Land quality changes following the conversion of the natural vegetation into silvo-pastoral systems in semi-arid NE Brazil

The presence of trees in grasslands frequently results in resource islands of higher soil quality. Therefore, some native trees are often preserved or agroforestry species are interplanted when land is cleared for pasture. We evaluated soil quality changes associated with the conversion of a native thorn forest (caatinga) into silvo-pastoral systems in semiarid NE Brazil. Soil nutrients, organic matter, microbial biomass and soil enzymes under native caatinga, the canopy of two preserved native (Ziziphus joazeiro and Spondias tuberosa) and one introduced tree species (Prosopis juliflora), and under a planted pasture of Cenchrus ciliaris were compared. The two preserved tree species maintained high nutrient and organic matter contents and high biological activity levels not only relative to the grass but also relative to the native caatinga. The non-deciduous Z. joazeiro was more effective than the deciduous S. tuberosa, and enriched surface soils by 100 mg P kg(-1). The complete replacement of natural caatinga with C. ciliaris pasture or a C. ciliaris-P. juliflora silvopastoral system had, after 14 years, decreased microbial biomass C and N contents, and ss-glucosidase activity, but did not affect soil nutrient or organic matter status. The biological activity under the trees of the tree-grass association was greater than under grass alone, although P. juliflora did not maintain higher nutrient or organic matter levels. The search for indigenous alternatives to the main agroforestry species may provide viable alternatives for improved land management and the conservation of biodiversity. Microbiological and biochemical parameters responded more readily to changes in land management than chemical fertility indices. They are important indices for the impact of vegetation changes and its associated land quality changes

Organic matter transformations and soil fertility in a treed pasture in semiarid NE Brazil

Planted silvo-pastoral systems are formed by sparing selected native trees when land is cleared for pasture establishment, or by planting selected species - often known agroforestry species - into the establishing pasture. Isolated trees within pastures and savannas are often associated with 'resource islands', characterized by higher fertility and organic matter levels under the tree canopies. We here examine the processes underlying the differences in fertility and organic matter in a buffel grass ( Cenchrus ciliaris L.) pasture that contained two tree species (Ziziphus joazeiro Mart., Spondias tuberosa Arruda Cam.) preserved from the native thorn forest and a planted agroforestry species (Prospois juliflora Swartz D. C). The objective is to distinguish effects of soil variability from those induced by the presence of trees or the planting of pasture. The delta(13)C signatures of the original ( largely C3) vegetation, the preserved and planted trees, and the planted C4 grass were used to distinguish the provenance of organic matter in the top soil (0 - 15 cm). This allowed the conclusion that all trees maintained C3 derived C at the original thorn forest level, while lower levels under pasture were due to mineralisation of organic matter. The net rates of forest-derived C loss under pasture varied with soil type amounting to between 25 and 50% in 13 years after pasture establishment. Only on Alfisol, C inputs from the pasture compensated for the C3-C losses. Analysis of organic and inorganic P fractions indicated Z. joazeiro and P. juliflora enriched the soil under their canopy with P, whereas S. tuberosa had no positive effect on fertility. A combination of ANOVA and spatial analysis and mapping was used to show vegetation effects

Broccoli yield in response to top-dressing fertilization with green manure and biofertilizer

The objective of this work was to evaluate the productive performance of broccoli under different top-dressing organic fertilizations. The experiment was conducted under protected cultivation, in a completely randomized design with four replications, with two plants per experimental unit. Broccoli seedlings were produced in a commercial substrate in styrofoam trays. The seedlings were transplanted to plastic pots containing 10.0 L of substrate made up of subsoil and organic compost at the ratio of 3:1 (v/v), respectively, which is equivalent to about 20.0 t ha-1 of organic compost at planting. After seedling establishment, the top-dressing fertilization treatments were applied: gliricidia biomass associated or not with liquid biofertilizer of cattle manure to the soil and bokashi. Two control treatments were established: one with mineral fertilization recommended for the crop and the other without top-dressing fertilization. The broccoli production was evaluated (commercial standard). Plants that received mineral fertilizer were more productive, however, they were not significantly different (p>0.05), by Dunnet test, from the plants fertilized with 2.5 t ha-1 gliricidiabiomass (dry mass) associated with liquid biofertilizer (2.0 L m-2) applied to soil. Top-dressing fertilizations with only gliricidia, at 2.5 and 5.0 t ha-1 of biomass (dry mass), resulted in no significant increase in production of broccoli inflorescence. The use of bokashi in addition to gliricidia biomass and liquid biofertilizer reduced the efficiency of the fertilization compared with plants that received only gliricidia and liquid biofertilizer

Biogeochemical cycling in terrestrial ecosystems of the Caatinga Biome

The biogeochemical cycles of C, N, P and water, the impacts of land use in the stocks and flows of these elements and how they can affect the structure and functioning of Caatinga were reviewed. About half of this biome is still covered by native secondary vegetation. Soils are deficient in nutrients, especially N and P. Average concentrations of total soil P and C in the top layer (0-20 cm) are 196 mg kg-1 and 9.3 g kg-1, corresponding to C stocks around 23 Mg ha-1. Aboveground biomass of native vegetation varies from 30 to 50 Mg ha-1, and average root biomass from 3 to 12 Mg ha-1. Average annual productivities and biomass accumulation in different land use systems vary from 1 to 7 Mg ha-1 year-1. Biological atmospheric N2 fixation is estimated to vary from 3 to 11 kg N ha-1 year-1and 21 to 26 kg N ha-1 year-1 in mature and secondary Caatinga, respectively. The main processes responsible for nutrient and water losses are fire, soil erosion, runoff and harvest of crops and animal products. Projected climate changes in the future point to higher temperatures and rainfall decreases. In face of the high intrinsic variability, actions to increase sustainability should improve resilience and stability of the ecosystems. Land use systems based on perennial species, as opposed to annual species, may be more stable and resilient, thus more adequate to face future potential increases in climate variability. Long-term studies to investigate the potential of the native biodiversity or adapted exotic species to design sustainable land use systems should be encouraged