Changes in soil organic carbon and its chemical fractions under different tillage practices on loess soils of the Guanzhong Plain in north-west China

Over the past 20years, conservation tillage has been used on the loess plateau of north-west China to improve the sustainability of local agriculture. There had been particular concern about loss of soil organic matter associated with traditional tillage. We examined the influence of four tillage treatments: conventional tillage (CT), subsoiling tillage (SST), rotary tillage (RT) and no-tillage (NT), with two straw residue management treatments (return and removal) on the distribution with soil depth (0-20cm, 20-40cm) of total organic carbon, labile organic carbon (KMnO4-C) and bound organic carbon. The study was carried out on a Loutu soil (Earth-cumuli-Orthic Anthrosol) over seven consecutive years of a winter wheat (Triticum aestivum L.)-summer maize (Zea mays L.) crop rotation. By the end of this period, conservation tillage (SST, RT and NT) led to greater storage of soil organic carbon (SOC) (22.7, 14.9 and 16.3% with straw return in contrast to 21.4, 15.8 and 12.3% with no straw return, respectively) compared with CT in the surface soil (0-20cm). The reduced tillage treatments (SST and RT) both increased significantly the highly labile organic carbon (HLOC) content of the surface soil (50% in both SST and RT) and mildly labile organic matter (MLOC) (49.4 in SST and 53.5% in RT) when straw was removed. The largest pool of bound carbon was observed in the Humin-C pool, and the smallest in the free humic acids C (FHA-C) in each tillage treatment. Conservation tillage led to an increased content of FHA-C and CHA-C. Results from correlation analyses indicate that SOC enrichment might have resulted from the increase in HLOC, MLOC, FHA-C and CHA-C over a short period. Labile organic carbon was associated with the organic carbon that was more loosely combined with clay (FHA-C and CHA-C). We conclude that both SST and RT are effective in maintaining or restoring organic matter in Loutu soils in this region, and the effect is greater when they are used in combination with straw return.