Organic matter fractions in soil under coffee with split applications of phosphorus and water regimes/ Fracoes da materia organica em Latossolo com cafeeiro sob parcelamentos de fosforo e regimes hidricos

Phosphorus fertilization and irrigation management are essential practices to increase coffee yields, though information is scarce about the effect of these practices on organic matter fractions of soils of the 'Cerrado' (savanna-like vegetation). The purpose of this study was to evaluate organic matter fractions of a clayey Oxisol under coffee with split applications of phosphorus (P) and water regimes. The experimental design was a randomized block with 3 x 2 factorial arrangement with three split applications of P (P1: 300 kg [ha.sup.-1] [P.sub.2][O.sub.5] applied annually of which 2/3 applied in September and 1/3 in December; P2: 600 kg [ha.sup.-1] [P.sub.2][O.sub.5] applied at planting and every two years, and P3: 1800 kg [ha.sup.-1] of [P.sub.2][O.sub.5] applied only at planting, corresponding to a 6-year requirement), two water regimes (with and without irrigation) and three replications. Soil from the 0-5 and 5-10 cm layers was sampled. The total organic carbon (TOC), labile carbon (LC), microbial carbon (Cmic), and carbon fractions of fulvic acid (FA), humic acid (HA) and humin (HU) were determined. The irrigation regime of coffee increased the TOC, LC and Cmic levels and the humified fractions of soil organic matter. In general, the form of P splitting had little influence on the fractions of soil organic matter. Key words: labile carbon phosphate fertilizer humic substances A adubacao fosfatada e o manejo da irrigacao sao essenciais para aumentar a produtividade do cafe; no entanto, ha poucas informacoes sobre o efeito dessas praticas nas fracoes da materia organica de solos de Cerrado. O objetivo deste trabalho foi avaliar fracoes da materia organica de um Latossolo cultivado com cafeeiro sob parcelamentos de fosforo (P) e regimes hidricos. O delineamento experimental foi em blocos ao acaso em arranjo fatorial 3 x 2, com tres parcelamentos de P (P1: 300 kg [ha.sup.-1] de [P.sub.2][O.sub.5] por ano, sendo 2/3 aplicados em setembro e 1/3 em dezembro; P2: 600 kg [ha.sup.-1] de [P.sub.2][O.sub.5], aplicada no plantio e a cada dois anos e P3: 1.800 kg [ha.sup.-1] de [P.sub.2][O.sub.5] aplicada somente no plantio, necessaria para seis anos); dois regimes hidricos (com e sem irrigacao) e tres repeticoes. A amostragem de solo foi feita nas camadas de 0-5 e 5-10 cm. Foram determinados o carbono organico total (COT) carbono labil (CL), carbono microbiano (Cmic) e o carbono das fracoes acido fulvico (AF), acido humico (AH) e humina (HU). O regime hidrico