Effects of maize stovers returning by mulching or deep tillage on soil organic carbon sequestration in Mollisol

Maize stovers returning is an effective approach to improve soil fertility of Mollisol, thereby to maintain safety of regional environment. However, the understanding of maize stovers returning with no tillage or deep tillage is still limited on Soil Organic Carbon (SOC) sequestration and distribution of aggregates associated Organic Carbon (OC). In this study, a six-year field experiment was carried out, which located in the middle of Mollisol region in northeast China. Three treatments were selected as follow: conventional tillage (CK), stovers mulching with no tillage (SM), and stovers incorporation with deep tillage (SI). Soil samples were collected from 0-10, >10-20, >20-30, and >30-40 cm depth. In addition, the specific parameters were estimated, including the SOC content, bulk density, aggregate size distribution and aggregate associated OC content, annual carbon input, SOC stock, and soil carbon sequestration rate. Compared with CK, SMR significantly increased by 22.4% SOC content at 0-10 cm soil depth, but no notable change in the SOC content at 10-40 cm soil depth. SI significantly increased by 18.1%-41.5% SOC content at 0-40 cm soil depth, with the greatest increasement at >20-30 cm soil depth. The SOC content in CK and SMR treatment was in a decreasing trend as soil depth increased, but no significant change was found in the SOC content among 0-10, >10-20 and >20-30 cm soil depths in SIR. The SOC stock changed in the same way as its content. The SMR had a high SOC stock at 0-10 cm soil depth than SIR, however, greater SOC stock at >20-30 cm soil depth was found in SIR than that in SMR. In the initial condition, the amount of carbon sequestration decreased as follow: SM>SI>CK at topsoil (0-20 cm), and SI>SM>CK at subsoil (>20-40 cm). The soil carbon sequestration rate at topsoil and subsoil were 1.34 and 0.77 Mg/(hm2•a) in SM treatment, and 0.85 and 1.74 Mg/(hm2•a) in SI treatment, respectively. Aggregate distribution and aggregate associated OC content were strongly affected by different stovers returning treatments. The SM significantly increased the proportion of macroaggregate and Mean Weight Diameter (MWD) at topsoil, and the SI significantly increased the aggregate stability at 0-40 cm soil layer, compared with CK. The SIR treatment had higher MWD at 10-40 cm soil layer than the SM. The 2-0.25 mm aggregate accounted for 48.3%-55.0% of the SOC, indicating the crucial role of macroaggregate in SOC sequestration in the Mollisol. Compared with CK, higher OC contents of >2 and <0.053 mm aggregated at 0-10 cm layer were observed in SM, where the SI resulted in greater OC content of >2 mm at 0-10 cm layer, as well as OC contents in each size of aggregate at 10-40 cm soil depth. The SOC content was significant positive correlations with MWD, OC contents of >2 mm and 2-0.25 mm aggregate, respectively, indicating that more exogenous organic carbon has been preserved in macroaggregates. In the Mollisol region, maize stovers returning can be used to improve the SOC sequestration and aggregate stability, where the positive effect of maize stovers mulching on SOC retention mainly focused on 0-10 cm soil depth. Furthermore, the maize stovers incorporation with deep tillage can contribute to great SOC sequestration and physical structure at 0-40 cm soil depth.