Long-term Cropping Systems and Tillage Management Effects on Soil Organic Carbon Stock and Steady State Level of C Sequestration Rates in a Semiarid Environment

A calcareous and clayey xeric Chromic Haploxerepts of a long-term (19 years) experimental site in Sicily (Italy) with different land use management and cropping systems were sampled (0-15 cm depth) to study their effect on soil aggregate stability and associated organic carbon (SOC). The experimental site had three tillage managements (no till [NT], dual-layer [DL] and conventional tillage [CT]) and two cropping systems (durum wheat mono-cropping [W] and durum wheat/faba bean rotation [WB]). The whole site covered an area of 4440 m2 with individual plots having 370 m2. The soil was ploughed during the experiment. Samples (2 – 4 kg each) were taken before and after the experiment. The annually sequestered SOC with W was 2.75 times higher than with WB and higher SOC concentrations were measured. The NT management system was the most effective in SOC sequestration whereas with DL no C was sequestered. The differences in SOC concentrations between NT and CT were surprisingly small. Cumulative C input of all cropping and tillage systems and the annually sequestered SOC indicated that a steady state occurred at a sequestration rate of 7.4 Mg C ha-1 yr-1. Independent of the cropping systems, most of the SOC was stored in the silt and clay fraction. This fraction had a high N content which is typical for organic matter interacting strongly with minerals. Macro-aggregates (> 250µm) were influenced by the treatments whereas the finest fractions were not. DL reduced the SOC in macro-aggregates while NT and CT gave rise to much higher SOC contents.