Biochar application alleviates unbalanced nutrient uptake caused by N deposition in Torreya grandis trees and seedlings

Foliar nitrogen: phosphorus (N:P) ratio is a valuable indicator of nutrient limitation in forest ecosystems. The ratio is currently dramatically affected by the increasing anthropogenic N deposition in China. Biochar, a soil amendment, has profound benefits for improving soil N and P availability. However, little is known about N and P uptake and nutrient balance of plants growing on biochar-amended soils under N deposition. Here, we conducted a two-year N addition and biochar application factorial experiment with Torreya grandis trees and seedlings, and the responses of concentrations of foliar N and P and the foliar N:P ratio were evaluated. N addition alone increased foliar N concentration, but did not change foliar P concentration, and thus increased foliar N:P ratio. Biochar application alone increased concentrations of both foliar N and P but did not cause any change in the N:P ratio. Most importantly, when applying N and biochar together, the concentration of foliar P increased and the foliar N:P ratio decreased, relative to each N treatment alone. This novel result suggests that biochar application can serve as an effective forest management tool, which can not only enhance the availability of N and P nutrients, but also alleviate the imbalanced uptake of nutrients indicated by the high foliar N:P ratios under increasing N deposition.