Interações bióticas e nitrogênio amoniacal no crescimento inicial de Dimorphandra wilsonii Rizz

Abstract

Deforestation of the Brazilian Cerrado for agriculture caused the tree species Dimorphandra wilsonii to become isolated and restricted to private properties occupied by pastures of Urochloa spp. Thus, the studies that comprise this thesis aim to evaluate the influence of N-NH4+, symbiotic associations with N2-fixing bacteria (NFB) and arbuscular mycorrhizal fungi (AMF) and Urochloa decumbens (African grass) in the initial growth of D. wilsonii. The hypotheses were that the N- NH4+ is the preferred source of inorganic N for this species, that the presence of symbionts favours the growth of D. wilsonii and facilitates its coexistence with U. decumbens, but despite this, the African grass competes with this Cerrado tree species. To verify these hypotheses two experiments were conducted under controlled conditions, by growing D. wilsonii seedlings under increasing concentrations of N-NH4+ (1.5 mM limiting concentration; 3 mM non limiting; and 10 mM high) inoculated or not with symbionts (strain BHCB8.5/Bradyrhizobium sp. isolated from nodules D. wilsonii; and an AMF species/Glomus etunicatum) and in the presence or not of U. decumbens, summing up to four (Chapter I) or five treatments (under the 3 and 10 mM doses a positive control for nitrification inhibition was used Nitrapyrin) (Chapter II). D. wilsonii biomass accumulation increased with the increasing N-ammonium availabilities, and was positively correlated with the ammonium availability in the substrate and negatively with that of nitrate. The plants that retained a greater fraction of the total N added were those cultured with 1.5 and 3 mM N-ammonium, especially those inoculated with the symbiont, highlighting their role in increasing N acquisition. Although Bradyrrizobium sp. strain that was used had been isolated from D. wilsonii nodules, there was no nodule formation. However, the N contained in plant biomass suggested the occurrence of N2 fixation in the plants inoculated and grown with 1.5 mM of N-ammonium and also those inoculated and coexisting with U. decumbens receiving 3 mM of N-ammonium. Thus, biological N2 fixation appears to be modulated by selective pressures imposed by low N availability and competition for this nutrient. Colonization by G. etunicatum was only observed in the treatment with 3 mM of N-ammonium that D. wilsonii coexisted with U. decumbens, significantly increasing the P content in the biomass of the tree species. The presence of U. decumbens affected the growth of D. wilsonii by direct competition for nutrients and / or indirectly by in vitro inhibiting the growth of the bacteria. Data suggest that D. wilsonii prefers N-ammonium as a source of inorganic N, the symbiotic relationship with Bradyrhizobium sp. and G. etunicatum can benefit the tree species by contributing to N and P acquisition and minimizing the negative effects imposed by the presence of U. decumbens and that this African grass can inhibit D. wilsoniis growth and impair its regeneration through seedlings by inhibiting the growth of N2-fixing bacteria and / or compete for nutrients. The information obtained in this study may contribute to the in situ and ex situ conservation of the D. wilsoniis germoplasm, a critically endangered species. However, more studies are needed to confirm what was observed, especially under field conditions.