Carbon and nitrogen storage in soil and greenhouse gases emission in areas cultivated with Jatropha spp

Jatropha cultivation is pointed as a multipurpose option whether is used for biodiesel production reducing greenhouse gases (GHG) emissions when replaces the need for fossil fuels, and also for storing C in the soil. The overall objective of this work was to evaluate the C sequestration potential of soil cultivated with Jatropha, as well as the variations in soil organic matter (SOM) quality. Therefore, C and N stocks were determined and GHG fluxes were measured in Jatropha cultivation. The difference between results from soil C storage and GHG emissions were used to calculate the C sequestration rates, defined as the C-equivalent (C-eq). SOM quality was assessed by isotopic analysis, physical fractioning, C management index (CMI), humufication degree (HFIL) and microbial C and N content. Land use change from native vegetation to agroecosystems altered the amount and composition of soil organic matter located in biomes of Cerrado, Atlantic Forest and Caatinga, especially within the surface layers. Jatropha cultivation preserved soil N and C stocks and contents, regardless of previous land use management (pasture, maize or native vegetation), proning to increase with cultivation time. Additionally, the isotopic analysis of C and N showed changes in SOM after two years with Jatropha cultivation. After 7 years of cultivation, the contribution of carbon derived from Jatropha residues to the total amount of the element stored within the 0-30 cm layer of soil reached 11.5%. The Jatropha cultivation increased the C contents in SOM, the CMI and microbial C and N contents along with cultivation time, which highlights the potential of Jatropha cultivation to improve SOM quality in the long term. HFIL of the areas with Jatropha cultivation showed lower values compared to the native vegetation. This indicates the increments of C contents for the areas planted with Jatropha are associated to the increase of the less stable portion of SOM and that the selective preservation is not the main mechanism responsible for C accumulation in soils under Jatropha cultivation. The N2O emission factors ranged from 0.21 to 0.46% for the doses up to 150 kg ha-1. Considering the average N fertilization rates applied annually in comercial Jatropha cultivation (75 kg ha-1 yr-1), the crop is responsible for the emission of 0.0362 Mg ha-1 yr-1 of C-eq. Moreover, the annual balance between soil C storage and GHG emission indicated that Jatropha cultivation for 7 years is able to contribute to the carbon sequestration accounting for 0.6 Mg ha- 1 yr-1 of C-eq stored in the soil. This study is pioneer in Brazil and the results generated in this research are basis for life cycle analysis of the Jatropha as a feedstock for biodiesel production in Brazil. (AU)