Soil macroaggregates: The hotspots driving emission or mitigation of greenhouse gases according to the management system adopted

Abstract

The association of no-tillage (NT) and legume cover crops has shown positive results to organic carbon (SOC) accumulation in subtropical soils. However, soil dynamics of powerful greenhouse gases (GHG) such as nitrous oxide (N2O) and methane (CH4) are not fully understood at a microscale under these systems. Thus, our objective was to evaluate net cumulative emissions (NCE, in CO2 equivalent [eq.]) of GHG considering fluxes of N2O and CH4 and SOC accumulation in three soil aggregates classes. The soil was sampled in a 30-year experiment in a subtropical Acrisol under conventional tillage (CT) and NT, combined with cropping systems with or without legume cover crops. Large (9.51 to 2 mm) and small (2 to 0.25 mm) macroaggregates and microaggregates (<0.25 mm), obtained by wet sieving, were incubated under 400 g kg−1 (aerobic condition) and 700 g kg−1 (anaerobic condition) of volumetric water for six months. Under anaerobic conditions, NCE were positive due to the high emission of N2O and CH4, making all the soil aggregates a GHG source to the atmosphere. Under aerobic conditions, NT and legume cover crops greatly contributed to reducing NCE: for each kilogram of C accumulated in macroaggregates, greater C from the atmosphere was taken up by NT than CT (−69.4 versus 57.1 mg CO2eq. kg−1 SOC) and by cropping systems with than without legume cover crops (74.7 versus 51.8 mg CO2eq. kg−1 SOC). Management systems did not impact GHG emissions of soil microaggregates. By containing labile organic matter, soil macroaggregates acted as hotspots of soil GHG emission/mitigation. Despite favoring N2O emission, NT and legume cover crop lead to GHG mitigation by promoting CH4 uptake and C accumulation in soil macroaggregates.