Data-based assessment of environmental controls on global marine nitrogen fixation

Abstract

There are a number of hypotheses for the environmental controls on marine nitrogen fixation (NF). Most of these hypotheses have not been assessed against direct measurements on the global scale. In this study, we use ~ 500 depth-integrated field measurements of NF covering the Pacific and Atlantic Oceans to test whether the spatial variance of these measurements can be explained by the commonly hypothesized environmental controls, including measurement-based surface solar radiation, mixed layer depth, sea surface temperature, surface nitrate and phosphate concentrations, surface excess phosphate (P^*), atmospheric dust deposition and surface wind speed, as well as minimum dissolved oxygen in upper 500 m to identify possible subsurface denitrification zones. By conducting simple linear regression and stepwise multiple linear regression (MLR) analyses, solar radiation and/or sea surface temperature as well as subsurface dissolved oxygen are identified as the predictors explaining the most spatial variance in the observed NF data, while dust deposition and wind speed do not appear to influence the spatial patterns of NF on global scale. Our study suggests that marine NF is coupled to regional loss of fixed nitrogen induced by subsurface low oxygen concentration, with its magnitude constrained by solar radiation or temperature. By applying the MLR-derived equation, we estimate the global-integrated NF at 71 (error range 49–104) Tg N yr^−1 in the open ocean, acknowledging that it could be substantially higher as the ¹⁵N₂-assimilation method used by most of the field samples underestimates NF. Our conclusion suggests that marine NF will increase in the future if subsurface nitrogen-losses increase as a consequence of developing deoxygenation with the global warming, a projection that will be modulated by other factors such as warming, elevated carbon dioxide, and changes in macro- and micro-nutrient distributions. More field NF samples in the Pacific and Indian Oceans, particularly in the oxygen minimum zones, are needed to reduce uncertainties in our conclusion

Understanding predicted shifts in diazotroph biogeography using resource competition theory

“…In particular, the model suggests that in any province, the locally limiting nutrient will be uniformly drawn down to a low, subsistence concentration. Thus, we do not anticipate any correlation between nutrient concentrations themselves and diazotroph biomass or nitrogen fixation: indeed no such correlation was found in the study of Luo et al (2014) which looked a compilation of observed nitrogen fixation and observed nutrient concentrations. The theory suggests that strong gradients of nutrient concentrations occur between provinces.…”

“…We note that the theory predicts relationships between fluxes of nutrients and not between nutrient concentrations. In particular, it does not suggest clear relationships between nitrogen fixation rate and iron or phosphate concentrations and indeed such relationships are not observed (Luo et al, 2014). Numerical Model: we find that the diazotroph biomass and nitrogen fixation rates are related to the net excess supply of Fe or P (Fig.…”

High levels of heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation

“…N 2 fixed by diverse diazotrophic taxa may have different fates within the marine environment (Glibert and Bronk 1994;Mulholland, 2007;Foster et al, 2011;Karl et al, 2012;, and therefore characterisation of the composition of active N 2 -fixing assemblages, combined with size-fractionated N 2 fixation rates, is necessary to determine the differential contribution of newly fixed N to pelagic ecosystems. Here we report changes in the biogeographical distribution and activity of diazotrophs across a broad tropical region, which has been identified as a potential global 'hotspot' for marine N 2 fixation (Montoya et al, 2004;Monteiro et al, 2010;Luo et al, 2014).…”

“…High rates of marine N 2 fixation have been observed in Australia's tropical waters (Montoya et al, 2004), and N 2 fixation rate models predict rates sometimes 4100 μmol m − 2 day − 1 in this region (Luo et al, 2014). Additionally, ecosystem models predict cyanobacterial diazotrophs will be abundant in northern Australian waters (Monteiro et al, 2010).…”

“…N 2 fixation has been found to be a significant biogeochemical feature of this important region of the ocean (Montoya et al, 2004;Luo et al, 2014), but we observed substantial physicochemical variability, manifest in differential temperature and salinity signatures and nutrient availability between the ATS and Coral Sea, which may influence the relative importance of diazotroph activity, particularly given the highly dynamic nature of diazotroph communities (Robidart et al, 2014). By combining nifH sequencing and sizefractionated 15 N 2 rate measurements, we assessed spatial and temporal patterns in the diversity and activity of N 2 -fixing bacteria across this region with the aim of characterising the dynamics of diazotrophy within this putative global N 2 fixation hotspot.…”