4.3 Effects of increasing stand ages on ecosystem P dynamics
Increasing stand ages increased the P concentrations in litter and
sustained a stable P concentration in the leaves of C. hystrixplantations, which resulted in declining N:P ratios in litter and
plants. Stable leaf P concentrations and increasing litter P
concentrations in C. hystrix plantations under increasing stand
ages led to decreased P resorption (Figure S1), and lower P resorption
indicated that the leaf P concentrations met the growth requirements forC. hystrix (He et al., 2011; Du et al., 2020). Plant available P
concentrations mainly result from the decomposition of organic P in
tropical forests (Crews et al., 1995; Lie et al., 2019). The enhanced
litter biomass and litter P concentrations caused by increasing stand
ages lead to higher total P concentrations in the soil (Jara et al.,
2009; Martinez–Garza et al., 2016), which led to stable P
concentrations in the leaves of C. hystrix (Huang et al., 2013).
In addition, C. hystrix requires less P than most other species
in tropical China. Huang et al. (2012) measured 71 species in a monsoon
evergreen broad-leaved forest that had been well protected for more than
400 years in southern China and found that the leaf P concentrations of
most species were higher than that of C. hystrix .
In the present study, it was found that increased soil total P
concentrations and MBP rather than plant P resorption provided a higher
P supply to plants with increasing stand ages. Increased MBP in the top
soil layer with increasing stand ages may be attributable to increased
litter P concentrations (Zhao et al., 2009; Bai et al., 2021). Lower AP
activity in the deep layer (20–30 cm) suggested that P limitation was
decreased under increasing stand ages in C. hystrix plantations
(Feng et al., 2017). It was found that the soil available P
concentrations in the top layer (0–10 cm) were lowered under increasing
stand ages, which may be related to enhanced leaching (Tiecher et al.,
2020) and P uptake by fine roots (Wang et al., 2021). In addition,
reduced AP activity in C. hystrix plantations induced by
increasing stand ages may contribute to decreased soil available P
concentrations in the top layer (Wang et al., 2019). Many studies have
found that soil total P concentrations are lower in older forests than
in younger forests (Feng et al., 2017; Chen et al., 2018; Yang et al.,
2021), which may be driven by higher P accumulation in plants as a
result of increasing plant growth (Kitayama et al., 2000; Vitousek et
al., 2010; See et al., 2015). P limitations in subtropical forests are
exacerbated by increasing stand ages due to declining total and
available P concentrations in the soil, as demonstrated by other studies
(Kitayama et al., 2000; See et al., 2015). In contrast, the present
study found that increasing stand ages led to higher total P
concentrations in the soil of C. hystrix plantations, which may
be related to the lower P requirement for maintaining the growth ofC. hystrix (Huang et al., 2012). Crews et al. (1995) and Frizano
et al. (2002) showed that soil P concentrations increased with stand
ages, which might be associated with increasing P accumulation in plant
biomass and due to litter returns to the soil (Vitousek et al., 2010).