Exogenous and endogenous nitrogen differentially affect the decomposition of fine roots of different diameter classes of Mongolian pine in semi-arid northeast China
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Nitrogen (N) addition could affect litter decomposition through its direct effects on soil N availability and indirect effects on initial litter chemistry. The aim of this study was to evaluate the relative contribution of these direct and indirect effects to the decomposition of fine roots with different diameter classes.
A two-year reciprocal replant–transplant field experiment was conducted in a Mongolian pine (Pinus sylvestris var. mongolica) plantation to examine the relative effect of exogenous and endogenous N enrichment induced by N addition (10 g N m−2 yr.−1) on the decomposition of fine roots with different diameter classes: < 0.5 mm (small fine root, SFR) and 0.5–2 mm (large fine root, LFR).
The LFR had significantly higher decomposition rates (k: 0.315–0.397 yr.−1) than the SFR (0.245–0.274 yr.−1) after 2 years of incubation. Exogenous N (i.e., increased soil N availability due to N addition) had no significant effect on the decomposition rates of fine roots, whereas endogenous N (i.e. increased N concentration in litter due to N addition) inhibited and accelerated the decomposition of SFR and LFR, respectively. Endogenous N decreased the net release of N but both endogenous and exogenous N increased the net release of phosphorus (P) from SFR. By contrast, exogenous and endogenous N decreased the net release of N and P from LFR.
Our results suggest that N addition affected fine root decomposition indirectly by changing the chemical traits of fine roots rather than directly through changing soil N availability. Elevated input and decreased net N release of fine roots might be a potential mechanism explaining the increases of total organic carbon and total N in the semi-arid forest soil under N addition. Our study also suggests that SFR may be a more important source of stable soil organic matter relative to LFR.
KeywordsDecomposition Nitrogen addition Fine root Root diameter class Pinus sylvestris var. mongolica
This study was funded by the National Natural Science Foundation of China (31870603, 41877341). We thank Gui-Yan Ai for help with the laboratory work and the CAS Key Laboratory of Forest Ecology and Management for funding Qun Gang’s visit to Canada. We also thank three anonymous reviewers for their comments and suggestions that greatly improved an earlier version of the manuscript.
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