New, rapid techniques to quantify the different pools of soil organic matter (SOM) are needed to improve our understanding of the dynamics and spatio-temporal variability of SOM in terrestrial ecosystems. In this study, total organic carbon (TOC) and oxidizable organic carbon (OCWB) fraction were calibrated and predicted by mid- and near-DRIFT spectroscopy in combination with partial least squares (PLS) regression method. PLS regression is a multivariate calibration method that can decompose spectral data (X) and soil property data (Y) into a new smaller set of latent variables and their scores that best describe all the variance in the data. Oxidizable organic carbon content was measured by a modified Walkley-Black method, and total organic carbon was measured by the carbon analyzer. The floodplain and Blackland Prairie soils in Texas were used for prediction of TOC and OCWB using mid- and near-DRIFT spectroscopy. Floodplain soil is mainly composed of quartz and kaolinite, whereas Blackland Prairie soils contain high concentrations of smectitic clays and low to high concentrations of carbonate minerals. The total organic carbon of 68 soil samples from two Texas sites varied between 0.19 and 4.36 wt.% C, and the oxidizable organic carbon of 26 samples from floodplain soils was in the range of 0.05 to 1.33 wt.% C. TOC and OCWB of soil were successfully calibrated and predicted by the PLS regression method using mid- and near-DRIFT spectroscopy. The correlation using mid-IR spectra for TOC (r = 0.96, RMSEV = 0.32 for calibration; r = 0.93, RMSEP = 0.44 for prediction) was about the same as the near-IR result (r = 0.95, RMSEV = 0.37; r = 0.93, RMSEP = 0.42). Therefore, we can also use mid-infrared region for quantification of total organic carbon in soils. The PLS1 regression model (r = 0.92) for prediction of OCWB using mid-IR spectra was more accurate than the PLS2 regression model (r = 0.90). PLS models showed better correlation with spectral data than the univariate least square regression method(r = 0.83) with TOC measured by the carbon analyzer. This study shows that the partial least squares (PLS1) method using mid-and near-IR spectra of neat soil samples can be used to predict both total organic carbon and oxidizable carbon fraction as a fast and routine quantitative method.

Phosphorus is a resource in finite supply. Use of organic amendments in agriculture can be a sustainable alternative to inorganic P, provided it can meet crop requirements. However a lack of consistent knowledge of plant P availability following application of organic amendments, limits its potential. Studies suggest chemical extraction procedures, may not reflect plant available P. The Diffusive Gradients in Thin-films (DGT) technique is based on natural diffusion of P via a hydrogel and sorption to a ferrihydrite binding layer; which should accurately represent soil P (CDGT) in a plant available form. The aim of this research was to evaluate changes in soil P availability, following the addition of organic amendments, cattle farmyard manure (FYM), green waste compost (GW), cattle slurry (SLRY) and superphosphate (SP) using Olsen P and DGT. The research included incubation, and glasshouse studies, using ryegrass (Lolium perenne L.). Soils with a history of application of the aforementioned organic amendments were used (Gleadthorpe), as well as a soil deficient in P (Kincraigie). The hypotheses were as follows H1 A build-up of P available by diffusive supply, from historic treatment additions and subsequent availability from fresh treatment additions will be demonstrated by DGT. H2 Historical treatment additions are more important at determining yield and P uptake than fresh additions. H3 DGT can detect changes in P available by diffusive supply following addition of different treatments and subsequently following lysis of microbial cells on a soil deficient in P. H4 DGT will provide a more accurate indication of plant P availability than organic amendments in a soil deficient in P. H5 P measurements using DGT will be lower from organic amendments than superphosphate.H6 DIFS simulations of soil kinetic parameters will provide additional information about how treatments influence P resupply from solid phase to solution following DGT deployment. Cont/d.

Management practices that may increase soil organic matter (SOM) storage include conservation tillage, especially no till (NT), enhanced cropping intensity, and fertilization. My objectives were to evaluate management effects on labile [soil microbial biomass (SMB) and mineralizable, particulate organic matter (POM), and hydrolyzable SOM] and slow (mineral-associated and resistant organic) C and N pools and turnover in continuous sorghum [Sorghum bicolor (L.) Moench.], wheat (Triticum aestivum L.), and soybean [Glycine max (L.) Merr.], sorghum-wheat/soybean, and wheat/soybean sequences under convent ional tillage (CT) and NT with and without N fertilization. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochepts) in southern central Texas was sampled at three depth increments to a 30-cm depth after wheat, sorghum, and soybean harvesting. Soil organic C and total N showed similar responses to tillage, cropping sequence, and N fertilization following wheat, sorghum, and soybean. Most effects were observed in surface soils. NT significantly increased SOC. Nitrogen fertilization significantly increased SOC only under NT. Compared to NT or N addition, enhanced cropping intensity only slightly increased SOC. Estimates of C sequestration rates under NT indicated that SOC would reach a new equilibrium after 20 yr or less of imposition of this treatment. Labile pools were all significantly greater with NT than CT at 0 to 5 cm and decreased with depth. SMB, mineralizable C and N, POM, and hydrolyzable C were highly correlated with each other and SOC, but their slopes were significantly different, being lowest in mineralizable C and highest in hydrolyzable C. These results indicated that different methods determined various fractions of total SOC. Results from soil physical fractionation and 13C concentrations further supported these observations. Carbon turnover rates increased in the sequence: ROC < silt- and clayassociated C < microaggregate-C < POM-C. Long-term incubation showed that 4 to 5% of SOC was in active pools with mean residence time (MRT) of about 50 days, 50% of SOC was in slow pools with an average MRT of 12 years, and the remainder was in resistant pools with an assumed MRT of over 500 years.

Recalcitrant soil carbon is a poorly understood component of total soil organic carbon (SOC). Although the turnover rate of the recalcitrant fraction is slow, warming temperatures are expected to speed the decomposition of recalcitrant SOC resulting in an increase of atmospheric CO₂ in the future. Several studies show that the oldest SOC is associated with the smallest mineral particles (clays), making direct spectroscopic analysis of old carbon difficult. To overcome the difficulty of analyzing natural samples, we created synthetic soils to examine the association between clay surfaces and specific biomolecules based on the hypothesis that clays with higher surface charge will more strongly bond organic molecules, and also that certain molecules will be better stabilized by clay. We used kaolinite, montmorillonite, or quartz (sand) as a synthetic soil inside 12 mL septum-capped vials, added either dissolved glucose or vanillic acid to each mineral, inoculated with soil microbes, and then purged the vials with a CO₂-free atmosphere. We incubated them and measured the concentration and [delta]¹³C of CO₂ that accumulated in the vials. Respiration rates were significantly higher in experiments containing vanillic acid than in those containing glucose. Respiration rates were lowest in experiments containing montmorillonite. We repeated the experiment using dilute H₂O₂ as an oxidant, and adding vanillic acid, glucose, or glycine. Vials with montmorillonite showed lower rates of CO₂ accumulation than kaolinite, and both glycine- and glucose-containing experiments had less CO₂ than vanillic acid-experiments. We conclude that the montmorillonite protected the organic matter from oxidation better than sand or kaolinite. Both clays protected organic matter better than sand. In all experiments with clay, the respired CO₂ had lower [delta]¹³C values than bulk substrate. This carbon isotope fractionation is likely due to preferential desorption, followed by oxidation, of 12C- as opposed to 13C- bearing organic molecules. The mineral-organic interaction is a strong bond that explains the old age of labile organic compounds in soils. These results indicate that the clay fraction of soils must be considered for accurate prediction of future land-atmosphere carbon fluxes. / text

As the largest carbon reservoir in ecosystems, soil accounts for more than twice as much carbon storage as that of vegetation biomass or the atmosphere. The goal of this study is to examine spatial patterns of soil organic carbon (SOC) in Canadian forest area at an eco-region scale and to explore its relationship with different ecological variables. In this study, the first Canadian forest soil database published in 1997 by the Canada Forest Service was analyzed along with other long-term eco-climatic data (1961 to 1991) including precipitation, air temperature, Normalized Difference Vegetation Index (NDVI), slope, aspect, and elevation. Additionally, an eco-region framework established by the Environment Canada was adopted in this study for SOC distribution assessment. Exploratory spatial data analysis techniques, with an emphasis on spatial autocorrelation analysis, were employed to explore how forest SOC was spatially distributed in Canada. Correlation analysis and spatial regression analysis were applied to determine the most dominant ecological factors influencing SOC distribution in different eco-regions. At the national scale, a spatial error model was built up to adjust for spatial effects and to estimate SOC patterns based on ecological and ecosystem property factors. Using the significant variables derived in the spatial error model, a predictive SOC map in Canadian forest area was generated. Findings from this study suggest that high SOC clusters tend to occur in coastal areas, while low SOC clusters occur in western boreal eco-region. In Canadian forest area, SOC patterns are strongly related to precipitation regimes. Although overall SOC distribution is influenced by both climatic and topographic variables, distribution patterns are shown to differ significantly among eco-regions, thus verifying the eco-region classification framework for SOC zonation mapping in Canada.

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Soils represent an important terrestrial carbon (C) sink, storing up to three times the amount of atmospheric C, however climate and land use changes may transform soils into C sources. River corridor (RC) soils and associated C are at risk to become mobilized by erosion such as bank failure and scour events. Once soil-derived organic C is transferred into the stream, microbial processes and photodegradation of the dissolved, labile (or bioavailable) fractions can lead to the production of CO2, which can evade and increase atmospheric CO2 levels. Because predicted increases in heavy precipitation will likely increase this type of riverine erosion, it is important to better understand the potential for the release of bioavailable C from RCs. One objective of this thesis was therefore to identify and characterize representative samples of soils from a typical Vermont RC for common land covers and simulate the production of dissolved organic matter (DOM) during riverine soil erosion. Field sites representative of typical agricultural and forested land uses were selected based on the analysis of 106 existing samples and resampled multiple times over the summer of 2015. Production of DOM from riverine erosion was simulated using aqueous soil extracts (ASE), where soil and water were shaken at fixed ratios followed by the separation of the extract. To study the characteristics of these extracts (which serve as analogue of stream water after erosion), water extractable C (WEOC) concentrations, water extractable nitrogen, fluorescence properties of DOM, and bioavailability were determined. Results indicated a common, dominantly terrestrial source material for all land covers, but C concentrations and fluorescence properties differed. High but variable amounts of soil organic C and WEOC were observed in agricultural riparian and agricultural stream bank samples, and lower concentrations in agricultural field, forest, forest riparian, and forest stream banks. WEOC bioavailability was high in all agricultural land covers and low in forested land covers. Because this study is the first in which ASE are used as analogues for stream water after riverine erosion, a second objective was to test laboratory methods used in this study for their effect on WEOC, fluorescence properties, and bioavailability. Specifically, the effects of soil drying, soil storage, and the effects of the extraction solution were tested. For this, ASE were prepared from soils that were field moist, dried, and after two years of storage. In addition, dried soils were extracted using different solutions including a salt solution, river water, and double deionized (DDI) water. Results indicated WEOC concentration and microbial humic-like fluorescence from extracts of dried soils were higher than those in extracts of field moist soils, while WEOC concentration and microbial humic-like fluorescence was highest in extracts of soils stored long term. In addition, the bioavailability of WEOC was higher in dried soils than field moist soils. The extraction solutions of DDI water and river water produced DOM with similar fluorescence properties, while the salt solution extracted a different, less humified pool of C. Overall, the ASE methods used in this study are effective in simulating stream bank erosion and subsequent C release into stream water, however the effects of drying the soils need to be considered when assessing DOM.

abstract: Drylands (arid and semi-arid grassland ecosystems) cover about 40% of the Earth's surface and support over 40% of the human population, most of which is in emerging economies. Human development of drylands leads to topsoil loss, and over the last 160 years, woody plants have encroached on drylands, both of which have implications for maintaining soil viability. Understanding the spatial variability in erosion and soil organic carbon and total nitrogen under varying geomorphic and biotic forcing in drylands is therefore of paramount importance. This study focuses on how two plants, palo verde (Parkinsonia microphylla, nitrogen-fixing) and jojoba (Simmondsia chinensis, non-nitrogen fixing), affect sediment transport and soil organic carbon and total nitrogen pools in a dryland environment north of Phoenix, Arizona. Bulk samples were systematically collected from the top 10 cm of soil in twelve catenae to control for the existence and type of plants, location to canopy (sub- or intercanopy, up- or downslope), aspect, and distance from the divide. Samples were measured for soil organic carbon and total nitrogen and an unmanned aerial system-derived digital elevation map of the field site was created for spatial analysis. A subset of the samples was measured for the short-lived isotopes 137Cs and 210Pbex, which serve as proxy erosion rates. Erosional soils were found to have less organic carbon and total nitrogen than depositional soils. There were clear differences in the data between the two plant types: jojoba catenae had higher short-lived isotope activity, lower carbon and nitrogen, and smaller canopies than those of palo verde, suggesting lower erosion rates and nutrient contributions from jojoba plants. This research quantifies the importance of biota on influencing hillslope and soil dynamics in a semi-arid field site in central AZ and finishes with a discussion on the global implications for soil sustainability. / Dissertation/Thesis / Masters Thesis Geological Sciences 2018

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