Effect of biochar addition amount on nitrogen loss during composting process of Elodea Nuttallii

Abstract: Biochar has the potential to reduce nitrogen (N) loss during composting process mainly through mitigating ammonia or water-soluble ammonium. However, until now there is still less quantitative information on the impact of biochar amendments on the N loss as well as on the optimum addition amounts and mixture ratios of biochar and compost. To fill in this knowledge gap, in the present study, a laboratory-scale static aerobic reactor was employed in the composting process of aquatic plants (Elodea nuttallii), in which different amounts of biochar amendment were applied. This experiment was composed of 6 treatments: 1) without biochar addition under traditional operation and composting condition, named as CK; and 2) with 5 different levels of biochar addition, the addition amount were 6%, 18%, 30%, 42% and 54% of the composting dry basis given in CK treatment, named as B1, B2, B3, B4 and B5, respectively. All the treatments applied the same amount of straw of aquatic plants (Elodea nuttallii) and rice as the composting material and had consistent environmental control conditions. The dynamics of accumulated temperature, rate of ammonia volatilization, pH value as well as the content of ammonium-N, nitrate-N and soil organic carbon were observed during the process of composting. Our study revealed the following findings: 1) Compared with the conventional composting process, adding biochar could significantly increase the composting temperature and prolong the duration days of high temperature, and therefore higher cumulative temperature and shortened composting period were obtained with the application of biochar. Furthermore, there was a significant logarithmic correlation (P<0.01) between the number of reduced days in composting cycle and the amount of biochar addition. When the biochar addition was below 50%, every 5% additional biochar amendment could shorten the composting period of approximately 0.4 d. 2) During the composting process, with application of biochar treatment (biochar content of 30%, 42%, and 54%), the accumulated NH3 volatilization was significantly lower than that of CK treatment (P<0.05). However, compared to CK treatment, accumulated NH3 volatilization was increased by 26.58% in B1 treatment (biochar content of 6%) and by 6.34% in B2 treatment (biochar content of 18%), respectively, and the nitrogen loss rate was higher than that of CK treatment. The rate of nitrogen loss rate during the composting process and the content of biochar addition showed a significant correlation of cubic curve (P<0.05). The appropriate ratio of biochar to composting was from 27% to 45% (dry biomass). In addition, when the content of biochar addition was in the range of 30%-55%, every 5% additional biochar applied could reduce the rate of nitrogen loss by 4.97% on average. Under biochar amendment, different impact factors that influenced the nitrogen loss were also analyzed using standard partial regression, and it was found the effect decreased gradually in the order of total nitrogen, ammonium nitrogen and organic carbon. Overall, this study highlights that reasonable application of biochar would have a great potential to reduce nitrogen loss and enhance working efficiency in composting process, which is a promising technology in composting.