Carbon, nitrogen and sulfur (CNS) status and dynamics in Amazon basin upland soils, Brazil

Abstract. Given the dimensions of the Amazon basin (7.5 million km²), its internal dynamics, increasing anthropogenic strain on this large biome, and its global role as one of two continental biospheric tipping elements, it appears crucial to have data-based knowledge on carbon and nitrogen concentrations and pools as well as on possible intra-annual dynamics. We quantified carbon (C_t, C_org), nitrogen (N) and sulfur (S) concentrations in litter (ORG) and mineral soil material (TOP 0–20 cm, BOT 30–50 cm) of upland (terra firme) oxisols across Amazonas state and present a first pool calculation. Data are based on triplicate seasonal sampling at 29 sites (forest and post-forest) within the binational project EcoRespira-Amazon (ERA). Repeated sampling increased data accuracy and allows for interpreting intra-annual (seasonal) and climate-change related dynamics. Extreme conditions between the dry season in 2016 and the subsequent wet season (ENSO-related) show differences more clearly. Median CNS in the Amazon basin TOP soils (C_t 1.9, C_org 1.6, N 0.15, S 0.03 wt-% under forest canopy) as well as C_org / N ratios show concentrations similar to European soils (FOREGS, GEMAS). TOP C_t concentrations ranged from 1.02 to 3.29 wt-% (median_Forest 2.17 wt-%; median_Post-Forest 1.75 wt-%), N from 0.088 to 0.233 wt-% (median_Forest 0.17 wt-%; median_Post-Forest 0.09 wt-%) and S from 0.012 to 0.051 wt.-% (median_Forest 0.03 wt.-%; median_Post-Forest 0.02 wt-%). C_org / N ratios ranged from 6 to 14 (median 10). A first pool calculation (hectare-based) illustrates forest versus post-forest changes. The elements are unevenly distributed in the basin with generally higher CNS values in the central part (Amazonas graben) as compared to the southern part of the basin. Deforestation and drought conditions lead to C and N losses – within 50 years after deforestation, C and N losses average 10 to 15 %. Regional climate change with increased drought will likely speed up carbon and nitrogen losses.