A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

ALTERATIONS TO NITRATE AND AMONIUM CONCENTRATIONS INPASTURE SOILS SUBJECTED TO TILLINGJanaina Braga do Carmo 1 , Carlos Clemente Cerri 2 , Christopher Neill 3 , Marisa deCássia Piccolo 2 and Diana Garcia 31 Doutoranda do Curso de Solos e Nutrição de Plantas- ESALQ-USP, Av. Pádua Dias s/n caixa postal 9CPG-Solos e Nutrição de Plantas, Piracicaba-SP, Brasil. Email jbcarmo@esalq.usp.br2 Centro de Energia nuclear na agricultura (CENA-USP).3 The Ecosystems Center, Marine Biological Laboratory.We examined the effect of tilling on soil extractable ammonium and nitrateconcentrations on an Ultisol at Nova Vida Farm (10°10`05” e 62°49`27”W) in centralRondônia. Tilling was conducted as part of an investigation of biogeochemicalchanges during different treatments designed to restore a degraded Brachiariabrizantha pasture. We made intensive measurements of soil NH + 4 -N and NO - 3 -Nconcentrations before and for 25 days after soils were tilled to 30 cm depth in earlyOctober. Soil was collected at 0-5 and 5-10 cm depths and immediately prepared andextracted with KCl 2N. Extractable NH + 4 -N and NO - 3 -N concentrations increaseddramatically in the tilled treatment 7 days after tilling. Extractable NH + 4 -N and NO - 3-N concentrations during the same time remained unchanged in the control treatment.NH + 4 -N concentrations were almost always higher than NO - 3-N concentrations in-both treatments. An increase in NO 3 concentrations after 18 days suggested anincrease in nitrification rate at that time. An increase in soil moisture 7 d after plowingmay have improved conditions for microbial activity, increasing the mineralization ofthe soil organic matter. Higher NH + 4 concentrations found in the tilled pasture can bea consequence of the slow transformation of NH + 4 to NO - 3 and absence of plantuptake in these pastures. More than 3 weeks after tilling, increases of NH + 4 (1.18 to24.75 µg g -1 dry soil) and NO - 3 (0 to 14.83 µg g -1 dry soil) in the soil could lead tohigher losses of the gaseous forms of N (N 2 , NO x and N 2 O), loss of N throughleaching, as well as enhanced availability of this element to plant growth.