4.1 Nutrient limitations and productivity in C. hystrixplantations
Consistent with the hypothesis, increasing stand ages reduced plantation
productivity, which might have been caused by increased N consumption
rather than possible P limitations. As forest stands age, P increasingly
accumulates in aboveground biomass or in the soil in recalcitrant
organic and inorganic forms, which further decreases the biological
availability of P (Vitousek & Farrington, 1997; Lambers et al., 2008;
Wardle et al., 2008). During forest succession, species use different
strategies to acquire P, including resorption from foliage or/and
absorption from the soil via rhizosphere processes.
The alternations in nutrient concentrations and stoichiometry caused by
increasing stand ages alter forest properties and functions, such as
lowering the capacity and rate of C fixation and productivity in
plantations (Fan et al., 2015, Ma et al., 2017). Therefore, knowing the
effects of increasing stand ages on nutrient status would be helpful to
take measures to maintain ecosystem function in plantations, such as
adding fertilizer (Waterworth et al., 2007; Markewitz et al., 2012;
Zhang et al., 2022). Most studies have shown that P limitation is
strengthened by increasing stand ages in subtropical plantations due to
accumulation in plants (Fan et al., 2015; Yang et al., 2021).
Additionally, lower N:P ratios have been found in older C.
hystrix plantations (leaf N:P < 20) than younger ones (leaf
N:P > 20), suggesting that increasing stand ages increase N
limitations but reduce P limitations in C. hystrix plantations
(Güsewell, 2004; Bui and Henderson, 2013; Liu et al., 2013; Yang et al.,
2021). Overall, increasing stand ages intensify N limitations but not P
limitations in C. hystrix plantations, and reduced leaf N
concentrations may further limit the growth of C. hystrix . Thus,
N fertilizer should be added rather than P fertilizer in C.
hystrix plantations to ensure their sustainable ecosystem function in
the future.
4.2 Effects of
increasing stand ages on ecosystem Ndynamics
In the present study, it was found that increasing stand ages decreased
the N concentration in leaves, which may have been due to N dilution as
a result of enhanced C. hystrix growth (Cregger et al., 2014;
Hayes et al., 2014). Additionally, lower litter N concentrations
resulted in declining soil available N concentrations, which further
decreased the N concentrations in leaves (Feng et al., 2018; Feng et
al., 2019b). Higher C:N ratios in leaves and litter should be due to
increasing C concentrations and decreasing N concentrations (Li et al.,
2013).
Although higher litter inputs induced by increasing stand ages were
found in the present study, reduced litter N concentrations might lead
to declining soil available N concentrations in C. hystrixplantations (Chen et al., 2017; Feng et al., 2019b).
In contrast to the findings of the
present study, several studies reported that the soil available N
concentrations were enhanced with increasing stand ages due to higher
litter inputs and the accelerated decomposition of organic matter
(Vitousek et al., 2010; Chen et al., 2017;
Feng et al., 2019b). Higher PhOx,
NAG, and CBH activities in the soil would help decompose organic matter
to increase the soil available N concentrations (Lucas–Borja et al.,
2016; Feng et al., 2017; Yang et al., 2021). MBN in the top layer
was reduced with increasing stand
ages, which may have been driven by declining litter N concentrations
(Feng et al., 2019b). The reduced soil available N concentrations and
MBN caused by increasing stand ages may not sustain the enhanced growth
of C. hystrix (Feng et al., 2019a). The soil C:N and MBC:MBN
ratios were enhanced with increasing stand ages, indicating that the
mineralization rate of organic matter was reduced and that N in the soil
might become a limiting element for plant growth (Yang et al., 2021). In
contrast, previous studies have demonstrated that N may be not a
limiting factor in subtropical forests due to the high level of N
deposition (Reich and Oleksyn, 2004; Huang et al., 2013;
Yang et al., 2021).