Elevated CO2 alters aggregate‑carbon and microbial community but does not affect total soil organic C in the semi-arid tropics

Abstract

Elevated CO2 increases biomass productivity, however, the retention of added C under such condition is less known in the arable soils of the Tropics. Hence, an experiment was conducted in free-air atmospheric carbon-dioxide enrichment (FACE) rings to assess the impacts of eight years of elevated CO2levels (550 ppm) on soil aggregation, soil organic carbon (SOC), microbial community and greenhouse gas (GHG) emission in a sandy loam soil. Aboveground biomass productivity of all crops increased significantly under elevated CO2as compared to the ambient CO2, however, soils under elevated and ambient CO2 had similar total SOC in all layers, up to 0–90 cm profile. Soils under elevated CO2 had ~54 % higher large macroaggregates, but had ~14 % fewer microaggregates than ambient CO2 in the 0–5 cm layer. Elevated CO2 led to ~25 % less microaggregate-associated C when compared to the ambient CO2 in the topsoil. Elevated CO2 led to 10 % more large macroaggregates than ambient CO2 in the 5–15 cm soil layer. β-glucosidase activity within microaggregates in the topsoil under elevated CO2 was 25 % higher than ambient CO2. Moreover, soil macro- and microaggregates under elevated CO2 had ~31 and 35 % higher phenol oxidase activities, respectively, than those under ambient CO2in the topsoil. Microaggregates under elevated CO2 also contained ~13 and 21 % more total glomalin in the topsoil and 5–15 cm soil layers, respectively, as compared to the ambient CO2. The functional diversity (0–15 cm soil) was significantly higher under elevated CO2 than ambient CO2. Soils exposed to elevated CO2 had higher relative abundances of fungi than ambient CO2. A 23.4 % higher qCO2 and higher global warming potential was observed in elevated CO2 exposed soil as compared to ambient. These results reveal that under elevated CO2, carbon sequestration is hindered by higher fungal activity and higher activities of soil C degrading enzymes.