Impacts of forest conversion and pasture management on carbon conservation in Eastern Amazonian soils

In the Amazon, the conversion of forests into pastures and the use of fire as management are still common, and although these practices are emitting, it is essential to understand their consequences. The study aimed to understand how land use conversion to pasture and the prolonged use of fire influence the proportion of C3 and C4 in the soil. The work was conducted in the municipality of Moju, Pará, Brazil, under an Am climate. The soils were classified as Yellow Latosols, with a medium texture. Three land use systems were analyzed: forest under selective logging, pasture of Urochloa humidicola cv. quicuio managed with the use of fire for 30 years and newly established intensive pasture (10 months) of Megathyrsus maximus cv. BRS Zuri. Four trenches (1 m³) were opened in each system, with samples collected at eight depths. The proportion of C3 and C4 was estimated based on the δ¹³C values, using isotope mass spectrometry (CF-IRMS). Statistical comparisons were made using ANOVA and significance was verified using the F test at 0.05. The abundance of ¹³C (‰) in the soil under the forest area varied from -29.08‰ in the surface layer of 0-5 cm to -a -25.74‰ in the deeper soil layer. The fire-managed pasture showed lower ¹³C abundance (p < 0.05) than the forest at all depths, ranging from -19.09 to -24.22. The recently converted pasture showed the lowest abundance of ¹³C (p < 0.05) only in the upper layers (0-20 cm), ranging from -26.91 to -27.31 at this depth. In terms of the proportion of C4 and C3, Zuri showed values of 21%, 8% and 9% C4, respectively, in the surface layers of the soil (0-5, 5-10 and 10-20 cm). The pasture managed with fire had higher proportions of C4 in the surface layers (0-5 and 5-10 cm), with values of 67% and 42%, respectively. However, significant proportions of C4 were still observed in deeper layers (10 to 100 cm), ranging from 31% to 28%. Pastures managed with fire deplete carbon from the forest (C3), so this practice should be avoided. Newly converted pastures can preserve C3, as long as minimal soil management is carried out. Such as: removal of wood for other uses, girdling of the remaining forest material and minimal use of burning to reduce the remaining vegetation in the forest.