Predictions of available nitrogen content in soil profile depth using available nitrogen concentration in surface layer

The Rolling and Flat Pampas are vast, plains which contribute with more than half of the Argentine wheat production. In the 80's three extensive studies were preformed in these regions, which determined nitrate content in the 0-20, 20-40, and 40-60 cm soil layers, comprising a wide range of soil, climatic and cultural variables. More recently, we determined the mineral-N (N) content at 0-30 and 30-60 cm depths, in a set of farmer's plots under different tillage systems and previous crop. These datasets were used in order to establish simple equations to predict N availability in the 0-60 cm stratum, from the N availability in the upper layer. Nitrate content at wheat presowing time presented a clear stratification pattern with depth. For the fist dataset, strong relationships were obtained between the nitrate content at different soil depths, with previous crops of corn, sunflower or sorghum (R-2 = 0.52 to 0.90, P < 0.05), but low adjustments were obtained for soybean. In the second set of data, mineral-N measured in the 0-30 and 30-60 cm layers were also linearly correlated, for each tillage treatment (plow, disk and no-till) and previous crop (R-2 > 0.67). Two general equations were obtained that relates N availability in the upper layer with the availability in the 0-60 cm layer (R-2 > 0.89, P < 0.001), comprising a wide range of management and environmental conditions. As N fertilizer recommendations in these Pampean Regions required soil nitrate content to a depth of 50 cm, close predictions of N availability in the this stratum from the upper layer can facilitate sampling and extent the use of the diagnosis methodologies by farmers.