Nitrogen Stock of Pampean Soils.

Soil nitrogen stocks under contrasting vegetation types and land uses have not been assessed in the Pampas. Our objective was to establish the pattern of variation of soil organic nitrogen (SON) stored in surface and deep soil layers of the Argentine Pampas as affected by environmental conditions and land use. Eighty two farms widespread over the region were sampled during 2007-2008. At each farm paired treatments were sampled representing common land uses: trees, un-cropped controls, seeded pastures, cropped fields, and periodically flooded areas. The SON content and bulk density were determined up to one meter depth in layers of 25 cm. Rainfall and temperature were obtained from climatic records. Significant differences in SON contents were detected between treatments. Average SON stocks up to one meter were: 14.4 t ha^-1 for trees, 12.0 t ha^-1 for un-cropped controls, 10.8 t ha^-1 for seeded pastures, 10.5 t ha^-1 for cropped fields and 8.4 t ha^-1 for flooded lands.  In all cases around 50 % of N was stored in the first 25 cm of soil profile. The stratification pattern of SON was not affected by the treatments, implying that land use impacted the SON sequestered in soils but not its allocation in depth. Cropping determined a significant reduction of SON up to 50 cm of the soil profile. We developed an artificial neural network model (ANN) for SON estimation at county scale based on climate, soil properties, vegetation type and land use (R² = 0.73). This model, linked to satellite image classification information, allowed estimating Pampean SON stock. Soil surveys performed during 1960-1980 were used to make comparisons. Counties with SON content greater than 12 t ha^-1 up to one meter depth lost nitrogen while increases below this threshold prevailed.