Soil carbon and nitrogen stocks following forest conversion to long-term pasture in Amazon rainforest-Cerrado transition environment

Abstract

The conversion of Amazon rainforest to grasslands has significant implications for regional and global carbon cycling. The description of affected organic pools and their extent is crucial for accurate prediction of forest-to-pasture conversions. We assessed soil C and N stocks in granulometric fractions of soil organic matter (SOM), particulate organic matter (POM) and mineral-associated organic matter (MAOM), following forest-to-pasture conversion in transition environments of the Brazilian Amazon rainforest and Cerrado biomes. Five study situations (SS) were selected in the Lontra River basin, Tocantins state, Brazil. The situations were established by the association between vegetation and soil classes: OFAr (Ombrophilous Forest on Orthodystric Arenosol); OFPt (Ombrophilous Forest on Petric Plinthosol); DCFr (Dense Cerrado on Acric Petroplinthic Ferralsol); DCAc (Dense Cerrado on Haplic Acrisol); and CSAr (Cerrado Sensu Stricto on Orthodystric Arenosol). In each SS, soil samples were collected in pasture areas with different planting times (between 10 and 30 years), in addition to a reference area with natural vegetation. DCFrP30 (30-y-old pasture) and DCFrNV (natural vegetation area) were statistically similar, both having the highest mean C stocks (0–20 cm: 41.5 ± 6.6 t C/ha; 0–40 cm: 64.6 ± 12.3 t C/ha; 0–60 cm: 85.0 ± 14.8 t C/ha; 0–100 cm: 102 ± 21.8 t C/ha) and N stocks (0–20 cm: 3.5 ± 0.6 t C/ha; 0–40 cm: 5.3 ± 0.9 t C/ha; 0–60 cm: 6.9 ± 1.1 t C/ha; 0–100 cm: 8.1 ± 1.5 t C/ha), especially in MAOM. The highest C-C3 (δ13C ≈ −27‰) losses occurred in newer pastures of OFAr and CSAr and in DCFr, while C-C4 (δ13C ≈ −14‰) gains were greater in POM and in 20-y-old pastures of OFPt and DCFr. Our results demonstrate significant change in stocks and distribution of soil C and N in SOM granulometric fractions following the conversion of forest to pasture. Moreover, the results give important insight into the development of strategies focused on the recovery and conservation of environmental quality and mitigation of climate change, particularly for the Amazon region where Cerrado-Amazon rainforest transition areas have been incorporated into agricultural land in recent years.