Does nitrogen fertilization impact nonstructural carbohydrate storage in evergreen Podocarpus macrophyllus saplings?

Abstract

Nonstructural carbon (NSC), which represents the relationship between the carbon source and carbon sink, is an important factor that reflects the functions and performance of a tree. However, little is known regarding the time-series responses of NSC storage in evergreen species to different nitrogen (N) fertilization regimes. This study, which was based on a pot experiment, examined the response of the NSC (soluble sugars and starch) storage to different N addition intensities [light N addition (LN): 6.5 g N m−2 a−1; moderate N addition (MN): 13.0 g N m−2 a−1; and heavy N addition (HN): 26.0 g N m−2 a−1)] in saplings of the evergreen species Podocarpus macrophyllus. Our results showed that the net photosynthetic rate (Pn) under MN was significantly higher than that under LN, but was comparable to that under HN. Moreover, saplings subject to MN had a significant higher leaf biomass than that to LN and HN. These results indicated that the C supply via photosynthesis under MN was greater than that under LN and HN. The NSCs reserve under MN was considerable with that under LN, which suggested that saplings in MN group consumed higher and stored lower properties of NSCs than those in LN group. However, saplings under HN stored higher properties of NSCs than those under MN considering that no difference in NSCs pools was found between the two treatments. The leaf N concentrations were found in the increasing sequence of LN < MN < HN, whilst the leaf chlorophyll concentration under HN was obviously lower than that under MN. The growth rate under MN was higher than that under LN and HN. We concluded that the NSCs allocation between consumption and reserve in P. macrophyllus saplings depended on soil N availability, and an excessive N addition to soil favors the storage rather than the consumption of NSCs by plants.