Does fertilization help or hinder? Nutrient inputs shape competition dynamics.

Invasive grasses such as Urochloa brizantha pose a major challenge to tropical forest restoration, often outcompeting native species and altering successional dynamics. Success in forest restoration often depends on initial cultivation practices and the competitive dynamics between native species and invasive grasses. This study evaluated the interaction between Senegalia polyphylla, a native tree species recommended for restoration, and Urochloa brizantha, an invasive grass, under different mineral and organic fertilization regimes. A greenhouse experiment was conducted using a completely randomized design with a factorial arrangement involving three cultivation types (monocultures of each species and interspecific competition), six fertilization treatments combining different proportions of mineral and organic fertilizers and a control without fertilization, and four replicates. Biomass production, nutrient content, and nutrient use efficiency were assessed.Our results indicate that, in the absence of fertilization, S. polyphylla facilitates the growth of U. brizantha, promoting a 96% increase in grass biomass production. However, nutrient addition transforms facilitation into competition, particularly with organic fertilization. Although S. polyphylla showed near-optimal phosphorus uptake even without fertilization, competition with U. brizantha limited its efficient use of nutrients, particularly P and K. Moreover, the invasive grass demonstrated more efficient nitrogen use across all fertilization regimes, highlighting its competitive potential. S. polyphylla biomass was not significantly redistributed aboveground as a strategy to overcome light competition, suggesting that belowground competition was equally intense. We conclude that fertilization, while beneficial for initial revegetation, must be carefully planned in restoration projects, as it may favor invasive grasses and hinder native tree growth.

fertilizacao.pdf