Examination of the negative correlation between leaf δ15N and the N:P ratio across a northeast-southwest transect in China

Abstract

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 (δ¹⁵N) can provide another proxy for nutrient limitations along with the N:P ratio, but the negative relationships between N:P and δ¹⁵N were mainly limited to fertilization experiments. It will obviously benefit the study of the nature of nutrient limitations if the relationship could be explained more generally. We analyzed leaf δ¹⁵N, N, and P contents across a northeast-southwest transect in China. Leaf δ¹⁵N 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 δ¹⁵N in indicating the shift of nutrient limitations across the whole N:P range still requires more validated field investigations. Notably, negative relationships between δ¹⁵N 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 δ¹⁵N 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 δ¹⁵N in reflecting shifts in nutrient limitations, depending on the plant nutrient limitation range, is general. We examined the relationships between leaf δ¹⁵N and the N:P ratio across an extensive transect, providing references for the widespread use of leaf δ¹⁵N in reflecting shifts in nutrient limitation.

Keywords: N:P ratio; Nitrogen isotopes; Nutrient limitation; Nutrient stoichiometry.