HISTORICAL NITROGEN, PHOSPHORUS, AND SULFUR BALANCE OF THE PAMPAS

Resumen

A surface balance for nitrogen, phosphorus, and sulfur was performed for the Argentine Pampas during the 1870-2010 time interval, which comprises the agricultural expansion period in the region. Nitrogen inputs accounted for in the balance were atmospheric deposition, symbiotic fixation, and fertilization. Outputs included were grain harvest and livestock products. Phosphorus and sulfur balances included atmospheric deposition and fertilization as inputs and the same outputs that the nitrogen balance. Cumulative balances were calculated and also an annualized output/input ratio. National censuses and official statistics were used to determine the outputs meanwhile atmospheric deposition, symbiotic fixation, and fertilizer used were estimated. Nitrogen input was of 202 Mt, atmospheric deposition (36%) and symbiotic fixation (58%) represented the main components. The output was of 76 Mt, with grain harvest as main factor (83%), thus resulting in a positive nitrogen balance of 126 Mt. This nutrient flow is equivalent to one quart of the soil nitrogen stock to 1 m depth. Previous studies showed that soil nitrogen stock did not changed between 1960-1980 and 2007-2008, and therefore a loss of 26 kg ha^-1 yr^-1 due to gas emission and leaching was calculated during this period. Phosphorus input was 4.2 Mt, with the major contribution caused by the fertilization (67%), and the output was 12.2 Mt, generated mainly by grain harvest (76%), determining a negative balance of -8.0 Mt. Sulfur input was 3.9 Mt, mainly generated by atmospheric deposition (81%) and the output was 5.6 Mt, largely due to grain harvest (82%) resulting also in a negative balance of -1.7 Mt. These results indicate phosphorus and sulfur losses from soils stocks. The nitrogen output/input ratio varied from 0.2 to 0.7 along the study period, while the phosphorus and sulfur ratios have been larger than 1 since decades.