Changes in soil organic carbon and aggregate stability after conversion to conservation tillage for seven years in the Huang-Huai-Hai Plain of China

Soil aggregate stability and organic carbon（OC） are regarded as effective indicators of soil structure and quality. A longterm field experiment was established in 2006 to examine the influence of tillage systems on soil aggregation and OC in a sandy loam soil in the Huang-Huai-Hai Plain of China. The study involved eight treatments： plowing every year with（TS） and without residue（T）, plowing every 2 years with（2TS） and without residue（2T）, plowing every 4 years with（4TS） and without residue（4T）, and no plowing with（NTS） and without residue（NT）. In 2013, soil samples were collected at depths of 0–5, 5–10 and 10–20 cm, and separated into three aggregate-size classes： macroaggregates（〉250 μm）, microaggregates（53–250 μm） and the silt＋clay fraction（〈53 μm） using wet sieving method. Soil parameters measured were water-stable aggregates, geometric mean diameter（GMD）, mean weight diameter（MWD） and OC concentrations in different aggregate-size fractions and in bulk soil. The tillage treatments significantly（P〈0.05） influenced soil aggregate stability and OC distribution. Higher MWD and GMD were observed in 2TS, 4TS and NTS as compared to T. With increasing soil depth, the amount of macroaggregates and MWD and GMD values were increased, while the proportions of microaggregates and the silt＋clay fraction were declined. The OC concentrations in different aggregate fractions at all soil depths followed the order of macroaggregates〉microaggregates〉silt＋clay fraction. In the 0–5 cm soil layer, concentrations of macroaggregateassociated OC in 2TS, 4TS and NTS were 14, 56 and 83% higher than for T, whereas T had the greatest concentration of OC associated with the silt＋clay fraction in the 10–20 cm layer. Soil OC concentrations under 4TS and NTS were significantly higher（P〈0.05） than that of T in the 0–10 cm layer. Residue retention promoted formation of macroaggregates, increased macroaggregate-associated OC concentrations and thus increased total soil OC stock. The macroaggregate-associated OC was positively correlated（R^2=0.96） with soil OC concentration, while the silt＋clay fraction-associated OC was negatively correlated（R^2=0.82） with soil OC concentration. The concentration of soil OC was positively correlated with MWD（R^2=0.94） and GMD（R^2=0.92）. We concluded that increasing tillage intensity led to a loss of carbon（C）-rich macroaggregates and an increase of C-depleted silt＋clay fraction. The conservation tillage system, especially NTS and 4TS, increased soil aggregate stability and promoted OC accumulation in macroaggregates, provided the potential to improve soil C sequestration and soil structure in the Huang-Huai-Hai Plain of China.