Afforestation with tropical N-fixing species in the Brazilian Atlantic rainforest: carbon and nitrogen in the soil profile

Abstract

The Atlantic Rainforest biome is one of the most threatened in the world by deforestation where afforestation programs are urgently needed. N-fixing species should be prioritized in re-establishing forest covers as they can enhance soil C and N and stimulate cycling of other nutrients. Yet, tropical ecosystems play a key role in global warming and remain underrepresented in global biogeochemical assessments. To better understand the effects of tropical N-fixing species on soil C and N pools after pasture conversion we selected three 27-year-old monospecific stands comprising Plathymenia reticulata, Hymenaea courbaril, and Centrolobium tomentosum. We evaluated soil organic carbon (SOC), nitrogen (STN), and the natural abundance of ¹³C and ¹⁵N in the soil profile up to 100 cm depth. SOC was higher for P. reticulata, but that was not true when considering soil layers up to 30 cm soil depth. Meanwhile, STN was similar across species and δ¹⁵N values showed enrichment at intermediate soil layers indicating ¹⁴N gaseous loss. Most of the SOC originated from the planted trees rather than the former pasture, except beneath C. tomentosum where C₄ derived C is decreasing at a slower rate. This study presents novel insights in understanding how tropical N-fixing tree species may be impacting soil C and N pools where specific-species leaf traits appear to be mediating SOC retention to the mineral soil.