addition presented in N-limited grassland ecosystem, coinciding with what has been observed in macroorganisms (Aber et al 1998;Bai et al 2010). The N deposition rate in this area was estimated to be 2 g N m −2 year −1 .…”

“…The N deposition rate in this area was estimated to be 2 g N m −2 year −1 . Closely to the threshold that N-induced AOB community change happened, 2-4 g N m −2 year −1 in this study, while the threshold for plants in this region was found to be approximately 10.5 g N m −2 year −1 (Bai et al 2010). This also implied that AOB was a potential sensitive indicator for assessing N addition effect.…”

“…Such ecosystem is therefore likely to be highly sensitive to N input. For the aboveground plant communities, Bai et al (2010) identified the critical threshold of N loading of below 1.75 g N m −2 year −1 for N-induced species loss to mature Eurasian grasslands and of 10.5 g N m −2 year −1 for the changes in aboveground biomass, species richness, and plant functional group composition to both mature and degraded ecosystems. However, the impact of N loading on the important functional microbes mediating N transformations underground, ammonia oxidizers in this area, is unknown.…”

Nitrogen enrichment and grazing accelerate vegetation restoration in degraded grassland patches

“…Several plausible mechanisms may explain these observations. First, by favoring the initial establishment of fast-growing plant species (Bai et al, 2010;Stevens et al, 2004), N addition can apparently promote the biodiversity in degraded grassland patches. Furthermore, the encouragement of fast-growing species by N addition caused significant decreases in species similarity in the RV and OV quadrats.…”

Testing mechanisms of N-enrichment-induced species loss in a semiarid Inner Mongolia grassland: critical thresholds and implications for long-term ecosystem responses

“…Experimental studies in grasslands have demonstrated N addition consistently decreases species richness (SR) and promotes a few species with particular traits that enable them to exploit the additional N [7][8][9][10]. Such reductions in SR and shifts in functional groups composition can have profound impacts on multiple ecosystem functions, such as N retention, carbon sequestration, above-and below-ground biomass, and stability [11][12][13][14].…”