Fixação e transferência de nitrogênio em cultivos consorciados e solteiros de gramíneas e leguminosas

Abstract

The production of several crops in most tropical soils is limited by the need for nitrogen fertilization. Biological nitrogen fixation (BNF) has the potential to reduce the use of nitrogen fertilizers in agricultural systems with low input use, common in several areas of Northeast Brazil. Leguminosae and grasses are plants capable of fixing N in association with diazotrophic bacteria and in intercropped crops part of the N fixed in the legumes can be transferred to the grasses, contributing significantly to the increase of N stocks in the system. The objective of this work was to evaluate BNF and N transfer between plants in intercropped and single crops of C4 grasses (maize, Zea mays L., sorghum, Sorghum bicolor (L.) Moench .; and millet, Pennisetum glaucum (L. ) and legumes (cowpea and cowpea), as well as the effect of these crops on soil physical and hydraulic attributes. A greenhouse experiment was conducted using samples from the surface layer of a Planosols collected in the city of Belo Jardim, semiarid region of Pernambuco. The treatments consisted of single and intercropping cultures of three grass species and three intercalary species, with a factorial arrangement using a randomized block design, with four replications. Each experimental plot consisted of a vase, always with two plants, that could be of the same species or of different species, according to the treatment. Sunflower was inserted in the experiment to be used as an N-fixing nonbinding intermediate species, as well as pots grown with cotton and castor bean, to be used as reference plants for BNF estimates using the δ15N (‰) method. Grasses presented biomass ranging from 23.91 to 41.76g / plant, and the consortia did not increase productivity in relation to isolated crops. The presence of compatible rhizobial populations in the soil was demonstrated by the natural nodulation of legumes. It was verified that the grass species does not alter the nodulation of legumes. Grasses did not get nitrogen from the atmosphere. The legumes fixed high proportions of atmospheric nitrogen, varying between 67% and 87% of the N absorbed by the plant. There was no nitrogen transfer from the legumes to the associated grasses. The soil collected from each vase in the volumetric rings presented PR varying from 0.90 to 1.36 MPa and being within the ideal limit for the root development of the plants. The percentage of macropores was higher in the consortium with cowpea (8.46%) than in single crop (5.42%). Macroporosity and total soil porosity may have influenced natural nodulation and nodule biomass in legumes.