Salt Marshes on Substrate Enriched in Organic Matter: The Case of Ombrogenic Atlantic Salt Marshes

Ombrogenic Atlantic salt marshes are defined as areas of halophytic, terrestrial vegetation which are periodically flooded by the tide and have a predominant underlying organic substrate comprising of wood and/or Sphagnum peat that formed under freshwater conditions. The objective of this study was to determine to what extent salt marsh plant ecology and, specifically, vegetation composition and zonation relate to this underlying substrate of organic matter (peat). A vegetation survey was carried out on nine salt marshes, three on peat substrate and two on sand, mud and sand/mud, respectively. In parallel, key edaphic variables were measured including pH, conductivity, organic content, moisture content and nutrients: ammonium, nitrate and phosphorus. Salt marshes on peat substrate are distinct. Ammonium content was twice the maximum reported in other salt marsh studies, while the vegetation composition of salt marshes on peat substrate was significantly different from that of other salt marshes. Salt marshes on peat substrate were found to be higher in species diversity and richness and characterised by a predominantly forb and rush community. However, some common salt marsh species, such as Atriplex portulacoides and Spartina anglica were absent from salt marshes on peat. Ordination analysis revealed that zonation was primarily associated with conductivity on peat substrates. In contrast, moisture plays a greater role in zonation within non-peat salt marshes. The findings confirm that the high organic matter content of ombrogenic Atlantic salt marshes is associated with distinct vegetation composition.     

Organic solids produced by electrical discharge in reducing atmospheres: Tholin molecular analysis

The complex dark brown solid of a class called tholins, produced on passage of an electrical discharge through a roughly equimolar mixture of methane and ammonia with 2.6% water vapor, is analyzed by vacuum pyrolysis followed by gas chromatography and mass spectrometry. Pyrolyzates include a wide range of aliphatic and aromatic nitriles, alkanes, aromatic hydrocarbons, pyrrole, and pyridine. This tholin is remarkably stable to 950°C. It and its degradation products are candidate constituents of planetary aerosols in the outer solar system and of the grains in the interstellar medium.     

Enriched H2, CH4, and He concentrations in hydrothermal plumes associated with the 1996 Gorda Ridge eruptive event

The rapid field response in March and April 1996 to diking-eruptive events on the Northern Gorda Ridge allowed the first detailed co-measurement of ³He, H₂, and CH₄ in plumes associated with a submarine eruption, and examination of the chemical transformation of these volatile species during aging of the plumes. The diking-eruptive event generated extensive event plumes with a low ³He/heat ratio of ∼0.4×10^-12 cm³ STP gm^-1 °C^-1, while chronic plumes had ³He/heat ratios of 2.1–6.2×10^-12 cm³ STP gm^-1 °C^-1. Hydrogen concentrations in the initial event plume (EP96A) reached a maximum value of 47 nM, 235 times that of ocean background concentration (0.2 nM). In contrast, roughly one month later H₂ concentrations, although still elevated, had drastically dropped to 40 times background values. Methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments, reaching a maximum value of 7 nM. The intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high, localized concentrations of H₂. The data indicate, however, that maximum rates of H₂ production during event plume formation could have lasted a few days at most.     

Experimental hydrothermal alteration of a martian analog basalt: Implications for martian meteorites

Abstract— A number of martian meteorite samples contain secondary alteration minerals such as Ca‐Mg‐Fe carbonates, Fe oxides, and clay minerals. These mineral assemblages hint at hydrothermal processes occurring in the martian crust, but the alteration conditions are poorly constrained. This study presents the results of experiments that examined the alteration of a high‐Fe basalt by CO₂‐saturated aqueous fluids at 23 and 75 °C and by mixed H₂O‐CO₂ vapors at 200 and 400 °C and water‐rock ratios of 1:1 and 1:10. Results indicate that observable alteration of the basalt takes place after runs of only seven days. This alteration includes mobilization of silica into phases such as opal‐CT and quartz, as well as the formation of carbonates, oxides, and at some conditions, zeolites and hydrous silicates. The degree of alteration increases with run temperature and, in high‐temperature vapor experiments, with increasing water content of the vapor. The degree of alteration and the mineralogy observed in the martian meteorites suggests that none of these samples were exposed to aqueous fluids for long periods of time. Nakhla and Lafayette probably interacted with water for relatively brief periods of time; if so, silica may have been leached from the parent rocks by the altering fluids. Allan Hills 84001 shows possible evidence for very limited interaction with an aqueous fluid, but the overall slight degree of alteration described for this meteorite strongly suggests that it never interacted extensively or at high temperature with any water‐bearing fluid. Elephant Moraine A79001 may not have been altered by aqueous fluids at all. The results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models that invoke long‐term reaction with large volumes of water. Our experiments studied alteration of a high‐Fe basalt by dilute, CO₂‐saturated, aqueous solutions at 23 and 75 °C and by mixed H₂O‐CO₂ vapors at 200 and 400 °C. The results suggest that alteration of the parent rock takes place even after very short reaction times of seven days. All experiments produced carbonate minerals, including calcite, and in some cases, magnesite, siderite, and ankerite. A free silica phase, either opal, quartz, or hydrated silica, formed in most experiments. More altered experiments also contained minerals such as zeolites and hydrous phyllosilicates. Clay minerals were not observed to form in any experiments. In aqueous fluids, higher temperature corresponded with a higher degree of alteration, whereas changing fluid composition had no observable effect. In high‐temperature vapors, the degree of alteration was controlled by temperature and the proportion of H₂O to CO₂, with water‐rock ratio also playing a role in transport of silica. Application of these results to martian meteorites that contain secondary alteration minerals suggests that none of the martian rocks underwent extensive interaction with aqueous fluids. Nakhla and Lafayette contain clay minerals, which suggests that they interacted with water to some extent, possibly at elevated temperatures. Although ALH84001 shows possible evidence of very limited interaction with aqueous fluids, EETA79001 does not. These results support models for the alteration of these meteorites that do not invoke long‐term interaction with water or reaction with large volumes of water. Except for some models for alteration of ALH84001, this conclusion agrees with most of the literature on alteration of martian meteorites.     

America's Changing Farmscape: A Study of Agricultural Tourism in Michigan*

Agricultural tourism incorporates visits to farms for the purposes of on‐site retail purchases, enjoyment, and education. Long popular in the European Union (EU), agritourism is gaining popularity throughout the United States. Interest has grown as a result of stagnant grain prices, rising farm costs, and growing international competition. For rural areas seeking new economic options, the potential of these operations to generate new sources of income through sales and horizontal linkages to other tourism‐based activities has sparked interest beyond the farm gate. This article, based on a survey and a statistical analysis of 300 agritourism operations in Michigan, summarizes factors associated with successful operations.     

The Bioavailability of Effluent-derived Organic Nitrogen along an Estuarine Salinity Gradient

Total maximum daily loads for nitrogen (N) are currently being established for the Chesapeake Bay watershed. While we know inorganic N is bioavailable in the environment and therefore its input contributes to cultural eutrophication, the bioavailability of organic N is unclear. Using bioassay experiments, we examined the impact of effluent-derived organic nitrogen (EON) from wastewater treatment plants on natural water samples collected along an estuarine/salinity gradient within the lower Chesapeake Bay watershed. All of the inorganic N and between 31% and 96% of the EON was removed during biotic bioassays within the first 2 days. Further, there was substantial abiotic reactivity of effluent N when it was added to natural water samples. Results demonstrate that organic and inorganic N in effluent is removed to support the growth of microbial communities. These are the first results aimed at assessing the reactivity of EON in natural waters along an estuarine/salinity gradient.     

Soil degradation assessment in mixed farming systems of southern Africa: use of nutrient balance studies for participatory degradation monitoring

Soil degradation assessments for mixed farming systems of the Molopo Basin (North West Province, South Africa and Southern District, Botswana) are provided from farmer–based research, designed to quantify nutrient fluxes across the farming system and to analyze the social, economic, political and environmental factors affecting nutrient management practices. This paper discusses the practical difficulties of how to use participatory farmer–led studies to assess soil degradation extent and its causes, and of then disseminating this information to farming communities and policymakers. Nutrient balance studies show that land degradation is evident on arable fields as soil nutrient depletion (the main threat to poorer manure–reliant farmers) and soil acidification (the main threat to wealthier farmers who have become dependent on inorganic fertilizer additions). Integrated nutrient management strategies involving both compound fertilizer additions and regular manure inputs can mitigate most soil degradation even on the sandy infertile Kalahari soils, but remain infrequently practised. The need to retain nutrient flows through the livestock sector from rangeland to arable land is thus vital to environmental sustainability and offers an applicable entry point for agricultural development initiatives and support. Factors identified as threatening the flow of nutrients from rangelands to arable lands include policy settings in terms of the different support programmes offered to communal farmers, village–level extension advice, household poverty levels and labour constraints.     

Biogeochemical studies of a Native American runoff agroecosystem

Research on soil fertility is presented in the context of runoff agriculture, a venerable farming system that has been used for millennia in arid to semiarid regions, where water is a major limiting resource for crop production. The agroecology of runoff farming was studied with the Zuni to evaluate nutrient and hydrologic processes, management, maize productivity, and soil quality in some of the oldest recognized fields in the United States. This ancient Southwest agriculture has functioned without conventional irrigation or fertilization by tapping into biogeochemical processes in natural watersheds connected to fields. Carefully placed fields are managed on alluvial fans and other valley margin landforms to intercept runoff and associated sediment and organic debris transported from adjoining forested uplands. We report on research to evaluate and link nitrogen and phosphorus, two key nutrients for crop production, in watershed, soil, and crop components of this agroecosystem. Nutrient data have been collected by observational and experimental methods for each component and the transport of nutrients from watershed to field to maize. The condition of Zuni agricultural soils suggests that their knowledge and management of soils contributed to effective conservation. This study and others indicate the need for further long‐term monitoring and experimental research on watersheds, runoff processes, field soils, and crops across a range of arid to semiarid ecosystems. © 2007 Wiley Periodicals, Inc.