Soil microbial biomass and mineralization of aggregate protected carbon in fallow-maize systems under conventional and no-tillage in Central Zimbabwe

Soil organic carbon has been reported to increase soil microbial activities in soils. However, very few studies have evaluated soil microbial biomass carbon (SMBC), aggregate protected carbon, and mineralizable carbon, SOC associated with sand, silt and clay and aggregate stability under fallow-maize rotation systems. The objectives of this study were to determine SMBC, aggregate protected carbon, and mineralizable carbon, SOC associated with sand, silt and clay and aggregate stability under conventional tillage (CT) and no-tillage (NT) in a fallow-maize rotation system. We hypothesized that improved fallow systems, which are fast growing leguminous trees grown during the fallow phase, had greater SMBC, aggregate protected carbon, SOC in the different soil fractions and aggregation when compared to natural fallow or continuous maize systems. Treatments studied were improved fallows of Acacia angustissima (A. Angustissima), Sesbania sesban (S. Sesbania), natural fallow and continuous maize. Samples were collected from 0–50, 50–100 and 100–200 mm depths 1 year after fallow termination in plots subjected to CT and NT. Total SOC was determined by wet combustion, SMBC by fumigation extraction, aggregate protected SOC by wet sieving and sedimentation into sand, silt and clay after 21-day incubations of intact and crushed macroaggregates of size <250 mm. Improved fallows had greater SOC stocks when compared to natural fallow or continuous maize, SOC of the 0–50 mm depth was 5.8, 5.3, 5.4 and 4.5 Mg ha−1 C under CT and 6.0, 6.1, 5.8 and 4.5 Mg ha−1 C under NT for A. Angustissima, S. Sesbania, natural fallow and continuous maize, respectively. Soil microbial biomass carbon for the 0–50 mm depth was 802, 740, 563, and 534 mg kg−1 under NT compared to 667, 490, 452 and 456 mg kg−1 under CT for A. Angustissima, S. Sesbania, natural fallow and continuous maize, respectively. Improved fallows also had greater macroaggregate protected carbon, aggregate protected carbon than natural fallow and continuous maize. When mineralized SOC was expressed as a fraction of the total SOC, continuous maize lost a greater percentage of total SOC through mineralization than fallows. Improved fallows also had greater SOC associated the various soil fractions when compared to natural fallow and continuous maize. It was concluded that improved fallowing maintained greater levels of SMBC, aggregate protected carbon and SOC associated with various soil fractions. No-tillage resulted in greater SOC sequestration because of improved aggregation which protected carbon from mineralization compared to CT.