In drought conditions, forest soils can serve as a small but surprisingly persistent sink for the greenhouse gas nitrous oxide. The effect highlights a research avenue necessary for predicting Earth's climate.
Increasing amounts of reactive nitrogen1 are entering the environment through human agency. One consequence is increased production of the powerful greenhouse gas nitrous oxide — N2O — by microorganisms in soils. We do not understand the intricate dynamics of N2O production and consumption in soils, prompting research such as that reported in Global Change Biology2 by Goldberg and Gebauer. They tracked N2O fluxes in European spruce-forest soils under experimental conditions of a predicted climate pattern — increasing episodes of drought followed by heavy rainfall.
The burning of fossil fuel, planting of crops associated with bacteria that can capture atmospheric dinitrogen (N2), and use of increasing amounts of fertilizer, all result in more nitrogen in Earth's biological cycles1. Atmospheric concentrations of N2O have risen by 18% since the middle of the eighteenth century, in part because of these activities3. In the atmosphere, N2O lasts for an average of 114 years before undergoing reactions resulting in its destruction. However, this atmospheric N2O sink accounts for only about 71% of known sources, leaving 5.2 million tonnes 'missing' from the atmosphere annually3. This discrepancy means that we are either overestimating N2O sources or underestimating N2O sinks. Goldberg and Gebauer offer evidence that addresses this point. Reports of soil N2O fluxes typically describe net emissions to the atmosphere4, but Goldberg and Gebauer conclude that forest soils may serve as net N2O sinks to a greater extent than previously thought.
Nitrous oxide is both produced and consumed in soils by a complex suite of microbially mediated processes. For example, denitrifiers perform the step-by-step chemical reduction of nitrate (NO3−) to N2, producing N2O as an intermediate by-product. Some N2O escapes from the soil, but under anaerobic conditions some is completely transformed into N2. The denitrification pathway is thus associated with both N2O production and consumption. Moreover, other groups of microorganisms4,5,6 can also transform nitrogen in ways that produce and consume N2O.
Moisture availability can promote the production of N2O (ref. 7), but investigators often presume that N2O consumption — transformation into N2 — is also highest when soil moisture levels are high because of the anaerobic nature of this process4. There are relatively few field studies that report net N2O consumption at the soil surface, but those that do describe comparatively small fluxes that show no predictable relationship with soil moisture. These small fluxes are often dismissed as an indication of the challenges of quantifying N2O levels near detection levels, and not robust evidence of soil N2O sink strength4. Thus, the influence of soil moisture on N2O production and its ultimate fate — release or consumption — remains unclear.
Goldberg and Gebauer2 have tackled this issue by inducing soil drought in a spruce forest in Germany, and measuring net N2O fluxes at the soil surface and concentrations within the soil profile. They found that net N2O consumption at the soil surface was enhanced by drought, and, surprisingly, that it occurred across much of a forest's growing season. Deep within the soil profile, concentrations of N2O were orders of magnitude greater than atmospheric levels. During drought, soil N2O concentrations near the surface declined to sub-atmospheric levels, promoting the diffusion of N2O from the atmosphere into the soil. With rewetting, soils quickly resumed their role as a net source of N2O to the atmosphere. Both net uptake and production rates of N2O were small — within the range that other investigators have viewed as unreliably close to zero4. However, net N2O consumption during the drought was sufficient in both magnitude and duration for the soil profile to require almost four months for cumulative fluxes to reflect net N2O production again.
To help identify the processes governing these N2O fluxes, Goldberg and Gebauer2 quantified the rare, stable isotopes 15N and 18O in the N2O within the profile. Because these isotopes are relatively heavy, microbial processing of them tends to be slower than that of the more common, lighter 14N and 16O. As a result, quantifying 15N2O and N2 18O can provide clues about the processes governing N2O production and consumption8,9,10.
Trends in N2 18O were possibly confounded by the exchange of oxygen with soil water, but the 15N2O data are revealing. Near-surface soil N2O exhibited high amounts of 15N2O relative to deeper values. This is consistent with microbial consumption of N2O dominating N2O dynamics throughout the profile, because microorganisms preferentially consume 14N2O, leaving remaining N2O relatively enriched in 15N. Importantly, soil drought increased the relative abundance of 15N2O throughout the profile. The authors2 suggest that the source strength of N2O within the profile declined with drought, whereas rates of N2O transformation into N2 remained constant.
Goldberg and Gebauer's study2 is an example of the kind of research required to understand the global N2O budget — measurement of N2O fluxes and soil-profile concentrations, combined with isotopic analyses — but they do not elaborate on the mechanisms governing the isotopic shifts they observed. If, as they suggest, N2O sources declined and consumption was maintained with drought, the dominant microbial source must have shifted to one that generates greater relative amounts of 15N2O. This could explain increasing 15N2O throughout the profile with drought, because different microbial pathways can generate N2O exhibiting distinct abundances of 15N. Alternatively, rates of consumption of N2O could have increased while the N2O source strength and path were maintained; such a change would result in a shifting 'front' of 15N2O with drought, similar to that observed.
Studies such as this are essential for predicting Earth's future climate. Atmospheric concentrations of N2O are low compared with those of carbon dioxide — 319 parts per billion compared with 379 parts per million (figures as of 2005)3. On a 100-year timescale, however, each molecule of N2O has some 300 times the radiative forcing power of CO2 (ref. 11), making it an important driver of Earth's climate.
Future research must clarify the mechanisms that generate net soil N2O uptake in different soil types under varying environmental conditions. Research will particularly need to focus on the influence of soil moisture on N2O dynamics, given the predicted increase in drought frequency and rainfall variability in many regions. Advances8,12 in quantifying the relative abundances of two types of 15N2O (15N14NO and 14N15NO) promise to help in distinguishing between the paths by which N2O is produced. Such work will help us further constrain the global N2O budget and, ultimately, understand critical features of the two-way feedbacks between climate and N2O.