Biochar addition inhibiting nitrification of reclaimed soils in coal-mining subsidence area

Abstract: The soil structure was severely destroyed, and the content of organic matter and fertility was extremely low in reclaimed soil in the coal-mining subsidence area. How to improve the soil fertility and recover soil productivity was a critical problem for sustainable agriculture in this area. Nitrogen fertilizer was one of the largest amounts of fertilizers used in agricultural production, and the nitrification of ammonium to nitrate was an important nitrogen transformation process in dryland soil. Numerous evidences showed that biochar could improve soil fertility and affect nitrogen transformation in soil. However, little attention was paid to compare the difference in nitrogen transformation affected by biochar addition in soils with different fertility levels, especially in the extremely infertile reclaimed soil in the coal-mining subsidence area. In this paper, the effect of biochar addition on the nitrification in the reclaimed soil from a coal-mining subsidence area was examined by using a laboratory aerobic incubation method with constant temperature and controlled humidity under different nitrogen supply levels. In addition, vegetable soil with high fertility was used as the comparation. The experimental factors included three levels of biochar (0, 1.5% and 3%), two levels of nitrogen (at the rates of 200 and 400 mg/kg, respectively), and two kinds of soil (vegetable soil and reclaimed soil with extremely low fertility). In addition, soils without any biochar or nitrogen addition were cultured at the same condition to provide background nitrification information. Nitrate nitrogen and ammonium nitrogen concentration in soils were monitored during the cultivation period. The variation of nitrate nitrogen concentration in the cultivated soils was simulated by Logistic function and the significance values of the model were analyzed. It showed that, compared with the vegetable soil, the nitrification was slow in the reclaimed soil. The maximum rate of nitrification in the reclaimed soil was only 17.32% of that in the vegetable soil, and the time the maximum nitrification rate occurred delayed 4.2 days in the reclaimed soil compared with the vegetable soil. The inhibition of biochar addition on the nitrification was obvious especially in nitrification acceleration stage, and was affected interactively by nitrogen supply level and soil type. Compared with low nitrogen supply, biochar exerted greater inhibition on the nitrification so that the nitrification acceleration stage extended 6 days (for the reclaimed soil) or 11 days (for the vegetable soil) in the soil with high nitrogen supply. However, the nitrification acceleration stages were all terminated before the 18th day under both of the two nitrogen supply levels. In the reclaimed soil, especially under high nitrogen supply, the time the maximum nitrification rate occurred was obviously delayed as the increasing of biochar amount, but the maximum rate of nitrification was not affected by biochar and nitrogen supply level. However, in the vegetable soil, the value of maximum rate of nitrification was significantly affected by nitrogen or biochar supply. Hence, biochar addition could suppress soil nitrification, and this suppression was especially obvious in the nitrification acceleration stage, which could be affected interactively by nitrogen level and soil type. The outcome of this research will provide a theoretical guide for improving the fertility of the reclaimed soil in the coal-mining subsidence area.