Soil organic matter dynamics and physical quality changes associated with the agricultural expansion in the Matopiba region, Brazil

Author(s)

Rafael Silva Santos

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2023-01-24

Examining board members

Carlos Eduardo Pellegrino Cerri; Dener Marcio da Silva Oliveira; Carlos Gustavo Tornquist

• Advisor: Carlos Eduardo Pellegrino Cerri

Abstract

Land use change (LUC) and soil mismanagement have significant effects on soil quality, which may compromise soil\'s capacity to promote ecosystem services, including food production and climate regulation, depending on the extent to which soil attributes are modified. In the so-called Matopiba region in northeastern Brazil, large areas of the Brazilian savannah (Cerrado biome) were converted to agricultural areas (e.g., soybean and cotton production) in recent years. However, little is known about the long-term impacts of this land-use change on soil physical properties and related soil functions as well as soil organic matter (SOM) dynamics. For this purpose, we carried out a field experiment in three consolidated agricultural areas (∽23 years old) across a 1000-km transect within the Matopiba region, Brazil\'s new agricultural frontier, to assess the extent to which the land-use change from native vegetation (NV) to agricultural areas under no-tillage (NT) has impacted soil physical quality and SOM dynamics. Specifically, we i) quantitatively assessed the impacts that the conversion from the Cerrado to agriculture has imposed on soil physical quality; ii) quantitatively and qualitatively assessed water extractable organic matter (WEOM) dynamics; and iii) used the DayCent model to predict how the current management adopted in the Matopiba region can affect soil C stocks over a 50-yr period based on the current climate and projected climate change scenarios, as well as how the intensification of this agricultural system (e.g., integrated crop-livestock - ICL) can contribute to increasing soil C sequestration. We observed that the conversion from NV to agriculture under NT increased the compaction process and reduced total soil porosity, unbalancing the proportion between soil water and air storage to critical levels. The soil physical quality index (SPQI) was reduced by ∽33% in NT, indicating detrimental effects of agriculture expansion on soil functionality. Also, water availability and air diffusion were the soil functions most affected by LUC according to the SPQI. In addition, we observed that although soil organic C decreased along the soil profile after NV (4.2-20.7 g kg-1) conversion to NT (3.8-14.2 g kg-1), water-extractable organic C levels (3.679.3 mg L-1) were similar between land uses. WEOM had lower aromaticity and molecular weight in NT than NV; and a higher decomposition of aliphatic and polysaccharides than aromatic and amine/amide was observed after LUC. The DayCent model indicates that the agricultural intensification through the conversion of the soybean-cotton rotation to integrated crop- livestock (ICL) systems increased soil organic C stocks compared to the NV (36.6 Mg ha-1), irrespective of the systems used. Climate change had little effect on C stocks under the simulated ICL systems, which trend was the same as the current climate. Finally, our findings indicate the need to improve the NT practices currently used in the Matopiba region towards alleviating soil compaction and improving soil structure as well as the importance of tailoring ICL systems to the Matopiba region as a strategy to increase soil C. These concerted actions are paramount to alleviate the anthropogenic pressure on the environment and to restore soil functionality, ensuring sustainable food production and provision of ecosystem services. (AU)

• FAPESP's process: 19/17576-9 - Organic matter dynamics under the influence of land use change: effects on dissolved organic carbon
• Grantee: Rafael Silva Santos
• Support Opportunities: Scholarships in Brazil - Doctorate