Stoichiometric ratios in the soil of native and forage areas in a seasonally dry tropical forest

Abstract

The availability of potassium (K), calcium (Ca) and magnesium (Mg) in the soil affects the nutrition and productivity of the plant cover. The Ca:Mg, Ca:K and Mg:K stoichiometric ratios indicate the nutrient load balance in the soil. However, limited attention has been paid to providing a scientific basis for how changes in land use, especially the cultivation of forage, impacts the availability and reserves of base cations in semi-arid regions, since, in these environments, fertiliser is not used. Therefore, the aim of this study was to quantify the concentrations of soil K, Ca and Mg in areas of preserved Caatinga (PC), and areas where buffel grass (BG) has been cultivated for 40 years and Leucaena grass (LE) for 11 years. Four 100 x 100 cm trenches were opened in each type of plant cover and collections were made in the 0-5, 5-10, 10-15 and 15-20 cm layers. The concentrations of K, Ca and Mg and their ratios were determined and compared using the Mann-Whitney test. The soil under buffel grass showed higher concentrations of K and Mg, and a lower concentration of Ca. Compared to PC, LE presented a lower Ca content in the 0-5 cm layer only. BG had the lowest values for the Ca:Mg and Ca:K stoichiometric ratios due to depletion of the Ca. Despite being soluble and easily leached, K remained in the soil under each type of cover, favoured by the semi-arid climate. Soil Ca, which is exported due to grazing of the produced biomass, should be replaced so as not to compromise the support capacity of seasonally dry tropical environments through the cultivation of forages.