Effect of conversion of natural grassland to cropland on nitric oxide emissions from Venezuelan savanna soils.: A four-year monitoring study

Conversion of "natural" savanna to agricultural soil is proceeding at a very fast rate. In this work, nitric oxide (NO) emissions were measured immediately after conversion of a "natural" grassland to fertilized agricultural fields (corn, sorghum and cultivated pasture), and also during the second and fourth year after conversion. Large fluxes were observed after plowing grassland soil, and almost no difference was observed between ammonium-fertilized and unfertilized plowed grassland soils. Soil water content and pH related negatively to NO emissions. NO emissions were positively correlated with soil nitrate concentrations and carbon content (corn only) whereas little or no correlation was found with ammonium concentrations. This suggests that in these savanna soils, the NO emitted from soil is mainly denitrification-derived. The disturbed soils emitted on average similar to 7 times more NO than the original grassland. During the second and fourth year of cultivation the emissions from corn and sorghum fields (plowed every year) were around 10 times higher than from the control grassland soils. The fertilizer-induced emission (FEI) values for NO emission estimated in this work show that similar agricultural practices could lead to dramatically different FEI values depending on the water content of the soils. These FEI results corroborates that modeling approaches to determine the global agriculture-derived NO emissions should take into consideration the main factors that regulate the NO emission at the scale of functional units with similar climate, soil and management conditions instead of expressing it as a percentage of the fertilizer applied. In our sandy loam savanna soils these factors are inorganic nitrogen content, pH and WFPS.