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Contributed Talk

Session: : Biogeochemistry: New Paradigms In Biogeochem Cycling 2

COS 29-6 - New global estimates of terrestrial biological nitrogen fixation and its modification by agriculture

Monday, August 7, 2023
4:45 PM – 5:00 PM PDT
Location: B116
Abstract: Biological nitrogen fixation (BNF) is considered the primary source of “new” nitrogen (N) to unmanaged terrestrial ecosystems and a critical source of N in agricultural systems globally. As N availability is one of the main factors regulating primary production, BNF plays a fundamental role in global terrestrial productivity and the biosphere’s response to climate change. There is, however, great uncertainty associated with estimates of global terrestrial BNF, which limits our ability to project future productivity and carbon storage. The uncertainty of global terrestrial BNF estimates is partly due to sampling bias. BNF studies tend to occur in areas with abundant N-fixing organisms, and efforts to upscale these results have used a variety of poorly-supported assumptions. Our main goal was to address this bias and upscale global terrestrial BNF based on N-fixing activity and rigorously quantified abundances of N-fixing organisms across multiple niches (i.e., trees, shrubs, herbs, soil, litter, wood, biocrusts, ground moss, and epiphytic lichen) and biomes. We conducted a systematic search of the literature and compiled a dataset with more than 1,200 BNF rates (kg N ha-1 y-1) and N-fixer abundance data from more than 900 sites and 328 studies in natural biomes. Our dataset included BNF rates originally reported and rates we converted from other types of BNF data using consistent spatial and temporal methods. The study sites comprised unmanaged/undisturbed ecosystems, those regenerating from disturbance, and managed plantations and non-intensive pasturelands. Using this dataset, we derived niche-specific estimates of BNF activity per unit of N-fixer. We aligned the per-N-fixer BNF estimates with robust datasets of N-fixer abundance for each niche. Preliminary analyses indicate that natural biomes fixed 85 Tg N y-1 while cultivated pasturelands fixed 30 Tg N y-1 and managed croplands fixed 34 Tg N y-1 in 2019 (based on Food Agriculture Organization‘s grain production data). In natural biomes, vascular plants represented 43% of total BNF, whereas all other “cryptic“ niches represented 57% of total BNF. Our results add to the evidence that global terrestrial BNF in natural biomes is lower than previously thought. Furthermore, the conversion of natural biomes to croplands and pasturelands has increased global BNF by 60%, representing the largest BNF sources worldwide. Our analyses provide BNF summaries by niche and biome for improving estimates of natural and anthropogenic modification of the global N cycle, as well as BNF maps and raster datasets useful for benchmarking models of Earth system biogeochemistry.