Institute of Agriculture and Natural Resources

Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) catalyzes the first step in net photosynthetic CO2 assimilation and photorespiratory carbon oxidation. The enzyme is notoriously inefficient as a catalyst for the carboxylation of RuBP and is subject to competitive inhibition by O2, inactivation by loss of carbamylation, and dead-end inhibition by RuBP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Resolution of X-ray crystal structures and detailed analysis of divergent, mutant, and hybrid enzymes have increased our insight into the structure/function relationships of Rubisco. The interactions and associations relatively far from the Rubisco active site, including regulatory interactions with Rubisco activase, may present new approaches and strategies for understanding and ultimately improving this complex enzyme.

Abstract The mixed model equations as presented by C. R. Henderson offers the base for a methodology that provides flexibility of fitting models with various fixed and random elements with the possible assumption of correlation among random effects. The advantage of teaching analysis of variance applications from this methodology is presented. Particular emphasis is placed upon the relationship between choice of estimable function and inference space.

This chapter reviews numerous definitions of drought to determine those characteristics scientists consider most essential for a description and understanding of the phenomenon. It discusses the far-reaching impacts of drought on society. The chapter suggests that definitions of drought are typically simplistic and, in that way, often lead to a rather poor understanding of the dimensions of the concept. It also suggests that definitions of drought should not be formulated in a narrow sense, but rather should incorporate both physical and social measures that have a local or regional significance. Drought is frequently defined according to disciplinary perspective. Drought's impacts must be seen as dynamic, resulting from interactions between supply and demand. The inadequate understanding of the concept of drought and the lack of appreciation of its physical and social impacts by the scientific community and governments has serious worldwide implications for the future as the difference between food production and consumption narrows.

Abstract Global-change investigations have been hindered by deficiencies in the availability and quality of land-cover data. The U.S. Geological Survey and the University of Nebraska-Lincoln have collaborated on the development of a new approach to land-cover characterization that attempts to address requirements of the global-change research community and others interested in regional patterns of land cover. An experimental 1 -kilometer-resolution database of land-cover characteristics for the coterminous U.S. has been prepared to test and evaluate the approach. Using multidate Advanced Very High Resolution Radiometer (AVHRR) satellite data complemented by elevation, climate, ecoregions, and other digital spatial datasets, the authors define 152, seasonal land-cover regions. The regionalization is based on a taxonomy of areas with respect to data on land cover, seasonality or phenology, and relative levels of primary production. The resulting database consists of descriptions of the vegetation, land cover, and seasonal, spectral, and site characteristics for each region. These data are used in the construction of an illustrative 1:7,500,000-scaIe map of the seasonal land-cover regions as well as of smaller-scale maps portraying general land cover and seasonality. The seasonal land-cover characteristics database can also be tailored to provide a broad range of other landscape parameters useful in national and global-scale environmental modeling and assessment.

Dune fields in parts of northern China contain important stratigraphic records of late Quaternary change in the East Asian monsoon. In this study, 33 new optically stimulated luminescence (OSL) ages and other measurements from aeolian sediment sections are used to reconstruct the timing of wet‐dry climate variation in the Mu Us and Otindag dune fields of north China. The results indicate dune activity and dry climate in the last few hundred years, 14 ka to about 7–8 ka, and 50 ka to 60 ka. The dunes were mainly stable, implying a wetter climate, between about 7–8 ka and 2.4 ka. These results imply a lag of several thousand years between peak summer insolation at 10–11 ka and high summer monsoon rainfall after 7–8 ka. In the investigated regions, the monsoon climate may not respond directly to orbital forcing over millennial time scales. Land surface feedbacks may account for lagged dune field response.

The numbers of glutathione S-transferase, cytochrome P450 and esterase genes in the genome of the hymenopteran parasitoid Nasonia vitripennis are about twice those found in the genome of another hymenopteran, the honeybee Apis mellifera. Some of the difference is associated with clades of these families implicated in xenobiotic resistance in other insects and some is in clades implicated in hormone and pheromone metabolism. The data support the hypothesis that the eusocial behaviour of the honeybee and the concomitant homeostasis of the nest environment may obviate the need for as many gene/enzyme systems associated with xenobiotic metabolism as are found in other species, including N. vitripennis, that are thought to encounter a wider range of potentially toxic xenobiotics in their diet and habitat.

Research Article| April 01, 1995 Dune-dammed paleovalleys of the Nebraska Sand Hills: Intrinsic versus climatic controls on the accumulation of lake and marsh sediments David B. Loope; David B. Loope 1Department of Geology, University of Nebraska, Lincoln, Nebraska 68588 Search for other works by this author on: GSW Google Scholar James B. Swinehart; James B. Swinehart 2Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, Nebraska 68588 Search for other works by this author on: GSW Google Scholar Jon P. Mason Jon P. Mason 1Department of Geology, University of Nebraska, Lincoln, Nebraska 68588 Search for other works by this author on: GSW Google Scholar Author and Article Information David B. Loope 1Department of Geology, University of Nebraska, Lincoln, Nebraska 68588 James B. Swinehart 2Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, Nebraska 68588 Jon P. Mason 1Department of Geology, University of Nebraska, Lincoln, Nebraska 68588 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1995) 107 (4): 396–406. https://doi.org/10.1130/0016-7606(1995)107<0396:DDPOTN>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David B. Loope, James B. Swinehart, Jon P. Mason; Dune-dammed paleovalleys of the Nebraska Sand Hills: Intrinsic versus climatic controls on the accumulation of lake and marsh sediments. GSA Bulletin 1995;; 107 (4): 396–406. doi: https://doi.org/10.1130/0016-7606(1995)107<0396:DDPOTN>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Although running water is the dominant geomorphic agent on Earth, eolian processes can gain ascendancy in regions where the climate is arid, vegetation is sparse, and abundant sand is available for transport. With climate change, the boundaries between fluvial-dominated and eolian-dominated areas may shift. Although there have been few reports in the North American literature of river systems blocked by dune sand, our work in the Nebraska Sand Hills provides evidence of multiple episodes of such blockage events. During prolonged arid intervals in latest Pleistocene and middle Holocene time, eolian dune sand blocked two large valley systems in western Nebraska. These blockages raised the water table of the High Plains aquifer as much as 25 m over an area of 7000 km2 and created over one thousand lakes. Wetlands far removed from the discharge points of the buried paleovalley system are strongly alkaline (exceeding 250 000 mg/L total dissolved solids [TDS]). Relatively fresh (280 mg/L TDS), flow-through lakes are present at the distal end of the system where the gradient of the water table is steep and the cross section of the buried valley is large. Anomalously thick marsh and lake sediments accumulated in deep paleovalleys upstream of dune dams near the southern margin of the Sand Hills. Our cores and radiocarbon dates from Blue and Crescent Lakes reveal their histories to be quite distinct from adjacent Swan Lake; these differences are best explained by multiple blockage events. Our work explains why lakes are most abundant in the driest part of the Sand Hills. It also provides another line of evidence for major dune activity in the Sand Hills region during Holocene time and shows that factors other than regional climate, specifically location, height, and hydraulic conductivity of dune dams, can control the rise and fall of the ground-water table and the chemistry of lakes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Significant progress has been made in using remote sensing as a means of acquiring information about wetlands. This research provides a brief review of selected previous works, which address the issues of wetland identification, classification, biomass measurement, and change detection. Suggested new research emphases include compiling basic spectral‐reflectance characteristics for individual wetland species by means of close‐range instrumentation, analyzing canopies architectures to facilitate species identification, and assessing the impact on composite spectral signatures of wet soils and variable depths of standing water beneath emergent canopies. These research foci are justifiable when considered in the context of environmental change / variability and the production of trace gases.

BACKGROUND: Osteopetrosis is a skeletal disorder of humans and animals characterized by the formation of overly dense bones, resulting from a deficiency in the number and/or function of bone-resorbing osteoclast cells. In cattle, osteopetrosis can either be induced during gestation by viral infection of the dam, or inherited as a recessive defect. Genetically affected calves are typically aborted late in gestation, display skull deformities and exhibit a marked reduction of osteoclasts. Although mutations in several genes are associated with osteopetrosis in humans and mice, the genetic basis of the cattle disorder was previously unknown. RESULTS: We have conducted a whole-genome association analysis to identify the mutation responsible for inherited osteopetrosis in Red Angus cattle. Analysis of >54,000 SNP genotypes for each of seven affected calves and nine control animals localized the defective gene to the telomeric end of bovine chromosome 4 (BTA4). Homozygosity analysis refined the interval to a 3.4-Mb region containing the SLC4A2 gene, encoding an anion exchanger protein necessary for proper osteoclast function. Examination of SLC4A2 from normal and affected animals revealed a approximately 2.8-kb deletion mutation in affected calves that encompasses exon 2 and nearly half of exon 3, predicted to prevent normal protein function. Analysis of RNA from a proven heterozygous individual confirmed the presence of transcripts lacking exons 2 and 3, in addition to normal transcripts. Genotyping of additional animals demonstrated complete concordance of the homozygous deletion genotype with the osteopetrosis phenotype. Histological examination of affected tissues revealed scarce, morphologically abnormal osteoclasts displaying evidence of apoptosis. CONCLUSIONS: These results indicate that a deletion mutation within bovine SLC4A2 is associated with osteopetrosis in Red Angus cattle. Loss of SLC4A2 function appears to induce premature cell death, and likely results in cytoplasmic alkalinization of osteoclasts which, in turn, may disrupt acidification of resorption lacunae.

In order to determine the prevalence and distribution of the human pathogen, Escherichia coli O157:H7, in free-ranging deer, hunters were asked to collect and submit fecal samples from deer harvested during a regular firearm season (14-22 November 1998). Prior to the season, 47% of the hunters with permits in the southeastern Nebraska (USA) study area indicated a willingness to participate in the study. Approximately 25% of successful hunters in the area submitted deer fecal samples. Escherichia coli O157:H7 was cultured from four (0.25%) of 1,608 total samples submitted. All of the fecal samples that were properly identified (1,426) and all that were positive for E. coli O157:H7 were from white-tailed deer (Odocoileus virginianus). We were unable to detect a statistically significant geographic distribution pattern of E. coli O157:H7. The presence of E. coli O157:H7 in the feces of free-ranging deer has implications not only for hunters, consumers of venison, and others in contact with deer or deer feces, but also for the development of strategies aimed at reducing and/or controlling this pathogen in water sources and domestic livestock.

Gross and microscopic lesions and distribution of virus were studied in specific pathogen-free calves (SPF) 10 days post-inoculation (PI) with bovine viral diarrhea virus (BVDV). To investigate possible differences in tissue tropism between BCDV isolates, two clinically and antigenically different noncytopathic (ncp) isolates of BVDV were compared in the study. Four calves were exposed to noncytopathic (ncp) BVDV 7937, and four to ncp-BVDV 126. Two additional calves that were not exposed to virus served as controls. Both ncp-BVDV 7937 and ncp-BVDV 126 induced mild disease characterized by variable fever and anorexia. Lymphoid depletion was evident in Peyer's patch of four calves and the thymus of two calves exposed to BVDV. Differences between these isolates in the distribution of BVDV or BVDV antigen in tissues of inoculated calves were not found. High concentrations of BVDV and BVDV-specific antigen were detected in the thymus, Peyer's patch, and mesenteric lymph node of all exposed calves. BVDV was shown to infect cells of the bone marrow without causing microscopic lesions. High concentrations of BVDV were recovered from the bone marrow of all calves exposed to BVDV and BVDV-specific antigen was demonstrated at this location in six of these calves. Platelet counts of calves exposed to BVDV were significantly reduced during infection, which resulted in thrombocytopenia in one calf. Focal areas of necrosis were observed in squamous epithelial cells of the tonsil and ruminal mucosa. BVDV-specific antigen was found in and adjacent to these foci. Calves exposed to ncp-BVDV 7937 had slightly more severe clinical signs than those exposed to ncp-BVDV 126.

Three winter treatments were cross classified with 2 spring treatments to create 6 feeding and grazing systems utilizing Nebraska sandhills range and subirrigated meadow forage. Systems were evaluated with multiparous crossbred beef cows over 4 years (240 head beginning year 1). Systems were: 1) owing range during winter; 2) grazing subirrigated meadow during winter; and 3) fur feed of meadow bay during winter; in combination with either: a) full feed of subirrigated meadow hay during May, or b) grazing subirrigated meadow during May. From June through November all cows grazed range. The feeding and grazing systems were compared with selected linear contrasts and evaluated with respect to variable input prices. Some differences in cow body weight and body condition occurred but differences were considered small. Throughout the study, cows on all systems generally maintained a body condition score of about 5 (1 to 9 scale) year long. Inputs of hay were reduced by grazing range or subirrigated meadow during winter and during May without affecting pregnancy rate. Weaning weight of calves was increased 5.0 kg by grazing meadow during May compared to feeding hay during May. When opportunity costs were included in the analysis, the most profitable system involved grazing subirrigated meadow during winter and during May. Grazing subirrigated meadow during May enhanced the profitability of all wintering systems.

Abstract Soils surrounding former munitions production facilities are highly contaminated with 2,4,6‐trinitrotoluene (TNT). Long‐term availability and fate of TNT and its transformation products must be understood to predict environmental impact and develop appropriate remediation strategies. Sorption and transport in surface soil containing solidphase TNT are particularly critical, since nonlinear sorption isotherms indicate greater TNT availability for transport at high concentrations. Our objectives were to determine long‐term sorption and bound residue formation in surface and subsurface Sharpsburg soil (Typic Argiudoll). Prolonged equilibration of 14 C‐TNT with the soil revealed a gradual increase in amount sorbed and formation of unextractable (bound) 14 C residues. The presence of solid‐phase TNT did not initially affect the amount of 14 C sorbed during a 168‐d equilibration. After 168 d, 93% of the added 14 C was sorbed by uncontaminated soil, while 79% was sorbed by soil containing solid‐phase TNT. In the absence of solid phase, pools of readily available (extractable with 3 m M CaCl 2 ) and potentially available (CH 3 CN‐extractable) sorbed TNT decreased rapidly with time and coincided with increased 14 C in soil organic matter. More 14 C was found in fulvic acid than in the humic acid fraction when no solid‐phase TNT was present. After sequential extractions, including strong alkali and acid, 32 to 40% of the sorbed 14 C was irreversibly bound (unextractable) in Sharpsburg surface and subsurface soil. Results provide strong evidence for humification of TNT in soil. This process may represent a significant route for detoxification in the soil‐water environment.

Past disposal of wastewaters containing 2,4,6-trinitrotoluene (TNT) at the former Nebraska Ordnance Plant has resulted in numerous acres of TNT-contaminated soil. Examining the microbial population of these soils revealed several TNT-tolerant Pseudomonas spp. We selected one species, P. savastanoi, to determine its ability to transform TNT. Pure culture experiments were performed in pseudomonas minimal medium containing 0.31 mM TNT (70 mg TNT . L(-1)) under varied nutrient and cell density regimes. Experiments with TNT as a sole C or N source showed that P. savastanoi has the ability to denitrate TNT, as evidenced by production of 2,4-dinitrotoluene (2,4-DNT) and NO2- with time. TNT denitration and formation of 2,4-DNT were enhanced by removing NH4+ and adding NO2- to the growth medium. In all experiments, 2-amino-4,6-dinitrotoluene (2-ADNT) and 4-amino-2,6-dinitrotoluene (4-ADNT) appeared as incidental reduction products. Glucose addition to the medium enhanced 2-ADNT and 4-ADNT production and decreased denitration of TNT. Mid-log phase cells rapidly transformed [ring-14C(U)]TNT but were unable to mineralize significant quantities of TNT, as evidenced by conversion of less than 1% of the label to 14CO2. These results indicate that P. savastanoi is a TNT-tolerant pseudomonad that can promote TNT degradation through reductive denitration and nitro moiety reduction.

Abstract Geochemical and geomorphic data from dune fields in southwestern Nebraska provide new evidence that the Nebraska Sand Hills once migrated across the North and South Platte rivers and dammed the largest tributary system to the Missouri River. The Lincoln County and Imperial dune fields, which lie downwind of the South Platte River, have compositions intermediate between the Nebraska Sand Hills (quartz-rich) and northeastern Colorado dunes (K-feldspar-rich). The most likely explanation for the intermediate composition is that the Lincoln County and Imperial dunes are derived in part from the Nebraska Sand Hills and in part from the South Platte River. The only mechanism by which the Nebraska Sand Hills could have migrated this far south is by complete infilling of what were probably perennially dry North Platte and South Platte river valleys. Such a series of events would have required an extended drought, both for activation of eolian sand and decreased discharges in the Platte River system. A nearby major tributary of the North Platte River is postulated to have been blocked by eolian sand about 12,000 14 C yr B.P. We propose that an eolian sand dam across the Plattes was constructed at about this same time.

The impact of 16 years (1988-2003) of management practices on high groundwater nitrate concentrations in Nebraska's central Platte River valley was assessed in a 58,812-ha (145,215-ac) groundwater quality management area intensively cropped to irrigated corn (Zea mays L.). Crop production and groundwater nitrate data were obtained from approximately 23,800 producer reports. The terrace, comprising approximately 56% of the study area, is much more intensively cropped to irrigated corn than the bottomland. From 1987 to 2003, average groundwater nitrate concentrations in the primary aquifer beneath the bottomland remained static at approximately 8 mg N/l. During the same period, average groundwater nitrate concentrations in the primary aquifer beneath the terrace decreased from 26.4 to 22.0 mg N/l at a slow, but significant (p < 0.0001), rate of 0.26 mg N/l/year. Approximately 20% of the decrease in nitrate concentrations can be attributed to increases in the amount of N removed from fields as a consequence of small annual increases in yield. During the study, producers converted approximately 15% of the approximately 28,300 furrow-irrigated terrace hectares (approximately 69,800 ac) to sprinkler irrigation. The conversion is associated with about an additional 50% of the decline in the nitrate concentration, and demonstrates the importance of both improved water and N management. Average N fertilizer application rates on the terrace were essentially unchanged during the study. The data indicate that groundwater nitrate concentrations have responded to improved management practices instituted by the Central Platte Natural Resources District.

Abstract Dissolved pesticide concentrations in 168 water samples collected from two closely‐spaced lakes between early May 1990 and mid‐June 1991 indicated large differences in impacts from watershed nonpoint source inputs. Pesticide levels in Maskenthine Lake, a small impoundment of 34 ha, increased in response to spring and early summer runoff events. Twelve pesticides [atrazine, 2‐chloro‐4‐ethylamino‐6‐isopropylamino‐ s ‐triazine; alachlor, 2‐chloro‐ N ‐(2,6‐diethylphenyl)‐ N ‐(methoxymethyl)acetamide; metolachlor, 2‐chloro‐ N ‐(2‐ethyl‐6‐methylphenyl)‐ N ‐(2‐methoxy‐1‐methylethyl)acetamide; cyanazine, 2‐[[4‐chloro‐6‐(ethylamino)‐ s ‐triazin‐2‐yl]amino]‐2‐methylpropionitrile; EPTC, S ‐ethyl dipropylthiocarbamate; butylate, S ‐ethyl diisobutylthiocarbamate; propachlor, 2‐chloro‐ N ‐(1‐methylethyl)‐ N ‐phenylacetamide; trifluralin, α,α,α‐trifluoro‐2, 6‐dinitro‐ N,N ‐dipropyl‐ p ‐toluidine; simazine, 2‐chloro‐4,6‐bis(ethylamino)‐ s ‐triazine; propazine, 2‐chloro‐4,6‐bis(isopropylamino)‐ s ‐triazine; fonofos, O ‐ethyl S ‐phenyl ethylphosphonodithioate; metribuzin, 4‐amino‐6‐(1,1‐dimethylethyl)‐3‐(methylthio)‐1,2,4‐triazin‐5(4H)‐one] and two atrazine metabolites, deethylatrazine, 2‐amino‐4‐chloro‐6‐isopropylamino‐ s ‐triazine (DEA) and deisopropylatrazine, 2‐amino‐4‐chloro‐6‐ethylamino‐ s ‐triazine (DIA) were detected. Atrazine, cyanazine, DEA and DIA levels were greater than 1 µg/L. Atrazine remained above the maximum contaminant level for potable water of 3 µg/L throughout the period of investigation. The pesticide response to spring and summer runoff events was much less pronounced at Willow Lake, a 284 ha impoundment. Ten of 12 pesticides and two atrazine metabolites in Maskenthine Lake were also detected in Willow Lake; however, concentrations did not exceed 1 µg/L. After the spring flush of pesticides in May and June, concentrations decreased until the following spring. Ratios of DEA to atrazine and DIA to atrazine remained relatively constant suggesting that appreciable amounts of biodegradates were not produced. Apparent half‐lives were calculated from observed concentration decreases that appeared to follow first‐order kinetics. Atrazine was the most persistent agrichemical ( t 1/2 = 193, t 1/2 = 124) introduced in the spring runoff events. Extracted pesticide levels were higher hi bottom cores from Maskenthine Lake than from Willow Lake. Differences in pesticide levels in the two lakes were related to watershed slope, soil drainage capacity, landuse, and rainfall.

Abstract The reflectance spectra of suspended sediments derived from a silty soil were analyzed in an effort to measure suspended sediment concentration (SSC) in surface waters. Twenty levels of SSC (50 to 1000 mg/l) were created in a large tank filled with 7510 liters of clear water. Reflectance measurements were made using a high resolution spectroradiometer. The spectroradiometer data were integrated into the band widths of Landsat‐TM. The simulated TM bands were then used to develop regression models for estimating SSC. We found that the spectroradiometer data, integrated into the band width of Landsat‐TM 4, when used in second‐order regression models, is the best estimator of SSC.

Drought relief has become an expected response of the federal government to periods of widespreaddrought in the United States. A wide range of emergency, short-term and long-term drought programs wereformulated to deal with the extreme drought of the 1930's. By 1977 there were 40 separate programsadministered by 16 different federal agencies. This paper traces the development of federal drought reliefbureaucracy, including drought designation criteria and procedures. Two obvious deficiencies of past drought relief efforts in the United States are noted. First, drought relieforganizations and programs have been hastily assembled during periods of crisis. This has resulted in reducedprogram effectiveness as implementation has often been delayed pending program formulation and congressional approval. Second, previous efforts have had no adequate and systematic provision of timely information on drought conditions and impacts to persons or agencies involved in administering drought programs. The design, implementation and evaluation of a near real-time drought surveillance and early warningsystem is mentioned as one way atmospheric scientists can contribute to improved drought response in theUnited States.

Abstract Dredged sediments from the Gulf Intracoastal Waterway near Galveston, Tex., were used as a substrate material in the construction of an experimental intertidal salt marsh. The objective of this research was to compare selected substrate properties of the experimental site with those of established marshes. Clay mineralogical properties of the experimental marsh were compared with those of three nearby natural marshes. A sequential chemical extraction procedure was used to obtain data on the partitioning of micronutrients and heavy metals among selected marsh substrate fractions. Clay minerals found in the sediments of the experimental marsh were equivalent to those identified in the natural marshes. Total elemental substrate concentrations of Cu, Ni, Cr, Zn, Mn, and Fe averaged 7.9, 8.6, 25.5, 25.2, 123, and 12,200 µ g/g, respectively, over the four marsh sites. Copper, nickel, zinc, and chromium displayed only minor variations in substrate partitioning between the experimental and natural marsh samples. Micronutrients and heavy metal concentrations in the exchangeable and water‐soluble fraction were low compared with other fractions. Approximately 30% of the total substrate Cu, Ni, and Zn was associated with the organic matter fraction. Metals fixed within the lattice structures of clay and silicate minerals ranged from 20% Mn for experimental marsh samples to 90% Cr for one of the natural marshes. Major differences in Mn and Fe substrate partitioning were observed when the experimental marsh samples were compared with those of the natural marshes.