Carbon stock and quality in forest and agricultural environments

Abstract

The present study aimed (i) to quantify the carbon stocks (C) in the biomass, litter and soil reservoirs in forest and agricultural ecosystems, (ii) to quantify the stocks and carbon quality in the soil, (iii) to evaluate the effects of changes in land use in the C reservoirs, and (iv) to quantify and evaluate the seasonal variation of carbon flows through litter input in forest ecosystems in Dry Tropical Forest and Tropical Rainforest environments in Northeast Brazil. The biomass of leaves, wood/manioc/stem and total biomass of species with the highest absolute density in forest environments and in agricultural crops of cassava and sugarcane were estimated. In addition, the accumulated litter biomass in forest environments was estimated. Soil samples from six layers up to 1.0 m deep were collected to determine the carbon content and density of the soil and to estimate the soil's organic carbon stock. The carbon content and stocks of each compartment were determined by species and by the type of land use in each area. Deformed and undisturbed soil samples were collected at 0.00 - 0.05; 0.05 - 0.10; 0.10 - 0.20 m deep layers. The contents of total organic carbon, carbon of humic substances (fulvic acids, humic acids and humine), labile carbon and fractions of oxidizable carbon were estimated. Carbon stocks, as well as carbon recovery and depletion rates were calculated. Contribution of litter was evaluated using 20 collectors of 1.0 m², installed in the center of the plots. Material deposited in the collectors was taken at 45; 90; 135; 180; 225; 270 and 315 days after its installation in April 2017 and in April/2018, totaling 18 collections per area. The litter was separated into leaves, branches+bark and miscellaneous material. The dry and total litter biomass of each fraction was determined, as well as the carbon content. The land-use change in the dry and humid environment represented significant effects for the carbon stock in the soil up to 1.0 m deep. Conversion of forest to agricultural cultivation in the humid environment altered carbon content and stock in the soil, especially in the top layers, which was mainly related to a decrease in litter supply and agricultural cultivation practices. Changes in land use reduced the carbon recovery rate from fractions of fulvic acids, humic acids, humine and total humic substances in the surface layers in the dry environment. The leaf biomass contributed the most to total carbon in litter among years in the humid environment (~70% of the total C contributed). The largest contributions of C trough litter occurred during the period of water limitation and is related to the leaf senescence of Caatinga species. The peaks of C input through litter in the rainforest occured during winter and spring, the driest period.