Imprint of denitrifying bacteria on the global terrestrial biosphere

Abstract

Loss of nitrogen (N) from land limits the uptake and storage of atmospheric CO₂ by the biosphere, influencing Earth's climate system and myriads of the global ecological functions and services on which humans rely. Nitrogen can be lost in both dissolved and gaseous phases; however, the partitioning of these vectors remains controversial. Particularly uncertain is whether the bacterial conversion of plant available N to gaseous forms (denitrification) plays a major role in structuring global N supplies in the nonagrarian centers of Earth. Here, we use the isotope composition of N (¹⁵N/¹⁴N) to constrain the transfer of this nutrient from the land to the water and atmosphere. We report that the integrated ¹⁵N/¹⁴N of the natural terrestrial biosphere is elevated with respect to that of atmospheric N inputs. This cannot be explained by preferential loss of ¹⁴N to waterways; rather, it reflects a history of low ¹⁵N/¹⁴N gaseous N emissions to the atmosphere owing to denitrifying bacteria in the soil. Parameterizing a simple model with global N isotope data, we estimate that soil denitrification (including N₂) accounts for ≈1/3 of the total N lost from the unmanaged terrestrial biosphere. Applying this fraction to estimates of N inputs, N₂O and NO_x fluxes, we calculate that ≈28 Tg of N are lost annually via N₂ efflux from the natural soil. These results place isotopic constraints on the widely held belief that denitrifying bacteria account for a significant fraction of the missing N in the global N cycle.