Carbon and Nitrogen Stocks and Soil Organic Matter Persistence under Native Vegetation along a Topographic and Vegetation Gradient in the Central Amazon Region

Abstract

The Amazon Forest has a soil organic carbon stock (SOCS) potential of 126 to 141 Tg year−1 and it depends on soil organic matter (SOM) accumulation factors and stabilization mechanisms. This study aimed to evaluate SOCS, soil nitrogen stocks (SNS), SOM fractions under the Amazon Forest along a topographic and vegetation gradient (Terra Firme, River Plain, and Terraces), and to evaluate the main mechanisms responsible for SOM stabilization. The study was developed using 35 study points (35 profiles) in Coari County, Amazon State, Brazil. In each profile, soil samples were collected from soil horizon for soil analysis. Of the 35 soil profiles, 10 were selected to evaluate the contribution of free light fractions (FLF) and intra-aggregate light fractions (ILF), C and N contents, and SOCS and SNS up to 1 m soil depth. SOCS and SNS are influenced by topographic and vegetation gradient, being statistically equal in the Terra Firme and River Plains areas (median of 92.5 and 92.2 Mg C ha−1, respectively), but Terraces presented a greater median (157.9 Mg C ha−1). There are relationships between SOCS and SNS and C, N, Al, clay content, t value, FLF, and ILF. SOCS, SNS, and SOM stabilization in Amazon soils are influenced by soil properties and landscape position. SOCS in the Terrace is mainly in FLF form. If vegetation cover loss continues, an amount of up to 98.05 Mg C ha−1 of FLF can be lost, causing soil degradation and global warming.