Effects of Nutrient Additions on the Diversity of the Herbaceous-Subshrub Layer of a Brazilian Savanna (Cerrado)

摘要

The anthropogenic increase of nutrient availability in natural ecosystems is related to different impacts as soil acidification and loss of biodiversity. The present work investigated the effects of nutrient additions on the diversity of the herbaceous-subshrub layer of a Brazilian savanna. The experimental design consisted of an unfertilized control and four fertilization treatments as follows: +N (100 kg N ha~(-1) year~(-1) as (NH_4)_2SO_4), +P (100 kg of P ha~(-1) year~(-1) as Ca(H~2PO_4)_2 + CaSO_4.2H_(2O), +NP (100 kg of N+100 kg of P ha~(-1) year~(-1) as (NH_4)_2SO_4 plus Ca(H_2PO_4)_2 + CaSO_4.2H_2O) and + Ca (4,000 kg of dolomitic limestone and CaSO_4.2H_2O). Nutrient treatments were applied twice yearly between 1998 and 2006. Soil physic-chemical analyses were performed and related to the results of vegetation surveys performed in April 2009. Calcium (Ca) addition increased the soil pH in all Ca fertilized plots, while the additions of nitrogen (N), nitrogen and phosphorus (NP) and phosphorus (P) decreased soil pH. The floristic diversity was high in all treatments but differed significantly between them (p<0.01). NP plots presented the lowest richness and the control plots showed the highest richness. Species cover and environmental variables were correlated to axes 1 and 2 in the Canonical Correspondence Analysis (F= 1.14; p = 0.035). In our study the invasive grass Melinis minutiflora (African C4 grass) responded to P and N+P treatments but was absent in the N plots, probably due to P limitation. The low cover of M. minutiflora in control and in Ca plots may be due to competition with native species, since these plots had a high H' index for the herbaceous-subshrub layer and were probably also P limited. The combination of N+P was especially detrimental to diversity, inducing the invasion by M. minutiflora. The invasion seems to be strongly limited by P and interespecific competition with native species.