Differential responses of needle and branch order-based root decay to nitrogen addition: dominant effects of acid-unhydrolyzable residue and microbial enzymes
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Background and aims
Both chemical differences between foliage and different orders of fine roots and their contrasting decomposing microenvironments may affect their decomposition. However, little is known about how foliage and branch order-based root decomposition responds to increased N availability and the response mechanisms behind.
The effects of different doses of N addition on the decomposition of needles and order-based roots of Pinus elliottii (slash pine) were monitored using the litterbag method for 524 days in a subtropical slash pine plantation in south China. The acid-unhydrolyzable residue (AUR) concentration and microbial extracellular enzymatic activities (EEA) in decomposing needles and roots were also determined.
Our results indicate that the responses of needle and order-based root decomposition were N-dose-specific. The decomposition of both needles and lower-order roots was inhibited under the high N dose rate. The retarded decomposition of lower-order roots could be explained more by the increased binding of AUR to inorganic N ions, while the retarded decomposition of needles could be explained more by the reduced microbial EEA. Further, in contrast to lower-order roots, N addition had no effect on the decomposition of higher-order roots.
We conclude that the decomposition of foliage and fine roots may fail to mirror each other at ambient conditions or in response to N deposition due to their contrasting decomposition microenvironments and tissue chemistry. Given the differential effects of N addition on order-based roots, our findings highlight the need to consider the tissue chemistry heterogeneity within branching fine root systems when predicting the responses of root decomposition to N loading.
KeywordsDecomposition Fine root Foliage Microorganism Nitrogen deposition
This research is financially supported by the grants from the National Natural Science Foundation of China (No. 31130009) and the National Key Project of Scientific and Technical Supporting Program (No. 2013BAC03B03). The authors acknowledge the contributions of the anonymous reviewers.
Compliance with ethical standards
The authors declare no conflict of interest. All authors have been personally and actively involved in this work and have consented to this submission.
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