Data from field experiments performed in the humid portion of the Pampas (rainfall 800-1000 mm yr-1), where fallow effects on nitrate nitrogen levels of soils had been tested, were integrated to study management effects on nitrate nitrogen accumulation before wheat sowing. We compiled data from 3 published papers, where nitrate nitrogen levels were reported after fallow periods of different durations in commercial production fields with soils contrasting in organic matter contents and agricultural histories (n = 153). Data were analyzed by surface regression techniques to develop a model suitable for estimation of nitrate nitrogen production during the fallow periods of wheat crops using the R2 as the decision criterion for model selection. A model could be developed that accounted for ca. 40 % of nitrate nitrogen variability at the end of the fallow period (Figure 1). The independent variables in the model were years since the last pasture, soil organic matter level and fallow duration. Nitrate nitrogen decreased 2.5 kg ha-1 by each year since the last pasture, and increased ca. 0.5 kg ha-1 by day of fallow duration and 14 kg ha-1 by 1 % increase in soil organic matter in the 0-20 cm soil layer. The model may be used for predictions of nitrate nitrogen accumulation under different production scenarios in the Pampas in relation to agricultural management.

Figure 1. A: Relationship between nitrate nitrogen contends in the 0-60 cm soil layer before wheat sowing observed in production fields and estimated by the model:

Nitrate nitrogen = - 2.5 YSLP + 0.46 FD + 14 OM where:

Nitrate nitrogen = nitrogen in nitrates 0-60 cm alter fallow (kg N ha-1) YSLP = years since the last pasture in the filed FD = fallow period (days) OM = soil organic matter in the 0-20 cm layer (%)

B: Nitrate nitrogen produced during fallow period for some possible scenarios in the Pampas estimated using the model.