Massey Documents by Type

Permanent URI for this communityhttps://mro.massey.ac.nz/handle/10179/294

Browse

Has files: YesSubject: search.filters.subject.Soil biochemistry

Search Results

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    The long-term effects of elevated CO₂ on soil organic carbon sequestration, partitioning and persistence in a grazed pasture : a thesis presented in partial fulfilment of the requirements for the degree of Doctor in Philosophy in Soil Science at Massey University, Manawatu, New Zealand
    (Massey University, 2024-03-04) Gonzalez Moreno, Marcela Angelica
    The increased concentration of atmospheric carbon dioxide (CO₂) is a significant driver for climate change and also influences the cycling of soil organic carbon (OC) in ecosystems. Despite the importance of grassland soils as a sink for CO₂, the effect of long-term exposure to elevated CO₂ (eCO₂) on OC sequestration, partitioning and persistence in grazed grassland soils is poorly understood. This thesis aimed to investigate the effect of eCO₂ on soil OC stocks, the partitioning of OC in soil fractions and persistence in a grazed legume-based pasture at the New Zealand (NZ) Free Air CO₂ Enrichment (FACE) facility. The NZ-FACE, established in 1997, is the only FACE experiment worldwide that includes the influence of grazing practices on the above- and below-ground components of the OC cycle. The effect of eCO₂ on soil OC persistence and stability was assessed by measuring changes in soil OC stock, in the distribution of soil OC in the soil fractions by wet fractionation analysis and in the soil OC decomposition pathways by determining the molecular composition of soil organic matter (OM) by pyrolysis analysis followed by gas chromatography/mass spectroscopy (GC-MS) and thermally assisted hydrolysis methylation-GC-MS (THM-GC-MS) analysis to a soil depth of 250 mm. In Chapter 3, we assessed OC storage and persistence in the soil fractions in a grazed legume- based pasture exposed to eCO₂ for 22 years on Pukepuke soil (Mollic Psammaquent) at three soil depths. Our study revealed that after 22 years of exposure to eCO₂ there were no significant changes in the stocks of OC and N as well as the partitioning of OC within different soil fractions in the Pukepuke soil. Interestingly, in the last 10 years at the NZ-FACE facility, there has been a sharp reduction of OC and N stocks in the Pukepuke soil, independent of the CO₂ treatment. We suggest that in the sandy Pukepuke soil under conditions of warmer temperatures and a wetter system, the deficiency that has emerged in soil nutrient availability, the environmentally enhanced plant growth and the larger amounts of fresh OM input has caused a positive priming effect, mainly in the labile fraction. Even though eCO₂ did not change the soil OC stocks nor OC content in the soil fractions in any soil layer, it did modify the soil nutrient status (phosphate in particular) and did increase polysaccharides and aliphatic proportions in the coarse particulate organic matter and micro-aggregates indicating that the priming was further enhanced in eCO₂ soils with this effect being especially prominent in the 50 – 150 mm soil layer (Chapter 3). In Chapter 4, the hypothesis that grazing animals, by returning nutrients in urine, dung, and plant litter trampled into the soil surface, would contribute to an increase in soil OC and N stocks under eCO₂ was investigated and rejected. Despite not finding any interaction effect between eCO₂ and defoliation treatment on the soil OC stocks and partitioning in the soil fractions, the presence of an interaction effect in the soil OM molecular composition suggests that distinctly different OC decomposition pathways exist depending on pastures management under eCO₂. Our study showed that under grazing there was an accumulation of lignin-derived OM, which reveals a higher proportion of shoot-derived rather than root- derived OC under eCO₂. In Chapter 5, the influence of the inherent properties of a soil – which might enhance or limit the effects of eCO₂ on soil OC persistence and stability – was examined in two contrasting soils (Pukepuke and Stratford; a Entic Dystrandept) in mesocosms installed at the NZ-FACE in May 2005 and extracted after 15 years. Our results showed that over the course of the mesocosm study, OC and N contents and stocks (to 150 mm soil depth) in the Pukepuke soil declined by 16 t ha-1 under ambient CO₂ atmosphere, possibly as a result of soil disturbance during the establishment of the mesocosms. In the Stratford soil, with the ability to strongly preserve OM through mineral associations, the decline in soil OC was much smaller (5 t ha-1). Elevated CO₂ interacted with soil type and after 15 years of exposure to eCO₂, the Pukepuke soil had 6.5 t ha-1 more OC stocks, compared to the same soil under ambient CO₂ conditions, while no differences were found in the OC stocks of the Stratford soil (Chapter 4). These findings indicate that in the Pukepuke soil the eCO₂ treatment might have (i) helped overcome the impact of disturbance by favouring plant growth and generating a larger plant detritus input to the soil that enabled the partial replenishment of the OC loss at the time of mesocosm establishment, or (ii) limited the impact of disturbance, as eCO₂ often improves soil aggregation. It is crucial to consider that (i) the Stratford soil was subjected to ~50% less precipitation at the NZ-FACE compared to its original location and (ii) the mesocosm might have introduced new variables due to physical barriers. Thus, extrapolating the findings to field conditions at the NZ-FACE facility and elsewhere requires cautious interpretation. The findings presented in this thesis contribute significantly to enhancing our understanding of the mechanistic processes underlying the influence of eCO₂ on the stabilization and mineralization of soil OM. These insights have direct implications for the development of sustainable agricultural management practices in response to a changing environment.
  • Thumbnail Image
    Item
    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
    (Massey University, 2023) Siregar, Idri Hastuty
    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.
  • Thumbnail Image
    Item
    Studies on the dynamics of organic sulphur and carbon in pastoral and cropping soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 2000) Singh, Bhupinder-Pal
    Soil organic matter (SOM) can be depleted or regenerated by altering land management practices. Soil tests capable of reporting the size of dynamic SOM fractions may be useful for indicating the environmental cost of landuse and management practices. Information on the effect of land management practices on soil organic S content and turnover is scarce. This study evaluated the ability of a sequential chemical fractionation procedure to characterise changes in soil S and C organic fractions on a range of pasture and cropping soils with different management histories. The fractionation involved an initial extraction with ion exchange resins followed by dilute (0.1 M NaOH) and concentrated (1 M NaOH) alkali. In addition, recently rhizodeposited 14C (root+exudate derived) produced during a short-term (one week) 14CO2 pulse-labelling study of intact soil cores growing ryegrass/clover pastures, was used to trace the fate of root-derived C in both chemical and density fractionation procedures. In pasture and cropped topsoils, the major amounts of soil S and C were either extracted in 0.1 M NaOH (49-69% S and 38-48% C) or remained in the alkali-insoluble residual fraction (17-38% S and 46-53% C). These two fractions were more sensitive to change caused by different landuse and management practices than the resin and 1 M NaOH fractions. With a large amount of dynamic soil C remaining in the residual fraction it was concluded that increasing strengths of alkali were not capable of sequentially fractionating S and C in SOM into decreasingly labile fractions. The chemical fractionation allocated recent root and root-released 14C amongst all the fractions. Again, most root 14C appeared in the 0.1 M NaOH and residual fractions. Although small in amount, C of higher specific activity (more recently synthesised root C) was preferentially extracted by resin and 1 M NaOH extracts. Density separation was not capable of recovering recent root and root-released 14C in a single fraction. Root-derived 14C was distributed between light (mostly fibrous root debris) (42%) and heavy (organics attached to clay and silt) (45%) fractions. The dispersing reagent soluble fraction recovered <13% of the 14C. An anaerobic incubation and various acids and oxidising agents were tried, in order to recover a greater proportion of root and root-released 14C as a single identity. These were not very successful in either extracting or increasing the alkali solubility of the root C fraction. A 30% H2O2 pretreatment of soil plus roots, or hot 1 M HNO3 treatment of the residual fraction, were more efficient extractants of the root C fraction and should be investigated further to check their ability to better characterise soil organic S and C fractions with a change in management practices. The 14CO2 pulse labelling study of pasture swards showed a greater allocation of recently photo-assimilated 14C to the topsoil layer with a greater proportion of 14C recovered in roots than in the soil. An in situ soil solution sampling technique with mini Rhizon Soil Moisture samplersTM effectively monitored the rapid appearance of a 14CO2 pulse in soil water at various depths. A comparison of the 14CO2 pulse labelling study under light and dark conditions indicated that, in the light lysimeters, 14CO2 photo-assimilation/translocation/rhizosphere respiration was the main pathway for CO2 generation at various soil depths. In the dark lysimeters, 14CO2 diffusion was the main mechanism and 14C assimilation (either photo-assimilation or assimilation by chemolithotrophs in rhizosphere soil) was small. The 14CO2 activity in soil water from four soil depths of dark and light soil cores, and a CO2 diffusion model, were used to identify the 14CO2 contribution from rhizosphere respiration in the light lysimeters. A model was developed, but the unknown geometry of the air-filled pore space in the undisturbed soil cores made it impossible to precisely calculate the contribution made by root respiration to soil water 14CO2 activity.