NITRATE LIMITATION OF N2O PRODUCTION AND DENITRIFICATION FROM TROPICAL PASTURE AND RAIN-FOREST SOILS

Nitrous oxide production was measured in intact cores taken from activ e pasture and old-growth forest Inceptisols in the Atlantic Lowlands o f Costa Rica. Following additions of aqueous KNO3 or glucose, or the t wo combined amendments, the cores were incubated in the laboratory to determine if N2O production rates were either N-limited or C-limited i n the two land use types. Differences in rates of denitrification (N2O + N-2 production) among amended forest and pasture soils were determi ned by addition of 10% C2H2. The forest soils were relatively insensit ive to all amendment additions, including the acetylene block. Forest N2O production rates among the treatments did not differ from the cont rols, and were consistently lower than those of the pasture soils. Wit h the addition of glucose plus nitrate to the forest soils, production of N2O was three times greater than the controls, although this incre ase was not statistically significant. On the other hand, the pasture soils were definitely nitrogen-limited since N2O production rates were increased substantially beyond controls by all the amendments which c ontained nitrate, despite the very low N level (5 mg N kg(-1) soil) re lative to typical fertilizer applications. With respect to the nitrate plus glucose plus acetylene treatment, denitrification was high in th e pasture soils; N2O production in the presence of C2H2 was 150% of th e rate of N2O production measured in the absence of the acetylene bloc k. The results are discussed in relation to the effects of agricultura l land use practices and subsequent impacts of disturbance on N2O rele ase.