Fertilizantes de leguminosas: autossuficiência de nitrogênio em sistemas orgânicos de produção

Abstract

The growing concerns of society with the environment and quality of life have increased the demand for more sustainable agricultural systems, as practiced by organic agriculture. However, it is not possible to spread exponentially organic agriculture worldwide due to the limit availability of organic fertilizers. Legume fertilizers – products derived from cutting, dehydration and milling of leguminous (Fabaceae) biomass with high potential for biological N2 fixation, N accumulation and easiness of management – are promising sources of N to organic agriculture, and possibly effective alternative for replacement of synthetic sources of N. This thesis is organized into three chapters. In the first study, we evaluated the production of seedlings of tree legumes most cultivated worldwide, gliricidia (Gliricidia sepium) and leucaena (Leucaena leucocephala), non-inoculated and inoculated with mycorrhizal fungi (AMF - Glomus clarum and Scutellospora heterogama) in styrofoam trays with non-sterilized substrate. Some of these seedlings were used in a field experiment, establishing bank of legume (25 000 plants ha-1) for the production of fertilizers. Over 30 months, we evaluated the performance of the legume banks, the importance of using seedlings inoculated with mycorrhizal fungi and the effect of two doses of rock phosphate on yield of biomass and accumulation of wood in the trunks remaining in the field below cutting height (1.2 m high). In subsequent chapters, application of gliricidia fertilizers were compared to urea fertilization in maize (Zea mays) and citronella grass (Cymbopogon nardus). The objective was to evaluate the potential of these fertilizers in the provision of N to crops, as well as the impacts of nutrients on soil quality attributes. The gliricidia and leucaena seedlings inoculated with AMF had higher dry matter production and accumulation of N, P, Ca and Mg in shoots compared to those non-inoculated, showing the benefit of effective inoculation with AMF. The average productivity of gliricidia bank (20 Mg ha-1 yr-1 dry matter) was significantly higher (P <0.05) than leucaena bank (9 Mg ha-1 yr-1), no effect of phosphorus levels or the use of seedlings inoculated with the AMF on the annual average of the variables occurred. Gliricidia and leucaena banks enable concentrated biomass production for each cutting without replanting. Furthermore, the trunks remaining in the field accumulate considerable amount of wood, representing effective removal of atmospheric CO2 and potential raw material production for various uses. As nitrogen source, gliricidia fertilizer is capable of replacing the urea in the provision of N to crops, with environmental advantages over the synthetic source. While urea fertilizer acidify the soil and does not increase the soil levels of total-C and total-N, the application of gliricidia fertilizer enable significant increase (P <0.05) of pH, total-C and total-N. Furthermore, the N losses through volatilization of ammonia (NH3) from urea fertilizer are more than twice the losses from gliricidia fertilizer. As practical recommendation, in the first application, the dose of gliricidia fertilizer can be twice the dose of urea-N to achieve the same productivity levels. However, in subsequent application the dose of the legume fertilizer may be reduced due to the improvement of the soil quality attributes.