Objectives Understanding the decomposition characteristics and driving factors of straw and biochar in the reclaimed soil from coal mining could provide a theoretical basis for the rational utilization of resources and soil fertility improvement in the areas.
Methods Here, we tested soils collected from the early (first year, R1), middle (10 years, R10) and long-term (30 years, R30) stages of reclamation in the coal mining area in Shanxi Province. Three organic materials, including maize straw (MS), wheat straw (WS), and biochar (BC), were selected for the decomposition experiment under the three stages of soil reclamation. The treatment with no organic material was designated as the control (CK). All the organic materials (8 g carbon) were mixed with 200 g of soil in a nylon mesh bag (0.38 µm aperture) and buried up to 15 cm in soil depth. On days 12, 23, 55, 218, 281, and 365, soil samples were collected from the bags to analyze soil microbial biomass carbon and nitrogen (SMBC and SMBN), the total and dissolved organic carbon carbon (C) and nitrogen (N) contents, and the relationship between each measured parameter and the decomposition rate of the organic materials was analyzed.
Results 1) The humification coefficient of straw (46.2%) was lower compared to biochar (86.6%), and the decomposition rate of the former was (P<0.05) higher than that of biochar. Soil reclamation year did not affect the humification coefficient of the organic materials. The straw decomposition rate in R30 was higher than that in R1 during 0−12 days, and that became similar among the three reclamation soils after 12 days. 2) The proportion of easily decomposed C pools of straw and biochar was 55% and 12%, and that of stable C pools was 43% and 87%, respectively. 3) Adding straw (P<0.05) increased the contents of SMBC, SMBN, DOC, and DON in the soil reclamation stages. The highest increase was found in R1, while R10 and R30 had statistically similar values. Adding biochar did not affect the reclaimed soil’s active C and N content. 4) During 0−23 days, straw decomposition rate was correlated with soil active C, N and the straw lignin content. After 23 days, straw decomposition rate was correlated with soil SMBN and straw lignin content (P<0.05). Biochar decomposition rate was not affected by soil properties. During the entire decomposition period, there was a significant positive and linear relationship between DOC in the reclaimed soil and the decomposition rate of straw.
Conclusions The humification coefficient of straw and biochar was not related to soil reclamation year. The decomposition rate of straw was higher than that of biochar, mainly depending on the proportion of easily decomposed C pool in the organic material. In the reclaimed soil, straw decomposed rapidly within 12 days after burying it in the soil, and the active soil nutrients increased significantly. Biochar was stable and decomposed slowly, and the active soil nutrients did not change. In summary, promoting the return of straw to the reclaimed soil could substantially increase the reclaimed soil’s active C and N contents; biochar should be used in long-term reclaimed soil to stabilize the C and N pools.
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