How does increasing humidity in the environment affect soil carbon and nitrogen stocks and the C/N ratio in tropical drylands? Evidence from northeastern Brazil

Abstract

Drylands represent 45 % of the Earth’s surface and can contain up to 32 % of the organic carbon (C) of all soils on the planet. In this study, we used 91 soil profiles (from the surface to the rock) to estimate the total stocks of C and nitrogen (N) accumulated in the profiles and the surface and subsurface C/N ratio along a gradient of aridity, ranging from 30.81 (semi-arid) to 60.92 (dry sub-humid), in order to understand the effects of increased humidity in the environment on the dynamics of C and N accumulation in tropical drylands. For data analysis, we used linear regression, non-parametric tests (Wilcoxon-Mann-Whitney and Kruskal-Wallis tests) and principal components analysis (PCA). We concluded that the increase in the humidity of the environment positively influenced the C stock and no influence on N stock and C/N ratio. We observed an increase of 24.7% in organic C accumulation under the more humid climate, which was accompanied by greater aboveground plant biomass, with a predominance of deciduous and subdeciduous forests and an increase in of 48.0% in the thickness of the profiles (1.75 ± 0.21 m), when compared with profiles under semi-arid climate (1.18 ± 0.56 m). Under dry sub-humid climate, soils under the influence of deciduous forests, presented an average of 45.4% and 79.4% more C than soils under the influence of hypoxerophytic and hyperxerophilous forests respectively, predominant vegetation of semi-arid climate. Individually, the aridity index did not show any relationship with the stocks of C and N and with the C/N ratio of the studied soils, which indicates the need to carry out comprehensive and integrated studies that address several environmental variables together, to better understand the C and N dynamics of soils in tropical drylands.