THE EFFECT OF DIVERSIFIED CROP ROTATIONS ON SOIL ORGANIC CARBON DYNAMICS IN A CLAYEY TROPICAL SOIL

This study assessed the impact of crop diversification within no-till crop rotations on soil organic carbon and total nitrogen stocks, as well as on labile (Carbon and Nitrogen in the particulate organic matter) and persistent (Carbon and Nitrogen in the mineral-associated organic matter) Organic matter fractions. The objective was to identify practical indicators for monitoring public policies promoting low carbon emission agriculture. Field experiment was conducted in 2014/2015 cropping season using a complete random block design. Seven treatments were evaluated: soybean/cotton (CONTROL); maize/soybean (M/Sb); soybean/maize+brachiaria (Sb/M+Br); soybean/millet+brachiaria/crotalaria spectabiliscotton (Sb/Mt+Br/CrsCt); soybean/cotton/common beans/millet+ brachiaria (Sb/Ct/Cb/Mt+Br); millet-cotton/ soybean/maize/crotalaria spectabilis (Mt-Ct/Sb/M/Crs); crotalaria-cotton/soybean/ sorghum+brachiaria/crotalaria ochroleuca+ brachiaria (Cr-Ct/Sb/Sg+Br/Cro+Br). Sampling was done in May 2020 on an Oxisol in a neotropical savanna of the Central West region of Brazil (Capivara Experimental Research Farm of Embrapa Rice and Beans, Santo Antônio de Goiás, Goiás State, Brazil). Treatment comparisons were made after correction for equivalent soil mass per soil layer. The C-POM, N-POM, C-MAOM, and N-MAOM fractions were obtained through granulometric physical fractionation. Total SOC and SOC stocks were inadequate indicators of the impact of crop rotations on SOC. However, the distribution of C and N among the soil organic matter (SOM) fractions (C-POM, N-POM, C-MAOM, and N-MAOM) was influenced by crop rotations. Rotations with greater crop diversity, including gramineae, had higher concentration of C and N in the particulate SOC (C-POM and N-POM). Differences in rotation composition also affected the C to N ratio, particularly in the POM fraction, which was higher in rotations involving brachiaria grass and maize. Most diversified rotations contributed to maintaining higher C-POM stocks.