Atributos agrofisiológicos do feijão-guandu e biológicos do solo sob manejos de adubação nitrogenada

Abstract

The combined effect of nitrogen fertilization and biological nitrogen fixation is still poorly understood concerning the agrophysiological aspects of pigeon pea and the microbiological attributes of the soil after its cultivation. Thus, the objective of this research was to evaluate the physiological and agronomic responses of pigeon pea (Cajanus cajan (L) Millsp.) and soil biological quality as a function of nitrogen fertilization and inoculation with Rhizobium tropici. The study was carried out in a greenhouse in a randomized complete block experimental design, with three replications, in a 3x5 factorial scheme, where the first factor corresponded to the evaluated pigeon pea genotypes BRS03, BRS04 and IAPAR 43. The second factor corresponded to the evaluated pigeon pea genotypes BRS03, BRS04 and IAPAR 43, fertilization and inoculation managements: i) only base fertilization; ii) base fertilization with nitrogen and inoculant; iii) base and nitrogen fertilization; iv) base and inoculant fertilization and; v) without the application of fertilizer or inoculant. The base fertilization consisted of micronutrients and macronutrients, except nitrogen. Nitrogen fertilization was performed with Urea P.A. The experimental plots consisted of one plant per pot with a volume of 10 dm3. Physiological responses related to photosynthesis were evaluated, including the internal carbon concentration; transpiration rate; stomatal conductance; net photosynthetic rate at 21, 37 and 60 days after seedling emergence, with the aid of a portable infrared gas analyzer. At full flowering, the soil was collected at a depth of 0-20 cm, close to the root system, to evaluate the biological quality of the soil, and the total organic carbon; total nitrogen; carbon and nitrogen from microbial biomass; basal soil respiration; β-glucosidase and urease enzyme activity was determined. The data obtained were submitted for analysis of variance and the means were compared by the Tukey test, at 5% probability, additionally, a multivariate analysis of the data was performed. At 60 days after emergence, the base fertilization associated with a nitrogen source allowed photosynthetic gains, with an average of 26.77 μmol m-2 s-1, while the absence of fertilization was 8.17 μmol m-2 s-11. The genotypes BRS03 and BRS04 showed higher levels of chlorophyll a and b and, consequently, higher net photosynthetic rate, at 21 days of cultivation. At 60 days, these same genotypes showed pigment values very close to those of cultivar IAPAR 43. Treatments containing macronutrients and micronutrients, regardless of nitrogen supply, provided higher values for the characteristics chlorophyll a and b, net photosynthetic rate, plant height and mass, and root drought. Canonical analysis showed a high positive correlation between stomatal conductance, photosynthesis and transpiration, while the internal carbon concentration was negative. The IAPAR 43 genotype flowered at 73 days, BRS03 and BRS04, at 169 and 171 days, respectively. This behavior influenced plant height, number of nodes and main root length. The application of mineral N and inoculation of pigeon pea increased the total N, the activity of β-glucosidase and urease enzymes in the soil so that the association of these sources favored the mineralization of this nutrient. The genotypes BRS03 and BRS04 required longer flowering time and obtained greater plant height, number of nodes on the main stem, aerial dry mass, main root length and increase in microbial carbon and soil microbial quotient. The base fertilization contributed to increases in chlorophyll a and b, net photosynthetic rate, plant height and root dry mass. The BRS03 and BRS04 genotypes showed higher levels of organic and microbiological carbon. Based on the results obtained, it was found that the BRS03 and BRS04 genotypes showed potential for use as green manures, since they presented higher biomass production, while the IAPAR 43 genotype may have greater aptitude for grains, as its cycle is shorter. The study showed that the supply of nitrogen sources for pigeon pea can be effective for its growth and production, either via inoculation or mineral fertilization.