Spatial patterns and determinants of nitrogen composition in the trunk xylem sap of tree species from tropical to temperate forests

Trunk xylem sap nitrogen (N) composition can provide key information for interpreting tree N transport stra-tegies in a changing environment. However, there remains poorly explored to properly depict the spatial patterns and determinants of the xylem sap N composition in different tree species across climate zones. Here, we analyzed the trunk xylem sap N of dominant tree species from tropical to temperate forests in eastern China. The random forest (RF) approach was used to quantify the relative importance of biotic and abiotic variables in driving the spatial patterns of the trunk xylem sap N composition. The concentration ranges of total N (TN), ammonium (NH4+) and nitrate (NO3-) in the trunk xylem sap were 0.37-37.23, 0.01-2.60 and 0.01-0.49 mM, respectively. Dissolved organic nitrogen (DON) was the main component of TN in the xylem sap of forest trees, accounting for 83.4 +/- 3.6%, followed by NH4+ (11.8 +/- 3.2%) and NO3- (4.8 +/- 1.3%). In addition, the TN and NH4+ concentrations increased from the tropical rainforest and then decreased to the northern temperate forest, with a peak in the subtropical deciduous forest in the middle-latitude climatic transition zone. The NO3- con-centration in the temperate forest was higher than that in other forests, regardless of coniferous and broad-leaf forests. Tree species was the key factor affecting the spatial TN patterns, whereas soil pH primarily and conversely determined the spatial NH4+ and NO3-; patterns. Together, tree characteristics and soil properties were the most important factors affecting the spatial patterns of xylem sap N composition. These findings provide new insights into N cycling within forest trees and have implications for better understanding the adaptation to global changes in forest ecosystems.

Automated summary

This study reveals the spatial patterns and determinants of trunk xylem sap nitrogen composition in different tree species across climate zones. Tree species and soil properties were found to be the key factors affecting the spatial patterns of nitrogen composition.