The Dynamics of Soil Organic Carbon in Response to Vegetation and Agricultural Practices in Parent Material of a Mollisol.

Soil parent material is low in fertility and "starved" for soil organic carbon (SOC) but they can be potential arable land resources for agricultural utilization. Due to the extremely limited available nutrients in most parent materials, vegetation and fertilization management is important for providing necessary available nutrients with the aim of enhancing the pedogenic process of the parent materials. A ten year field experiment was established based on the exposed parent material (PM) of a Mollisol. The content, physical and chemical fractions of SOC were tracked from 2004-2013 under natural fallow (NatF), alfalfa (Alfa), or soybean-maize (S-M) rotations with and without fertilization and/or crop residues returned, compared to an arable Mollisol (M1C0) without organic C input. Over the 10 year of soil parent material restoration, SOC content increased from 4.8 g kg^-1 to 7.5-12.7 g kg^-1 in the field treatments, with higher SOC in soils following larger amount of organic C input, accounted for 43% of the SOC in M1C0 (29.4 g kg^-1). The content of SOC in the occluded light fraction (oLF) increased significantly, indicating that oLF fraction was more sensitive than the free light fraction (fLF) and heavy fraction (HF) to detect changes in soil due to agricultural practices at the initial formation stage of soil. For the chemical fraction of SOC, the proportion of fulvic acids (FA) was greater than humic acids (HA). The C contents ranged from 15 to 19%, and 21 to 42%, respectively. The C content in humin showed a positive relationship with cumulative C input, showing that C inputs were incorporated into stable SOC in a relatively short time. The dynamics of C accumulation was depicted by using three distinct indices of SOC properties, providing insight into the mechanisms how SOC accumulates in soil.