Plant N:P stoichiometric-nonstructural carbohydrate linkages are age-dependent in extremely overfertilized apple production systems

Background and aims Plant N:P stoichiometry and nonstructural carbohydrates (NSCs) can characterize ecosystem responses to global changes, yet their patterns, drivers, and interrelations in fertilized forest ecosystems are poorly documented. Background and aims We conducted large-scale sampling of apple orchard system at five stand ages (8-28 years) in six sites along a precipitation gradient (355-675 mm yr(-1)) on China's Loess Plateau. Results Foliar N, P, and N:P ratios had divergent responses to stand age among sites but responses were uniformly quadratically correlated with precipitation, shifting from N-to P-limitation as precipitation increased. Sugar, starch, and NSCs responded consistently to precipitation across stand ages, whereas responses to stand age differed among sites. Foliar starch and NSCs were highest in rain-rich sites (> 550 mm), whereas branch values were highest in < 450 mm sites. Age-induced changes in starch, NSCs, and sugar:starch ratios varied greatly among sites. Precipitation, fertilization, and soil properties coregulated NSCs, N:P and sugar:starch, with their contributions age-dependent. PLS-PM illustrated that plant N:P ratios and NSCs linkages were age-dependent. Foliar N:P stoichiometry directly modulated leaf NSCs and sugar:starch ratios in young orchards, directly regulated leaf NSCs but indirectly affected branch NSCs and sugar:starch by mediating branch N:P ratio in mature orchards, and directly regulated NSCs and sugar:starch in both leaf and branch in old orchards. Conclusions Overall, this study systematically deciphered the patterns and drivers of plant N:P stoichiometry and NSCs and their interrelations in fertilized apple orchard system, thereby better understanding plant survial-strategies to concurrent climate change and anthropogenic activities.