The impact of agroecological practices on carbon sequestration of an avocado plantation

The increase of the world population up to 9.5 billion by 2050 will require an increase on food demand of the order of 30% (Lal 2015). Agroecological practices which are considered Nature-based Solutions (NbS) (Maragkaki et al. 2024) aim to produce significant amounts of food and can improve sustainability of agroecosystems while being based on various ecological processes and ecosystem services such as nutrient cycling, N fixation, natural regulation of pests, soil, water and biodiversity conservation and, carbon sequestration (Wezel et al. 2013). Carbon credits is a financing mechanism for farmers who implement such practices in order to increase soil organic carbon (SOC) and thus produce higher quantities of food. Carbon credits are economically viable for farmers as they receive an additional income for every ton of CO 2 sequestered (Liu 2022). The objective of this study was to assess the impact of agroecological practices on avocado plantations (2019-2024) and forecast the carbon accumulation until 2028 using the Carbon, Aggregation and Structure Turnover (CAST) model (Stamati et al. 2013). The study area is located in the valley of Koiliaris River Basin (Latitude: 35.43717, Longitude: 24.1427, Elevation: 15 m). The avocado plantation consists of 25 six-year-old trees (Avocado 1) and 45 four-year-old ones (Avocado 2) irrigated through drip irrigation with a piping system of 25 and 15 drips, respectively. The agroecological practices that have been applied to the field are manure addition, mulching and grass incorporation in the soil and sustainable irrigation practices (Maragkaki et al. 2024). The soil is silty-clay (67-68%) and the organic matter is 2.5% to a depth of 50 cm. The avocado plantation instrumentation consists of a meteorological station, NDVI (Normalized Difference Vegetation Index) and PRI (Photochemical Reflectance Index) cameras, soil moisture probes located at different soil depths and flow meters. The infrastructure comprises of wireless, autonomous monitoring stations that transmit data to a remote database server (Lilli et al. 2024). The CAST model is a model that incorporates the physical protection of SOC within the soil aggregates. It is based on the theory that macro-aggregates are formed in the presence of particulate organic matter (POM), after colonization of the latter by communities of decomposer microbes and encrustation of POM by interaction with clay particles and micro-aggregates to form macro-aggregates. The decomposition of the POM fragment results in disruption of aggregated particles and the release of micro-aggregates (Andrianaki et al. 2017, Stamati et al. 2013). In this study, the CAST model was initialized and calibrated using chronosequence data (2019, 2023 and 2024). The model simulated the SOC and the organic carbon (OC) contained in macro-aggregates (AC3), micro-aggregates (AC2) and silt-clay sized micro-aggregates (AC1) during a 10-year period (2019-2028). The addition of 11 tC/ha/yr and 13tC/ha/yr for Avocado 1 and 2 respectively increased the SOC with the C accumulated in the soil for the six-year-old trees (Avocado 1) being 2.8 tC/ha/yr (25%) and for the four-year-old ones (Avocado 2) 3.0 tC/ha/yr (23%). Moreover, it was observed that in May 2024 the OC contained in macro-aggregates (AC3) decreases and the OC contained in micro-aggregates (AC2) increases (Fig. 1). It was assumed that drought (due to the limited precipitation during January to March of 2024) resulted in a decline of OC in macro-aggregates (>250μm) and an increase of OC in micro-aggregates (53-250μm). Several studies conducted in forests and agricultural lands found out that drought can have serious consequences for the structural stability of the soil, which influences the protection of the organic matter stored in the soils and hence its mineralization (Quintana et al. 2023, Su et al. 2020, Zhang et al. 2018, Zhang et al. 2019). However, to confirm this assumption, soil samples will be collected from the field in February 2025 and will be analyzed to determine the organic carbon distribution.