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 ( 15 N/ 14 N) to constrain the transfer of this nutrient from the land to the water and atmosphere. We report that the integrated 15 N/ 14 N of the natural terrestrial biosphere is elevated with respect to that of atmospheric N inputs. This cannot be explained by preferential loss of 14 N to waterways; rather, it reflects a history of low 15 N/ 14 N 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 Ϸ1/3 of the total N lost from the unmanaged terrestrial biosphere. Applying this fraction to estimates of N inputs, N2O and NOx fluxes, we calculate that Ϸ28 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.denitrification ͉ isotope ͉ nitrogen ͉ ecosystem ͉ microbe

Microbial denitrification dominates nitrate losses from forest ecosystems

“…This approach similarly yields lower denitrification fluxes than our method, which is based on NO 3 − isotope composition (i.e., 2.0-9.8 kg of N ha −1 ·y −1 ; Table 1). This difference is consistent with the idea that bulk soil δ 15 N data yield a lower bound for denitrification rates from the land biosphere (9), because this approach misses zones of complete NO 3 − consumption and isotope effects of denitrification that can be diluted at larger scales (8,9).…”

“…For the purpose of comparison, we estimated denitrification rates using the N 2 O/(N 2 O + N 2 ) ratio (24) and total gaseous N loss rate by soil 15 N/ 14 N enrichment (9,21). Soil N 2 O emission rates were 0.1-4.7 kg of N ha −1 ·y −1 (Table S1), and an average N 2 O/(N 2 O + N 2 ) ratio of 0.5 (24) was used.…”

“…As an alternative, natural composition of N and oxygen (O) isotopes in NO 3 − provides nonintrusive, quantitative, and integrative constraints on denitrification across a myriad of spacetime scales (8,9). This approach takes advantage of the biological imprint of soil denitrification on the N cycle and the tendency for kinetic isotope effects to elevate 15 N/ 14 N and 18 O/ 16 O of NO 3 − systematically in forests (8).…”

Aridity threshold in controlling ecosystem nitrogen cycling in arid and semi-arid grasslands

“…Therefore, the positive correlation between AI and soil d 15 N observed in areas with AIo0.32 may be mainly due to increase of f gas . Changes of d 15 N input , e G and e p may also be the potential reasons, however, their variations are within a certain range so that they alone may not be enough to cause such a significant change and correlation 17,22 . In fact, we observed higher abundance of nitrification and denitrification genes with increasing AI (Fig.…”

Biological nitrogen fixation: rates, patterns and ecological controls in terrestrial ecosystems

“…The gaseous loss flux from land (GL) is more difficult to determine directly, but calculations based on N isotopes make it possible to estimate the fraction of total losses that are gaseous ( f G ) at steady state [28,31]. The ratio of stable N isotopes ( 15 N/ 14 N) in the terrestrial biosphere depends on the ratio of 15 N/ 14 N in inputs, the degree to which the different loss pathways (hydrologic versus gaseous) fractionate against the heavier isotope and the relative magnitudes of the different loss pathways.…”