Effect of biochar application depths on leaching of nitrate and crop growth

Abundant use of chemical fertilizer causes reactive nitrogen load to the environment. Leaching of reactive nitrogen occurs mainly in nitrate (NO₃−N) form, which moves into groundwater and other water bodies causing human health hazard and harming ecosystems, especially in tropical islands. Biochar application, which is known as a measure of carbon sequestration, can mitigate the leaching of NO₃−N. We have accumulated knowledge regarding the effective application rate of biochar. However, the information on the effect of biochar application depth remains unclear, although it would affect the leaching of NO₃−N and crop growth. The objective of this study was to evaluate the effect of biochar application depth on the leaching of NO₃−N and crop growth. Using biochar made from bagasse, which is a major organic waste in tropical islands, we conducted a pipe experiment with upland rice (NERICA). We set four treatments: control (no biochar application); surface application (0−5 cm); plow layer application (0−30 cm); subsurface application (25−30 cm). Regarding leaching of NO₃−N, the result under surface application showed a 12% decrease, while that under plow layer application showed an 11% increase against that under the control. Whereas the leaching of NO₃−N was the same under subsurface application as that under the control. Total nitrogen uptake by crop was the highest under surface application, whereas those under plow layer and subsurface applications were smaller than those under the control. By comparing the leaching of NO₃−N with the total nitrogen in the root, we obtained a clear relationship that the higher the total nitrogen in the root was, the lesser the leaching became. The result of the matric potential head in each pipe revealed that soil water condition was stressless for crops under the surface application. On the other hand, dry stress occurred more frequently under plow layer and subsurface applications. These results indicated that, depending on biochar application depth, soil water stress conditions differed and affected root growth positively/negatively. Consequently, crop growth and the leaching of NO₃−N were changed. The surface application can be considered as an effective application, which mitigates the leaching and promotes crop growth simultaneously. We believe the finding of this study encourages the establishment of sustainable agriculture.