NITROGEN MINERALIZATION AND SOIL FERTILITY IN THE DRY CHACO ECOSYSTEM (ARGENTINA)

Grassland productivity of Dry Chaco ecosystems in Argentina is often decreased by the invasion of shrubs (Larrea spp.). To better understand the reasons for this degradation and how to reverse or avoid it, we studied soil fertility and N-mineralization rates under four vegetation types predominant in the region: trees, shrubs, interspaces between trees and shrubs, and pure grassland. Two types of trees were studied: leguminous (Prosopis flexuosa DC) and non-leguminous (Aspidosperma quebracho-blanco Schlecht.). Net N-mineralization rates were significantly higher under Prosopis (150 mg N kg-1 yr-1) than under Larrea (95 mg N kg-1 yr-1), the interspaces (66 mg N kg-1 yr-1) and the pure grassland (68 mg N kg-1 yr-1). Mineralization rates under Aspidosperma (115 mg N kg-1 yr-1) were not significantly different from Prosopis. Nitrogen produced in the wet season (spring-summer months) represented 90% of the accumulated N for 1 yr. Nitrogen mineralization was positively correlated (P < 0.05) with both soil moisture and temperature. Nitrates comprised the highest proportion of mineralized N, with a trend to some NH4+ accumulation in the dry season. At the 0- to 10-cm depth, organic C and total N were significantly higher under Prosopis than under Aspidosperma, Larrea, or the interspaces. At deeper soil depths, organic C and extractable P were higher under and between Larrea and under the pure grassland than under and between tree canopies. Results indicate that the highest soil fertility and N-mineralization rates were under leguminous trees and the lowest in the interspaces and under Larrea.