Saturday, 15 July 2006

Ammonia Volatilization from Nitrogen Fertilizers in Pampean Agroecosystems of Argentina.

Roberto Alvarez and Haydee Steinbach. Facultad de Agronomía, Univ de Buenos Aires, Av. San Martin 4453, Buenos Aires, 1417, Argentina

Ammonia volatilization may be an important way of nitrogen loose from soils. Despite this process is more intense in soils with pH higher than 7, it can also take place in acid soils when urea is used as fertilizer because of the high pH it generates in the surrounding of solubilizing granules. The Pampean Región of Argentina is a vast plain of around 60 Mha with temperate climate where Mollisols of pH ca. 6 predominate. Nitrogen fertilization practice, with urea as the main fertilizer used, had widespread in recent years, with rates ranging from 40-100 kg N ha-1, but information about possible nitrogen losses by volatilization is scarce. We compiled published results from 6 papers where ammonia volatilization from applied urea was measured under field conditions across the Pampean Region, generating 32 paired data from comparisons of volatilization between fertilized and non fertilized scenarios. Papers were selected only when nitrogen rate and application technique were clearly defined and when ammonia volatilization was measured for at least some weeks after fertilization or until rainfall occurred. Temperature during the period of 30 days after urea application was estimated for each experimental site using climate records. Data were analyzed by response surface regression techniques using the R2 as the decision criterion for developing a model suitable for volatilization prediction. A model could be developed that accounted for around 90 % of ammonia volatilization variability during wheat and corn crops (Figure 1A). Volatilization increased as nitrogen rate and temperature after application increase, and also when fertilizer was broadcast. The model could be used for estimation of ammonia volatilization under different scenarios usually founded in Pampean Agroecosystems (Figure 1B). For example, volatilization during the initial stages of wheat crops (air temperature 8-10 ºC) would produce very low nitrogen losses no greater than 5-6 % of fertilizer nitrogen for common rates applied in the Pampas. Nitrogen losses increase during corn crops because of higher temperatures during the growing cycle but it would not be expected to be greater than 10-12 % of nitrogen fertilizer when applied at V6 (air temperature ca. 18-20 ºC) for the common rates used. Figure 1: A) Relationship between nitrogen volatilization from urea applied in corn (full circles) and wheat crops (empty circles) observed and estimated using the model: N volatilized (kg N/ha) = - 3.37 + 0.00566 * N rate * Temp. + 0.0255 * N rate * Apl. Where: N rate = nitrogen rate (kg N/ha), Temp. = mean air temperature 30 days after fertilization (ºC), Apl. = fertilizer application, incorporated 0 and broadcast 1. B) Nitrogen volatilization estimated using the model for some common scenarios founded in the Pampas, incorp.: fertilizer incorporated, broad.: fertilizer broadcast.

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