NET NITROGEN MINERALIZATION AND NET NITRIFICATION ALONG A TROPICAL FOREST-TO-PASTURE CHRONOSEQUENCE

Soil inorganic nitrogen pools, net mineralization and net nitrificatio n rates were compared during the dry season along a chronosequence of upland (terra firme) forest, 3-, 9- and 20-year-old pastures in the we stern Brazilian Amazon Basin state of Rondonia to investigate the infl uence of forest conversion to pasture on soil nitrogen cycles. Surface soil (0 to 10 cm) from forest had larger extractable inorganic nitrog en pools than pasture soils. In the forest, NO3- pools equaled or exce eded NH4+ pools, while pasture inorganic N pools consisted almost excl usively of NH4+. Rates of net N mineralization and net nitrification i n seven - day laboratory incubations were higher in the seven - day fo rest than in the pastures. Net N mineralization rates did not differ s ignificantly among different-aged pastures, but net nitrification rate s were significantly lower in the 20-year-old pasture. Higher net N mi neralization and net nitrification rates were measured in laboratory a nd in situ incubations of sieved soil, compared with in situ incubatio ns of intact soil cores. Rates calculated in seven-day incubations wer e higher than determined by longer incubations. Sieving may increase N mineralization and/or decrease N immobilization compared with intact cores. We concluded that 7-day laboratory incubation of sieved soil wa s the most useful index for comparing N availability across the chrono sequence of forest and pasture sites. High net nitrification rates in forest soils suggest a potential for NO3- losses either through leachi ng or gaseous emissions.