Negative effects of nitrogen override positive effects of phosphorus on grassland legumes worldwide

Abstract

Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in low-nutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents. Nitrogen addition reduced legume cover, richness, and biomass, particularly in nitrogen-poor soils, while cover of non–nitrogen-fixing plants increased. The addition of phosphorous, potassium, and other nutrients enhanced legume abundance, but did not mitigate the negative effects of nitrogen addition. Increasing nitrogen supply thus has the potential to decrease the diversity and abundance of grassland legumes worldwide regardless of the availability of other nutrients, with consequences for biodiversity, food webs, ecosystem resilience, and genetic improvement of protein-rich agricultural plant species.

Nitrogen But Not Phosphorus Addition Affects Symbiotic N2 Fixation in Grasslands Located on Four Continents

“…Our results suggest that continuous anthropogenic N enrichment of grasslands can lead to a decrease in legume biomass production, which in the long-term can limit symbiotic N 2 xation. The most plausible explanation for reduced rates of N 2 xation per unit area with N addition is that higher soil N availability allows grasses and forbs to outcompete legumes (via competition for light) and reduce legume biomass (Suding et al 2005; Soussana and Tallec 2010; Tognetti et al 2021). The size of the response of symbiotic N 2 xation per area to N addition was affected by several abiotic factors.…”

“…Addition of N reduced N 2 xation per area more strongly at sites with higher soil pH, atmospheric N deposition, MAT, and MAP (Table 2, Table S6). The reasons for this could be that i) legumes and rhizobium strains from neutral and alkaline sites are less tolerant to soil acidi cation caused by urea addition (Hungria and Vargas 2000), and that ii) N addition has a larger effect at sites where the antrophogenic N input through atmospheric N deposition is already large, and iii) N 2 xation is more sensitive to N addition at sites where the N 2 xation is not constrained by temperature or water availability (Houlton et al 2008;Tognetti et al 2021).…”

“…The %Ndfa often declines with availability of both ammonium and nitrate in soil (Leidi and Rodríguez-Navarro 2000; Peoples et al 2012), because symbiotic N 2 xation is energetically expensive, and legumes take up reactive N, if available. In addition, grasses and non-leguminous forbs often displace legumes at high N availability due to their higher competitiveness for light (Soussana and Tallec 2010;Tognetti et al 2021). Thus, anthropogenic N inputs to grasslands, can lead to decreased N 2 xation by legumes affecting the grassland functioning.…”

“…Furthermore, it has been observed that simultaneous additions of N and P (NP) can offset the negative effect of N addition on N 2 xation resulting in higher symbiotic N 2 xation compared to only N addition in tropical leguminous trees (Zheng et al 2016). However, a recent study revealed that P addition enhances legume abundance in grasslands but does not mitigate the negative N effect when both elements are added simultaneously (Tognetti et al 2021). Nevertheless, the extent to which the NP addition affects the N 2 xation rates in grasslands remains to be studied.…”

“…Therefore, a standardized and globally replicated experiment is needed to gain insight into N 2 xation. Recent work using a standardized and globally replicated experiment has shown that legume biomass production declines with N addition (Tognetti et al 2021), but the extent to which element addition affects symbiotic N 2 xation rates was not evaluated.…”