Can N₂O emissions offset the benefits from soil organic carbon storage?

Abstract

To respect the Paris agreement targeting a limitation of global warming below 2°C by 2100, and possibly below 1.5°C, drastic reductions of greenhouse gas emissions are mandatory but not sufficient. Large‐scale deployment of other climate mitigation strategies is also necessary. Among these, increasing soil organic carbon (SOC) stocks is an important lever because carbon in soils can be stored for long periods and land management options to achieve this already exist and have been widely tested. However, agricultural soils are also an important source of nitrous oxide (N2O), a powerful greenhouse gas, and increasing SOC may influence N2O emissions, likely causing an increase in many cases, thus tending to offset the climate change benefit from increased SOC storage. Here we review the main agricultural management options for increasing SOC stocks. We evaluate the amount of SOC that can be stored as well as resulting changes in N2O emissions to better estimate the climate benefits of these management options. Based on quantitative data obtained from published meta‐analyses and from our current level of understanding, we conclude that the climate mitigation induced by increased SOC storage is generally overestimated if associated N2O emissions are not considered but, with the exception of reduced tillage, is never fully offset. Some options (e.g. biochar or non‐pyrogenic C amendment application) may even decrease N2O emissions.

The importance of nitrogen for net carbon sequestration when considering natural climate solutions

“…However, most studies that report ER potentials of natural climate solutions (NCS) do not account for the additional nitrogen demand of these activities (Griscom et al, 2017;Lal et al, 2018;Roe et al, 2019). In this issue of Global Change Biology, Guenet et al (2020) begin to address this by providing an important review of the potential implications for greenhouse gas (GHG) mitigation when accounting for nitrogen requirements to store carbon within agricultural NCS.…”

“…When we look towards CO 2 sequestration from NCS to help mitigate climate change, we often overlook the fact that carbon and nitrogen cycles are coupled (see figure 1 in Guenet et al, 2020). In their paper, Guenet et al (2020) describe this relationship in the context of how agricultural management practices may lead to an increase in soil organic carbon (SOC), and highlight that this SOC gain can be at least partially negated by increased N 2 O emissions (see figure 2 in Guenet et al, 2020). By combining data from multiple meta-analyses, it is clear that there is a considerable uncertainty when it comes to the ER benefit of different management activities within agricultural NCS, highlighting the need for longer-term studies that account for the system as a whole.…”

“…By combining data from multiple meta-analyses, it is clear that there is a considerable uncertainty when it comes to the ER benefit of different management activities within agricultural NCS, highlighting the need for longer-term studies that account for the system as a whole. Guenet et al (2020) demonstrate that, at least for agricultural systems, efforts to store carbon can and do lead to increased emissions of N 2 O. While there is still a lot of uncertainty, it will be important to consider these relationships and whether emissions will increase or decrease for all NCS activities.…”

“…Given the prominent role that NCS are being given in government strategies to achieve their Nationally Determined Contribution ambitions, it is critical to improve our understanding of the interactions between C and N cycles, and the impact of land management on the processes that are involved in the emission of N 2 O. which we can influence these stores will inevitably be a major factor that ultimately determines the overall carbon balance and ER potentials associated with NCS. In their paper, Guenet et al (2020) outline some of the ways in which we can influence this carbon balance, with addition of biochar as one potentially important mechanism to both store carbon in stable forms and reduce the emissions of N 2 O. When biochar is applied to soil, it can modify pH, water holding capacity and N availability through adsorption and desorption (Sun et al, 2015), that ultimately leads to a reduction in N 2 O emissions.…”

“…Furthermore, only a few relatively short-term studies have estimated biochar stability in-situ with residence time estimates from decades to thousands of years (Gurwick et al, 2013;Wang et al, 2016). The key outcome from Guenet et al (2020) is that the associated N 2 O debt acquired from managing land to increase SOC does not outweigh the overall GHG reduction benefit for most evaluated scenarios. In the future, if agricultural soils do progress towards carbon saturation, then N 2 O emissions will gain more importance, highlighting the need for effective mitigation strategies.…”