Abstract
Differences in nitrogen (N) acquisition patterns between plant species are often reflected in the natural 15N isotope ratios (δ15N) of the plant tissues, however, such differences are poorly understood for co-occurring plants in tropical and subtropical forests. To evaluate species variation in N acquisition traits, we measured leaf N concentration (%N) and δ15N in tree and understory plant species under ambient N deposition (control) and after a decade of N addition at 50 kg N ha−1 yr−1 (N-plots) in an old-growth subtropical forest in southern China. We also measured changes in leaf δ15N after one-year of 15N addition in both the control and N-plots. The results show consistent significant species variation in leaf %N in both control and N-plots, but decadal N addition did not significantly affect leaf %N. Leaf δ15N values were also significantly different among the plant species both in tree and understory layers, and both in control and N-plots, suggesting differences in N acquisition strategies such as variation in N sources and dominant forms of N uptake and dependence on mycorrhizal associations among the co-occurring plant species. Significant differences between the plant species (in both control and N-plots) in changes in leaf δ15N after 15N addition were observed only in the understory plants, indicating difference in access (or use) of deposited N among the plants. Decadal N addition had species-dependent effects on leaf δ15N, suggesting the N acquisition patterns of these plant species are differently affected by N deposition. These results suggest that co-occurring plants in N-rich and subtropical forests vary in their N acquisition traits; these differences need to be accounted for when evaluating the impact of N deposition on N cycling in these ecosystems.
Highlights
Enhanced atmospheric nitrogen (N) deposition into forest ecosystems from increased anthropogenic emissions of reactive N [1] has several cascading negative ecological effects on the forest ecosystems including increased N leaching to waters, soil acidification, increased N gas losses from soil and changes in biodiversity [2]
We found that the two ectomycorrhizal plants (ECM) trees (Syzygium acuminatissimum and Castanopsis chinensis), on average, have significantly lower leaf C:N ratio in both control and N-plots (Table S4) and are more 15 N-enriched than the three arbuscular mycorrhizal plants (AM) trees (Cryptocarya chinensis, Memecylon ligustrifolium and Syzygium rehderianum) in both control and N-plots (Figure S2)
We found that the change in leaf δ15 N was more pronounced in the ECM trees than in the AM trees (Figure S2), as reported by a recent tracer study [70], indicating that the ECM trees may have more access to deposited N than AM trees, which in turn suggests that N acquisition of ECM plants may be more sensitive to N deposition than that of AM trees
Summary
Enhanced atmospheric nitrogen (N) deposition into forest ecosystems from increased anthropogenic emissions of reactive N [1] has several cascading negative ecological effects on the forest ecosystems including increased N leaching to waters, soil acidification, increased N gas losses from soil and changes in biodiversity [2]. Forests 2019, 10, 991 carbon (C) and N cycles, and enhanced N deposition is likely to have rapid and deleterious effects on these ecosystems that are naturally regarded as N-rich [3]. In old-growth forests with multiple co-occurring species in subtropical monsoon climates, the effects of individual species on soil processes are difficult to assess due to the aforementioned species interactions the functional diversity of these ecosystems has an implication for below- and above-ground processes [10]. There is a need to understand how these ecosystems respond to global change factors such increased N deposition, which is a recognized threat to plant diversity [11]
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