**4. Discussion**

The interspecific patterns of leaf- and twig-leached DOM production differed between broadleaf and coniferous trees. Specifically, broadleaf trees produced greater amounts of DOC and soluble nutrients in the leaf leachates but had lower DOC quantity in the twig leachates than coniferous trees (Table 3). Previous studies also observed greater amounts of DOM for broadleaf tree leaf litter than for coniferous tree leaf litter in temperate and subtropical forests [16,19,35]. In general, plant-leached DOM production is controlled by litter physical and chemical traits such as non-structural carbohydrates and lignin concentration [15,16,28,36]. Thus, these inconsistent results would be explained by the differences in interspecific patterns of leaf and twig traits between broadleaf and coniferous trees. These findings sugges<sup>t</sup> that the interspecific patterns of fresh plant tissue-leached DOM production between broadleaf and coniferous trees is largely dependent on organ type in subtropical plantations.

In this study, the greater DOM production of broadleaf trees relative to coniferous trees would be explained by the following mechanisms. First, broadleaf trees have higher soluble sugars, starch, and polyphenols concentrations in fresh leaves than coniferous trees (Table 1). Second, compared with coniferous trees, broadleaf trees had greater specific leaf area and water holding capacity [37], which would enhance leaching of soluble compounds from fresh leaves. However, coniferous tree twigs had lower tissue density than broadleaf tree twigs (Table 1), which would favor the entry of waters into plant tissues [14]. Accordingly, DOC production was greater for coniferous tree twigs than for broadleaf tree twigs.

During leaching, broadleaf tree leaves had greater DTN and DTP productions than coniferous tree leaves, despite no significant difference between broadleaf and coniferous tree twigs (Table 3). These inconsistent interspecific patterns would be explained by the differences in nutrient concentrations in fresh tissues between broadleaf and coniferous trees. In forest soils, the soluble nutrient leached from plant litter is the primary nutrient source for microbial growth and activity [3,4,38]. Accordingly, the interspecific difference of fresh tissue-leached DTN and DTP productions would partly help explain the spatial variations of soil microbial-mediated ecological processes between broadleaf and coniferous tree plantations in subtropical regions.

Tree fresh tissue-leached DOC, DTN, and DTP productions negatively correlated with both C:N ratio and C:P ratio (Table A1). This result was contrary to the previous studies which did not observe any significant relationship between litter-leached DOM production and C:N:P stoichiometry [35,39]. These inconsistent findings indicate the contrasting roles of C:N:P stoichiometric ratios in regulating DOM production between fresh and senesced tree tissues. In plant tissues, the higher C:N and C:P ratios represent the lower metabolic activities and associated smaller amounts of non-structural compounds and nutrients [40,41]. In this study, plant tissues with higher C:N and C:P ratios were observed to have lower soluble sugars and starch (Table 1). Thus, fresh tree tissue-leached DOM production increased with decreasing C:N and C:P ratios.

For both leaves and twigs, broadleaf trees had greater SUVA254 and SUVA350 values in the leachates than coniferous trees (Figure 1), indicating that broadleaf tree tissue-leached DOM had greater amounts of aromatic compounds than coniferous tree tissue-leached DOM [26,27]. Moreover, both SUVA254 and SUVA350 values correlated negatively with C:N and C:P ratios, but correlated positively with soluble sugars, starch, and polyphenols (Table A1). Hagedorn and Machwitz [15] also found that the molar UV absorptivity at 285 nm of litter-derived DOM exhibited negative relationship with C:N and lignin:N ratios. In this study, broadleaf trees had higher soluble organic compounds in the leaves and twigs than coniferous trees (Table 1), and thus produced DOM with greater aromaticity in subtropical plantations.

Notably, broadleaf trees had lower fresh leaf- and twig-leached DOM biodegradability than coniferous trees during 28-day incubation (Figure 1). This interspecific pattern of fresh tissue-leached DOM biodegradability was contrary to the previous studies reporting greater senesced leaf-leached DOM biodegradability for broadleaf trees than for coniferous trees

in forests [18,19,42]. These inconsistent results indicate that the interspecific variations of plant-leached DOM biodegradability differ between fresh and senesced tissues in forests. In general, DOM biodegradability is influenced by DOM aromaticity and C:N:P stoichiometry because microbial activity is limited by the availability of energy and nutrients [17,19,28,43]. The initial SUVA254 value, SUVA350 value, and DOC:DTN ratio were also observed to correlate negatively with DOM biodegradability in this study (Figure 2). In these forests, coniferous tree tissues often had lower aromatic C content and DOC:DTN ratio in the leachates than broadleaf tree tissues (Figure 1). Therefore, coniferous trees had higher fresh tissue-leached DOM biodegradability than broadleaf trees. These results imply that the interspecific variation of fresh tissue-leached DOM biodegradability is co-regulated by C quality and N availability in subtropical plantations.

Tree fresh leaves had greater DOC, DTN, and DTP productions and DOM aromaticity, but had lower DOM biodegradability than tree fresh twigs (Table 3 and Figure 1). Kammer and Hagedorn [44] also found that senescent leaves produced greater quantity of DOC with higher refractory components than senesced twigs in a temperate forest. In these forests, tree fresh leaves had higher soluble sugars and starch contents and lower tissue density than tree fresh twigs (Table 1). In contrast, tree leaves could allocate a greater amount of defensive compounds (i.e., condensed tannins and polyphenols) to resist herbivores than tree twigs due to the relatively higher nutrient concentrations [45]. In this study, higher polyphenol concentration was also observed for tree leaves than tree twigs (Table 1). Therefore, tree fresh leaves produced higher amounts of DOM containing greater aromatic compounds than tree fresh twigs in subtropical plantations. Due to the lower DOM aromaticity and DOC:DTN ratio, fresh tissue-leached DOM biodegradability was higher for tree twigs than for tree leaves in these plantations.

In forests, litter-leached DOM strongly influences soil organic matter formation and stabilization [1,9]. In these plantations, tree fresh leaves produced much greater amounts of DOM with higher biodegradability than tree senesced leaves [19]. These observations imply that tree fresh leaf-leached DOM would promote the accumulation of mineralassociated C fractions via the microbial pathway [9,38]. Considering that the intensity and frequency of extreme weather events exhibit an increasing trend as a result of global climate change [46], our findings highlight the crucial role of plant fresh tissue in regulating C budget in subtropical forest ecosystems.
