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To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkali-extractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice (Oryza sativa L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.

Chemical characteristics of dissolved organic matter (DOM) extracted from an ombrotrophic bog in northern Minnesota by two methods – XAD-8 and DEAE-cellulose – were compared using 13C NMR spectroscopy, excitation–emission matrix fluorescence spectroscopy (EEMS), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). DOM extracted with XAD-8, a relatively hydrophobic sorbent, had a larger 13C NMR signal in the aromatic carbon region, but DOM extracted by DEAE-cellulose, a weak anion exchanger, had a larger signal in the carboxylic carbon region. DOM extracts prepared by the two methods were similar in their overall chemical characteristics that serve as proxies for organic matter sources. For example, only small differences were observed in the fluorescence index (FI) values of the extracts, and all values were within the range expected for terrestrially-derived DOM. EEMS spectra of both extracts had “humic-like” peak A areas smaller than that of standard reference Suwannee River fulvic acid. Of 2801 distinct chemical formulas assigned during FT-ICR MS analysis of the two extracts, 66% were present in both; 15% were unique to the XAD-8 extract, and 19% were unique to the DEAE-cellulose extract. Van Krevelen plots showed that the DEAE-cellulose extract had more tannin-like and condensed aromatic entities and formulas with higher O/C ratios, whereas the XAD-8 extract had more lignin-like material and formulas with higher H/C ratios. Overall, differences in chemical characteristics of the extracts reflect the mechanisms by which the extractants operate.

Lake Pepin, a natural impoundment on the Upper Mississippi River, has water quality problems of high sedimentation rates and elevated phosphorus (P) levels. The majority of sediments in Lake Pepin come from river banks consisting of fine tills in the Minnesota River Basin. Since 1850, inorganic P concentrations in lake sediments have continuously increased. This study explored whether the increase in inorganic P concentrations can be explained through selective transport of fine particles combined with in-stream P adsorption. The measurements included total P (TP) content, P adsorption/desorption isotherms, and the solution equilibrium P concentration at zero adsorption (EPC) for various bank materials. Results showed that till bank materials are inherently high in TP (>400 mg kg), have strong P binding ability, and have low EPC (

Restoration of wetland hydrology can produce ecological benefits but may have unintended consequences. We examined effects of altered water level on release of dissolved reactive phosphorus (DRP) and greenhouse gases (GHG) in soil cores from a marsh being evaluated for restoration. We also measured field concentrations of DRP and other constituents in wetland porewater. Intact cores from a sampling location with higher Fe and lower calcium carbonate (CaCO(3)) contents released more DRP than another location, and displayed higher DRP under completely saturated compared to partly drained conditions. Porewater samples collected from the high-Fe location also contained higher DRP levels. Chemical data suggest that redox-driven reactions largely controlled DRP levels at the high-Fe site, while CaCO(3) adsorption was more important at the low-Fe site. Over the long term, water table elevation may attenuate P draining from the wetland due to decreased mineralization. However, such measures may increase P release in the short term. Raising the water level in soil cores resulted in decreased nitrous oxide (N(2)O) emissions, increased methane (CH(4)) emissions, and an overall increase in total global warming potential (GWP). The proportion of total GWP contributed by N(2)O decreased from 14% to < or = 1% as water level was raised, while the proportion contributed by CH(4) increased from 10 to 20% to 60 to 80%. Restoration of hydrology in the Rice Lake wetland has the potential to affect both local water quality and global air quality. These combined effects complicate the cost-to-benefit analysis of such wetland restoration efforts.

The binding of Hg{sup 2+} in organic matter of soils and waters controls the transport and transformations of Hg in terrestrial and aquatic ecosystems. The authors developed a competitive complexation method using the strong complexation of Hg{sup 2+} by Br{sup {minus}} for determining the Hg{sup 2+} binding strength in organic soils at native and elevated Hg concentrations. The distribution coefficients determined in KBr suspensions for sorption of native HG{sup 2+} to soil organic carbon (SOC) (K{sub soc}) are in the range of 10{sup 22} to 10{sup 23}. The K{sub soc} significantly decreased with increased additions of Hg{sup 2+} and with decreasing pH. Using data for reduced organic S concentrations determined by x-ray absorption near-edge structure spectroscopy (XANES), the authors calculated surface complex formation constants on the order of 10{sup 32} for a model site having acidity constants of mercaptoacetic acid. This value is in fair agreement with the tabulated value of 10{sup 345} for Hg{sup 2+} binding in mercaptoacetic acid. At native Hg concentrations, formation constants and K{sub soc} values were similar for different types of soil organic matter along transects from uplands into wetlands, despite varying concentration of Hg and reduced organic S. Their adsorption data are consistent withmore » the conclusions from their previous extended x-ray absorption fine structure spectroscopy (EXAFS) study that in a humic acid and soil, Hg{sup 2+} ions bond in two-fold coordination involving one reduced S and one O or N.« less

The need to understand the biological reactivity of polymers has increased in response to the integration of biological waste treatment systems (e.g., composting and soil application) into waste management plans of communities worldwide. The purpose of this study was to evaluate the compostibility and fate of a commonly used, synthetic polymer that is present in most solid waste streams. This was accomplished using a laboratory-scale composting system, simulated solid waste, and crosslinked 14 C-polyacrylate. Commercial preparations were composed primarily of a high-molecular-weight fraction (>500 kDa) that was resistant to degradation. Only the unpolymerized monomers and low-molecular-weight oligomers (totaling about 8% of the commercial preparation) were readily degraded during composting. Polymer crosslinkages were also degraded rapidly. Degradation of the low-molecular-weight fractions was considered significant, because these fractions have demonstrated a greater mobility potential in soils. Molecular size distribution and infrared (IR) spectra of extractable organic fractions confirmed that the structural integrity of the polymer chains was maintained during composting. These residual, high-molecular-weight polyacrylates could improve soil structure, and should continue to degrade slowly leaving no toxic residues.

Boron is an essential plant nutrient having a narrow range between concentrations that cause deficiency or toxicity symptoms in plants. Boric acid (B[OH] 3 ) and silicic acid (Si[OH] 4 ) have many chemical similarities and both are thought to be associated with organic matter in soils. The objectives of this study were to investigate the variation in boric and silicic acid adsorption and desorption in low ash humic acid over a range of B and Si concentrations and pH values. Borate adsorption by humic acid increased significantly from pH 3.67 to 8.70. The data fit the Freundlich equation better than the Langmuir model at high pH ; however, both fit well at pH 10. The constant capacitance model predicted B adsorption and the pH maximum using catechol as the adsorbing ligand and published stability constants. The predicted adsorption envelope was narrower than experimental data. This could be due to variation in stability constants that result from variation in substitution on other sites on the aromatic ring. Inclusion of glucose and salicylate in the adsorption model would also broaden the adsorption envelope by increasing adsorption at pH > 9 and < 9, respectively. Desorption of B from humic acid appears to show hysteresis effects at pH 7.6 but not at pH 8.15 and 8.7. Generally, B will be easily released from HA as solution concentrations decrease. Silicic acid did not adsorb in humic acid. The stability constant of Si-catechol humate complexes is apparently too low for adsorption of Si to occur at the pH and solution concentrations used in this experiment.

▪ Pollution control workers have proposed composting as a bioremediation method for the cleanup of contaminated soil and for the detoxification of hazardous organic chemicals. However, decomposition (mineralization to CO2 and H2O) may not always be enhanced by composting, and transformation products of unknown risk may accumulate during composting. This paper presents a scientifically based scheme to determine the ultimate fate of synthetic materials in a composting environment, and for studying factors that may control the degradation of specific chemicals including herbicides (e.g., substrate bioavailability, enzyme concentrations, environmental conditions).

The use of organic residues as soil additives is increasing, but, depending on their composition and application methods, these organic amendments can stimulate the emissions of CO(2) and N(2)O. The objective of this study was to quantify the effects of management practices in irrigated sweet corn (Zea mays L.) on CO(2) and N(2)O emissions and to relate emissions to environmental factors. In a 3-yr study, corn residues (CR) and pasteurized chicken manure (PCM) were used as soil amendments compared with no residue (NR) under three management practices: shallow tillage (ST) and no tillage (NT) under consecutive corn crops and ST without crop. Tillage significantly increased (P < 0.05) CO(2) and N(2)O fluxes in residue-amended plots and in NR plots. Carbon dioxide and N(2)O fluxes were correlated with soil NH(4) concentrations and with days since tillage and days since seeding. Fluxes of CO(2) were correlated with soil water content, whereas N(2)O fluxes had higher correlation with air temperature. Annual CO(2) emissions were higher with PCM than with CR and NR (9.7, 2.9, and 2.3 Mg C ha(-1), respectively). Fluxes of N(2)O were 34.4, 0.94, and 0.77 kg N ha(-1) yr(-1) with PCM, CR, and NR, respectively. Annual amounts of CO(2)-C and N(2)O-N emissions from the PCM treatments were 64 and 3% of the applied C and N, respectively. Regardless of cultivation practices, elevated N(2)O emissions were recorded in the PCM treatment. These emissions could negate some of the beneficial effects of PCM on soil properties.

The chemical composition of the soil solution provides used information on the feasibllity of amending agricultural land with municipal and industrial waste, because the soil solution is the medium for most soil chemical reactions, the mobile phase in soils, and the medium for mineral absorption by plant roots. The soil solutions studied in tits research were from plots in a 4-yr field experiment conducted to evaluate the effects of the trace metals and P in sewage-sludge incinerator ash. Treatments compared ash with equivalent P rates from triple-superphosphate fertilizer and a control receiving no P application. Ash and phosphate fertilizer were applied annually at rates of 35, 70, and 140 kg citrate-soluble P ha -1 . Cognitive ash applications during 4 yr amounted to 3.6, 7.2, and 14.4 Mg ash ha -1 . Soil solutions were obtained by centrifugation-immiscible liquid displacement using a fluorocarbon displacing agent. Following chemical analysis, a chemical speciation model was used to determine possible solubility-controlling minerals for trace metals and P, and correlations between solution composition and plant uptake were analyzed. Ash increased soil solution pH, Cd, and Zn, but bad no significant effect on solution concentrations of other trace metals. Ash increased soil solution P and S, but P increases were less than those from equivalent citrate-soluble P rates of phosphate fertilizer. Soil solution Ba appeared to be in equilibrium with barite (BaSO 4 ). Solubility data did not indicate that any discrete general phases controlled Cd, Zn, Cu, Ni, Pb, or P solubility. Soil solution P concentration was strongly correlated (r=0.92) with P accumulation in sweet corn (Zea mays L.) plants, but solution trace metal concentrations were either weakly correlated (r=0.49 for Zn and 0.36 for Cd) or not significantly correlated (r=0.09 for Ni and -0.25 for Cu) with plant accumulation

A simple semiempirical model for the prediction of changes in soil pH and base saturation with acidic deposition was developed. In steady-state ecosystems acidic deposition results in losses of basic cations from exchange sites. In the model, depletion of exchangeable bases is calculated from the difference between input acidity and output of H/sup +/ and Al/sup 3 +/ in water percolating through the solum. A correction for the decrease in bicarbonate weathering is made but sulfate adsorption is not considered. Estimates including the effect of increased mineral weathering with decreased pH can be made if the natural weathering rate is known and if the order with respect to H/sup +/ is known for the rate expression for soil weathering. The model predicted laboratory data for the acidification of samples for three soil horizons. For a fourth horizon, that was formed in contact with bedrock and high in weatherable minerals, the model overestimated the effect of the acid. Model predictions using soil data showed a slow decrease in soil pH and base saturation until the pH region of Al buffering was attained and then a new steady-state was achieved. The pH and base saturation in the Al buffering region was similarmore » to that found in very acid soils under forest vegetation.« less