Nitrous oxide emissions from soils: how well do we understand the processes and their controls?

Abstract

Although it is well established that soils are the dominating source for atmospheric nitrous oxide (N2O), we are still struggling to fully understand the complexity of the underlying microbial production and consumption processes and the links to biotic (e.g. inter- and intraspecies competition, food webs, plant-microbe interaction) and abiotic (e.g. soil climate, physics and chemistry) factors. Recent work shows that a better understanding of the composition and diversity of the microbial community across a variety of soils in different climates and under different land use, as well as plant-microbe interactions in the rhizosphere, may provide a key to better understand the variability of N2O fluxes at the soil-atmosphere interface. Moreover, recent insights into the regulation of the reduction of N2O to dinitrogen (N2) have increased our understanding of N2O exchange. This improved process understanding, building on the increased use of isotope tracing techniques and metagenomics, needs to go along with improvements in measurement techniques for N2O (and N2) emission in order to obtain robust field and laboratory datasets for different ecosystem types. Advances in both fields are currently used to improve process descriptions in biogeochemical models, which may eventually be used not only to test our current process understanding from the microsite to the field level, but also used as tools for up-scaling emissions to landscapes and regions and to explore feedbacks of soil N2O emissions to changes in environmental conditions, land management and land use.

Keywords: N2O; environmental controls; modelling; processes.

Figure 1.

Drivers and processes of soil N₂O emissions across temporal and spatial scales. Different colours indicate the level of understanding. Underlying grey boxes show different measuring techniques (enzymatic, chamber, eddy covariance (EC)/micrometeorological measurements) commonly used for identifying N₂O production and consumption processes and soil surface fluxes.

Figure 2.

Biotic and abiotic processes of nitrous oxide (N₂O). Processes potentially leading to N₂O formation and consumption, involved N compounds, their reaction pathways as well as their oxidation states are shown. According to current knowledge, anaerobic ammonia oxidation does not contribute to N₂O formation or consumption. By contrast, N₂O may at least serve as a substrate for biological dinitrogen fixation. Processes predominantly requiring anaerobic (or micro-aerobic) conditions are underlined by grey illuminated segments. Norg/R-NH₂, monomeric organically bound N forms; NH₄⁺, ammonium; NH₃, ammonia; NH₂OH, hydroxylamine; NO₂⁻, nitrite; NO₃⁻, nitrate; NO, nitric oxide; N₂O, nitrous oxide; N₂, molecular dinitrogen. DNRA, Dissimilatory Nitrate Reduction to Ammonium.

Figure 3.

Schematic of process models used for simulation of N₂O emission with different degrees of complexity: (a) simplified, (b) conceptual, (c) complex. Black arrows and components are accounted for in the models, grey arrows and components are optional, red arrows indicate exchange of components between anaerobic (denitrification) and aerobic (nitrification) micro-sites in the soil. Simplified process models use potential denitrification rates which are decreased by reduction factors related to soil environmental conditions for calculation of N₂O emission. In addition, conceptual models also include N₂O emission from nitrification mostly by use of fixed fractions. However, both simplified and conceptual models follow the theory that N₂O production in the soil equals N₂O flux at the soil–atmosphere interface. Complex process models calculate N turnover via nitrification and denitrification considering the dynamics of microbes. Nitrification and denitrification N turnover is weighted by calculation of anaerobic-aerobic volume fractions as function of soil oxygen concentrations. For this complex process models take into account diffusion processes which also determine the N₂O flux at the soil–atmosphere interface, thus in contrast to simplified and conceptual models emission is not equal to production.