Abstract
This study aimed to investigate the effects of long-term nitrogen fertilization on non-structural carbohydrates (NSC) and nitrogen (N) status and their interaction in mature trees at the whole-tree scale. Ten g N m−2 yr−1 of ammonium nitrate fertilizer were applied to 26-year-old Larix gmelinii Rupr. (larch) and Fraxinus mandschurica Rupr. (ash) trees in Northeastern China from 2002 to 2012. NSC, total carbon (C) and total N concentrations in different compartments were examined. For both species, concentrations of NSC and their components (soluble sugars and starch) tended to increase in aboveground organs but decrease in fine roots following N fertilization, with significant (p < 0.05) changes only observed in ash stems and larch roots. N fertilization increased N concentrations and decreased the C:N ratio in all organs, especially in foliage and roots, while the effects of fertilization on total C concentrations varied with tree species and organs. Concentrations of NSC (mainly reflected in soluble sugar) were generally negatively correlated with N concentration in fine roots but positively related to N concentration in aboveground woody organs in both control and fertilized treatments. However, fertilization strengthened this correlation in fine roots and weakened this relationship in aboveground organs. This study provides a decade-long insight into the effect of currently increasing N deposition on tree growth and function.
Highlights
Introduction iationsCarbon (C) storage in trees is key for biogeochemical processes of terrestrial ecosystems [1,2]
C allocation is mediated by non-structural carbohydrates (NSC), which are the sum of starch and low-molecular soluble sugars [1,3]
Except for significant decreases of soluble sugar concentration in the bark and starch concentration in the pith for larch, nonstructural carbohydrates (NSC) concentrations in other compartments generally increased in both species in response to N fertilization
Summary
Carbon (C) storage in trees is key for biogeochemical processes of terrestrial ecosystems [1,2]. NSC are the most important components in C storage in quantitative terms, because they are building blocks for the synthesis of important compounds (such as proteins and amino acids), but they can serve as the energy source for metabolic activities such as growth, nutrient assimilation and other life maintaining processes [1,4,5]. NSC play a central role in tree resistance to environmental changes [6,7] because starch, the temporarily immobile forms of NSC, can be mobilized and re-allocated to metabolic processes when plants encounter C deficits (C demand exceeds concurrent C acquisition by photosynthesis) [4,8,9,10].
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