Impact of long-term conservation tillage on soil aggregate formation and aggregate organic carbon contents

【Objectives】 Conservation tillage is widely applied as its function in improving soil organic carbon (SOC) contents and the stabilization of soil aggregation. The objectives of this study were to reveal changes of SOC and soil aggregate organic carbon under long-term tillage systems, the influences of different tillage treatments on the formation of different sizes of aggregates, and the sustainable tillage system for the loess hilly region of China.【Methods】 A long-term tillage experiment, started in 1999, was used for the study. The tillage treatments included: no-tillage (NT), sub-soiling and mulch tillage (SM), and conventional tillage (CT). Soil samples were collected at depths of 0-10 cm and 10-20 cm in 2002 and 2011, soil aggregates were separated into＞2000 μm, 250-2000 μm, 53-250 μm and＜53 μm using wet sieving method. The SOC concentrations were measured by potassium bichromate titrimetric method. 【Results】 The tillage treatments affect SOC contents more significantly in surface soil (0-10 cm) than in sub-surface (10-20 cm). Compared with CT, SOC contents in the 0-10 cm soil layer are significantly increased by 33.47% and 44.48% in the NT and SM treatments after 13 years, respectively. Compared with 2002, the SOC contents in NT and SM in 2011 are increased by 1.92% and 8.59% respectively, while that in CT decreased by 18.97%. The NT and SM play a role in improving soil structure which could improve the contents of soil macroaggregates and water stability of aggregate in surface soil. Compared with CT, the contents of water-stable macroaggregates (＞2000 μm) in NT and SM are significantly increased by 40.71% and 106.75% respectively, and the soil aggregate mean weight diameters (MWD) by 20.55% and 39.68% respectively, while the contents of microaggregates (53-250 μm) are significantly decreased by 19.72% and 22.53% respectively. NT and SM significantly improve soil aggregate organic carbon contents in surface soil, especially those in macroaggregates of ＞2000 μm in size. Compared with CT, the organic carbon contents in macroaggregates of ＞2000 μm in NT and SM are significantly increased by 40.00% and 27.60%. Macroaggregates organic carbon contents in NT and SM are increased with time, and microaggregates organic carbon contents are decreased reversely. Macroaggregates (＞2000 μm) organic carbon contents in the year of 2011 in NT and SM are increased by 23.93% and 7.12% respectively compared with the year of 2002, and microaggregates (53-250 μm) organic carbon contents in NT and SM are decreased inversely by 19.58% and 13.27%. 【Conclusions】 The long-term no-tillage and sub-soiling and mulch tillage, significantly improve surface soil structures through increasing water-stable macroaggregates contents, and improve aggregate organic carbon contents in all sizes aggregates. The macroaggregates organic carbon contents in the conservation tillage are increased with the elongation of experiment, these might explain the higher SOC content in the conservation tillage. In conclusion, the long-term conservation tillage improves soil structure along with SOC content and is a more sustainable tillage system for the loess hilly region of China.