Intensification and diversification of degraded pasture areas in Brazil: potential of soil C accumulation and mitigation of the climate changes

Abstract

Intensification and diversification of degraded pasture areas in Brazil: potential of soil C accumulation and mitigation of the climate changesPasture improvements in Brazil, where pastures occupy ~167 million ha, have great potential for soil carbon (C) accumulation and to mitigate of greenhouse gases (GHG), such as the carbon dioxide (CO2). However, soil degradation on current pasture areas (> 50% are degraded) is an issue to be solved through sustainable management. The aim of our current field study was to quantify soil C stocks and to elucidate the processes, involved in the soil C accumulation in areas subjected to different intensive and diversified systems of pasture management in Brazil. Treatments included fertilized pasture (FP), integrated crop-livestock (ICL) and integrated livestock-forest (ILF), compared to the extensive form (conventional management system) under contrasting climatic conditions (tropical humid, tropical mesic and subtropical). To meet this goals, there were avaluaded i) the soil C stocks changes and the soil organic matter (SOM) quality; ii) the relationships between the soil C and N stocks with the different phosphorus (P) fractions; iii) the soil chemical and biochemical properties and, the soil bacterial community structure to assess the controlling factors on soil C accumulation and, iv) the use mathematical modeling to predict the soil C changes and GHG emissions. In general, the adoption of more intensive and diversified systems of pasture management in areas previously used with extensive management systems increased the soil C stocks and the soil C lability. Likewise, there was an improvement in the soil chemical properties related to soil fertility, especially in soil P. The increase in the labile P contents was directly proportional to the increase of the more labile SOM fractions. Furthermore, the conversion of conventional management system to more intensive and diversified pasture management systems, besides changing the soil chemical and biochemical properties and the soil bacterial community structure, also modified the mechanisms that control the soil C accumulation. According to the structural equation modeling, the improvement in the soil chemical properties was the factor that most influenced in soil C accumulation. In addition, through long-term predictions performed with the DayCent model, it was found that while extensively managed pastures were a GHG source to the atmosphere, systems of pasture management, such as FP, ICL and ILF were GHG sinks. Thus, for the conditions tested in this study the DayCent model proved to be an a efficient and costeffective tool to predict soil C pool changes and to monitor GHG emissions. Finally, it is believed that the results found in this thesis can assist national initiatives aimed at restoring degraded pasture areas (e.g., "ABC Plan"), as well as fits the scope of the Brazil's NDC (Nationally Determined Contribution) for mitigating GHG emissions (reduce emissions 37% by 2025 and 43% by 2030) through the increase of the soi...