Effects of subsoiling and straw return on soil labile organic carbon fractions in continuous rotary tillage cropland

Soil organic carbon (SOC) and its labile carbon fractions (LOC) including particulate organic carbon (POC), readily oxidized organic carbon (ROC), microbial biomass carbon (MBC), dissolved organic carbon (DOC), can sensitive response to tillage method change and organic matters input. We compared the contents changes of SOC, POC, ROC, MBC, DOC and their correlations, and analyzed change in the proportion of LOC fractions in soil organic carbon (LOC/SOC), based on a 7 years old experiment for tillage method change and straw management including the rotary tillage with straw removal (RT), rotary tillage with straw return (RTS), rotary tillage conversion to subsoiling with straw removal (RT-DT) and rotary tillage conversion to subsoiling with straw return (RTS-DTS). The results showed that SOC contents in the depths of 0-10 and >10-20 cm under RTS-DTS increased by 16.9% and 20.0% compared with those of RTS treatment, respectively; while there had no significant difference on SOC in the depth of >10-20 cm between RTS-DTS and RTS treatments. Compared with the RT and RT-DT treatments, SOC content was increased 6.1%-15.6% and 19.1%-32.3% by the RTS and RTS-DTS treatments in three soil layers. POC contents increased after rotary tillage conversion to subsoiling awith straw return, its contents under RTS-DTS treatment in the 0-10, >10-20 and >20-30 cm soil depths were high 53.8%, 30.7% and 13.6% than those of the RTS treatment, respectively. POC contents were increased 36.8%, 24.5% and 34.0% by RTS-DTS in comparison to RT-DT. The highest POC content was generally observed at the 0-10 cm soil depth in each treatment. These differences of POC contents were significant affected by soil tillage (23.8%-63.0%), straw management (24.2%-72.4%) and their interaction effect (2.6%-25.1%). Compared with the RTS treatment, whereas, the contents of ROC, MBC and DOC were decreased by RTS-DTS in three soil depths. Soil ROC in the 0-10, >10-20 and >20-30 cm soil depths under RT-DT treatment were declined 19.9%, 13.0% and 67.9% than those of RT treatment, and their contents under RTS-DTS treatment were declined with 6.6%, 10.0% and 4.6% than those of RTS treatment, respectively. These decreases were significant related with the tillage method change and straw return (P<0.05). The straw return increased ROC contents in the 0-30 cm soil layers and the content was decreased with the deepening of soil depth. Compared with the RTS treatment, soil MBC contents in three soil depths were decreased 23.9%, 30.6% and 23.8% by RTS-DTS, respectively. Although soil DOC content was increased after crop straw return, the contents were significant decreased by the tillage method change (P<0.05). For example, DOC content was declined 8%-41% by the RTS converting to RTS-DTS. There had significant positive correlations between SOC and POC (r=0.87, P<0.01), ROC (r=0.82, P<0.01), DOC (r=0.55, P<0.05), MBC (r=0.68, P<0.05). Meanwhile, there were significant positive correlations among soil POC and ROC (r=0.75, P<0.01), POC and MBC (r=0.66, P<0.05), ROC and DOC (r=0.75, P<0.01), ROC and MBC (r=0.77, P<0.01), DOC and MBC (r=0.65, P<0.05). The proportions of LOC/SOC were significant decreased by RT-DT and RTS-DTS. Rotary tillage conversion to subsoiling with straw return could increase the SOC and POC content while decrease ROC, MBC, DOC fractions and LOC/SOC proportions, which would advantage to accelerate accumulation and stability for SOC pool.