Argentina: Soil Organic Carbon Sequestration Potential National Map. National Report. Version 1.0. Year: 2021 Franco Daniel Frolla1, Marcos Esteban Angelini2, Marcelo Javier Beltrán3, Guillermo Ezequiel Peralta5, Luciano Elias Di Paolo4, Darío Martín Rodríguez2, Guillermo Andrés Schulz2, Carla Pascale Medina6 1INTA Bordenave - [email protected] 2INTA, Soil Institute - [email protected] 3INTA, Soil Institute - [email protected] 4 Global Soil Partnership Secretariat - FAO - [email protected] 5 Global Soil Partnership Secretariat - FAO - [email protected] 6 Global Soil Partnership - Argentinian National Focal Point - Carla Pascale Medina - [email protected] 1 Executive summary In the last decade’s agricultural land increased and soil organic carbon (SOC) stocks decayed in Argentina. Several farming practices may be used to restore or diminish the SOC loss and different SOC simulation models have been used to estimate and project SOC changes. However, these studies have mainly focused on specific regions and practices. The goal of this work was to apply the FAO-GSP Technical Specifications and Country Guidelines for Global Sequestration Potential Map v1.0 approach to produce a SOC potential sequestration map for Argentina at 1 km resolution using the best available national data. Specific objectives were 1) to estimate the SOC evolution under the business as usual (BAU) practices in twenty years (2040), 2) to estimate the absolute SOC sequestration of different sustainable soil management scenarios: 5% (SSM1), 10% (SSM2) and 20% (SSM3) increment in organic matter inputs, and 3) to calculate the differences in SOC sequestration between the BAU scenario and SSM scenarios (relative sequestration rates - RSR), as well as the differences between SSM scenarios in 2040 and the SOC stocks in 2020 (absolute sequestration rates - ASR). The results showed that average SOC sequestration in the BAU scenario would decrease at a rate of -0.089 t C ha-1 year-1 between 2020 and 2040. SSM1 and SSM2 projections also showed a negative SOC evolution. Only SSM3 generated absolute SOC increments. With respect to the BAU scenario, we found an average increment of 0.025 t C ha-1 yr-1 for SSM1, an increase of 0.053 t C ha-1 yr-1 for SSM2, and an increase of 0.106 t C ha-1 yr-1 for SSM3, for the period under study. Our results suggest that agricultural systems are currently a source of CO2 rather than a net sink at the national level, and that increasing C inputs by 5 to 10% would not be enough to achieve a positive C balance in the future. Nevertheless a sequestration potential of 4.2 to 16.7 Mt C yr-1 can be expected under SSM compared to BAU practices, indicating that the wide adoption of SSM practices could mitigate about 11-48% of current annual national agricultural emissions. Abbreviations C - Carbon CO2 - Carbon dioxide SOC - Soil organic carbon BAU - Business as usual SSM - Sustainable soil management RSR - Relative sequestration rate ASR - Absolute sequestration rate 2 1. Introduction Soil organic carbon (SOC) is a key factor affecting soil physical fertility, as it improves several soil properties such as infiltration, structural stability, porosity, aeration and structure. It also improves soil chemical fertility since C is part of the soil organic matter, which constitutes the main reservoir of nutrients for crops (nitrogen, sulfur, zinc, among others). SOC is positively correlated with soil microbial biomass that acts on nutrient cycling and metabolization processes of toxic molecules. The total SOC stock in topsoil (0-30cm) is about 19.7 Pg C (FAO-ITPS GSOC map, 2018). Thus, due to the size of the soil carbon pool, even small increments in the net soil C storage may represent a substantial C sink potential. Although agricultural greenhouse gas emissions (GHGs) contribute to an important share of Argentina GHG emissions (135.53 MtCO2eq, 37% of total country GHG emissions; SAyDS, 2019), increasing ASOC stocks through judicious land use and sustainable soil management (SSM) practices may represent an important strategy to reduce and mitigate GHG emissions. In Argentina, the total productive area is about 157 million hectares (INDEC, 2021). Agricultural area (croplands) is about 40 (forty) million hectares, predominantly under no tillage system (91% agricultural area; AAPRESID, 2020). Soybean is the main product (45 million tons in 17 million hectares), followed by corn (44 million tons in 6.3 million hectares), wheat (17 million tons in 6.5 million hectares), barley (4.1 million tons in 0.1 million hectares) and sunflower (2.7 million tons in 1.3 million hectares).The rest of the area (over 124 Million hectares) is occupied with grasslands and shrublands dedicated to livestock production, and other agricultural uses. In the last decade’s agricultural land increased and SOC content decayed. This process of land use change was explained by increasing soybean monoculture and displacing livestock area, reducing SOC content (Lavado & Taboada, 2009). There has been an intense expansion of agriculture at the expense of grasslands, native forests and other natural resources in semiarid, sub-humid and subtropical regions of the country (Volante et al., 2012). Currently, soils of the Chaco-Pampean region exhibit SOC levels between 40-70% of the contents of virgin soils (Alvarez & Steinbach, 2009; Sainz Rozas et al., 2011; Milesi Delaye et al., 2013). Several farming practices may be used to restore or diminish the SOC loss, reduce soil erosion, sequester atmospheric carbon dioxide (CO2) and improve the soil quality (Poffenbarger et al., 2020). Among these practices, the inclusion of cover crops (CC) during winter has been postulated as one of the most promising activities (Ruis & Blanco-Canqui, 2017). The inclusion of CC showed average SOC sequestration rates of 0.45 tC/ha/yr (± 0.03), in Argentina (Alvarez et al., 2017; Beltran et al., 2018; Romaniuk et al., 2018). Increasing nutrient availability, crop growth and residue returns by increasing fertilizer use showed an increment of SOC around 0.18 tC/ha/yr (± 0.03) (Duval et al., 2020; Restovich et al., 2019). The inclusion of cycles with perennial pastures in crop rotations showed average SOC sequestration rates of 0.76 tC/ha/yr (± 0.03), exhibiting the greatest potential to increase SOC stocks (Costantini et al., 2016; Gil et al., 2016). Sustainable soil management (SSM) practices (FAO, 2020) such as the above mentioned practices have demonstrated potential to increase SOC stocks in different agricultural systems in Argentina, and thus sequester atmospheric CO2 as SOC to mitigate GHG emissions. However, SOC sequestration from these practices show highly variable sequestration rates, depending on edapho-climatic conditions, land use and management, among other factors. It is therefore relevant to identify which regions, soils, climates and systems have a greater potential to increase SOC stocks, in order to establish priorities for research and implementation of private and public policies. In this 3 sense, the use of SOC models has shown in other countries and regions to be a powerful tool to identify these conditions (Lugato et al., 2014). In Argentina, different SOC simulation models have been used to estimate and project SOC changes. The IPCC (Intergovernmental Panel on Climate Change) empiric Tier 1 and Tier 2 approaches have been applied to estimate historic SOC stocks and flows in the Pampa Region at a county scale (Villarino et al., 2014). The AMG model (Andriulo et al., 1999) has been one of the most widely used, especially in agricultural lands of the Rolling Pampa Region , to project SOC stocks under different management scenarios (Irizar et al., 2015). The Century model has also been adjusted and used to simulate historic SOC changes in temperate grasslands (Piñeiro et al., 2006). Finally, the RothC model has also been adjusted and used to simulate SOC stocks under continuous cropping and mixed systems (Studdert et al., 2011; Montiel et al., 2019). However, these studies have mainly focused on specific regions, edapho-climatic conditions and practices. Coupling SOC dynamic models with empirical models and spatial data, such as soil data and climatic data, will enable the transition from site-specific SOC stocks estimations to spatial predictions and projections, and this in turn become a valuable tool to better identify conditions with higher potential to increase SOC stocks, as well as to detect hot-spots of SOC losses. During the previous year, FAO (2020) developed an approach to simulate SOC stocks and generate SOC sequestration potential national maps, using a spatialized version of the RothC model and georeferenced input data. The goal of this work is to apply the FAO (2020) approach to produce a SOC potential sequestration map for Argentina at 1 km resolution using the best available national data. The objective of the national SOC sequestration map is (1) to estimate the SOC evolution with the business as usual practices; (2) the absolute SOC sequestration of different sustainable soil management scenarios (3) the differences in SOC sequestration between the business as usual and sustainable management scenarios. The National Institute of Agricultural Technology (INTA) is the institution in charge of this process. 2. Methods 2.1. Study area Argentina is in southern South America with a total surface area of 2.8 million km2. According to the country’s total area, it is ranked the seventh among all world countries. Argentina’s climatic characteristics are very diverse because of its vast territory, with a wide range of rainfall, from 2000 mm in the northeast to 200 mm in the south region of the country. The temperature varies from a mean annual temperature of 24° C in the north to less than 5° C in the south region (Bianchi & Cravero, 2010; Rodríguez & de la Casa, 1990). With almost 40 million hectares of grain sown per year and more than 40 million heads of cattle, Argentina is a large net exporter of agricultural products such as soybean, wheat, corn, sunflower, sorghum, beef and milk.