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).