Objectives Soil organic carbon is the core of soil fertility, the fractions of organic carbon are highly heterogeneous. We studied the promotion effect of fertilization on different organic carbon fractions to provide theoretical basis for efficient remediation of coal mining soil.
Methods A field experiment was conducted in the coal mining reclamation soil in Shanxi Province since 2014. The planting system was rotation of one-year-soybean with two-year-maize. Five fertilization treatments included natural recovery (ZH), no fertilization (CK), chemical fertilizer (NPK), organic manure (M), manure combined with chemical fertilizer (MNPK). In the 2018 maize season, 0‒20 cm top soil samples were collected. Using the physical-chemical fractionation methods, the soil organic carbon were classified into four fractions: unprotected fractions, physical protective fractions, chemical protective fractions and biochemical protective fractions. The saturation trend of soil organic carbon by analyzing the correlation between the fractions carbon and the total organic carbon content.
Results Compared with natural reclamation, rotation significantly increased the soil total organic carbon content (P < 0.05). Soil total organic carbon increased by 11.8%, 24.0%, 38.8% and 49.3% respectively in CK, NPK, M and MNPK treatments, and M and MNPK trenaments were significantly higher than in CK and NPK treatments. Of the total organic C in M and MNPK treatments, the unprotected organic C were 2.11 g/kg and 1.68 g/kg and the physically protected organic C were 0.70 g/kg and 0.49 g/kg, respectively, which were significantly higher than in CK and NPK treatments; while the chemically protected organic C and biochemical protection organic C content were not significantly different among the four treatments. The quality ratios of each fraction in M and MNPK treatments were not significantly different with the CK and NKP treatments in the light fraction, silt and clay in the micro-aggregate, but significantly in the large aggregates (unprotected and physically-protected). The organic C in unprotected coarse and fine particles and the physically protected organic C were all significantly and positively correlated with the total organic C content (P < 0.01). According to the slope of the fitted equation, with the increase of total organic C per unit, the highest increase rate in the four fractions was the unprotected coarse particle organic C (39.7%), then were the unprotected fine particulate organic C (18.6%) and physically protected organic C (21.7%), respectively. The relationship between the silt and clay of chemical protection fractions C and the clay of biochemical protection fractions C were not significant with the total organic C.
Conclusions Fertilization and crop rotation are more effective than natural recovery in rapid increase of soil organic carbon contents, manure are more effective than pure NPK fertilization. Single and combined application of manure with NPK could increase the coarse aggregates contents, i.e. increase the friable unprotected coarse and fine particle C and physically protected C, and single manure application performs better than with NPK together. However, as the newly increased soil carbon are quite active, so long-term organic manure input is necessary for the recovery of soil function in coal mine reclamation.
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