**3. Results**

Tree species, organ type, and their interaction significantly affected DOC production, DTN production, DTP production, DOC:DTN ratio, and DOC:DTP ratio (Table 2). For each tree species, leaves leached greater amounts of DOC, DTN, and DTP than twigs (Table 3). Broadleaf tree (*L. formosana* and *S. superba*) leaves had greater DOC, DTN, and DTP productions than coniferous tree (*P. massoniana* and *P. elliottii*) leaves, whereas broadleaf tree twigs had lower DOC production than coniferous tree twigs (Table 3). Plantleached DOC, DTN, and DTP productions showed a negative relationship with the initial tissue C:N and C:P ratios, but exhibited a positive relationship with polyphenols, soluble sugars, and starch (Table A1). In the leachates, tree leaves often had higher DOC:DTN and DOC:DTP ratios than tree twigs (Table 3). In addition, broadleaf tree leaves had greater DOC:DTN ratio in the leachates than coniferous tree leaves, although there was no significant difference between broadleaf and coniferous tree twigs (Table 3).

**Table 2.** Results (*F*-values) of linear mixed models on the effects of tree species (S), organ type (O), and their interaction on tree tissue-leached DOM properties and biodegradability in subtropical plantations in southern China.


DOC, dissolved organic carbon; DTN, dissolved total nitrogen; DTP, dissolved total phosphorus; DOM, dissolved organic matter. \*\*\*, *p* < 0.001.

**Table 3.** Dissolved organic C (DOC), dissolved total N (DTN), and dissolved total P (DTP) productions, and stoichiometric ratios of tree tissue leachates in subtropical plantations in southern China.


DOC, dissolved organic carbon; DTN, dissolved total nitrogen; DTP, dissolved total phosphorus. The data in the parentheses are the standard errors of the means (*n* = 6). In the same column, the different uppercase letters indicate the significant differences among the tree leaf treatments (*p* < 0.05), and the lowercase letters indicate the significant differences among the tree twig treatments (*p* < 0.05).

Tree species, organ type, and their interaction produced significant effects on SUVA254 and SUVA350 values (Table 2). Irrespective of tree species, leaves had higher SUVA254 and SUVA350 values in the leachates than twigs (Figure 1). For both leaves and twigs, broadleaf trees had greater SUVA254 and SUVA350 values in the leachates than coniferous trees (Figure 1).

Tree species and organ type independently produced a significant effect on DOM biodegradability (Table 2). Regardless of tree species, twig-leached DOM had greater biodegradability than leaf-derived DOM (Figure 1). For both leaves and twigs, broadleaf

trees had lower DOM biodegradability than coniferous trees (Figure 1). In addition, tissueleached DOM biodegradability was negatively related to the initial SUVA254 value, SUVA350 value, and DOC:DTN ratio, but exhibited no significant relationship with DOC:DTP ratio (Figure 2).

**Figure 1.** Optical properties and biodegradability of tree litter-derived dissolved organic matter (DOM) in subtropical plantations in southern China. The error bars are the standard error of the means (*n* = 6). Different uppercase letters indicate the significant differences among the tree leaf treatments (*p* < 0.05), and different lowercase letters indicate the significant differences among the tree twig treatments (*p* < 0.05). For each tree species, \*, \*\*, and \*\*\* indicate the significant differences between leaf and twig at the levels of *p* < 0.05, *p* < 0.01, and *p* < 0.001, respectively.

**Figure 2.** Relationships between plant-leached dissolved organic matter (DOM) biodegradability and the initial properties in subtropical plantations in southern China.
