Combined application of high amount of rice straw and oil cake enhances tobacco productivity and soil carbon sink function in Panzhihua area

Objectives We studied the seasonal and residual effects of different organic fertilizers on soil carbon balance to improve the carbon sink function of tobacco planting soil in Panzhihua area.

Methods In 2017, 2018 and 2019, three field experiments were conducted in Panzhihua City, Sichuan Province. Flue-cured tobacco was used as the test crop. Under a blanket NPK fertilizer input, five organic fertilizers were tested: no organic fertilizer control (T1), rice straw (T2), sheep manure (T3), oil cake (rapeseed dregs, T4), sheep manure + oil cake (T5). In 2017, hole application was used. In the following year, hole application was used but the amount of organic fertilizer was doubled. In 2019, the amount of organic fertilizer was the same as that in 2018, but the method of hole application and strip application were adopted. During the growth period of flue-cured tobacco, the soil respiration rate was measured, and after the harvest of the tobacco, the amount of carbon returned to field through flue-cured tobacco roots was measured.

Results Fertilization and transplanting increased the rate of soil respiration and emissions rapidly, and then subsequently decreased rapidly in the short term. Excluding the effects of fertilization and transplanting, soil respiration intensity fluctuated slightly throughout the growth period, attaining the peak value on the 70^th day after transplanting and gradually decreased. During the growth season of flue-cured tobacco, the average CO₂ emission rate in 2017 to 2019 were in the range of 27.92–97.77, 11.36–145.82 and 28.46–166.82 CO₂ mg/(h·m²), and the total CO₂ emission were 1079–1775, 1577–2037 and 1441–3285 kg/hm². There was no significant difference in amount of carbon return to field by root between organic fertilizer treatment and control flue-cured tobacco in 2017, 2018, 2019. Soil carbon loss from the four organic fertilizer treatments in 2017 did not differ in 2018. Under conventional application rate (2017), the soil carbon balance of T1, T2, and T4 was deficient, and the soil carbon balance of T3 and T5 was slightly surplus due to the high amount of flue-cured tobacco root carbon returned to the field. Under increased amount of organic fertilizer (2019), except T1, the soil carbon input of the four treatments were in the order T2>T5>T3>T4, but the carbon loss of T2, T3, and T5 were similar, which was significantly greater than that of T4. The carbon surplus was T2>T3≈T5>T4. Path analysis showed that organic fertilizer carbon input was the main factor affecting soil carbon balance and soil respiration. Compared with T1, T3, T4 and T5 treatments significantly increased flue-cured tobacco yield and productivity, and T4 had the highest productivity, reaching 12930 yuan/hm² and 19598 yuan/hm² in 2017 and 2018, respectively.

Conclusions Organic fertilizer carbon input is the major driver of soil carbon balance. No application of organic fertilizer resulted in net soil carbon deficit, and the application of low amount of rice straw and oil cake also resulted in soil carbon deficit, thus the soil was carbon source. While high organic carbon input led to carbon surplus, the soil was carbon sink. Compared with sheep manure and oil cake, rice straw performs satisfactory in increasing the soil carbon sink function. Oil cake has advantages over other organic fertilizer in promoting the growth and yield, and the output value of flue-cured tobacco. Therefore, the combination of rice straw and oil cake is prospective for achieving high production efficiency and soil carbon sink function in the tested area.