Biochar addition reduced net N mineralization of a coastal wetland soil in the Yellow River Delta, China

• Biochar alone decreased N mineralization due to higher C:N ratio.
• Biochar addition alone decreased urease activity.
• Combined addition of biochar and reed stem had little effect on N mineralization.
• Adding N to biochar could be a promising strategy for remediating degraded soil.

Soil degradation has seriously threatened global soil and food security. Biochar application is a promising management option to remediate the degraded soils. However, extensive application of biochar is limited by lack of understanding the effects of biochar on nitrogen (N) mineralization in the degraded coastal wetland soils. Therefore, the individual or combined effects of biochar, reed stem and urea fertilizer application on N mineralization in a coastal wetland soil were investigated using a 150-days incubation experiment, and the underlying mechanisms were discussed. Biochar addition reduced net N mineralization, but no significant effect was observed between the treatments with different addition rates. The combined addition of the biochar and reed stem had little effect on net N mineralization because of the higher C:N ratio (45.5–49.3). However, biochar addition in combination with the urea fertilizer initially decreased net N mineralization, but slightly increased it later on. The biochar-induced reduction of net N mineralization was mainly ascribed to the increased C:N ratio and decreased urease activity. Therefore, adding N fertilizer to the biochar to enhance the delivery of N prior to its incorporation into soil, which may avoid N immobilization due to N deficiency, could be an effective strategy for remediating the degraded coastal wetland soils.

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