CARBON AND NITROGEN CHANGES IN DIFERENT COMPARTIMENTS OF THE ORGANIC MATTER UNDER AGRO-FORESTRY PASTURE SYSTEM

Abstract

Em um sistema agrossilvipastoril, que vem sendo implantado anualmente desde 1993, em Paracatu/MG, foi estudado o efeito de sua introdu\ue7\ue3o sobre as mudan\ue7as nos teores de Carbono e Nitrog\ueanio em diferentes compartimentos da mat\ue9ria org\ue2nica do solo. O sistema foi implantado em solo sob floresta de eucaliptos, abandonada ap\uf3s tr\ueas cortes. No primeiro ano, al\ue9m dos eucaliptos plantados em espa\ue7amento de 10 x 4 m, nas entre linhas e entre linhas, respectivamente, foi realizado um cultivo com arroz ( Oryza sativum sp), no segundo ano cultivou-se soja ( Glicyne max sp) e no terceiro ano, a braqui\ue1ria ( Brachiaria bizantina sp). Ap\uf3s o terceiro ano, a pr\ue1tica de pastejo da braqui\ue1ria por bovinos de corte foi iniciada, permanecendo na \ue1rea at\ue9 o final do ciclo do sistema, previsto para o 11\ub0 ano. Uma \ue1rea de cerrado e outra \ue1rea com eucaliptos em sistema de cultivo convencional foram usadas, como refer\ueancia \ue0 condi\ue7\ue3o natural e \ue0s condi\ue7\uf5es existentes antes da implanta\ue7\ue3o do sistema agrossilvipastoril, respectivamente. Foram coletadas amostras nas idades de 1, 2, 3, 6 e 10 anos do eucalipto. De acordo com os resultados, verificou-se que houve significativa diminui\ue7\ue3o nos teores e estoques de carbono e nitrog\ueanio totais, bem como na biomassa microbiana. Com o avan\ue7o da idade do sistema, iniciou-se uma tend\ueancia para recupera\ue7\ue3o dos teores e estoques dessas fra\ue7\uf5es. Entre as subst\ue2ncias h\ufamicas, a fra\ue7\ue3o \ue1cido f\ufalvico foi a menos afetada pela introdu\ue7\ue3o do sistema. Verifica-se que o sistema agrossilvipastoril foi desfavor\ue1vel a manuten\ue7\ue3o da mat\ue9ria org\ue2nica do solo nos primeiros anos, por\ue9m, apresenta potencial para recupera\ue7\ue3o da mesma com o avan\ue7o da idade.Since 1993, an agro-forestry pasture system has been annually implanted in Paracatu, Minas Gerais State. So, the effects from its introduction upon the changes in carbon and nitrogen at different compartments of the soil organic matter were studied. This system is implanted in soil under eucalypt forest that was abandoned after three removal cuts. In the first year, besides planting the eucalyptuses in interlines at spacing 10 x 4 m, the following crops are grown: rice in the first year; soybean in the second one; and the brachiaria in the third one. After the third year, the brachiaria pasture by meat cattle was begun and remaining in that area until the end of the system cycle, what is foreseen for the 11th year. A savannah area and another one with eucalypt planting under the conventional system were used as reference to natural condition and previous conditions to implantation of the agrosylvopasture system. Samples aged 1, 2, 3, 6, and 10 years were collected. The results show significant decrease in the contents and stocks of both total carbon and nitrogen, as well as in microbial biomass. As the system becomes older, it starts a tendency for the recuperation of the contents and stocks of those fractions. Among the humic substances, the fulvic acid fraction was less affected by the introduction of this system. Briefly, the results show the system is unfavorable to the maintenance of the soil organic matter at the first years, but presents a potential for recovering as its age advances