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Grassland ecosystems cover around 37% of the ice-free land surface on Earth and have critical socioeconomic importance globally. As in many terrestrial ecosystems, biological dinitrogen (N 2 ) fixation represents an essential natural source of nitrogen (N). The ability to fix atmospheric N 2 is limited to diazotrophs, a diverse guild of bacteria and archaea. To elucidate the abiotic (climatic, edaphic), biotic (vegetation), and spatial factors that govern diazotrophic community composition in global grassland soils, amplicon sequencing of the dinitrogenase reductase gene— nifH —was performed on samples from a replicated standardized nutrient [N, phosphorus (P)] addition experiment in 23 grassland sites spanning four continents. Sites harbored distinct and diverse diazotrophic communities, with most of reads assigned to diazotrophic taxa within the Alphaproteobacteria (e.g., Rhizobiales ), Cyanobacteria (e.g., Nostocales ), and Deltaproteobacteria (e.g., Desulforomonadales ) groups. Likely because of the wide range of climatic and edaphic conditions and spatial distance among sampling sites, only a few of the taxa were present at all sites. The best model describing the variation among soil diazotrophic communities at the OTU level combined climate seasonality (temperature in the wettest quarter and precipitation in the warmest quarter) with edaphic (C:N ratio, soil texture) and vegetation factors (various perennial plant covers). Additionally, spatial variables (geographic distance) correlated with diazotrophic community variation, suggesting an interplay of environmental variables and spatial distance. The diazotrophic communities appeared to be resilient to elevated nutrient levels, as 2–4 years of chronic N and P additions had little effect on the community composition. However, it remains to be seen, whether changes in the community composition occur after exposure to long-term, chronic fertilization regimes. 

Abstract Nutrient availability and grazing are known as main drivers of grassland plant diversity, and increased nutrient availability and long‐term cessation of grazing often decrease local‐scale plant diversity. Experimental tests of mechanisms determining plant diversity focus mainly on vascular plants (VP), whereas non‐vascular plants (NVP, here bryophytes) have been ignored. It is therefore not known how the current models based on VPs predict the rates of total (NVP + VP) losses in plant diversity.Here we used plant community data, including VPs and NVPs, from nine sites in Europe and North America and belonging to the Nutrient Network experiment, to test whether neglecting NVPs leads to biased estimates of plant diversity loss rates. The plant communities were subjected to factorial addition of nitrogen (N), phosphorus (P), potassium with micronutrients (K_+μ), as well as a grazing exclusion combined with multi‐nutrient fertilization (NPK_+μ) treatment.We found that nutrient additions reduced both NVP and VP species richness, but the effects on NVP species richness were on average stronger than on VPs: NVP species richness decreased 67%, while VP species richness decreased 28%, causing their combined richness to decrease 38% in response to multi‐nutrient (NPK_+μ) fertilization. Thus, VP diversity alone underestimated total plant diversity loss by 10 percentage points.Although NVP and VP species diversities similarly declined in response to N and NPK_+μfertilizations, the evenness of NVPs increased and that of VPs remained unchanged. NP, NPK_+μfertilization and NPK_+μfertilization combined with grazing exclusion, associated with decreasing light availability at ground level, led to the strongest loss of NVP species or probability of NVP presence. However, grazing did not generally mitigate the fertilization effects.Synthesis. In nine grassland sites in Europe and North America, nutrient addition caused a larger relative decline in non‐vascular plant (NVP) than vascular plant species richness. Hence, not accounting for NVPs can lead to underestimation of losses in plant diversity in response to continued nutrient pollution of grasslands. 

Abstract Background and aims The amount of nitrogen (N) derived from symbiotic N 2 fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known. Methods We evaluated symbiotic N 2 fixation in 17 natural and semi-natural grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the 15 N natural abundance method. Results N as well as combined N and P (NP) addition reduced aboveground legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Addition of N and/or P had no significant effect on the symbiotic N 2 fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials. Conclusion Our results reveal that N addition mainly impacts symbiotic N 2 fixation via reduced biomass of legumes rather than changes in N 2 fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N 2 fixation in grasslands, and these effects cannot be reversed by additional P amendment. 

Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically-limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin), i.e., the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here, we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory-based) and realized (field-based) soil net Nmin. Herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin. However, fertilization alone and in combination with herbivore removal consistently increased potential soil net Nmin. Realized soil net Nmin, in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net Nmin were contingent on site-specific soil and climatic properties. Fertilization effects on potential soil net Nmin were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net Nmin declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion, and alterations in future climatic conditions can negatively impact soil net Nmin across global grasslands under realistic field conditions. This is important context-dependent knowledge for grassland management worldwide. 

Abstract Eutrophication is a widespread environmental change that usually reduces the stabilizing effect of plant diversity on productivity in local communities. Whether this effect is scale dependent remains to be elucidated. Here, we determine the relationship between plant diversity and temporal stability of productivity for 243 plant communities from 42 grasslands across the globe and quantify the effect of chronic fertilization on these relationships. Unfertilized local communities with more plant species exhibit greater asynchronous dynamics among species in response to natural environmental fluctuations, resulting in greater local stability (alpha stability). Moreover, neighborhood communities that have greater spatial variation in plant species composition within sites (higher beta diversity) have greater spatial asynchrony of productivity among communities, resulting in greater stability at the larger scale (gamma stability). Importantly, fertilization consistently weakens the contribution of plant diversity to both of these stabilizing mechanisms, thus diminishing the positive effect of biodiversity on stability at differing spatial scales. Our findings suggest that preserving grassland functional stability requires conservation of plant diversity within and among ecological communities.