Effect of tillage method change and straw return on soil labile organic carbon

Abstract: Subsoiling is an important method to solve the problem of subsoil compaction in 15-30 cm depth due to the increase of bulk density or the decrease in porosity after long-term no tillage or rotary tillage. Although subsoiling is of growing concern in Chinese agriculture production system and it could significantly influence the different fractions in soil carbon pool, there are few information on soil labile organic carbon (LOC) and carbon pool management index response to the conversion from long-term no tillage and rotary tillage to subsoiling in the North China Plain. The objective of this study was to estimate the changes in soil LOC content, the ratio of LOC fraction to soil organic carbon (LOC/SOC), and the soil carbon pool management index (CMI) by six-year tillage method change experiment. The experimental treatments including rotary tillage with residue removal (RTA), rotary tillage with residue return (RTS), rotary tillage converting to subsoiling with residue removal (RTA-STA), rotary tillage converting to subsoiling with residue return (RTS-STS), no tillage with residue removal (NTA), no tillage with residue return (NTS), no tillage converting to subsoiling with residue removal (NTA-STA), no tillage converting to subsoiling with residue return (NTS-STS). The results showed that tillage method changes significantly impacted soil LOC, LOC/SOC and soil CMI in this study. In comparison to RTS treatment, RTS-STS increased the content of soil LOC in the 0-30 cm depth, but the LOC/SOC ratio and soil CMI declined. However, soil LOC content was decreased in the 0-10 cm layer while increased in the 10-20 cm layer. Meanwhile, soil LOC contents under the RTS-STS and NTS-STS treatments were significant higher than that of the RTA-STA and NTA-STA treatments. Tillage and residue factors played a key role for the change of soil LOC content in different layers (P<0.05). Soil LOC/SOC also presented a significant difference after tillage method changed. For the RTA-STA and RTS-STS treatments, the ratio of LOC to SOC declined in the 0-30 cm layer (P<0.05). Although there was no significant difference on soil LOC/SOC between NTA-STA and NTS-STS treatments in the 0-10 cm layer, the LOC/SOC value under NTS-STS treatment was great higher than that of NTA-STA treatment in the 10-30 cm layer (P<0.05). Observations of soil CMI declined with the deepening of soil layers after RTS was converted to RTS-STS, but the opposite result was showed after NTS was converted to NTS-STS. Tillage, crop residue, experiment years and their interactions significantly affected the soil LOC after tillage method changed according to a multi-way ANOVA (analysis of variance) analysis (P<0.05). The conclusions are drawn that soil LOC, LOC/SOC and soil CMI can be decreased by long-term RT converting to RT-ST, which will promote soil carbon sequestration, but the adopting of NT converting to NT-ST might enhance the activity of soil carbon pool in the 10-30 cm depth.