Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The Okiep copper district in the north-western corner of South Africa is a region that has been mined for over 150 years. Most mining operations have ceased, but years of mining has left the area scattered with abandoned mining sites. Acids (as used in ore processing) together with acid mine drainage generated from tailings exposure, collectively referred to as AMD hereafter, are a contamination risk to water resources and the biodiversity of this arid area. This study focused on an abandoned copper processing pond located close to the town of Nababeep. The leaching pond is unlined and has been excavated in the shallow colluvial soils. The natural soils of the area are shallow (60 cm) (WRB – Arenosol; SA – Oakleaf). Formations of corroded granite-gneiss boulders are an indication of the corrosiveness of the AMD collecting in the pond. The AMD was collected from the pond in the dry season at its most concentrated form and the AMD had exceptionally high concentrations of Al (26.9 g/l), Fe (42.9 g/l), Mg (20.5 g/l), Cu (3.8 g/l) and Mn (3.4 g/l). Melanterite (FeSO4·7H2O), a soluble ferrous compound, was found to play an important role in the immediate release of Fe and sulfates. The pristine soils have a sandy texture (2–5.2 %clay). The minerals detected in the clay phase include illite, kaolinite, montmorillonite and quarts. Pristine soils show some degree of contamination with low pH (4.38–4.77) and high Cu and sulfate contents. Soils located in the processing pond, which have been exposed to AMD for an extended period of time, showed poorly crystalline phases to be present (indicated by a broadening of the XRD peaks for clay minerals). Saturation indices (SI) were determined for saturated paste extracts of the pond soils and the obtained SI values support the notion of dissolution of silicate clays, as the obtained SI values ranged between –1.3 and –11.77 for illite and –4.76 to 0.58 for kaolinite. Jarosite, a new phase, formed in the contaminated soil and is a sink for K. Long term weathering experiments of pristine soils exposed to AMD indicated that clay minerals are significantly weathered and altered, which was identified by observing the broadening of the clay XRD peaks. Fourier transform infrared (FTIR) spectra were generated by scanning clay samples of the weathered soil. Amorphous phases were confirmed by structured water bands with wavenumber values of 3700 and 3300 cm−1 for acid treated soil. Micrographs showed a more amorphous and corroded morphology in the acid treated soil. Metal retention experiments were conducted by exposing the pristine soil to AMD repeatedly. Iron was the predominant metal attenuated in the soil. Metals such as Al, Mn, Na, K, Ca and Co were released by the soil into solution. Removal of Si is associated with the dissolution of clay minerals. The pristine soil shows limited capacity to neutralize acidity and low capacity to retain metals when leached with AMD. Metals were predominantly extracted in the water soluble phase of the long term weathering treatments. Aluminium was the most mobile fraction, being extracted predominantly from the water soluble fraction (2035 mg/kg). Exchangeable and acid soluble fractions did not retain significant quantities of metals. In the soil from the processing pond, the reducible fraction had a high concentration of reducible Fe (21175 mg/kg) and Si (3070 mg/kg). The reducible fraction also had the highest concentration of Cr (15.85 mg/kg), Cu (41.53 mg/kg), Pb (8.0 mg/kg) and Zn (10.65 mg/kg) compared to the other fractions of this soil. For the control experiment, the concentration of Cu (77.3 mg/kg), Pb (10.8 mg/kg) and Zn (24.1 mg/kg) were higher than contaminated soil yet lower for Cr (6.05 mg/kg). From these experiments, it can be concluded that the pristine soils studied have a limited ability to retain heavy metals in the non-bioavailable fraction, and, due to the nature of AMD, they are not effective in retaining metals sufficiently. The findings of this study suggest that the capacity of these pristine soils to buffer pH and retain metals is greatly limited. This could be as a result of (1) the low clay content, (2) the low concentration of secondary carbonates and (3) the low pH of the soil. The leaching of AMD from the pond is thus not regulated by the soils and poses a risk for nearby water resources. / AFRIKAANSE OPSOMMING: Die Okiep Koper Distrik, geleë in die noord-westelike hoek van Suid-Afrika, is ‘n streek waar mynbou al vir meer as 150 jaar plaasvind. Die meeste mynbou aktiwiteite is gestaak, maar jare van mynbou het die streek nagelaat met verskeie verlate mynbouterreine. Sure, wat in die prosessering van erts gebruik word, en suurmynwater wat gegenereer word uit die mynuitskot (gesamentlik verwys na as suurmynwater) is ‘n besoedelingsrisiko vir waterbronne en biodiversiteit in hierdie dorre area. Die studie fokus op ‘n verlate koper prosesserings aanleg naby die dorpie Nababeep. Die logingsdam is nie geseël nie en is uitgegrawe in vlak, kolluviale grond. Die natuurlike gronde van hierdie area is vlak (60 cm) (WRB – Arenosol; SA – Oakleaf). Formasies van weggevrete graniet-gneis rotse in die opgaardam is ‘n aanduiding van die bytende potensiaal van hierdie suurmynwater. Suurmynwater was versamel in die dam gedurendie die droë seisoen in die mees gekonsentreerde vorm. Die suurmynwater het besonderse hoë konsentrasies van Al (26.9 g/l), Fe (42.9 g/l), Mg (20.5 g/l), Cu (3.8 g/l) en Mn (3.4 g/l). Melanteriet (FeSO4·7H2O) is ‘n ysterhoudende verbinding en, alhoewel dit oplosbaar is, speel dit ‘n belangrike rol in die onmiddelike vrylating van Fe en sulfate. Die onversteurde grond het ‘n sand tekstuur (2–5.2 % klei). Die minerale wat in die klei fraksie ge-identifiseer is sluit illiet, kaoliniet, montmorilloniet en kwarts in. Die ongerepte gronde dui egter op ‘n mate van besoedeling deurdat dit ‘n lae pH (4.38–4.77) enhoë Cu en sulfaat inhoud het. Die grond wat geleë is naby die prosesseringsaanleg en ook blootgestel is aan suurmynwater vir ‘n verlengde tyd, dui daarop dat swak kristallyne fases teenwoordig is in die grond. Dit word bevestig deur ‘n verbreding van die XRD pieke van kleiminerale. Versadiging indekse (VI), wat bepaal is in versadigde grondekstraksies van die damgrond, ondersteun die oplossing van die silikaatkleie en word gereflekteer deur VI waardes wissel tussen –1.3 en –11.77 vir illiet en –4.76 tot 0.58 vir kaoliniet. Jarosiet is ‘n nuwe fase wat gevorm het in die besoedelde grond en is ‘n sink vir K. Langtermyn verweringseksperimente wat gedoen is deur die onversteurde grond bloot te stel aan suurmynwater wys beduidende verwering en verandering van klei-minerale deur verbreding van die XRD pieke. Fourier transform infrarooi (FTIR) spektra is op kleimonsters van die verweerde grond gegenereer. Amorfe fases is bevestig deur gestruktureerde waterbindings met frekwensies tussen 3700 en 3300 cm−1 vir suurbehandelde grond. Metaal-vasleggings eksperimente is uitgevoer deur herhaaldelik die onversteurde grond aan die suurmynwater bloot te stel. Yster is die metaal wat hoofsaaklik in die grond vasgehou is. Metale soos Al, Mn, Na, K, Ca en Co was vrygestel in oplossing deur die grond. Die vrylating van Si deur die grond word geassosiëer met die oplossing van kleiminerale. Die onversteurde grond toon beperkte vermoë om suur te neutraliseer en metale te bind in die grond wanneer dit met suurmynwater geloog word. Metale was hoofsaaklik ge-ekstraëer in die wateroplosbare fase vir die langtermyn verweringsbehandelings. Aluminium was die mees mobiele fraksie wat ontrek is van die water oplosbare fraksie (2035 mg/kg). Uituilbare en suuroplosbare fraksies het nie ‘n groot hoeveelheid metale vasgehou nie. Gronde wat versamel is naby die prosesseringsdam het die hoë konsentrasies vanFe (21175 mg/kg) en Si (3070 mg/kg) in die gereduseerde fraksie gehad. Die reduserende fraksie het ook die hoogste konsentrasie van Cr (15.85 mg/kg), Cu (41.53 mg/kg), Pb (8.0 mg/kg) en Zn (10.65 mg/kg) gehad in vergelyking met ander fraksies in die grond. Vir die beheer eksperiment was die konsentrasie van Cu (77.3 mg/kg), Pb (10.8 mg/kg) en Zn (24.1 mg/kg) hoër as in die besoedelde grond en laer vir Cr (6.05 mg/kg). Dus kan daar van hierdie eksperimente afgelei word dat die onversteurde grond beperkte kapasiteit het om swaar metale in grond vas te hou in die nie-biobeskikbare fraksie. As gevolg van die aard van die suurmynwater, is die grond nie voldoende om die metale effektief in grond te behou nie. Die bevindinge van hierdie studie dui daarop dat die kapasiteit van die ongerepte grond om pH te buffer en metale in grond te behou baie beperk is. Dit kan toegeskryf word aan die lae kleiinhoud, lae konsentrasie van sekondêre karbonate en die lae pH van die grond. Die loging van suurmynwater van die logingsdam is dus nie gereguleer deur die gronde nie en stel die naasliggende waterbronne in gevaar.

The exploitation of metallic sulfide ores produces vast quantities of fine-grained wastes hosting potentially toxic elements (PTEs). There are concerns that, if improperly disposed of and managed, waste mineral particles can behave as vectors that disperse PTEs via aeolian and fluvial transport, subsequently contaminating soils and crops used to support human populations. The importance of these particles, as sources and influencers of PTE biogeochemistry in productive soils, has received limited research. Long-term (365 d) batch incubation experiments, field weathering experiments and phytoavailability trials, were performed to establish the rate, patterns and factors limiting PTE (Cd, As, Hg) release from grains of sphalerite (Zn(Fe,Cd)S), arsenopyrite (FeAsS) and cinnabar (HgS) into soil matrices (0.1 % mineral:soil m/m), and the bioavailability of the liberated PTEs to important food crops (Tricitum aestivum, wheat and Oryza sativa, rice). All three of the ores underwent chemical weathering in oxic agricultural soils of both temperate and sub-tropical provenance, during which nonessential PTEs (cadmium, mercury, arsenic) were released in bioavailable forms, at rates relevant to agricultural production. Sphalerite weathered at a rate of 0.6 to 1.2 % a-1 (Cd basis) in the experimental soils, releasing 0.5 to 1 μmol Cd g-1 ZnS a-1 into the soil matrix. Cinnabar weathering reached a maximum of 12.0 – 13.5 % (Hg basis) after 90 days exposure in oxic soils, whereas arsenopyrite weathering was rapid and extensive, reaching 56 to 66 % (S basis) after 180 days. The PTE concentrations accumulated in edible grains of wheat and rice grown in the sulfide-contaminated soils were higher than international food safety limits by factors of 8 (Cd in rice), 10 – 30 (Hg in wheat and rice) and 8 – 12 (As in wheat and rice). The primary geochemical factors controlling PTE release and bioavailability were solid-phase associations (i.e. PTEs complexed by clays, metal oxyhydroxides and organic matter) and the precipitation of secondary mineral phases. Weathering arsenopyrite grains were passivated from further oxidation by secondary iron-arsenate phases, which also co-precipitated arsenic liberated from the ore. Secondary phase formation was identified as the cause of decreasing extractable Hg (liberated from cinnabar) after mercury release from cinnabar peaked (≤ 90 days exposure). For sphalerite, the evidence indicates that secondary sulfide phases formed under flooded (sulfate-reducing) soil conditions (paddy rice), limited the bioavailability of cadmium previously liberated under oxic conditions. These key findings demonstrate a potential human health hazard relating to the dispersal of PTE-hosting sulfide ore particles produced by mining activities into soils supporting human populations via crop contamination. This work also highlights differences in ore geochemistry, showing the need for additional research on different ore minerals and their alteration products.

Les plantes hyperaccumulatrices de métaux ont développé des mécanismes spécifiques de prélèvement de la fraction disponible des métaux du sol, conduisant à leur accumulation dans les parties aériennes. L’utilisation agronomique de ces plantes pourrait être une voie de dépollution des sols contaminés en métaux. Ce travail a pour objectif de contribuer à la compréhension des mécanismes chimiques et biologiques qui conditionnent l’accumulation du Ni par les plantes hyperaccumulatrices, en réponse à la disponibilité du métal dans le sol. Après observation du fonctionnement naturel du système sol ultramafique-plante hyperaccumulatrice, nous avons étudié, en laboratoire, l’altération de trois phases minérales nickélifères modèles communes des sols ultramafiques (chrysotile, smectite, goethite) soumises à l’influence de trois Brassicacées hyperaccumulatrices de Ni natives de sites de serpentine d’Europe (Leptoplax emarginata, Alyssum murale et Thlaspi caerulescens). Les résultats montrent que dans les sols ultramafiques, le Ni phytodisponible provient majoritairement de l’altération des minéraux nickélifères silicatés ferromagnésiens, hérités de la roche mère. Les hyperaccumulateurs ont un comportement différent en fonction de la disponibilité du Ni des sols. Dans les milieux à forte disponibilité du Ni, les hyperaccumulateurs accumulent le Ni proportionnellement à la disponibilité du Ni du milieu. Elles réduisent ainsi significativement la fraction de Ni initialement disponible du milieu, et ne semblent pas favoriser la dissolution des minéraux porteurs de Ni. A l’inverse, dans les milieux à faible disponibilité du Ni, la présence des hyperaccumulateurs accélère la dissolution de phases minérales nickélifères silicatées, en favorisant la solubilisation du métal. Dans ce cas la plante prélève la quasi-totalité du Ni solubilisé. Ces résultats sont essentiels à l’élaboration d’un modèle de culture qui permettra de faciliter l’application du procédé de phytoextraction du Ni à grande échelle / Metal hyperaccumulator plants developed specific mechanisms to extract available metals from the soil and consequently accumulate them in aerial parts. The agronomic use of these plants for the decontamination of metal polluted soils is under study worlwide. This work was undertaken to better elucidate the chemical and physiological mechanisms that influence Ni accumulation by hyperaccumulators in response of Ni availability in soils. To answer these questions we undertook two approaches. Firstly, a site study to determine the natural functioning of the ultramafic soil-Ni hyperaccumulators system. Secondly, a lab-scale set of experiments that were designed to study the weathering of three ultramafic Ni-minerals (chrysotile, smectite, goethite) in the rhizosphere of three Ni-hyperaccumulating species naturally growing on European serpentine soils (Leptoplax emarginata, Alyssum murale et Thlaspi caerulescens). Results showed that, in ultramafic soils, phytoavailable Ni derives from the weathering of primary nickeliferous ferromagnesian silicates. Hyperaccumulators show a different behaviour depending of Ni availability in the environment of the culture. In environments showing high available Ni, hyperaccumulators accumulate Ni proportionally to Ni availability. In this way, they can significantly reduce the initially available fraction of Ni but do not seem to enhance the weathering of unavailable Ni forms. Contrarily, in environments with extremely low Ni availability, hyperaccumulators may strongly enhance the dissolution of nickeliferous silicate minerals and so favour Ni solubilisation. If so, hyperaccumulators absorb the majority of Ni solubilised. These results are of considerable interest for the elaboration of a crop model designed for the optimisation of the phytoextraction concept and high scale application

The critical zone is the interface between abiotic and biotic constituents that spans from the vegetation canopy through the groundwater and represents an open system shaped by the climate, topography, and vegetation communities of a given environment. Four studies were completed to examine soil development, specifically mineral weathering and soil carbon storage, across semiarid sites spanning the Santa Catalina Mountain Critical Zone Observatory (SCM-CZO). The Santa Catalina Mountain Critical Zone Observatory is located along an environmental gradient in southern Arizona where co-varying climate and vegetation community properties have generated distinct changes in soil development across a relatively short distance (<20 miles). Soil, saprock, and parent rock were sampled on north-facing slopes from five climate-vegetation zones spanning desert scrub to mixed conifer forest. Within each climate-vegetation zone, samples were collected from two divergent summit and two convergent footslope landscape positions to account for topographic controls on mineral transformation. In the first study, the soil morphologic, physical, and chemical properties collected for all samples were combined with profile development indices to quantify soil variation with landscape position across the SCM-CZO. The results of this research demonstrated that climate and landscape position exert significant control on soil development in semiarid ecosystems, and that the profile development index is an effective tool for detecting these regional to hillslope scale variations in soil properties. The second study consisted of a cross-scale analysis of feldspar mineral transformation across the selected research sites to connect measures of pedon-scale soil development, depletions of feldspar and sodium in bulk soil, and elemental losses across feldspar grains at the microscale. Results indicated that greater soil development in the mixed conifer pedons corresponded to increased total feldspar and sodium losses. Desert scrub soils presented less evidence for feldspar transformation including lower profile development indices, gains in total feldspar percentages attributed to dust deposition, and less Na chemical depletion at the microscale. Greater soil development in convergent positions relative to adjacent divergent sites was consistent across all sites, with the highest degree of total feldspar depletion occurring in the conifer convergent locations. The third study focused on the physical distribution and mean residence time of soil organic carbon (SOC) in the SCM-CZO soils described for the first two studies. Surface (0-10 cm) and subsurface (30-40 cm) samples were collected from the aforementioned granitic regolith profiles. The soils were characterized using total C and N, δ¹³C, Δ¹⁴C, and radiocarbon derived mean residence time (MRT) estimates of bulk soil and physically separated C fractions to quantify SOC change with climate, vegetation, and landscape position. The results document a shift in SOC stabilization mechanisms across bioclimatically distinct ecosystems from mineral-associated SOC in the desert scrub soils to a mixture of mineral and occluded SOC in the conifer soils. Soils in the convergent landscapes concentrated the most SOC and typically exhibited the longest residence times across all locations. The fourth study examined the geochemical and mineralogical properties of the SCM-CZO soils across regional and hillslope scales of study to quantify soil development in semiarid environments. X-ray fluorescence and x-ray diffraction were used to characterize the elemental and mineralogical properties of the soils and parent material. Desert scrub dust samples were analyzed using x-ray fluorescence. The results indicate that mineral and base cation depletion were greatest in the convergent landscape positions at both sites and increased from the hot, moisture-limited desert scrub sites to the wetter, more productive conifer ecosystems. Enrichments in mica and select elements (i.e., Fe, Mg) suggested that dust deposition was a significant contributor to soil development across all sites. Geochemical estimates of dust fraction inputs confirmed this finding with dust composing up to 35% of the regolith material in the mixed conifer convergent soils. Clay mineral assemblage was dominated by halloysite and smectite minerals in the desert scrub site, reflecting complex climatic and mineral microtextural interactions in the dry, silica-rich desert environment. Clay minerals at the mixed conifer site exhibited the greatest degree of mineral transformation in the SCM, consisting of vermiculite, illite, kaolinite, and minor amounts of smectite and gibbsite. These findings confirm the interactive role of climate, vegetation, and landscape position in shaping the critical zone, where greater moisture availability and biological production are likely driving increased soil organic carbon storage and mineral weathering across various scales of study.

Inverse geochemical modelling has been used frequently in groundwater systems between wells along a known flowpath and between precipitation and stream waters in catchments. This research expands the use of inverse geochemical modelling through a reaction path model (RPM) between waters in an alpine catchment to determine the geochemical connections and disconnections within the catchment. The data for this study are from the Green Lake 4 catchment in the Colorado Front Range during the 1996 snowmelt season, which has been divided into discrete time intervals based on snowmelt hydrology. Unique combinations of geochemical connections occur during these time intervals, and they show a dynamic hydrologic system. RPM results show notable disconnections; soil water is not geochemically connected to any other end member. These changes reflect changes in weathering reactions in the catchment that are dependent on the duration and timing of snowmelt. Previously end-member mixture analysis (EMMA) models have been used to discern the water sources in catchments. The combination of RPM and EMMA approaches offers the opportunity to connect the source of water to the internal hydrologic structure of the catchment, to better understand how catchments might respond to changes in climate or atmospheric deposition.

Determining where, when, and how subsurface flow affects soil processes and the resulting arrangement of soil development along flow paths is challenging. While hydrologic regime and soil solution acidity are known to influence weathering rates and soil transformation processes, an integrated understanding of these factors together is still lacking. This dissertation explores the effects of subsurface flow on the mobility and distribution of dissolved organic carbon (DOC) and base cations to explain spatial patterns in chemical weathering in a forested headwater catchment. In the first chapter, relationships between hydrologic behavior, fluxes of weathered elements, and the extent of soil elemental loss across landscape positions are established. The second chapter investigates what specific groundwater behavior best explains spatial patterns in solution DOC concentrations during storm events. Lastly, in the third chapter, near surface saturation dynamics are examined to determine when and where DOC mobilization might be enhanced by subsurface flow. Results show that weathering extent was greatest in the upper reaches of the catchment, where O horizon saturation frequency and DOC concentrations are highest. Annual base cation fluxes, which were also greatest in these positions, could indicate where weathering is likely still enhanced. Additionally, while O horizon saturation occurred across the catchment, spatial differences in DOC concentrations suggest there are other sources of acidity to groundwater solutions other than just leaching from O horizons. Shallow organic soils, near bedrock outcrops at the top of the catchment is likely this additional C source, in which drainage water is transported downslope to nearby mineral soils when water tables are high and hydrologic connectivity between soils is increased. Spring and fall storm events were identified as times when groundwater most frequently reached O horizons during the snow-free year, providing insight into the timing of these processes throughout the year. This dissertation highlights how catchment structure mediates DOC flushing events, which in turn, influences the spatial architecture of soil development and chemical weathering processes across the landscape. / Doctor of Philosophy / This dissertation explores how the movement and chemistry of groundwater influences chemical weathering in forest soils. Chemical weathering is an important process in which rocks and soils are broken down into soil nutrients and water-soluble elements. The control of weathering processes by spatial and temporal differences in water behavior across landscapes is not well understood. To address these knowledge gaps, this dissertation measured groundwater fluctuations, solution chemistry, and nutrient fluxes across a mountainous forested landscape. Results from this work found that areas with more frequent flushing of organic matter-rich soil horizons increases groundwater acidity, which can enhance weathering processes. Flushing frequency of organic horizons and soil nutrient fluxes were greatest in the highest elevation portions of the landscape, where soils were most weathered (greatest loss of soil nutrients). This study revealed that flushing events occurred most frequently in spring and fall storm events during the snow-free year, shedding light on the when weathering might be most enhanced. Overall, this research demonstrates that topographic graphic position described differences in catchment groundwater behavior and solution acidity, which contributes to predictable patterns of weathering and soil development across the landscape.

Ce travail concerne l'étude des interactions bactéries /silicates grâce à de nouveaux matériaux ou gels de silice hybrides dopés en particules colloïdales de smectites (GSH). Les deux objectifs principaux sont la caractérisation physicochimique des GSH et l'amélioration des connaissances sur les mécanismes de l'altération minérale grâce aux GSH. Dans le premier volet de la thèse, nous avons ainsi pu mettre en évidence que le gel est constitué d'une matrice siliceuse particulaire de nature fractale dans laquelle les particules minérales sont dispersées. La structure des GSH est stabilisée par le traitement hydrothermal choisi (type autoclave). Dans le second volet de la thèse, l'altération minérale est étudiée pour des GSH à concentration variable en particules minérales, à teneur élevée (NAu-2) ou basse (SWy-2) en fer. Il est ainsi montré que la diffusion des acides organiques et des sidérophores dans le GSH, hors adhésion ou formation d'un biofilm en surface des minéraux, est suffisante pour altérer efficacement les particules minérales. La mise en évidence de gradients de protons locaux produits par la bactérie en interaction avec les GSH peut également expliquer l'efficacité biotique de la dissolution minérale par rapport à des conditions abiotiques. En conclusion, les points forts et les limites de l'utilisation d'un matériau hybride type GSH pour cibler les interactions minéral/microorganismes sont discutés / This work deals with the study of bacteria/silicate interactions by using new hybrid materials, i.e. hybrid silica gel dopped with colloidal smectite particles (HSG). The aims are (1) to characterize HSG physicochemical properties and (2) to get a better knowledge about mineral/bacteria interactions. In a first part of the thesis, it is shown that HSG is a fractal silica network containing well-dispersed mineral particles. Hydrothermal treatment is necessary to stabilize the HSG structure. In the second part, it is shown that diffusion of organic acids and siderophores is sufficient to explain the high efficiency of bacterial action compared to abiotic treatments. Preventing bacterial adhesion or biofilm formation at the mineral surface permits to enhance the influence of metabolites diffusion. To conclude, the advantages and limits of using HSG in environmental studies are discussed

Biomass energy is currently the largest renewable contributor to global energy supply and there is increasing demand for bioenergy feedstock. Consequently, the production of purpose-grown woody bioenergy crops, such as short rotation coppice (SRC) willow, is expected to proliferate. Although the economic and environmental benefits associated with SRC willow production are well documented, systematic assessments of nutrient cycling within these plantations are rare. The objective of this study was to examine biomass production and biogeochemical cycling of nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca), and magnesium (Mg) during an initial four-year rotation of six willow varieties grown at four plantations along a 500 km north-south pedoclimatic gradient within Saskatchewan, Canada. Nutrient budgets were also calculated after quantifying various nutrient inputs (e.g., atmospheric deposition, soil mineral weathering, and organic matter mineralization), outputs (e.g., above- and below-ground biomass, leaching, and denitrification), and transfers (e.g., canopy exchange, leaf litter decomposition, and fine root turnover) affecting the plant available soil nutrient pool. Total stem, leaf litter, and below-ground (primarily fine roots) biomass production after four years averaged 19.0, 7.1, and 12.5 Mg ha-1, respectively, with corresponding soil nutrient budget deficits of 17, 39, 112, 271, and 74 kg ha-1 of N, P, K, Ca, and Mg, respectively, but a soil S surplus of 60 kg ha-1. Despite willow’s relatively low nutrient-demanding nature, negligible leaching and denitrification losses, and substantial nutrient cycling from leaf litter, the nutrient export in harvested biomass over multiple rotations will require soil nutrient amendments, particularly N and P, to maintain plantation productivity. Given the apparent eventual need for supplemental fertility to support adequate willow growth over the 22-yr plantation life span, the fate of broadcast 15N-labelled fertilizer was also examined. Though the willow accumulated less than ⅓ of the applied fertilizer N after one year, the majority of the residual fertilizer N (51%) remained available for willow uptake in subsequent years. Further research is needed to track the fate of applied fertilizers over multiple rotations to better understand fertilizer dynamics for optimizing SRC willow agronomy; thus helping to promote its viability as a biomass energy feedstock option.

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