Background The abandonment of cultivated land is a common phenomenon in China. Understanding the changes of soil quality after the cultivated land abandoned is very important for land resource protection and the reuse of abandoned cultivated land. Herein, the soil in arable land and abandoned cultivated land (for 6 years) under topsoil (0-15 cm) and subsoil (15-30 cm) were selected to explore the changes of soil quality after the cultivated land abandoned.

Methods The varieties of soil quality were studied from the soil aggregate stability and soil organic carbon (SOC) characteristics. The stability of soil aggregate was determined by wet sieving. The characteristics of SOC was determined through chemical-physical composition of soil organic carbon which obtained by humic fractionation and aggregate fractionation.

Results 1) The content of >0.25 mm water stable aggregate (WAS) in abandoned cultivated land topsoil was significantly higher than that in arable land topsoil, however, no significant difference was observed for subsoil in abandoned and arable land (P>0.05). 2) This was the result of that abandonment of cultivated land facilitated the increase of SOC content in the topsoil and had no influence in subsoil. The stability of aggregate was positively correlated with fractionation of SOC. 3) By grouping aggregate of SOC we found that SOC was dominantly stored in macro-aggregate (>0.25 mm), and by humic fractionation of SOC we found that HMr (insoluble humin residues) was the dominant components of SOC. 4) The result of SOC fractionation indicated that the SOC had a lower humification and a weak chemical protection for soils in the abandoned cultivated land. In addition, the proportion of SOC in macro-aggregate was higher for soils in the abandoned cultivated land than that in the forest land, indicating that the physical protection of aggregate to SOC was weaker for soils in abandoned cultivated land.

Conculsions Abandonment of cultivated land would increase soil aggregate stability and organic carbon content. Soils in the abandoned cultivated land had a higher content of organic carbon, but possessed a weak chemical and physical protection to soil organic carbon. It could be speculated that cultivation on the abandoned cultivated land at present time, could generate the destruction of macroaggregates and released a large amount of soil organic carbon which would be mineralized and decomposed by microorganism, and degrading soil structure. Therefore, it is essential to determine the optimal time for the restoration of soil structure and soil organic carbon on abandoned cultivated land for better recultivation in future.