EJP-Soil First call: Understanding tradeoffs and interactions between SOC stocks and GHG emissions for climate-smart agri-soil management

Soils store more carbon than vegetation and atmosphere together. However, soil organic carbon (SOC) is declining world-wide due to agricultural intensification, land use change, erosion, urbanisation and infrastructure development - with negative consequences for soil health and food security. Soil management methods targeted to increase SOC stocks can counteract this trend and, at the same time, absorb CO2 from the atmosphere. A common problem of this approach is that increased accumulation of organic carbon in soils can lead to increased emissions of nitrous oxide (n2o) and methan (CH4). By looking into the trade-offs of soil C management and non CO2-greenhouse gas (GHG) emissions in different plant production systems and pedoclimatic zones, TRUESOIL seeks to assess the “true” climate gains of C farming practices on a global level. A common strategy in carbon farming is to increase the input of plant-derived C to soils by adding organic biorests (plant litter, animal manures, digestates, biochar, etc.) or by growing cover crops or grass leys in rotation with cereals. Other practices such as reduced tillage or adapted grazing aim at reducing the microbial breakdown of organic carbon. Ongoing field trials organized in TRUESOIL investigate cover crops in cereal cropping (NOR), reduced tillage (GER), biochar addition to soil (POR) multispecies grass swards (FIN) and long-term application of organic versus mineral fertilizers (IR). In addition, rain exclusion shelters have been installed i 5 of the systems to investigate how drought stress affects carbon quality and stability in soil. In total 11 field trails are run in countries within (Germany, France, Spain, Portugal, Irland, Norway, Finland) and outside the EU (Ny-Zealand, Chile, Argentine, Indonesia og Ethiopia). The overall goal of TRUESOIL is to understand and model how local, site-specific factors regulate the balance between carbon accrual and CH4/ N2O emissions. For this, soil samples from the field trails have been sent to different partner laboratories to investigate soil C allocation patterns (GER), N2O production potentials (NOR), and microbial community composition (NZ). A highly prioritized question is why certain soils show ‘carbon-saturation’, i.e. an inherent upper limit of stabilized carbon in soil, while others do not. Research in TRUESOIL supports development of scientifically sound and locally adapted management strategies to maintain or increase soil carbon stocks without stimulating GHG emissions. Norway contributes to TRUSOIL with two field experiment at the NMBU research farm; one comparing different cover crops grown along with cereals, and another investigation carbon stabilization and N2O emissions along a natural hydrological/soil fertility gradient. To induce drought stress, nine rain exclusion shelters (~5 m2 plexiglass roofs diverting ca. 50% of rain) have been set up in 2023. At NMBU, we currently study soils from selected field trials in laboratory experiments using stable isotope tracing to elucidate how different carbon farming legacies affect the balance between stabilization of freshly added plant C and N2O emission potentials. Samples are then sent to New Zealand for metagenomic analysis. Results will be synthesized and serve for local extension on enhancing soil C sequestration and soil health.

Agricultural soils are depleted in soil organic carbon (SOC) and have the potential to sequester substantial amounts of C, which could be used in climate change mitigation. Common management practices for increasing SOC include the use of external or internally recycled OC inputs (organic amendments/fertilizers, biochar, plant residues), alternative cropping options (continuous green cover, cover crops) or measures that reduce OC losses (reduced tillage, adapted grazing). These practices have a potential to increase greenhouse gas (GHG) emissions by stimulating decomposition of previously sequestered C and N in soil. TRUESOIL assesses mechanisms and drivers behind increased GHG emissions in SOC-augmenting management practices and studies their interactions with increased SOC sequestration under different soil and climatic conditions (boreal, temperate, Mediterranean and semi-oceanic). Many C-augmenting management interventions are known, or have the potential, to modify soil N cycling, resulting in enhanced N2O emissions. To understand potential trade-offs between OC storage and GHG emissions, we combine intensive measurements of GHG fluxes in various cropping systems with carbon-nitrogen cycling studies and microbiological analyses. Comparison of soils that are SOC saturated with those that continue to accumulate SOC will aid to identify major drivers. Using rainfall exclusion experiments, we will also examine the future impact of reductions in precipitation on interactions between SOC accumulation and GHG emissions. TRUESOIL works towards an increased understanding of how environmental factors and management control OC sequestration, SOC persistence and stabilization and how this is linked to GHG emissions, opening for designing soil- and climate specific management strategies for climate smart crop production.