Fixação biológica de nitrogênio e micorrízação em gramíneas dos Campos Sulinos

Abstract

The knowledge of the level of association that occurs between diazotrophic bacteria and fungi arbusculares mycorrhizae (AMF) in grassland ecosystems may be important for the understanding of the changes caused by the addition of fertilizers containing phosphorus (P) and nitrogen (N), in the production and botanical composition of natural pastures. The objective of this study was to evaluate. (a) the occurrence of three genera of diazotrophic bacterial in the root system under fertilization with N and P, and determine the contribution of N via BNF, and (b) evaluate the behavior of the association between AMF and native grasses. Four most abundant grasses in natural grasslands of the Southern Campos in Rio Grande do Sul , Axonopus affinis, Paspalum notatum, Andropogon lateralis and Aristida laevis were grown in pots of 5 kg, in a greenhouse, two studies being conducted (A e B ). In A, two treatments were applied: 50 mg kg-1 soil P and 100 mg kg-1 of soil N (NP) and a control, being evaluated, the number of diazotrophic bacteria of the genera Azotobacter, Azospirillum and Herbaspirillum, and the contribution of BNF was determined by the technique of natural 15N abundance. In B, the treatments consisted of applying 50 mg kg-1 soil P (P), application of 50 mg kg-1 soil P and 100 mg kg-1 of soil N (NP), and a control, in both treatments mycorrhizal colonization was determined. For A, A. laevis demonstrate to be more dependent on biological N fixation than the other species. The grass P. notatum compared with other species demonstrated to be more efficient to absorb available soil N. The dry matter accumulation in shoots of the native species was higher with the application of NP. In B the mycorrhizal colonization was similar between the control, P and NP to the roots of A. lateralis and A. laevis, thus presenting a greater dependence on the mycorrhizal association. Differently, in A. affinis and P. notatum, the mycorrhizal colonization was lower when subjected to fertilization with P and NP, thus presenting a lower dependence. It is concluded for A that fertilization with N and P reduces diazotrophic colonization, increasing the production of dry matter and N content of the tissue. A. laevis showed the highest contribution of biological nitrogen fixation, since P. notatum showed higher N accumulation in soil. In relation to B, A. laevis and A. lateralis have a higher dependence on the mycorrhizal than A. affinis and P. notatum.