Dan C. Olk

Changes to soil organic N dynamics with leguminous woody plant encroachment into grasslands

Encroachment of nitrogen-fixing trees and shrubs into grasslands and savannas is a well-documented land cover change that occurs worldwide. In the Rio Grande Plains region of southern Texas, previous studies have shown woody encroachment by leguminous Prosopis glandulosa (mesquite) trees increases soil C and N, decreases microbial biomass N relative to soil N, and accelerates N mineralization and nitrification. We examined responses of the dominant organic N components in soil (amino acids and amino sugars) and two soil-bound protein-N acquiring enzymes (arylamidase and β-N-acetylglucosaminidase) along a grassland-to-woodland successional chronosequence to determine changes to soil N chemistry and extractability. The proportion of total N held within amino compounds was significantly lower in the woodlands (47xa0%) relative to the grassland soils (62xa0%). This increase in non-hydrolysable N was accompanied by increases in plant cell wall derived amino acids (e.g. hydroxyproline, serine) and losses of microbial amino sugars, indicating the woodland organic N pool was altered in composition and potentially in quality, either because it was more structurally protected or difficult to degrade due to polymerization/condensation reactions. Soil carbon-normalized activities of both soil-bound N-acquiring enzymes were significantly higher in woodland soils, consistent with changes in the biochemical composition of organic N. Although soil total N increases following woody encroachment, this additional organic N appears to be less extractable by chemical hydrolysis and thus potentially in more refractory forms, which may limit microbial N accessibility, slow the cycling of soil organic carbon, and contribute to observed soil C and N accrual in these systems.

Advanced solid-state NMR spectroscopy of natural organic matter

Solid-state NMR is essential for the characterization of natural organic matter (NOM) and is gaining importance in geosciences and environmental sciences. This review is intended to highlight advanced solid-state NMR techniques, especially a systematic approach to NOM characterization, and their applications to the study of NOM. We discuss some basics of how to acquire high-quality and quantitative solid-state 13C NMR spectra, and address some common technical mistakes that lead to unreliable spectra of NOM. The identification of specific functional groups in NOM, primarily based on 13C spectral-editing techniques, is described and the theoretical background of some recently-developed spectral-editing techniques is provided. Applications of solid-state NMR to investigating nitrogen (N) in NOM are described, focusing on limitations of the widely used 15N CP/MAS experiment and the potential of improved advanced NMR techniques for characterizing N forms in NOM. Then techniques used for identifying proximities, heterogeneities and domains are reviewed, and some examples provided. In addition, NMR techniques for studying segmental dynamics in NOM are reviewed. We also briefly discuss applications of solid-state NMR to NOM from various sources, including soil organic matter, aquatic organic matter, organic matter in atmospheric particulate matter, carbonaceous meteoritic organic matter, and fossil fuels. Finally, examples of NMR-based structural models and an outlook are provided.

Chemical Composition of Defatted Cottonseed and Soy Meal Products

Chemical composition is critical information for product quality and exploration of new use. Hence defatted cottonseed meals from both glanded (with gossypol) and glandless (without gossypol) cotton seeds were separated into water soluble and insoluble fractions, or water soluble, alkali soluble as well as total protein isolates. The contents of gossypol, total protein and amino acids, fiber and carbohydrates, and selected macro and trace elements in these products were determined and compared with each other and with those of soy meal products. Data reported in this work improved our understanding on the chemical composition of different cottonseed meal products that is helpful for more economical utilization of these products. These data would also provide a basic reference for product standards and quality control when the production of the cottonseed meal products comes to pilot and industrial scales.

Occurrence and abundance of carbohydrates and amino compounds in sequentially extracted labile soil organic matter fractions

PurposeThe study aimed to describe the carbohydrates and amino compounds content in soil, the light fraction (LF), the >53xa0μm particulate organic matter (POM), and the mobile humic acid (MHA) fraction and to find out whether the carbohydrates and amino compounds can be used to explain the origin of SOM fractions.Materials and methodsSoil samples were collected from two agricultural fields managed under organic farming in southern Italy. The LF, the POM, and the MHA were sequentially extracted from each soil sample then characterized. Seven neutral sugars and 19 amino compounds (amino acids and amino sugars) were determined in each soil sample and its correspondent fractions.Results and discussionThe MHA contained less carbohydrate than the LF or the POM but its carbohydrates, although dominated by arabinose, were relatively with larger microbial contribution as revealed by the mannose/xylose ratio. The amino compounds were generally less in the LF or the POM than in the MHA, while the fungal (aspartic and serine) and bacterial (alanine and glycine) amino acids were larger in the MHA than in the LF or the POM, underlining the microbial contribution to the MHA. Results from both sites indicated that total carbohydrates content decreased moving from the LF (younger fraction) to the MHA (older fraction), which seems to follow a decomposition continuum of organic matter in the soil-plant system.ConclusionsThe study showed that the MHA is a labile humified fraction of soil C due to its content of carbohydrates and concluded that the content of carbohydrates and amino compounds in the LF, the POM and the MHA can depict the nature of these fractions and their cycling pattern and response to land management.

Chemical composition of dissolved organic matter from various sources as characterized by solid-state NMR

This study used solid-state nuclear magnetic resonance techniques to characterize the chemical compositions of DOM from various sources and thereby explore the linkages of river DOM to its potential point and nonpoint sources. DOM samples were isolated from two streams draining watersheds of different land uses, landfill leachates of two ages, sequential (raw, facility- and wetland-treated) wastewaters in a wastewater treatment plant. Two sampling sites along the Missouri River were located upstream and downstream of the outlet of a tributary, to which the stream waters, landfill leachates, and wastewater effluents were discharged. Compared with the chemical composition of the stream DOM from a cropland-dominated watershed, the stream DOM from a grass/forestland-dominated watershed comprised 14xa0% more alkyl carbons (C), but ~5xa0% less O-alkyl C. The two leachate DOM samples from landfill cells of two ages showed similar area fractions of the carbon types, and their spectra exhibited a characteristic alkyl C peak at 37xa0ppm, potentially a molecular signature of the DOM of landfill leachate source. The DOM from the treated wastewater had slightly (~4xa0%) less alkyl C than that from the raw wastewater, and the DOM from the wetland-treated wastewater displayed decreased O-alkyl C but increased alkyl C, both to a small degree (by ~4xa0%), compared to the DOM in the facility-treated wastewater. The chemical composition of DOM in the Missouri River was affected by the local organic inputs, with the DOM from downstream the outlet displaying ~11xa0% higher alkyl C.

On-Farm Evaluation of a Humic Product in Iowa (US) Maize Production

The benefit to corn (Zea mays L.) production of a humic product derived from lignite was evaluated for 3 years under otherwise conventional crop management in Iowa farmers’ fields. A liquid extract, it was applied at a rate of 3.57 L ha−1, generally as a foliar spray mixed into routine pesticide applications during early stages of crop growth. In each of 3 years, hand-sampled corn plants collected at physiological maturity in 30–35 farmers’ fields across Iowa showed a significant increase in grain weight with product application in 70–80% of the cases, covering a range of soil types and grain yield levels. Mean increases were 630–940 kg ha−1, and these were inflated, as expected, compared to a limited number of yield increases estimated by mechanical combine, typically 310–630 kg ha−1, or about 5% of normal yield levels. Grain weight increases were associated with longer, thicker, and heavier cobs and slightly larger stover biomass. Plant nutrient concentrations were not affected at harvest. In-season measurements in a few intensively monitored farmers’ fields associated product application with slightly taller plants, increased leaf area, earlier onset of pollination, extended grain filling, and delayed senescence, i.e., extended duration of photosynthesis and decayed rotting of stems. Limited visual observations indicated great proliferation of roots, especially lateral roots. Ongoing data assessment will identify any environmental factors of product efficacy, an issue that to date remains unexplored in the humic product literature. Initial studies of alfalfa (Medicago sativa L.) found biomass increases with product application of 7–29%. A newly begun corn trial on nitrogen fertilizer response will estimate the amount of N fertilizer input that can be replaced by humic product application to save input costs and mitigate environmental degradation. The humic product increased economic yield in a large majority of cases by amounts that were agronomically modest but economically significant. Future work will expand to soybean (Glycine max (L.) Merr.) production.

Evaluation of a Proposed Standardized Analytical Method for the Determination of Humic and Fulvic Acids in Commercial Products

A commonly cited constraint to growth of the humic products industry has been the lack of a widely accepted procedure for determining humic and fulvic acid contents of commercial products. Several procedures have been used, but none is considered sufficiently quantitative, reproducible, and capable of discerning adulterants in the products.

Chemical nature of soil organic carbon under different long-term fertilization regimes is coupled with changes in the bacterial community composition in a Calcaric Fluvisol

Fertilization is an important factor influencing the chemical structure of soil organic carbon (SOC) and soil microbial communities; however, whether any connection exists between the two under different fertilization regimes remains unclear. Soils from a 27-year field experiment were used to explore potential associations between SOC functional groups and specific bacterial taxa, using quantitative multiple cross-polarization magic-angle spinning 13C nuclear magnetic resonance and 16S rRNA gene sequencing. Treatments included balanced fertilization with organic materials (OM) and with nitrogen (N), phosphorus (P), and potassium (K) mineral fertilizers (NPK); unbalanced fertilization without one of the major elements (NP, PK, or NK); and an unamended control. These treatments were divided into four distinct groups, namely OM, NPK, NP plus PK, and NK plus control, according to their bacterial community composition and SOC chemical structure. Soil total P, available P, and SOC contents were the major determinants of bacterial community composition after long-term fertilization. Compared to NPK, the OM treatment generated a higher aromatic C–O and OCH3 and lower alkyl C and OCH abundance, which were associated with the enhanced abundance of members of the Acidobacteria subgroups 6 and 5, Cytophagaceae, Chitinophagaceae, and Bacillus sp.; NP plus PK treatments resulted in a higher OCH and lower aromatic C–C abundance, which showed a close association with the enrichment of unclassified Chloracidobacteria, Syntrophobacteraceae, and Anaerolineae and depletion of Bacillales; and NK plus control treatments resulted in a higher abundance of aromatic C–C, which was associated with the enhanced abundance of Bacillales. Our results indicate that different fertilization regimes changed the SOC chemical structure and bacterial community composition in different patterns. The results also suggest that fertilization-induced variations in SOC chemical structure were strongly associated with shifts in specific microbial taxa which, in turn, may be affected by changes in soil properties.

Altered humin compositions under organic and inorganic fertilization on an intensively cultivated sandy loam soil

Humin is the most recalcitrant fraction of soil organic matter (SOM). However, little is known about quantitative structural information on humin and the roles of soil mircoorganisms involved in the humin formation. We applied advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy to provide deep insights into humin structural changes in response to long-term balanced fertilization on a Calcaric Fluvisol in the North China plain. The relationships between humin structure and microbiological properties such as microbial biomass, microbial quotient (qmic) and metabolic quotient (qCO2) were also studied. The humins had a considerable (35-44%) proportion of aromatic C being nonprotonated and the vast majority of O-alkyl and anomeric C being protonated. Alkyl (24-27% of all C), aromatic C (17-28%) and O-alkyl (13-20%) predominated in humins. Long-term fertilization promoted the aliphatic nature of humins, causing increases in O-alkyl, anomeric and NCH functional groups and decreases in aromatic C and aromatic CO groups. All these changes were more prominent for treatments of organic fertilizer (OF) and combined mineral NPK fertilizer with OF (NPKOF) relative to the Control and NPK treatments. Fertilization also decreased the alkyl/O-alkyl ratio, aromaticity and hydrophobic characteristics of humins, suggesting a more decomposed and humified state of humin in the Control soil. Moreover, the soil microbiological properties had strong correlations with functional groups of humins. Particularly, microbial biomass C was a relatively sensitive indicator, having positive correlations with oxygen-containing functional groups, i.e., COO/NCO and protonated O-alkyl C, and negative correlations with nonprotonated aromatic C. The qmic and qCO2 were also significantly positively correlated with NCH and aromatic CO, respectively. Our results deepen our understanding of how long-term fertilization impacts the structure of humin, and highlight a linkage between microbiological properties and recalcitrant fraction of SOM besides labile fraction.

Chemical Properties of Humic and Fulvic Acid Products and Their Ores of Origin

Commercial products consisting of humic and fulvic acids are thought to boost plant growth and economic yield in cropland agriculture. The specific mechanism for their benefit is unknown, in part because little information is available on the chemical nature of these products. We examined the chemical nature of eight humic acid products, three fulvic acids and seven of their source materials, lignite and leonardite ores, and humic shales. All samples were from the United States and Canada. Analysis by 13C cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy found that the humic acid products and the ores were dominated by large signals at aliphatic C (ca. 30 ppm) and aromatic C (ca. 130 ppm). These products were distinguished from one another by their relative proportions of aromatics and aliphatics and relative percentages of smaller signals for carboxyl C (approximately 175 ppm) and ketones/aldehydes C (ca. 200 ppm). These samples were also extracted for their concentrations of seven carbohydrates through anion exchange chromatography and pulsed amperometric detection. All ores and humic acid products had low concentrations of carbohydrates. The ores and products made without alkali extraction had slightly greater concentrations of carbohydrates than did extracted products, suggesting that alkali extraction for production of the humic acid products discriminated against carbohydrates. Results will also be discussed for sample concentrations of amino acids, two amino sugars, and lignin-derived phenols. Although these products share the same dominant chemical traits, they cannot be considered as chemically similar until the chemical mechanism for their effect on plant growth is identified and the corresponding compound(s) or functional group(s) is/are measured in these products and ores. Until then, field efficacy established for one product cannot be transferred to other products.