Long-term straw incorporation significantly reduced subsoil organic carbon stock in cinnamon soil

Objectives Straw incorporation, as an effective fertilization mode, has a significant effect on soil organic stock (SOC) sequestration in the surface soil, but its effect in the deep soil is still uncertain. We studied SOC stock change of the soil profile in cinnamon soil under different straw incorporation methods to provide a scientific basis for optimizing straw incorporation practices.

Methods The long-term field experiment of straw incorporation was started in 1992. A split plot design was used, with the main plots under chemical fertilizer application season (spring and autumn), while subplots were subjected to four straw incorporation modes No straw incorporation (CK), direct straw mulching (SM), straw incorporation after crushing (SC), and cattle manure produced by equal amount of straw (CM). After the spring maize harvest in 2013, the samples of 0–100 cm soil profile were collected to study the changes of SOC and nutrient content under different straw incorporation modes.

Results Regardless of the chemical fertilization season, SM, SC, and CM treatments significantly increased SOC content in 0–20 cm layer, while SM and SC treatments significantly decreased SOC content in 40–60 cm and 80–100 cm relative to the CK treatment. The SOC stock change in soil layers was significantly different among straw incorporation methods. Compared with CK, SOC stock in SM, SC, and CM treatments increased by 2.32, 5.42, and 12.60 t/hm² in the 0–20 cm soil, respectively, but decreased by 3.98, 6.99, and 3.76 t/hm² in the 40–100 cm layer, respectively. The increment of SOC stock in 0–20 cm layer under CM treatment was significantly higher than those under SM and SC treatments. Compared with CK, CM treatment increased SOC stock in 0–100 cm by 9.62 t/hm², while SM and SC treatments decreased SOC stock in 0–100 cm by 1.81 and 5.36 t/hm². Redundancy analysis showed that cumulative carbon input and soil nutrient explained 90.10% and 31.80% of SOC stock change in 0–20 am and 20–100 cm layer, respectively. The main factor determining SOC stock change in 0–20 cm soil depth was available P (80.10%) and in 20–100 cm soil depth was total N (25.28%).

Conclusions Long-term straw incorporation promoted soil organic carbon accumulation in surface soil by increasing carbon input and soil nutrients, but led to soil organic carbon depletion due to insufficient nitrogen supply below the surface soil. Cattle manure was the optimal mode of straw incorporation to improve fertility and crop yield in cinnamon soil.