Contemporaneous measurements are reported for nitrification, denitrification, and net sedimentwater fluxes of NH,+ and N03-in the mesohaline region of Chesapeake Bay. Seasonal cycles over a 2-yr period were characterized by a midsummer maximum in NH, + efflux to the overlying water and a May peak in NO,-. removal from water by sediments. Coherent temporal patterns for nitrification and denitrification were observed, with relatively high values in spring and fall and virtual elimination of both processes in summer. Indirect measurements indicate that nitrification was limited by the shallow 0, penetration (< 1 mm) here compared to reports for other marine sediments (2-6 mm). In addition, a strong positive correlation between the two processes suggested that denitrification was generally controlled by nitrification. Comparisons of NO,-fluxes and net nitrification rates (nitrification minus N03-reduction to NH,+) revealed that measurements of denitrification with the acetylene block method systematically underestimated actual rates. Rates of N, loss in denitrification were similar to NH,+ recycling fluxes to the overlying water in spring and fall, but in summer negligible denitrification contributed to enhanced NH,+ recycling. These results suggest that inhibition of denitrification in eutrophic estuaries such as Chesapeake Bay may reinforce the effects of nutrient enrichment by allowing increased rates of NH,' recycling.

Variations in sediment N:C ratios were correlated with water depth and season. 15NH,+ was used to measure the rates of NH,+ production (d) and incorporation into bacterial cells (i) in sediments from different stations, at different seasons. The validity of the rates d and i was indicated by the predicted correlation of cl:i ratios with N:C ratios of the sediment, and the predicted N:C ratio at which net NH4+ uptake occurred. There was also a correlation between rate cl and product (total NH,+). In the O-2-cm stratum correlations were also established between d, exchangeable NH4+ pool, ratio exchangeable NH?+ : porewater NH4+, flux of NH,+ from sediment, and flux of NH4+ into exchangeable pool. The NO:,-flux from sediment was correlated with nitrification rate and with season. Benthic infauna increased the flux of NHI' from the sediment by 50%. The rates of transfer of nitrogen (NO,-, NH?+, N,) from sediment to water were 44-66% of the net rates of organic nitrogen mineralization (d -i). Flux of NO,-+ NII,+ from the sediment could supply 30-82% of the nitrogen requirement of the planktonic primary producers.Available nitrogen occurs in sediment in the following major pools: organic N, porewater ammonium (NH,+pw), exchangeable ammonium ( NH4+ex), dissolved nitrate, and nitrogen gas. Our objective here was to measure these pools and the rates that connect them to each other and to the overlying water.The rate of organic-N mineralization, equivalent to the rate of NH,+ production (d), and the rate of NH,+ incorporation into cells (i), was measured by lsNH4+ dilution. Factors affecting the sediment organic N:C ratio, and the effect of the N: C ratio on the ratio d:i were investigated.The NH,+ that was not incorporated into cells (net ammonium production, di) had three possible fates. Some passed from the sediment to the overlying water, some was oxidized to nitrate, and the rest entered the sediment NH4+ pool. We examined the factors, mainly the ratio of NH4+ex : NH,+pw in the 0-2-cm stratum, which regulated the fate of NH,+. We also examined the factors, mainly seasonal changes, which regulated the loss of NO,-

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The global methane cycle includes both terrestrial and atmospheric processes and may contribute to feedback regulation of the climate. Most oxic soils are a net sink for methane, and these soils consume approximately 20 to 60 Tg of methane per year. The soil sink for atmospheric methane is microbially mediated and sensitive to disturbance. A decrease in the capacity of this sink may have contributed to the ∼1% · year−1 increase in the atmospheric methane level in this century. The organisms responsible for methane uptake by soils (the atmospheric methane sink) are not known, and factors that influence the activity of these organisms are poorly understood. In this study the soil methane-oxidizing population was characterized by both labelling soil microbiota with14CH4 and analyzing a total soil monooxygenase gene library. Comparative analyses of [14C]phospholipid ester-linked fatty acid profiles performed with representative methane-oxidizing bacteria revealed that the soil sink for atmospheric methane consists of an unknown group of methanotrophic bacteria that exhibit some similarity to type II methanotrophs. An analysis of monooxygenase gene libraries from the same soil samples indicated that an unknown group of bacteria belonging to the α subclass of the class Proteobacteria was present; these organisms were only distantly related to extant methane-oxidizing strains. Studies on factors that affect the activity, population dynamics, and contribution to global methane flux of “atmospheric methane oxidizers” should be greatly facilitated by use of biomarkers identified in this study.

Separation of slurry produces a solid fraction that is stored in manure heaps before being used as a fertiliser in crop production. Considerable amounts of ammonia (NH 3) and greenhouse gases may be emitted during storage, which has deleterious environmental effects. The emission levels can be expected to depend on oxygenation level inside the bulk of the stored manure and therefore on storage conditions. An experiment was thus set up to study gaseous emissions during storage of the solid fraction, and the effects of the oxygenation of manure heaps on emissions of NH 3 and various greenhouse gases. Emissions of NH 3 and the greenhouse gases methane (CH 4), nitrous oxide (N 2 O), and carbon dioxide (CO 2) from an uncovered and covered heap of solids separated from pig slurry were compared, and related to the oxygenation level inside the manure heap. Approximately 15% of the initial nitrogen content was lost when separated solids were stored uncovered. Of the initial nitrogen content, 4.8% was lost as N 2 O, 0.3% was lost as NH 3 , while the 9.6% unaccounted for was assumed lost as dinitrogen (N 2). Of the initial carbon content, 28% was lost during uncovered storage; the majority of this was emitted as CO 2 (25%), while 1.3% was emitted as CH 4. Oxygenation level inside the heap was found to influence the production and emission of greenhouse gases. Covering the heap with an airtight material delayed aeration of the bulk of the stored manure, which reduced the internal heat production, degradation of organic matter, and emission of NH 3 and greenhouse gases. Emissions of NH 3, N 2 O, and CH 4 were reduced by 12%, 99%, and 88%, respectively, when the manure heap was covered with an airtight material.