DOI: 10.5061/dryad.dbrv15f88
title: Data from: Disentangling the variability of symbiotic nitrogen fixation rate and the controlling factors

description: Symbiotic nitrogen (N) fixation (SNF), replenishing bioavailable N for
 terrestrial ecosystems, exerts decisive roles in N cycling and gross
 primary production. Nevertheless, it remains unclear what determines the
 variability of SNF rate, which retards the accurate prediction for global
 N fixation in earth system models. This study synthesized 1230 isotopic
 observations to elucidate the governing factors underlying the variations
 of SNF rate. The SNF rates varied significantly from 3.69 to 12.54 g N m-2
 yr-1 across host plant taxa. The traits of host plant (e.g., biomass
 characteristics and taxa) far outweighed soil properties and climatic
 factors in explaining the variations of SNF rate, accounting for 79.0% of
 total relative importance. Furthermore, annual SNF yield contributed to
 more than half of N uptake for host plants, which was consistent across
 different ecosystem types. This study highlights that the biotic factors,
 especially host plant traits (e.g., biomass characteristics and taxa),
 play overriding roles in determining SNF rate compared with soil
 properties. The suite of parameters for SNF lend support to improve N
 fixation module in earth system models that can provide more confidence in
 predicting bioavailable N changes in terrestrial ecosystems.
description: The peer-reviewed papers for this synthesis were searched up to
 September 7, 2022, using Web of Science and China National Knowledge
 Infrastructure Database. The SNF data were from the field measurements.
 The strings employed<strong> </strong>for searching papers
 are: ((“nitrogen fixation” or “N fixation” or
 “N<strong><sub>2</sub></strong> fixation” or
 “dinitrogen fixation” or “nitrogenase”) and field). The following criteria
 were used to sift out the peer-reviewed papers: 1.<strong>
 </strong>The SNF rate was measured using <sup>15</sup>N
 isotope method (e.g., <sup>15</sup>N natural abundance, and
 <sup>15</sup>N dilution) since these methods guarantee the
 accuracy of SNF across large spatial/temporal scale than acetylene
 reduction assay method (Davies-Barnard et al., 2020; Soper et al., 2021);
 2. The measurement must be given the unambiguous unit; 3. The studies that
 plant has been inoculated with specific strains of bacteria (e.g.,
 rhizobium) were excluded; 4. The SNF was measured in the field rather than
 the laboratory incubation or greenhouse cultivation, and the data from
 aqueous environments (e.g., river or lake sediments) were excluded.
 Finally, 112 peer-reviewed papers were eligible for the construction of
 SNF dataset.
description: The peer-reviewed papers for this synthesis were searched up to September
 7, 2022, using Web of Science and China National Knowledge Infrastructure
 Database. The SNF data were from the field measurements. The strings
 employed for searching papers are: ((“nitrogen fixation” or “N fixation”
 or “N2 fixation” or “dinitrogen fixation” or “nitrogenase”) and field).
 The following criteria were used to sift out the peer-reviewed papers:
 1.The SNF rate was measured using 15N isotope method (e.g., 15N natural
 abundance, and 15N dilution) since these methods guarantee the accuracy of
 SNF across large spatial/temporal scale than acetylene reduction assay
 method (Davies-Barnard et al., 2020; Soper et al., 2021); 2. The
 measurement must be given the unambiguous unit; 3. The studies that plant
 has been inoculated with specific strains of bacteria (e.g., rhizobium)
 were excluded; 4. The SNF was measured in the field rather than the
 laboratory incubation or greenhouse cultivation, and the data from aqueous
 environments (e.g., river or lake sediments) were excluded. The SNF rate,
 host plant taxa, biomass traits (i.e., the biomass of shoot, grain, and
 root), and soil properties (i.e., clay contents, bulk density, pH, cation
 exchange capacity, organic carbon, N and phosphorus contents, carbon:N
 ratio, and carbon:phosphorus ratio) were directly extracted from tables or
 graphs in peer-reviewed papers using GetData software. Climatic variables
 (i.e., mean annual temperature and precipitation) and soil microbial
 traits (i.e., carbon:N and carbon:phosphorus ratio of soil microbial
 biomass) were obtained from the climatic database
 ([http://www.worldclim.org](http://www.worldclim.org)) (Fick et al., 2017)
 and soil microbial biomass dataset (Wang et al., 2022), respectively,
 according to the geographical information.