agronomy Article Biochar Promotes Nitrogen Transformation and Tomato Yield by Regulating Nitrogen-Related Microorganisms in Tomato Cultivation Soil Lili Guo 1,2,3,† , Huiwen Yu 4,† , Wenquan Niu 1,3,5,6, * and Mourad Kharbach 7   Citation: Guo, L.; Yu, H.; Niu, W.; Kharbach, M. Biochar Promotes Nitrogen Transformation and Tomato Yield by Regulating Nitrogen-Related Microorganisms in Tomato Cultivation Soil. Agronomy 2021, 11, 381. https://doi.org/10.3390/ agronomy11020381 Academic Editors: José M. De la Rosa and Marina Paneque Received: 26 January 2021 Accepted: 18 February 2021 Published: 20 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 College of Water Resources and Architectural Engineering, Northwest A&F University, Weihui Road 23, Yangling 712100, China; 13279293652@163.com 2 Department of Plant and Environmental Science, Faculty of Science, University of Copenhagen, Højbakkegaard Alle 13, DK-2630 Taastrup, Denmark 3 Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A&F University, Yangling 712100, China 4 Department of Food and Science, Faculty of Science, University of Copenhagen, Rolighedsvej 26, DK-1958 Frederiksberg C, Denmark; huiwen.yu@food.ku.dk 5 Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, China 6 Institute of Soil and Water Conservation, CAS &MWR, Yangling 712100, China 7 Research Unit of Mathematical Sciences, University of Oulu, FI-90014 Oulu, Finland; mourad.kharbach@vub.be * Correspondence: nwq@nwafu.edu.cn † These two authors contributed equally. Abstract: Nitrogen (N) transformation in soil directly determines the effectiveness of N for plant growth. Biochar has received evermore attention because of its significant ability to improve soil. However, the effects of biochar on N-related microorganisms (Lycopersicon esculentum Mill.) in tomato cultivation soil, N transformation, utilisation of water and N fertiliser, and tomato yield remain unclear. The objective of this study was to investigate the responses of N-related microorganisms to biochar and N fertilisation in soil, along with the implications of biochar for altering N transformation, N uptake by tomatoes, and utilisation of water and N fertiliser. A two-year greenhouse experiment containing six biochar levels under drip irrigation (0, 10, 30, 50, 70, and 90 t ha -1 ) and two N fertiliser application rates (190 and 250 kg ha -1 ) was conducted in the northwest of China. The results showed that adding biochar significantly promoted urease activity, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and the number of amoA-type nitrifiers in the soil. The MBC:N ratio and the number of nirS-type denitrifiers were significantly inhibited when the added amount of biochar was greater than or equal to 30 t ha -1 . Moreover, biochar can increase the water content in the soil and can reduce the N lost to leaching. The inorganic N (NO 3 - and NH 4 + ) in the soil could be better maintained in the rootzone and better absorbed by tomato plants when adding 30, 50, and 70 t ha -1 of biochar. The amount of N fertiliser could be reduced by 24% without a significant loss of tomato yield when the amount of biochar added was over 30 t ha -1 . It was indicated that the yield of tomatoes and the net profits were quadratically related to the application rate of biochar. In the test area, 53 t ha -1 of biochar with 190 kg ha -1 of N and 44.6 t ha -1 of biochar with 190 kg ha -1 of N were calculated to be the best amounts from the perspectives of tomato yield and net profit, respectively. Thus, biochar promotes N transformation by regulating N-related microorganisms; hence, it increases the inorganic N in the roots of the plants, reduces N lost to leaching, and significantly promotes the N absorption of tomatoes. The results in this research are of great significance for the development of management strategies for tomato maintenance, environmental protection, and resource conservation. Keywords: biochar; urease activity; microbial biomass; amoA gene; nirS gene; tomato yield Agronomy 2021, 11, 381. https://doi.org/10.3390/agronomy11020381 https://www.mdpi.com/journal/agronomy