Biological nitrogen fixation is a key driver of global primary production and climate. Decades of effort have repeatedly updated nitrogen fixation estimates for terrestrial and open ocean systems, yet other aquatic systems in between have largely been ignored. Here we present an evaluation of nitrogen fixation for inland and coastal waters. We demonstrate that water column and sediment nitrogen fixation is ubiquitous across these diverse aquatic habitats, with rates ranging six orders of magnitude. We conservatively estimate that, despite accounting for less than 10% of the global surface area, inland and coastal aquatic systems fix 40 (30 to 54) teragrams of nitrogen per year, equivalent to 15% of the nitrogen fixed on land and in the open ocean. Inland systems contribute more than half of this biological nitrogen fixation.

The utility of vegetated ditch environments as nutrient sinks in agricultural watersheds is dependent in part on biogeochemical transformations that control plant uptake and release during decomposition. We investigated nitrogen (N) and phosphorus (P) uptake and release across four P enrichment treatments in ditch mesocosms planted with rice cutgrass (Leersia oryzoides) during the summer growing and winter decomposition seasons. Measured N retention and modeled denitrification rates did not vary, but P retention significantly increased with P enrichment. At the end of the growing season, root biomass stored significantly more N and P than aboveground stem and leaf biomass. Decomposition rates were low (<10% organic matter loss) and not affected by P enrichment. Nitrogen and P export during winter did not vary across the P enrichment gradient. Export accounted for <10% of observed summer N uptake (1363 mg m⁻²), with denitrification potentially accounting for at least 40% of retained N. In contrast, net P retention was dependent on enrichment; in unenriched mesocosms, P uptake and release were balanced (only 25% net retention), whereas net retention increased from 77% to 88% with increasing P enrichment. Our results indicate that vegetated ditch environments have significant potential to serve as denitrification sinks, while also storing excess P in agricultural watersheds.