Effect of tillage method changes on soil organic carbon pool in farmland under long-term rotary tillage and no tillage

Abstract: Subsoil compaction at 15-30 cm depth due to the increase of bulk density or decrease in porosity after long-term no tillage or rotary tillage is of growing concern. Subsoiling is generally regarded as an important method to reduce subsoil compaction due to long-term conservation tillage and thereby improve soil conditions. The sequestration and decomposition rates of soil organic carbon (SOC) pool would be changed by the tillage method conversion from long-term no tillage and rotary tillage to subsoiling. The objectives of this study were to estimate the SOC storage and the rate of SOC sequestration during 12-year (2002-2014) rotary tillage (RT) and no tillage (NT) experiment, and to analyze the changes of SOC content and SOC storage after RT and NT treatments converting to subsoiling (ST) that included rotary tillage-subsoiling (RT-ST) and no tillage-subsoiling (NT-ST) treatments (2008-2014). Two long-term tillage experiments were carried out in Tai'an. The results showed that the highest SOC content in different soil profiles under NT treatment was measured at the 0-10 cm depth, which was 20.1% and 48.4% higher than the 10-20 and 20-30 cm depth, respectively. Meanwhile, the SOC contents of the 0-10, 10-20 and 20-30 cm under NT were 33.4%, 46.8% and 51.4% higher than under RT treatment. A slight difference on SOC content was measured at 3 layers in the 0-30 cm depth under conventional tillage (CT) treatment, and the contents of SOC for NT and RT treatments were 37.7% and 4.6% lower than the CT treatment. These differences on SOC content between the treatments and soil profiles led to the difference for SOC sequestration during 12-year experiment. The SOC storage in the 0-30 cm soil depth was increased by NT treatment, because the mean content for SOC was increased by 0.32 g/kg during 2002-2014 according to the mean annual variation (MAV) index, and approximately 16.7 t/hm2 SOC was sequestrated under NT during 12 years and the SOC storage in 2014 increased by 38.3% in comparison to that in 2002. However, the SOC content decreased by 0.08 g/kg each year under RT treatment, which lost 9.8 t/hm2 SOC in the 0-30 cm depth from 2002 to 2014. The SOC storage under NT was 2.6 times higher than that under RT after 12 years. Although SOC storage was increased under NT in 2014 that was significantly higher than the CT, there was no significant increase for SOC storage under CT during 2002-2014. The content and sequestration rate of SOC were significantly changed by the tillage methods converting to subsoiling. Compared with the RT, SOC content of the 0-30 cm and SOC storage under the RT-ST treatment were increased by 32%-67% and 88.3% during 2008-2014, respectively. The soil lost 6.84 t/hm2 SOC under RT treatment but sequestrated 17.4 t/hm2 SOC when it was converted to RT-ST experiment that lasted 6 years. Although there was no significant difference on SOC content at 0-10 cm depth between the NT-ST and NT treatments except that in 2009, the SOC content in the 10-30 cm depth interval decreased significantly under the NT-ST treatment during 2012-2014, the SOC sequestration rate was decreased to 1.59 t/hm2 per year compared with the NT with 2.73 t/hm2 per year, and hence the SOC storage was deceased by 41.7% under the NT-ST treatment. Therefore, although a negative effect of the RT treatment on SOC sequestration was observed, the RT-ST could benefit SOC sequestration compared with the RT. Long-term NT plays an important role towards improving SOC level, but SOC sequestration decreasing is a potential risk under long-term NT-ST treatment, so the evolution for SOC sequestration of the different soil profiles in NT-ST system should be assessed with the long-term experiment in different regions.