Soil organic carbon sequestration efficiency and fractions as affected by organic fertilization rate in reclaimed cultivated land

Objectives  We studied the variation of carbon sequestration efficiency of reclaimed soil after long-term application of organic fertilizer rates, to provide a theoretical basis for rapid reclamation of soil fertility in the subsidence area of coal mines.

Methods The reclamation experiment, started in 2008, was located in the subsidence area of Shanxi coal mines. The treatments included no fertilizer input control (CK), chemical fertilizer (F), and low-level manure with chemical fertilizer (LMF) and high-level manure with chemical fertilizer (HMF). Before the harvest of maize in 2019, soil samples (0−20 cm) were collected for the measurement of total organic carbon (SOC). The SOC was further fractioned using a physical-chemical fractionation method for the quantitative analysis of SOC stock in different fractions.

Results After 11 years of reclamation, compared with CK, fertilization (F, LMF, and HMF) significantly increased the total SOC by 23.8%, 39.6%, and 82.1% (P<0.05), respectively. The sequestration rates of SOC under F, LMF and HMF treatments were 0.57, 0.83, and 1.28 t/(hm²·a), respectively. The carbon sequestration efficiency of the reclaimed soil was averagely 20.9%. Among the seven organic carbon fractions, the unprotected organic carbon fraction had the highest carbon sequestration efficiency of 9.0% and was the main form of soil carbon sequestration. Compared with CK, F treatment significantly increased the unprotected organic carbon fraction, the chemically protected fraction, and the biochemically protected clay fraction (37.1%, 52.3%, and 93.5%); LMF and HMF treatment significantly increased the content of unprotected coarse organic carbon fraction, physical protection fraction, chemical protection silt fraction and clay fraction, and biochemical protection clay fraction (66.1%, 179.6%, 59.7%, 48.6%, and 63.0%), and the increment effects of HMF on the above fractions were significantly higher than LMF (19.6%、32.1%、28.5%、5.3% and 7.3%). The annual average total carbon sequestration and that in each fraction were all positively and significantly correlated with the annual average organic carbon input (P<0.01).

Conclusions The annual input rate of organic carbon significantly affects the carbon sequestration efficiency, after 11 years of conservative application of manure, the reclaimed soil still has huge carbon sequestration potential, and the highest carbon sequestration efficiency is in the unprotected organic carbon fraction. Therefore, high-level organic fertilizer input is an effective measure to quickly restore the organic carbon content in the reclamation of coal mining soil.