Adubação nitrogenada mineral e fixação biológica de N em sistemas de produção pastagem-soja

Abstract

Integrated systems associated with fertilization management and biological N fixation are of great importance for nutrient cycling, reflecting the increase in crop productivity and soil conservation. The objective of the study was to evaluate the influence of the crop-livestock integration system with a consortium of legumes and nitrogen fertilization in the pasture, in the chemical and microbiological characteristics of the soil, and physiological parameters of the plant. The experiment was carried out in a dystrophic Red-Yellow Latosol, with a randomized block design, with four treatments: mombaça grass (Panicum maximum cv. Mombaça), with nitrogen application (M+N); mombaça grass, without nitrogen application (M-N); mombaça grass, in consortium with the pigeon pea legume (Cajanus cajan) (M+G); and mombaça grass, in consortium with the legume java (Macrotyloma axillare cv. Java) (M+J), and four replications. Pasture and soybeans were cultivated in three consecutive harvests, and soil samples were collected at the beginning and end of each cultivation for chemical and microbiological analysis. The volatilization of NH3 and CO2 emission were evaluated every 15 days during the experiment, simultaneously recording soil moisture and temperature. Forty trifoliate soybeans from the vegetative (V5) and reproductive (R3) stages were collected for physiological analyzes of carbohydrates, proteins, amino acids, ureids, chlorophyll, urease and nitrogen enzyme activity. The soybean grain yield was also evaluated. The consortium with legumes (M+G and M+J) over the years of management has contributed to increase the carbon and nitrogen stock of microbial biomass, activity of the enzymes dehydrogenase and urease. The highest C stocks occurred on the soil surface and during the cultivation of grasses, in proportion to the lower CO2 emissions. The highest CO2 emissions are associated with changes in soil moisture, with a positive correlation between moisture and soil CO2 flow. The largest stocks of N occurred on the soil surface and during soybean cultivation, in proportion to the lower volatility of NH3. During the three evaluated harvests there was an increase in the total N content, carbohydrates, chlorophyll, total free amino acids and urease enzyme activity in soybean leaves, demonstrating the improvement in the physiological response of soybeans, especially in the M+N and M+J managements. The FBN of the crop was able to meet the soybean requirement in N. The nitrogen fertilization in the pasture reduced the production of ureids in the soybean, but increased the grain yield. Introducing in the production system the integration with the fertilized pasture and in consortium with legumes, brought great gains in the soybean grain productivity and in the plant physiology.