1. Conceptual Design NOx emissions from tropical soils were determined from an integrated approach of field and laboratory measurements and model experiments. This was done in order to answer three basic questions: (a) what is the influence of soil moisture, temperature and soil substrates on production, consumption and diffusion of NO from the soil, (b) what are the differences in these relationships between various land use practices in Rondonia and (c) what are the intra- inter-seasonal trends. As part of the LBA-EUSTACH project, a comparison between NO field, modeled NO fluxes and soil N-pools and indices for both primary forest and established cattle pasture in Rondônia are presented with respect to objectives (a) and (b). Forest Soil Flux N-IndicesPasture Chamber Flux N-Indices Inverse model results show higher mineralization rates than those determined in the soil laboratory (above), and relatively stable nitrification rates for both land use classes. Forest soils indicate a build up of N-substrate prior to the rain event on day 288 and a significant decline in mineralization thereafter. Pasture inverse model results confirm the slow N-cycling found in old pastures. Inverse Model Similar soil physics and climatic conditions at both sites indicate that substrate availability (land use influenced) is responsible for the NO flux differences. The NGAS submodel (Parton et al., 1998), which uses empirically based parameters for nitrification and denitrication, was run in reverse to back out possible mineralisation and nitrification rates, which were responsible for the measured soil NO fluxes. Three degrees of freedom, namely measured net mineralisation, net nitrification and soil CO 2 where initialized with observed values and allowed to vary within a range of literature values. Measured soil diffusion rates ( 222 Rn), moisture, a pulsing factor,  d, temperature and NO flux were used as inputs in the optimization scheme. Method G.A.Kirkman, S.M. van Dijk, A. Gut, L. v Gatti, B. M. Gomes, P. Bahrmann, H. R. da Rocha, F.X. Meixner grant@mpch-mainz.mpg.de Pasture & Forest Nitric Oxide Soil Fluxes in Rondonia: Field, Laboratory and Model Integration Pasture and Forest 2 yrs. burn/rice 10 yrs. burn/graze 2 yrs. burn/beans 8 yrs. grazing clay 11 % pH 5.2  d 1.56 Mg m -3 C:N 12:1 open rain forest undisturbed seasonally dry 30 m canopy clay 13 % pH 3.6  d 1.22 Mg m -3 C:N 9:1 Soil N-Indices The procedure described by Hart et al. (1994) was used to quantify soil NH 4 and NO 3 pool sizes and net rates for soil samples from both the forest and pasture. Pasture soils show a dominant NH 4 pool, whereas forest pools tend to be equal or have a slightly higher NO 3 storage. Net rates for the forest soils are significantly lower than those from other studies in Brazil, while pastures older than 21 years show a considerable slowing in N cycling. The NH 4 pools of old pasture soils in Rondonia are on average a factor of 6 larger than the NO 3 pools. Empirical Model An empirical NO model was developed based on the Galbally & Johannson (1989) algorithm, where NO production and rate constant of NO consumption were determined in the laboratory as functions of soil moisture soil temperature. NO flux = (D eff  d k V m / M N ) 1/2 (P/k - [NO amb ] M N / V m ) where: P = f(T,  ) [ng N kg -1 s -1 ] NO production (laboratory) k = f(T,  ) [ng N kg -1 s -1 ppb -1 ] rate constant NO uptake (laboratory) T = soil temp.,  = WFPS, NO amb (field) D eff NO diffusion [m 2 s -1 ] ( 222 Rn method, field)  d = bulk density [Mg/m 3 ] (soil cores, field) M N = molar weight [kg/kmol] V m = molar volume [m 3 /kmol] F j = QA -1 (in j - out j ) M j V -1 Pasture Field & Model Flux Forest Field & Model NO Flux Nitric oxide fluxes were measured concurrently from a primary forest and a 22 yr. old pasture during the dry season in Rondonia. Fluxes were determined by automated dynamic chamber system and NO production and the rate constant uptake were measured with respect to soil temperature (10-38 ° C) and soil moisture (0-100% WFPS) in a laboratory. Compensation concentration (NO production/NO uptake) was measured continuously in the forest. Soil N-indices were also determined in the laboratory. LANDSAT ETM+, 6 August 1999 Path 231 Row 67, Pseudo Color, Bands 5, 4 & 2 USGS DEM, South America Pasture-Forest NOx Balance Results NO fluxes from forest soils were approximately an order of magnitude higher than from pasture soils. The minimum and maximum flux rates for pasture and forest in response to soil temperature are depicted in gray below. Forest NO fluxes show a positive diel trend. Extremely low fluxes at the pasture provide little indication of any temperature dependence. Laboratory results confirmed that the rate constant of uptake was comparable for forest and pasture but NO production varied considerably. Pasture soils were therefore a net sink of NOx during the September to November 1999. Forest N-Pools Forest N-Rates Pasture N-Pools Pasture N-Rates modeled observed modeled observed