Neste trabalho objetivou-se avaliar o efeito do monocultivo e do consórcio de macaúba e pastagem sobre a qualidade química e os compartimentos de carbono em Latossolo Vermelho-Amarelo no Cerrado Maranhense. Foram estudados quatro sistemas de uso do solo: macaúba, pastagem, macaúba consorciada com pastagem e vegetação nativa de cerrado. Nesses sistemas foram coletadas amostras de solo da superfície até 60 cm, para determinação dos atributos químicos e dos teores e estoques de carbono orgânico do solo. Os maiores valores de P, K, Ca e Mg foram observados nas áreas sob pastagem. Os estoques de carbono orgânico total e os teores de carbono lábil foram maiores no solo com vegetação nativa até 60 cm de profundidade, seguida pelo sistema macaúba associada com pastagem. Maiores valores de carbono da biomassa microbiana e de quociente microbiano foram observados no solo sob pastagem em todas as camadas. A presença da pastagem é importante no consórcio com macaubeiras pois aumenta os níveis de fertilidade do solo e os estoques de carbono favorecendo a melhoria da qualidade do solo nas condições do cerrado maranhense.

Legume green manure is a popular practice for sustainable agriculture in tropical environments. The use of grain, forage and purely green manure legumes in a subhumid tropical area in Northeast Brazil was investigated to determine the total C and N contents and estimate C-microbial biomass, soil basal respiration, microbial and metabolic quotients, before and after green manures and after the following corn. It was hypothesized that, although all legumes would increase microbial biomass and activity, there would be different effects in function of the main economic usage of the legume. The experimental design evaluated the soil before and after harvest of seven legumes: (i) purely green manures – sunn hemp (Crotalaria juncea L.) and velvet bean (Stizolobium aterrimum Piper & Tracy); (ii) forage – Campo Grande stylo (a physical mixture of 80% of Stylosanthes captata Vog. and 20% of Stylosanthes macrocephala) and calopo (Calopogonium mucunoides Dev.) and (iii) grain – common bean (Phaseolus vulgaris L.), cowpea (Vigna unguiculata L.) and peanut (Arachis hypogaea L.), followed by corn crop. After corn harvest, soils had the same total C and N contents before legume seeding, but C-microbial biomass was significantly higher in soils with grain and purely green manures, which produced higher shoot and root dry mass. Soil basal respiration decreased after corn harvest for all legumes comparing to control treatments, but metabolic stress increased in soils with forage species. Despite this, microbial quotient indicated an efficiency of the microorganisms in degrading the residual organic matter of all green manures used in corn.

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.