Phosphorus fractions in soil after successive crops of Pinus taeda l. without fertilization/Fracoes de fosforo apos cultivos sucessivos de Pinus taeda l. sem fertilizacao

Pinus cultivation without fertilization is a common practice in southern Brazil, which can induce a decline in the availability of phosphorus (P) in the soil. The purpose of this study was to evaluate the changes in phosphorus fractions in a Humic Cambisol subjected to continuous Pinus taeda L. cultivation without fertilization. Two forest stands were evaluated, after 16 years of Pinus cultivation (1st crop) and 49 years (3rd crop), when soil samples were collected (layers 0-10, 10-20, 20-40, 40-60, and 60-80cm) from six trenches per forest. In soil samples, the P contained in organic (Po) and inorganic (Pi) forms was determined by sequential chemical fractionation. Labile inorganic P fractions remained unchanged after the different cultivation periods. However, the labile organic fractions declined from the first to the third cycle (from 70.6 to 39.8mg [dm.sup.-3] in the 0-10cm layer), indicating that these forms influence the buffering capacity of labile Pi. The moderately labile organic P acted as a P drain, increasing its percentage of the total, from 34.7 to 56.3%, from the first to the third crop. Soil cultivation for 49 years with Pinus taeda resulted in a reduction of the organic P content, indicating that for this soil use, this P form should be used to diagnose P availability and fertilization requirements. Key words: forest rotations, successive crops, phosphorus fractionation, labile phosphorus. O cultivo de Pinus sem uso de fertilizacao e uma pratica comum no sul do Brasil e pode levar ao declinio da disponibilidade de fosforo (P) no solo. O objetivo deste estudo foi avaliar as modificacoes nas fracoes de fosforo em Cambissolo submetido a sucessivos cultivos de Pinus taeda L. sem fertilizacao. Foram avaliadas duas florestas, com 16 anos de cultivo de Pinus (1 cultivo) e 49 anos (3 cultivo), onde amostras de solo foram coletadas nas camadas de 0-10, 10-20, 20-40, 40-60 e 60-80cm, em seis trincheiras por floresta. Nas amostras de solo foram determinados os teores de P em formas organicas (Po) e inorganicas (Pi) usando esquema de fracionamento quimico sequencial. As fracoes inorganicas labeis de P nao sofreram alteracoes com os diferentes tempos de cultivo, no entanto houve declinio das fracoes organicas labeis do primeiro para o terceiro cultivo, passando de 70,6para 39,8mg [dm.sup.-3] na camada de 0-10cm, indicando que essas formas atuam no tamponamento do Pi labil. O P organico moderadamente labil atuou como um dreno de P aumentando seu percen