Soils with two different development stages have responded differently to ecological restoration practices in the present study. This is evident for example in changes of soil pH, mineral nitrogen, and microbial biomass C and P dynamics. Valuable comparisons are provided in the present study: (i) restoration site and reference mature forest indicate the differences between existing and past soil forming pathways; and (ii) differences between restoration site and unplanted grassland reveal the effects of ecological restoration on soil forming pathways. Ecological restoration is thought to re - supply ecosystem services by accelerating or reconstructing ecosystem succession (Prach and Walker, 2011). It has been argued that integration of soil pedogenesis into ecological restoration studies does help to predict future soil development of restored ecosystems (Moorhead, 2015). In addition, it was found that there were diverse responses to ecological restoration on soils developed from 3 different types of parent materials in a 9,000 ha degraded landscape (Abella et al., 2015). Therefore, knowledge of soil pedogenesis definitely needs to be considered when undertaking rehabilitation or resto ration of ecosystem across landscape consisting contrasting soils.
In terms of nutrient dynamics along a long-term soil chronosequence, it is known that soil nutrient dynamics transformed from N limiting at early stage to P limiting at later stage s of ecosystem development, will eventually lead to ecosystem retrogression if no major disturbance occurs (Peltzer et al., 2010). An accompanying increase in substrate N:P ratio occurs during this long-term process. The end-point is a reduction of standing plant biomass and ecosystem productivity in old regressive forest ecosystems (Wardle et al., 2004b). This soil nutrient transformation obviously has critical implications when ecological restoration practices are undertaken on large -scale landscapes consisting of distinct stages of soil development (e.g. newly developed versus highly weathered soils). Selecting appropriate native plant species that fit the corresponding soil nutrient status could be crucial for future native vegetation establishment and ecological restoration success. Vegetation composition changes along a long-term chronosequence: from angiosperm dominated to conifer dominated temperate rain forest in Franz Josef post glacial chronosequence (Richardson et al., 2004); and from conifer dominated to mixture of conifer-angiosperm temperate rain forest in Haast chronosequence (Turner et al., 2012); and in Metrosideros polymorpha (Myrtaceae) dominated tropical rain forest but with changes of other genera of trees and shrubs in Hawaii island chronosequence (Crews et al., 1995). These studies all suggested that plant community changes were mainly driven by soil nutrient
dynamics alongside long-term soil development. Pioneer N-fixing species are critical in the early stages, while plants adjusted to P-poor soils (e.g. mycorrhizal symbiosis) perform well in the late stages. Nevertheless, the extrapolation of the established knowledge is that soil P stocks in the present study site will face continuous losses as soil and ecosystem age under this super-humid climate, including in the restoration plots. However, unlike the initial proposed model by Walker and Syers (1976), a later study in Yang and Post (2011) indicated that soil labile organic P and secondary mineral P (Fe/Al) (non- occluded P in Walker and Syers’ model) continue to show their dynamics and proportional importance throughout different weathering stages. In addition, the accumulation of organic P during soil and ecosystem development consist of a variety of compounds which are subject to different availability to biochemical mineralization (Turner et al., 2007). These two soil P factions provide an investment for future ecosystem development and maintenance, and they will be re -available for uptake by plant via special P-acquisition strategies when the ecosystem enters P-limiting stage (Lang et al., 2016; Rosling et al., 2016). This is also applicable for the restored ecosystem. Furthermore, considering the bio- availability of soil P during ecological restoration, better soil nutrient management could potentially enhance ecosystem productivity and soil C sequestration, as well as minimize impacts on nearby water environment via reduction of N and P lost (Parfitt et al., 2005).
5.5
Conclusions
(1) Soil biogeochemistry, including soil P dynamics, responded differently to the trajectory of ecological restoration on old and young soils.
(2) Dynamics of soil organic matter (accumulation and decomposition) and increasing demand of N from fast-growing plants early in the restoration result in a rapid nutrient mineralization. (3) The results support Walker and Syer’s conceptual model of a loss of soil total P, increase of
occluded P and increasing importance of soil organic P as soils age.
(4) The present study is based on a relatively short-term soil chronosequence limiting the capacity to interpret soil nutrients dynamics in a longer-term ecosystem and period soil development. (5) The importance of incorporating knowledge of soil pedogenesis into ecological restoration was evident in the project for PCRP. This could be more critical when extrapolated from the PCRP site into larger-scale landscape restoration consisting of different soil types and highly weathered soils.
103 (6) The importance of organic P, progressive changes P fractions and loss of total soil P, help us to understand how to sustainably manage the restoration process and predict the fate of soil development of restored ecosystems. There is a diverse response to ecological restoration along the gradient.
(7) Based on the findings from the short-term soil chronosequence and ecological restoration in this study, not only soil pH, extent of soil organic matter accumulation, and N and P availability should be routinely monitored, but also soil parameters that inform the stage of soil development either soil P status (total P, organic P, secondary mineral P and primary mineral P) or soil Fe/Al minerals. A background study of a proposed restoration site should at least include a proper survey of soil types and soil descriptions, which is important for replanting species selection and ongoing vegetation development.
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