Active Soil Organic Carbon Fractions and Aggregate Stability Effected by Minimum Tillage and Crop Rotations on a Marginal Dryland Soil in Punjab, Pakistan

Conservation Agriculture (CA) is an important technique for enhancing soil organic carbon (SOC) content in the surface layer and improving structural stability. CA is not widely practiced in dryland soils of developing countries where marginal farming practices are extensively used. Therefore, a field study was conducted in dryland region of Punjab, Pakistan to compare minimum tillage and intensified cropping systems effects on active SOC fractions and aggregate stability. The experiment was laid out in a split-plot design having moldboard plough (MP) and minimum tillage (MT) as main plots, and crop sequences as sub-plots. The latter comprised of fallow–wheat (Triticum aestivum L.), (FW, control), mungbean (Vigna radiate L.) –wheat (MW), sorghum (Sorghum bicolor L.)–wheat (SW), green manure–wheat (GW) and mungbean-chickpea (MC) (Cicer arietinum L.). Tillage systems had more pronounced effects than cropping sequences on microbial biomass carbon (MBC), potentially minerlizeable carbon (PMC) and particulate organic carbon (POC). The PMC in second year was significantly more in the soil under MT than that under MP especially with SW, GW and FW sequences (448, 442 and 419 µg g-1 soil day-1, respectively). High MBC was also recorded under MT mainly with MW (361 µg g-1). POC was the highest under MP with MC sequence and was 6.41% more than that under MT. More water stable aggregate (WSA) was recorded in soil under MT plots sown with MC and GW (48.62% and 46.25%, respectively) than that under MP. The results indicate that MT with legume based-cropping sequences reduced breakdown of soil aggregates than the current MP and fallow-based systems in Pothwar, Pakistan.