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

Alternative title: Sann karbonlagring i jord: samspill og avveining mellom karbonlagring og klimagassutslipp ved klimasmart landbruk

Awarded: NOK 4.1 mill.

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 turn this trend and, in the best of all cases, net-absorb greenhouse gases (GHG) from the atmosphere. A common strategy in carbon farming is to increase the input of 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. No matter which strategy is chosen, C-farming affects the processes turning over and stabilizing organic matter in soil, with unknown consequences for the emission and uptake of other GHGs such as nitrous oxide (N2O) and methane (CH4). In the worst case, climate savings by increased SOC sequestration could be cancelled out by increased N2O and CH4 emissions. TRUESOIL quantifies the ‘true’ climate effect of management practices aimed at increasing SOC stocks by measuring SOC changes and CH4/N2O emissions simultaneously. Emissions are monitored in ongoing field experiments in 11 partner countries inside (Germany, France, Spain, Portugal, Irland, Norway, Finland) and outside the EU (Chile, Argentina, Indonesia and Ethiopia). Soils from partner sites have been sent to Norway to study abiotic and biotic factors controlling soil carbon sequestration and interactions with GHG emissions in laboratory experiments. 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 TRUESOIL with a field experiment at NMBUs research farm in which we measure nitrous oxide emissions along a hydrologic gradient under cereals. To induce additional drought stress, nine rain exclusion shelters (~5 m2 plexiglass roofs diverting ca. 50% of rain) were set up during the growing season 2023. Carbon sequestration will be studied in 2024 in a parallel stripe where maize was grown in 2023. The carbon-isotopic composition of maize roots and straw differs naturally from that of soil organic matter, which allows us to follow decomposition and stabilization of fresh litter in 2024 as affected by soil moisture. In addition, Norway leads work package 3 of TRUSOIL which studies carbon and nitrogen turnover in more detail by laboratory incubations, aiming at elucidating differences in underlying processes at the various partner sites, For this, soil samples are sent to Norway and a PhD student from New-Zealand will come to extract microbial DNA before and after the experiments, which will be sequences in New-Zealand.

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.

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