An investigation of soil carbon fluxes and pools in the thermo-sequence of Mt. Taranaki forest : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Manawatu, New Zealand

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Understanding the relationship between temperature and soil carbon (C) pools and fluxes is a key aspect in determining the feedback of the global C balance to rising temperature. The overall objective of this thesis was to investigate the influence of rising temperature in the net change of soil C stocks and fluxes in a thermo-sequence of Mt. Taranaki and explore the mechanisms underlying this change if any. Taranaki region has a ca. 3.2°C mean annual temperature (MAT) gradient with identical parent material, moisture (constant plant-available moisture), and vegetation type. The soil type under this study is alu-andic Andosol, which is the mineral soil group with the largest C content worldwide and characterised by its abundance in aluminium (Al)-organic matter (OM) complexes (e.g., Al³⁺-OM and allophane-OM complexes). Yet, there is considerable uncertainty as to how rising temperatures will affect the stability of organo-mineral complexes and their formation. Together with the unique thermo-sequence available across the Taranaki slope, they offer an excellent benchmark for investigating the potential responses of soil C storage to long-term warming. Firstly, we hypothesised that: (i) temperature rise influences the forms of reactive Al (i.e., short range order (SRO) constituents vs. organo-Al complexes), with a greater abundance of SRO constituents at warmer elevation sites as opposed to organo-Al complexes at colder elevation sites, where the weathering rate is slower; (ii) in warmer conditions, microbial-derived C is favoured, while in cooler conditions, plant fingerprints are more evident; (iii) as a result, the C preservation mechanism along the transect differs, with SRO constituents (and microbial-derived C) being more prevalent at warmer elevation sites and Al cations (and plant-derived C) being more abundant at colder elevation sites. To reveal how climatic and geochemical properties regulate soil C preservation, we conducted a field survey to investigate the changes in: (i) soil geochemical properties including; soil pH, reactive Al and Fe (i.e., SRO constituents and organo-Al complexes); (ii) total C content, stocks, and fractions, as well as composition of OM along the gradient, in which soils at four elevation sites were sampled down to 85 cm depth. Four sampling sites were selected, with elevations ranging from 512 to 1024 m above sea level (asl) and and a mean annual temperature (MAT) of 7.3, 8.2, 9.1, and 10.5 °C ( referred to as T7, T8, T9, and T10, respectively). The results showed that: (i) at colder elevation sites (T7 and T8), soil profiles were richer in well-preserved plant material and in organo-Al complexes, as opposed to (ii) warmer elevation sites (T9 and T10), which had a more microbial processed C and a higher fraction of protected C forms, along with a greater abundance of SRO Al constituents. The results revealed that climate (particularly temperature rise) and soil geochemistry interacted to regulate soil C preservation. While the study has revealed that the mechanisms that protect OM (particularly at depth) differ across the thermogradient, C stocks do not change with temperature., models projecting soil C changes over time under various climate scenarios should also consider the existing interaction with soil geochemistry (Chapter 3). After understanding the interaction between soil geochemistry and temperature rise in relation to the soil C preservation mechanism, we investigated the long-term effect of rising temperatures on the soil C fluxes (input and output) in a mature native forest along the thermo-sequence in Mt. Taranaki. We used specific molecular markers to monitor the changes in soil C abundance and composition (i.e., carbohydrates) in order to gain a deeper understanding of the effect of temperature rise on the turnover of soil C. We hypothesised that, in the absence of a water shortage, an increase in temperature would increase forest productivity and litterfall, which in turn would increase soil C inputs; this increase in organic substrate along with higher temperatures would, in turn, generate an additional soil CO₂ efflux; however, no net C loss would occur as the increased in soil C input would offset the increased soil CO₂ efflux. To test this, soil C pools, plant biomass C pools, soil C fluxes, and soil OM composition (i.e., carbohydrate abundance and composition), at four elevation sites along the Taranaki thermo-sequence were quantified. The outcome of this investigation demonstrated that above- and below-ground biomass C increased ca. 32% (significant at P <.05) when temperatures rose (from T7 to T10). This led to an increase in aboveground litterfall (29%), belowground C input (15%), soil respiration rate (16%, significant at P <.05), and decay intensity (as inferred from the carbohydrate preservation index (CPI)). The study showss that the increase in temperature along this quasi-thermo-sequence at Taranaki increases plant C input through enhanced net primary production, which counteracts soil C loss, resulting in no apparent detrimental effect on soil C storage (Chapter 4). This study highlights the importance of considering plant C input of an entire ecosystem along with soil OM decomposition when investigating the response to temperature rise. To understand the effect of warming on the temperature sensitivity (Q₁₀) of soil organic matter (OM) decomposition rate along the Taranaki thermo-sequence, we collected soils from four distinct elevation sites with four different depths and incubated them in the laboratory at temperatures of 5, 15, 25, and 35°C for 330 days (Chapter 5). The incubation data were then fitted with a three-component model to generate three C pools with distinct decomposition rates, followed by the calculation of their respective Q₁₀ values. Using multivariate analysis, these values were then combined with a complete set of soil geochemical characteristics and OM molecular composition data to gain mechanistic insights into the biogeochemical causes of Q₁₀ variations. The results of this study revealed that: (i) Q₁₀ of soil OM decomposition is inclined to attenuate over a centennial scale under elevated MAT; and (ii) Q₁₀ values of the bulk soil OM and the distinct C pools were differentially regulated by soil C availability, OM molecular composition, and OM-mineral interactions (Chapter 5). These results suggest that temperature affects the distinct C pools differently; with Q₁₀ values having a trend to decrease as temperature increases.All the results obtained in this thesis contribute to provide a mechanistic understanding about the effect of rising temperatures on soil C fluxes, and stocks as well as the underlying mechanism governing them, in order to accurately anticipate soil C dynamics in response to global warming.
Figures 2.3A (=Filimonova et al., 2016 Fig 8) and 2.3B (=Huang et al., 2016 Fig 10C) were removed for copyright reasons.
Soil biochemistry, Soils, Carbon content, Soils and climate, Temperate forests, New Zealand, Taranaki, Mount (N.Z.), Cimate