Nitrogen Stocks in Soil Classes Under Different Land Uses in the Brazilian Semiarid Region

Estimates of soil nitrogen (N) stocks are important for assessing potential nitrogen oxide emissions due to land-use changes and agricultural practices. No comprehensive data set exists for N stocks for the 850,000 km(2) of the semiarid Brazilian northeastern region, according to land use and soil classes. Changes in N stocks due to the conversion of caatinga to pasture or agriculture, in different soil classes, were quantified in 201 soil profiles down to 1 m deep, except in shallower soils (Leptosols, 50 cm; Luvisols, and Planosols, 80 cm). Acrisols, Arenosols, and Regosols had the larger N stocks (8.07 to 9.72 Mg ha(-1)) than Ferralsols (6.10 Mg ha(-1)), whose stocks were similar to those of the shallower soils (5.87 to 6.72 Mg ha(-1)). The conversion of dense caatinga to agricultural fields caused reductions in 23% in N stocks. The soils of the semiarid Brazilian region accumulate 612 Tg of N in the top 1 m in the different classes of soils and land uses, being 226.8 Tg under dense caatinga, 145 Tg under open caatinga, 168 Tg under pastures, and 70 Tg under crop fields. This study contributes to elucidating N dynamics in different soil classes and land uses, and it is the most complete cover of soil N stocks in the semiarid region of Brazil. The results highlight the importance of preserving native vegetation and the need to identify and foster the adoption of agricultural management practices to maintain and restore soil N stocks.