Examination of the negative correlation between leaf 815N and the N:P ratio across a northeast-southwest transect in China

Nitrogen (N) and phosphorus (P) are two crucial limiting mineral elements for terrestrial plants. Although the leaf N:P ratio is extensively used to indicate plant nutrient limitations, the critical N:P ratios cannot be universally applied. Some investigations have suggested that leaf nitrogen isotopes (815N) can provide another proxy for nutrient limitations along with the N:P ratio, but the negative relationships between N:P and 815N were mainly limited to fertilization exper-iments. It will obviously benefit the study of the nature of nutrient limitations if the relationship could be explained more generally. We analyzed leaf 815N, N, and P contents across a northeast-southwest transect in China. Leaf 815N was weakly negatively correlated with leaf N:P ratios for all plants, while there was no correlation between them for various plant groups, including different growth forms, genera, and species across the entire N:P range. This suggests that the use of leaf 815N in indicating the shift of nutrient limitations across the whole N:P range still requires more validated field inves-tigations. Notably, negative relationships between 815N and N:P hold for plants with N:P ratios between 10 and 20 but not for plants with N:P ratios lower than 10 or higher than 20. That is, changes in leaf 815N along with the N:P ratio of plants that are co-limited by N and P can exhibit variations in plant nutrient limitations, whereas plants that are strictly limited by N and P cannot. Moreover, these relationships are not altered by vegetation type, soil type, MAP, or MAT, indicating that the use of leaf 815N in reflecting shifts in nutrient limitations, depending on the plant nutrient limitation range, is general. We examined the relationships between leaf 815N and the N:P ratio across an extensive transect, providing ref-erences for the widespread use of leaf 815N in reflecting shifts in nutrient limitation.

Automated summary

Nitrogen and phosphorus are crucial limiting mineral elements for terrestrial plants. The leaf N:P ratio is commonly used to indicate plant nutrient limitations, but its universal application is limited. Leaf nitrogen isotopes (815N) have been suggested as another indicator, but the negative relationships between N:P and 815N are mainly seen in fertilization experiments. We analyzed leaf 815N, N, and P contents across a transect in China and found weak negative correlations between leaf 815N and N:P ratios for all plants but no correlation for different plant groups. The use of leaf 815N in reflecting shifts in nutrient limitations still requires more field investigations.