Biological nitrogen fixation by Prosopis juliflora (Sw.) DC stands in the Brazilian semi-arid region

Abstract

Prosopis juliflora was introduced to the Brazilian semi-arid region as a forage and firewood plant and spontaneously dispersed to valley areas and now forms almost pure stands in more than 500,000 ha of the region. Since eradication is not being considered, studies on the advantages and disadvantages are necessary. One of the potential advantages is the capability of symbiotically fixing atmospheric N 2 . We determined P. juliflora N fixation in six valley areas of Pernambuco state, Brazil. In each area, leaf and wood biomasses were estimated, N concentrations and amounts determined and N fixation in leaves estimated by the 15 N technique. Aboveground and leaf biomasses averaged 44.1 Mg ha −1 , and 2.1 Mg ha −1 , respectively. The nitrogen derived from the atmosphere (%Ndfa) in the leaves averaged 38.8%, corresponding to 24.2 kg ha −1 yr −1 of N fixation. An indirect rough estimation of the average N stock derived from fixation in the woody parts was based on the assumption that the 15 N values in the leaves and wood in the same plant was equal, and amounted to 74.1 kg ha −1 Considering the fixed N in the leaves as a stock, adding the estimated N stock in the wood, and extrapolating for the 113 thousand hectares covered by P. juliflora in Pernambuco, Paraíba and Rio Grande do Norte states would be equivalent to a stock of 11.1 Gg of N. Despite being a crude estimate, it indicates that P. juliflora can potentially contribute to a significant amount of fixed N in the plant-soil system in the Brazilian Northeast region.

Bibliographic Information