Interactive effects of sea-level rise and nitrogen enrichment on the decay of different plant residues in an oligohaline estuarine marsh

Sea-level rise (SLR) and nitrogen (N) enrichment are two critical issues that affect coastal marshes. They affect organic matter decomposition in soils and, thus, affect the marshes’ ability to sequester carbon. However, the impacts of SLR and N enrichment, especially their interactive effects, on the decomposition of different quality plant residues in soils remains unclear. Using in-situ weirs and litterbags, we examined the effects of SLR (three times the natural flooding duration), N enrichment (48 g N m−2 yr−1), and the combination of SLR and N enrichment, on the decomposition of buried Cyperus malaccensis shoot, and root and rhizome (R&R) residues over 549 days in the Minjiang Estuary, China. Individual SLR and N enrichment treatments significantly promoted shoot decomposition by promoting plant growth, but not for R&Rs because of their inability to increase porewater NH4+ levels. Interestingly, combined SLR and N enrichment significantly promoted both shoot and R&R decomposition. Moreover, we observed significant interactive effects between SLR and N enrichment on the decay of both residues, with antagonistic and synergistic effects on the decay of shoots and R&Rs, respectively. Although SLR or N enrichment, or their combined effect, affected plant residue decomposition differently, substrate quality appeared to be the chief factor governing organic matter decomposition. Overall, shoot decay rates (0.0023–0.0048 d−1) were significantly greater than those for R&Rs (0.0026–0.0031 d−1) and more sensitive to SLR, N enrichment, and combined treatments. Our results indicate that estimating the combined effect of SLR and N enrichment based on the addition of individual effects is often inaccurate and that substrate quality directs deviance. This study highlights the importance of quantifying the interactive effects of multiple global change factors and distinguishing substrate quality.