Mixing overstory tree- and understory fern-derived dissolved organic carbon produces non-additive effects on biodegradation in subtropical forests of southern China

Background and aims Litter-derived dissolved organic carbon (DOC) biodegradation is critical to forest ecosystem structure and function. However, it is unclear whether mixing overstory and understory litter-derived DOC could produce non-additive effects on biodegradation in forests. Methods We extracted DOC from leaf litter of two coniferous trees, four broadleaf trees, and four understory ferns in subtropical forests and used a laboratory incubation method to measure pure and mixed overstory and understory litter-derived DOC biodegradation. The purposes were to assess the interspecific patterns of litter-derived DOC biodegradation between overstory trees and understory ferns, and then examine the non-additive effects of mixing overstory and understory litter-derived DOC on biodegradation. Results Litter-derived DOC biodegradation of understory ferns was comparable to that of overstory broadleaf trees, but higher than that of overstory coniferous trees. Mixing overstory and understory litter-derived DOC produced non-additive effects on biodegradation in all 24 DOC mixtures. Specifically, synergistic effects were observed in 22 DOC mixtures, and antagonistic effects occurred in the other two DOC mixtures. Non-additive effects on DOC biodegradation were positively related to the interspecific differences in DOC:dissolved total phosphorus ratio and DOC aromaticity, respectively, and exhibited no significant relationship with the interspecific differences in DOC:dissolved total nitrogen ratio. Conclusions Non-additive effects are common during microbial degradation of overstory and understory litter-derived DOC mixtures, and trait dissimilarity among component DOC sources drives the directions and magnitudes of non-additive effects on biodegradation in subtropical forests.