ABSTRACT Brazilian semi-arid tropical is responsible for the production and export of fine table grapes, but the demand for phosphate fertilizers and other agricultural inputs increases the cost of production and threatens local water bodies by leaching of phosphorus (P). The objectives of this study were to quantify the organic and inorganic fractions of P and to determine chemical and microbiological attributes related to its mineralization in soils cultivated with vines in São Francisco Valley. It was hypothesized that P fractions and microbial activity would increase in areas fertilized with phosphate. Soil samples from vines were collected in cultivated areas and in fallow systems. P fractionation, macro-elements analysis, microbial biomass phosphorus (Pmic), and phosphatase enzyme activity were determined and correlated with each other and with the total proportion of synthesized glomalin. In both treatments, the inorganic P fractions exceeded the organic ones, and the total phosphorus available in those soils was associated with the most recalcitrant fractions, reducing risks of leaching of this element and contamination of water bodies. Pmic was higher in the winery areas due to the presence of phosphate fertilizers and organic matter, hence the activity of alkaline phosphatase enzymes and the production of total glomalin increased respectively in three and four evaluated areas, indicating that P-mineralized bacteria and arbuscular mycorrhizal fungi have been positively affected by the current agricultural system.

ABSTRACT Silvopastoral systems based on tree legumes intercropped with forage grasses can harbor a high diversity of rhizobia, and these bacteria are good indicators of soil quality in several management systems. The objective of this work was to evaluate the morphophysiological, genetic and symbiotic diversity of cowpea [Vigna unguiculata (L.) Walp] rhizobia from soils under silvopastoral systems based on shrub-tree legumes. The experiment was performed in a randomized block design with three treatments and three replications, consisting of signalgrass (Urochloa decumbens Stapf.) intercropped with sabia (Mimosa caesalpiniaefolia); signalgrass intercropped with gliricidia (Gliricidia sepium) and single signalgrass. The samples were collected in the legume row (0 meter) and 4 and 8 meters away. Later, cowpea was used as a trap plant to capture the rhizobia. All strains were phenotypically characterized, authenticated, and genetically identified. Phenotypical characterization of the 431 isolates showed high diversity forming 69 groups at 100% similarity, of which 60 were able to nodulate cowpea during the authentication, and 36 presented relative efficiency superior or equal to the recommended bacteria for the crop. Most of the sequenced strains belonged to Bradyrhizobium (67%) and Methylobacterium (9%). Leifsonia (9%), Cohnella (6%), Rhizobium (3%), Burkholderia (3%), and Paenibacillus (3%) were also represented. Soils under silvopastoral systems harbor efficient rhizobia populations in cowpea with a high genetic diversity, which can be recommended for agronomic efficiency assays.

ABSTRACT Some forage species, such as pangolão grass (Digitaria eriantha Steud. cv. Survenola), are resilient in tropical semi arid regions. A possible reason for this is the presence of endophytic and rhizospheric microorganisms. Thus, this study evaluated the diversity of associative bacteria in pangolão grass. Bacteria associated with the roots, culm, leaves, and rhizospheric soil were isolated and characterized in three municipalities of Pernambuco, Brazil. An initial phenotypic characterization was followed by a genotypic assessment by based repetitive extragenic palindromic-polymerase chain reaction (BOX-PCR) and partial sequencing of the 16S rRNA gene. We obtained 325 phenotypically-characterized isolates grouped into 243 strains with 100% similarity by BOX-PCR. The most diverse sampling environment was Araripina, and all factors affected bacterial diversity. There were 135 groups with 90% similarity, that were represented by a single strain each for sequencing. Among the sequenced strains, 118 showed 96.84–99.9% similarity with previously described strains, whereas 17 could not be identified. The following 18 genera were identified from three phyla, five classes, seven orders, and 13 families: Achromobacter, Agrobacterium, Bacillus, Burkholderia, Curtobacterium, Enterobacter, Herbaspirillum, Kosakonia, Ochrobactrum, Paenibacillus, Pantoea, Priestia, Pseudomonas, Rhizobium, Serratia, Shinella, Stenotrophomonas, and Variovorax. The diversity of endophytic and rhizospheric bacteria may contribute to the resilience of pangolão, as various strains of these genera have been described as plant growth promoters. This is the first evaluation of pangolão bacterial diversity under tropical semi arid conditions. Since several of the genera include strains known to promote plant growth, we propose further research to evaluate this on crops.

RESUMO Os solos afetados por sais ocupam extensas áreas em regiões áridas e semiáridas do mundo, e impactam a produção de alimentos, notadamente quando os processos de salinização ocorrem em áreas irrigadas. Nesse contexto, a possibilidade de uso de técnicas para recuperação desses solos precisa ser considerada. Entretanto, muitas dessas técnicas são inacessíveis para a maioria dos agricultores do semiárido brasileiro, face aos elevados custos, especialmente quando é necessária a instalação de sistema de drenagem subsuperficial. Há, portanto, necessidade de se avaliar outras estratégias de recuperação do solo, bem como a possibilidade de aproveitamento dessas áreas para outros propósitos. Nesta revisão são abordados tópicos relacionados à caracterização e impactos dos solos afetados por sais, a influência dos microrganismos do solo e as estratégias de recuperação, tais como o uso de condicionadores químicos e orgânicos, biodrenagem, fitorremediação e revegetação com espécies tolerantes. Os temas abordados nesta revisão indicam novas possibilidades para a recuperação e manejo dos solos afetados por sais no semiárido brasileiro. Também é apresentado um estudo de caso, que demonstra que o aproveitamento dos solos afetados por sais é definido por aspectos técnicos, sociais e econômicos.

ABSTRACT Salt-affected soils occupy extensive areas in arid and semi-arid regions around the world, and impact food production, particularly when salinization processes occur in irrigated areas. In this context, the possibility of using techniques for the reclamation of these soils needs to be considered. However, many of these techniques are inaccessible to most farmers in the Brazilian semi-arid region, given the high costs, mainly when it is necessary to install a subsurface drainage system. Thus, there is a need to evaluate other soil reclamation strategies as well as the possibility of using these areas for other purposes. This review covers topics related to the characterization and impacts of salt-affected soils, the influence of soil microorganisms, and strategies for soil reclamation, such as the use of chemical and organic conditioners, bio-drainage, phytoremediation, and revegetation with tolerant species. The topics addressed in this review indicate new possibilities for the reclamation and management of salt-affected soils in the Brazilian semi-arid region. A case study is also presented, which demonstrates that the use of salt-affected soils is defined by technical, social, and economic aspects.

RESUMO O feijão-caupi (Vigna unguiculata L. Walp) é econômica e socialmente importante nas regiões Norte e Nordeste do Brasil e estabelece simbiose com bactérias fixadoras de nitrogênio. Foi avaliada a compatibilidade simbiótica e eficiência de isolados rizobianos de solos do semiárido de Pernambuco e determinada sua estabilidade simbiótica com as cultivares IPA-206, BR 17-Gurguéia e BRS Novaera, selecionadas para diferentes ambientes. O experimento foi em casa-de-vegetação, avaliando um fatorial 3 × 28 (cultivares selecionadas para diferentes ambientes × inoculação com a estirpe atualmente recomendada, ausência de inoculação com e sem adição de nitrogênio mineral ou inoculação com 25 estirpes de solos do semiárido), em blocos casualizados com quatro repetições. Foram avaliados: número de nódulos, matérias secas da parte aérea, raiz e nódulos, matéria seca de nódulos por número de nódulos, nitrogênio acumulado na parte aérea pela matéria seca de nódulos, teor e acúmulo de nitrogênio na parte aérea, eficiência relativa com base no acúmulo de nitrogênio e eficiência relativa à estirpe recomendada baseada na matéria seca da parte aérea. No geral, as cultivares responderam diferentemente às diferentes linhagens e potencial de fixação biológica de nitrogênio da cultivar. As linhagens G7.85 e BR 3262 apresentaram potencial para fixação biológica de nitrogênio. A confirmação da BR 3262 foi adequada para inoculação do feijão caupi em diferentes cultivares.

ABSTRACT Cowpeas (Vigna unguiculata L. Walp) are an economically and socially important legume in northern and north-eastern Brazil and can establish effective symbiosis with nitrogen-fixing bacteria. We evaluated the symbiotic compatibility and efficiency of rhizobial strains from Pernambuco semi-arid soils and determined their symbiotic stability on the IPA-206, BR 17-Gurguéia, and BRS Novaera cultivars, selected for different environments. The experiment was conducted in a greenhouse to evaluate a 3 × 28 factorial arrangement (cultivars selected for different environments × inoculation with the currently recommended strain, uninoculated plants with or without mineral nitrogen, and 25 rhizobial strains from semi-arid soils) in a randomized block design with four replicates. We determined nodule number, shoot and root dry matter, nodule dry matter by nodule number, nitrogen accumulated in the shoot by nodule dry matter, nitrogen content and accumulation in the shoot, relative efficiency of the recommended strain based on nitrogen accumulation, and shoot dry matter. Overall, the cultivars responded differently to different strains and cultivar biological nitrogen fixation potential. Strains G7.85 and BR 3262 showed potential for biological nitrogen fixation. BR 3262 was confirmed to be adequate for inoculation of different cowpea cultivars.

ABSTRACT Biochar is a material produced by the pyrolysis of agro-industrial waste, which has become one of the most promising management tools to improve soil quality. The aim was to determine the effects of incorporating biochar from different coffee wastes in sandy soil, cropped with maize, on soil chemical and microbial attributes. The experiment followed a factorial design 2 × 3 + 1 with two types of biochar, including coffee ground (CG) or coffee husk (CH) in 3 doses (4, 8, and 16 t·ha-1) and a control fertilized solely with bovine manure (3 t·ha-1). The variables analyzed were soil organic carbon, chemical attributes, microbial biomass (C, N and P), soil basal respiration and microbial gene abundance (16S rRNA, 18S rRNA and nifH gene). Most chemical attributes were strongly increased by CH application, while CG at 8 t·ha-1 increased the soil C:N ratio (3.5 times), P (2.1 times) and K+ (7.9 times) and at 4 t·ha-1 increased the C content, microbial biomass C and N (3, 2.1 and 1.6 times, respectively). The application of CG biochar at 16 t·ha-1 showed trend to increase the abundance of bacteria, fungi and diazotrophic genes (11, 10 and 2%, respectively). Contribution of both coffee biochar types, but mainly CH, was more effective than the soil that received organic manure alone. Biochar from coffee wastes is a promising tool to improve sandy soil quality.

ABSTRACT 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.

RESUMO: Existem poucos estudos sobre a diversidade microbiana em solos de mamona no ambiente semiárido tropical. Os produtos de mamona têm sido amplamente utilizados em todo o mundo e justificam a importância comercial da ricinocultura no semiárido nordestino brasileiro. Como não há qualquer tipo de fertilização, ou manutenção de resíduos da cultura nos solos de mamona do presente estudo, presume-se que a comunidade de diazotróficas forneça o nitrogênio (N) nesses solos. Avaliou-se a estrutura e diversidade de comunidades de diazotróficas em solos de mamona do semiárido tropical em: i-Mamona com 50 anos de cultivo, consorciada com milho (CB-50); ii-Mamona com 10 anos de cultivo sem consórcio (CB-10) e iii-solo sob vegetação de Caatinga preservada (CAA). Realizou-se avaliação de redução do acetileno a etileno (ARA) e o perfil do gene nifH foi separado por eletroforese em gel de poliacrilamida com gradiente desnaturante (DGGE). A diversidade de diazotróficas foi estimada pelos índices de Shannon (H’) e Simpson (D). Com base na manutenção dos estoques de nitrogênio no solo e da biomassa microbiana ao longo do tempo, a atividade dos diazotróficos de vida livre foi determinante na entrada de N nesses solos. O gene nifH esteve presente e seus perfis agruparam os tratamentos CB-50 com CAA, reforçando a fixação de N pelos diazotróficos, os quais apresentaram maior diversidade nos solos cultivados, mesmo após a mudança de uso da terra para o plantio de mamona.

ABSTRACT: There are few studies of microbial diversity in castor bean soils in tropical semiarid environmental. Castor bean products have been widely used around the world justifying the commercial importance of ricinoculture in Brazil’ northeastern semiarid. There is no fertilization or maintenance of litter on the soils from the present study, so we hypothesized that the free-living diazotrophs microorganisms drive the nitrogen (N) input into these soils. We evaluated the communities’ structure and diversity of diazotrophs in tropical semiarid soils with i-Castor bean 50 years of cultivation intercropping with maize (CB-50); ii-Only Castor bean 10 years of cultivation (CB-10) and iii-soil under Caatinga vegetation (CAA). Nitrogenase enzyme activity was performed by the acetylene reduction assay (ARA) and the community profile of nifH gene was separated by denaturing gradient gel polyacrylamide electrophoresis (DGGE). Diazotrophs diversity was estimated by the Shannon (H’) and Simpson (D) indexes. Based on the maintenance of soil N stocks and N-microbial biomass over time, the activity of free-living diazotrophs was determinant in the N-input into these soils. The nifH gene was present and its profiles grouped the CB-50 with CAA treatments, so it reinforced the N-fixing by diazotrophs, which presented a greater diversity in the cultivated soils, even after the land use change for the castor bean implementation.

RESUMO O consórcio de leguminosas arbóreas com gramíneas forrageiras em sistema silvipastoril pode evitar a degradação das pastagens e recuperá-las. Neste sistema, o fornecimento de nitrogênio (N) ocorre através da simbiose entre leguminosas e bactérias. No entanto, a qualidade das pastagens permite a ação de bactérias diazotróficas de vida livre, que possuem o gene nifH que codifica a enzima nitrogenase. Além disso, na etapa da nitrificação, as bactérias amônio oxidantes (AOB) são avaliadas por regiões específicas do 16S rRNA de AOB. Assim, investigamos a influência da introdução das leguminosas arbóreas no sistema silvipastoril na estrutura da comunidade, na diversidade e na abundância de bactérias totais, bactérias diazotróficas e bactérias amônio oxidantes por DGGE (Eletroforese em Gel com Gradiente Desnaturante) e qPCR (PCR quantitativa em tempo real). O experimento teve nove parcelas de um hectare com Sabiá (Mimosa caesalpinifolia), Gliricídia (Gliricidia sepium) e Braquiária solteira (Brachiaria decumbens), em delineamento em blocos ao acaso com três tratamentos: I-Braquiária em consórcio com Sabiá; II-Braquiária em consórcio com Gliricídia e III - Braquiária solteira, com três repetições. A estrutura das comunidades de bactérias totais e bactérias amônio oxidantes foi influenciada pela introdução das leguminosas arbóreas, possivelmente por meio da modificação dos atributos químicos do solo. Bactérias totais, bactérias amônio oxidantes e bactérias diazotróficas foram mais abundantes nos solos sob leguminosas, que proporcionaram melhores condições para o crescimento microbiano quando comparado à braquiária solteira. O manejo silvipastoril com leguminosas arbóreas melhora a qualidade biológica do solo, favorecendo a comunidade bacteriana ligada ao ciclo do N.

ABSTRACT Intercropping tree legumes with forage grasses in a silvopastoral system can avoid pasture degradation benefiting the soil. In such a system, nitrogen (N) is supplied by symbiosis between legumes and bacteria. However, the pasture quality determines the action of free-living nitrogen-fixing bacteria, which possess nifH genes, which encode nitrogenase enzyme. Ammonium-oxidizing bacteria (AOB), involved in the nitrification step, can be evaluated by specific regions of the 16S rRNA corresponding to AOB. Thus, we investigated the influence of the introduction of tree legumes into a silvopastoral system on the community structure and abundance of total bacteria, diazotrophic bacteria and ammonium-oxidizing bacteria by DGGE (denaturing gradient gel electrophoresis) and real-time qPCR (quantitative PCR). The experiment involved nine plots of one hectare each, planted with sabia (Mimosa caesalpinifolia), a Gliricidia species (Gliricidia sepium), and a Brachiaria species (Brachiaria decumbens) in a randomized block design, forming three treatments: I-Brachiaria intercropped with sabia; II-Brachiaria intercropped with Gliricidia and III-Brachiaria only, with three replicates. The structures of the total bacterial and ammonium-oxidizing bacterial communities were influenced by tree legume introduction, possibly through modification of the soil chemical attributes. The copy numbers of total bacteria, ammonium-oxidizing bacteria and diazotrophic bacteria were higher in soils planted with legumes, which provided better conditions for microbial growth compared to planting with the Brachiaria species alone. Silvopastoral management with tree legumes improves the biological quality of soil, favouring the bacterial community linked to N-cycling.

RESUMO A seleção de rizóbios eficientes para inoculação em mucuna preta pode aumentar o uso dessa leguminosa como adubo verde, maximizando o incremento de nitrogênio (N) na cultura. O objetivo deste trabalho foi selecionar rizóbios que nodulam mucuna preta com eficiência superior às estirpes atualmente recomendadas, com potencial para compor um inoculante. Um experimento em casa de vegetação avaliou 39 estirpes em solo não esterilizado, seguido de um experimento de campo com as cinco estirpes mais efetivas sob condições de campo em um argisolo amarelo distrocoeso. Ambos os experimentos incluíram a inoculação com as estirpes atualmente recomendadas, sem inoculação e os controles fertilizados com N, avaliando-se assim os nódulos, a massa seca da raiz e da parte aérea, a concentração e acúmulo de N da parte aérea e eficiência relativa. A dose de N correspondente ao ganho de massa seca da parte aérea das plantas inoculadas foi também estimada. Em condições de campo, as plantas inoculadas com os isolados T2.19A e T1.17M tiveram concentração de N na parte aérea similar a aplicação de 80 kg ha-1 de N e significativamente superior aos demais tratamentos. Além disso, o acúmulo de N foi significativamente superior ao controle (112 e 104%), à inoculação com a mistura das estirpes recomendadas (99 e 91%) e à aplicação de nitrogênio mineral (58 e 52%), respectivamente. A inoculação com as estirpes T2.19A e T1.17M apresentou resultados promissores, mostrando o seu potencial para recomendação e inoculação em mucuna preta.

ABSTRACT The selection of efficient rhizobia for the inoculation of velvet bean may increase the use of this plant as green manure, maximizing the addition of nitrogen (N) to the crop. This study aimed to select rhizobia that nodulate velvet bean more efficiently than do strains currently recommended and for the potential of rhizobia to compose an inoculant. A greenhouse experiment evaluated 39 strains using non-sterile soil and was followed by a field experiment with the five most effective strains under field conditions on a dystrocohesive Yellow Argisol. Both experiments included non-inoculated and inoculated treatments with currently recommended strains and N-fertilized controls. Nodules, root and shoot dry mass, shoot N concentration and accumulation and relative efficiency were evaluated. The N dose corresponding to the shoot dry mass increase of the inoculated plants was also estimated. Under field conditions, the plants inoculated with the strains T2.19A and T1.17M had a shoot N concentration similar to that from the application of 80 kg ha-1 N, and the N concentration was significantly higher that of the other treatments; additionally, the N accumulation was significantly higher than that of the control (112 and 104% for the two strains, respectively), the recommended strain mixture inoculation (99 and 91%, respectively) and the mineral N application (58 and 52%, respectively). Inoculation with T2.19A and T1.17M presented promising results, showing the potential of these strains for recommendation and inoculation of velvet bean.

RESUMO: A maior produção brasileira de mamona (Ricinus communis L.) se concentra na caatinga do estado da Bahia, onde a mudança do uso da terra para o seu cultivo tem sido considerada uma estratégia de recuperação de áreas degradadas. No entanto, alterações no solo nativo proporcionam impactos ambientais, como emissões de gases de efeito estufa (GEE). Presume-se que elevadas emissões de GEE sejam provenientes da mudança do uso da terra para o cultivo de mamona e que os subprodutos dessa oleaginosa também participam dessas emissões. O objetivo deste trabalho foi realizar o inventário das emissões de GEE decorrentes do cultivo da mamona e seus subprodutos. Foi realizado um estudo na Fazenda Floresta, no município de Irecê (BA), onde foram avaliadas: a mudança no estoque de carbono (C) no solo cultivado com mamona e na floresta nativa local; a quantidade de folhas senescentes e fitomassa epígea; e as emissões de GEE provenientes dos subprodutos da produção agrícola de mamona. Os resultados mostraram que a soma das folhas senescentes e fitomassa epígea obtida pela poda da mamona é superior a 1,6 Mg ha-1ano-1 e o uso da torta de mamona no solo apresentou emissões de N2O próximas a 600 µg m-2. As emissões provenientes do uso do biodiesel de óleo de rícino representaram menos de 10% das emissões. A mudança de uso da terra foi responsável pelas elevadas emissões de GEE, representando 87% do total em CO2eq. Em comparação às emissões estimadas para o etanol de cana-de-açúcar, o biodiesel de mamona apresentou emissões 32% inferiores.

ABSTRACT: The largest production of castor bean (Ricinus communis L.) focuses on Bahia savanna, where the change of land use to their cultivation has been considered a strategy of degraded areas recovery. However, changes in the native soil can arise environmental impacts as greenhouse gases (GHG) emissions. We have assumed that higher GHG emissions come from a change of land use for castor bean cultivation and their sub-products contribute to GHG emissions. The objective of this study was to make the inventory of the emission of GHG resulting from the castor bean cultivation and their sub-products. It was done a study in the municipality of Irecê-BA and were evaluated: changes in carbon (C) soil stocks in cultivated areas with castor bean and Local native forest; a number of senescent leaves and the biomass produced epigeal; GHG emissions from the sub- products of castor bean. The results showed that the sum of senescent leaves and epigeal biomass obtained by castor bean pruning is more than 1.6 Mg ha-1ano-1 and the castor bean residues used in this soil presented N2O emissions close to 600 µg m-2. The emissions from the use of castor oil biodiesel represented less than 10% of the emissions. The change in land use handled high emissions of greenhouse gases, accounting for 87% of the total in CO2eq. Compared to emissions estimated for sugarcane ethanol, castor biodiesel showed emissions 32% lower.

Grande parte da produção brasileira de mamona encontra-se no Nordeste, como opção de cultura para a região semiárida no bioma Caatinga. O objetivo deste estudo foi avaliar as alterações nos estoques de C e N devido à mudança de uso do solo de vegetação natural para o cultivo de mamona no bioma Caatinga. Este trabalho foi realizado na Fazenda Floresta, no município de Irecê, no centro-norte baiano. O clima da região é do tipo BSwh (Köppen) - clima semiárido de altitude. O solo foi classificado em Latossolo Vermelho de textura argilosa. As situações avaliadas foram: três áreas cultivadas com mamona com diferentes tempos de implantação: (i) com 10 anos, (ii) com 20 anos e (iii) com 50 anos; e uma área de referência (vegetação nativa de Caatinga) contígua às situações avaliadas. Os estoques de C e N foram determinados em amostras de solo coletadas em cinco minitrincheiras, nas camadas de 0-5, 5-10, 10-20 e 20-30 cm. Os resultados mostraram que o constante aporte de resíduos vegetais na Caatinga promoveu a manutenção dos estoques de C (90 Mg ha-1) e N (10 Mg ha-1) para a camada de 0-30 cm. A mudança de uso da terra para o cultivo da mamona ocasiona redução em aproximadamente 50 % nos estoques de C e N do solo em relação à vegetação nativa nos primeiros 10 anos de implantação da cultura. A meia-vida da matéria orgânica do solo (MOS) calculada para essa situação na região do semiárido foi de 4,7 anos. O fator de emissão de C do solo, devido à mudança de uso da terra após 20 anos, conforme proposto pelo método do IPCC (2006), foi de 2,47 Mg C ano-1. Por meio do conjunto dos resultados, observa-se a fragilidade do solo do bioma Caatinga no que se refere à perda de MOS devido à mudança de uso da terra.

Much of the Brazilian castor bean (Ricinus communis L.) production is concentrated in the semi-arid region (Caatinga biome) of the Northeast, as an alternative crop. The purpose of this study was to evaluate changes in C and N pools due to land use change in the Caatinga biome by castor bean production. The study was conducted on the Fazenda Floresta in the municipality of Irecê (Mid -North region of Bahia, Brazil). The climate is type BSwh (Koppen) - Semi-arid climate. The soil was classified as clayey Oxisol. Three areas under castor bean were evaluated after different cultivation periods i) after 10 years; ii) after 20 years, and iii) after 50 years, and a reference area (native vegetation of the Caatinga) compared with the above situations. C and N stocks were determined in soil samples with five replications (layers 0-5, 5-10, 10-20 and 20-30 cm). The results showed that the constant plant residue input in the native vegetation ensured the maintenance of the C stocks (90 Mg ha-1) and N stocks (10 Mg ha-1). The land use change to castor bean production resulted in a reduction of about 50% in soil C and N stocks compared to Caatinga, in the first 10 years of crop establishment. The half-life of SOM of the semi-arid region was 4.7 yr. The C emission factor from soil, due to land use change after 20 years as proposed by the IPCC methodology was 2.47 Mg C yr-1. The results show the fragility of the soil in the Caatinga biome due to SOM loss caused by land use changes.