Changes in soil organic carbon fractions and residence time five years after implementing conventional and conservation tillage practices.

• Conservation tillage practices (CTP's) increased TOC compared to conventional tillage (CT). • CTP's increased OC stabilized by sands and aggregates + particulate OC compared to CT. • Reduced and minimum tillage reduced dissolved OC compared to CT. • Stubble cultivator increased the OC stabilized by clay and silt compared to CT. Tillage not only affects the quantity of soil organic carbon, SOC, but also its quality. Therefore, an investigation was conducted to see how a five-year (2011–2016) implementation of conventional tillage, CT, and conservation tillage practices, CTP's, under dryland vetch – wheat farming system can affect the total organic carbon, TOC, of soil and its fractions as well as its biological indicators including the half-life of organic materials, HL, and the mean residence time, MRT. The applied tillage practices included three CTP's of reduced tillage, RT, with chisel plow, minimum tillage, MT, with stubble cultivator, and no-tillage, NT, with direct seeding as well as one conventional tillage, CT, with moldboard plow. The results revealed that all applied CTP's (RT, MT, and NT) significantly increased the TOC of examined surface soils (0−25 cm) compared to CT. The labile fraction of SOC (dissolve organic carbon, DOC) under CTP's, excluding NT, showed a significant decrease compared to CT. While MT significantly increased the fraction of organic carbon stabilized by clay and silt particles, OC{c + s} compared to CT, NT and RT made no considerable change in OC{c + s} compared to CT. There was also a considerable (but insignificant) increase in the amount of organic carbon stabilized by sand and aggregates plus particulate organic matter, OC{S + A+POM}, under CTP's compared to CT. We recorded the greatest (228 ± 21 μg N g−1 2 h−1) and least (98 ± 27 μg N g−1 2 h−1) activity of urease under RT and CT systems, respectively, and the differences were significant. Based on the incubation data, the NT and RT systems significantly resulted in highest HL, (170 ± 9 days) for soil organic materials while CT had the lowest HL, (127 ± 5 days). While RT showed significantly the highest aggregate stability (55 ± 7 %), the CT and MT had the lowest aggregate stability (29 ± 4 %). In general, our results demonstrated the benefits of CTP's, and more specifically the benefits of RT and NT systems, in improving SOC, as primary indicator of soil quality, in surface soils under dryland agriculture in terms of quantity and quality of organic carbon. [ABSTRACT FROM AUTHOR]