Nutrient limitation of terrestrial free‐living nitrogen fixation

Abstract

Nitrogen (N) fixation by free-living bacteria is a primary N input pathway in many ecosystems and sustains global plant productivity. Uncertainty exists over the importance of N, phosphorus (P) and molybdenum (Mo) availability in controlling free-living N fixation rates. Here, we investigate the geographic occurrence and variability of nutrient constraints to free-living N fixation in the terrestrial biosphere. We compiled data from studies measuring free-living N fixation in response to N, P and Mo fertilizers. We used meta-analysis to quantitatively determine the extent to which N, P and Mo stimulate or suppress N fixation, and if environmental variables influence the degree of nutrient limitation of N fixation. Across our compiled dataset, free-living N fixation is suppressed by N fertilization and stimulated by Mo fertilization. Additionally, free-living N fixation is stimulated by P additions in tropical forests. These findings suggest that nutrient limitation is an intrinsic property of the biochemical demands of N fixation, constraining free-living N fixation in the terrestrial biosphere. These findings have implications for understanding the causes and consequences of N limitation in coupled nutrient cycles, as well as modeling and forecasting nutrient controls over carbon-climate feedbacks.

Global pattern and controls of biological nitrogen fixation under nutrient enrichment: A meta‐analysis

“…In cases that no SD, SE, or CI was given, we assigned SE as 1/4 of the means (Dynarski & Houlton, 2018). In cases that no SD, SE, or CI was given, we assigned SE as 1/4 of the means (Dynarski & Houlton, 2018).…”

“…Although anthropogenic N deposition has to some extent reduced BNF rates in natural ecosystems (Sullivan et al, 2014;Vitousek, Menge, Reed, & Cleveland, 2013), annual rates of terrestrial BNF still reach Tg N/year (Cleveland et al, 1999;Galloway et al, 2004;Meyerholt, Zaehle, & Smith, 2016;Vitousek et al, 2013). Given the N constraints of net primary productivity (NPP) in numerous natural ecosystems (LeBauer & Treseder, 2008;Vitousek et al, 2002), N inputs via BNF increase NPP (Dynarski & Houlton, 2018). Moreover, N fixers can increase soil mineral N in N-limited biomes and transfer the fixed N to non-N-fixing plants (Rousk, Sorensen, & Michelsen, 2017;Zanetti et al, 1997), thereby increasing carbon dioxide (CO 2 ) sequestration by enhancing photosynthesis (Lüscher, Hartwig, Suter, & Nösberger, 2000).…”

“…Despite the important role of BNF in terrestrial ecosystems, our current knowledge of the factors controlling terrestrial BNF remains poor (Dynarski & Houlton, 2018), which has impeded our ability to accurately understand, estimate, and project terrestrial N budgets and NPP in a changing world (Meyerholt et al, 2016;Wang, Houlton, & Field, 2007). Nutrient availability has long been considered a key regulator of BNF (Reed, Cleveland, & Townsend, 2011).…”

Bedrock Nitrogen Weathering Stimulates Biological Nitrogen Fixation

“…These ecosystems can also receive more than 38% of their N budget from bedrock sources (Houlton et al 2018), yet interactions between bedrock N weathering and N fixation have not been examined; such interactions have the potential to influence both nutrient cycling and C storage in temperate forest ecosystems. Recent studies of free-living N fixation have noted the existence of these multiple nutrient controls over N fixation on ecosystem (Reed et al 2013, Perakis et al 2017 to global scales (Dynarski and Houlton 2017), suggesting that the conventional wisdom of N fixation suppression by N availability is an incomplete model. However, this view does not consider interactions between bedrock N inputs and the cycling of other elements essential for N fixation, such as C, molybdenum (Mo), and phosphorus (P).…”

“…Such interactions between N and C may be important in regulating freeliving N fixation rates; as N-fixing bacteria are dominantly heterotrophic in temperate forests, relying on decomposing organic matter to fuel their metabolism and N fixation activity (Reed et al 2011), enhanced C accumulation can increase the energy supply available to N-fixers. Further, soil organic matter contributes to retention of Mo (Marks et al 2015) and organic P , which have been demonstrated to support free-living N fixation across diverse terrestrial ecosystems (Dynarski and Houlton 2017). N additions have also been found to increase P availability through stimulation of biological phosphatase enzyme production (Olander and Vitousek 2000, Vitousek et al 2010, Marklein and Houlton 2012.…”