Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

/pdf/climate-change-2007-the-physical-science-basis-1x93ttz9jt.pdf

Climate Change 2007: The Physical Science Basis.

Human production of food and energy is the dominant continental process that breaks the triple bond in molecular nitrogen (N2) and creates reactive nitrogen (Nr) species. Circulation of anthropogenic Nr in Earth’s atmosphere, hydrosphere, and biosphere has a wide variety of consequences, which are magnified with time as Nr moves along its biogeochemical pathway. The same atom of Nr can cause multiple effects in the atmosphere, in terrestrial ecosystems, in freshwater and marine systems, and on human health. We call this sequence of effects the nitrogen cascade. As the cascade progresses, the origin of Nr becomes unimportant. Reactive nitrogen does not cascade at the same rate through all environmental systems; some systems have the ability to accumulate Nr, which leads to lag times in the continuation of the cascade. These lags slow the cascade and result in Nr accumulation in certain reservoirs, which in turn can enhance the effects of Nr on that environment. The only way to eliminate Nr accumul...

http://bioeeos660-f12-bowen.wikispaces.umb.edu/file/view/Galloway+et+al.+2003.pdf

The Nitrogen Cascade

Rates and reactions of biogeochemical processes vary in space and time to produce both hot spots and hot moments of elemental cycling. We define biogeochemical hot spots as patches that show disproportionately high reaction rates relative to the surrounding matrix, whereas hot moments are defined as short periods of time that exhibit disproportionately high reaction rates relative to longer intervening time periods. As has been appreciated by ecologists for decades, hot spot and hot moment activity is often enhanced at terrestrial-aquatic interfaces. Using examples from the carbon (C) and nitrogen (N) cycles, we show that hot spots occur where hydrological flowpaths converge with substrates or other flowpaths containing complementary or missing reactants. Hot moments occur when episodic hydrological flowpaths reactivate and/or mobilize accumulated reactants. By focusing on the delivery of specific missing reactants via hydrologic flowpaths, we can forge a better mechanistic understanding of the factors that create hot spots and hot moments. Such a mechanistic understanding is necessary so that biogeochemical hot spots can be identified at broader spatiotemporal scales and factored into quantitative models. We specifically recommend that resource managers incorporate both natural and artificially created biogeochemical hot spots into their plans for water quality management. Finally, we emphasize the needs for further research to assess the potential importance of hot spot and hot moment phenomena in the cycling of different bioactive elements, improve our ability to predict their occurrence, assess their importance in landscape biogeochemistry, and evaluate their utility as tools for resource management.

/pdf/biogeochemical-hot-spots-and-hot-moments-at-the-interface-of-1vpqinkleu.pdf

Biogeochemical Hot Spots and Hot Moments at the Interface of Terrestrial and Aquatic Ecosystems

We present estimates of total nitrogen and total phosphorus fluxes in rivers to the North Atlantic Ocean from 14 regions in North America, South America, Europe, and Africa which collectively comprise the drainage basins to the North Atlantic. The Amazon basin dominates the overall phosphorus flux and has the highest phosphorus flux per area. The total nitrogen flux from the Amazon is also large, contributing 3.3 Tg yr-1 out of a total for the entire North Atlantic region of 13.1 Tg yr-1. On a per area basis, however, the largest nitrogen fluxes are found in the highly disturbed watersheds around the North Sea, in northwestern Europe, and in the northeastern U.S., all of which have riverine nitrogen fluxes greater than 1,000 kg N km-2 yr-1.

Regional nitrogen budgets and riverine N & P fluxes for the drainages to the North Atlantic Ocean: Natural and human influences

Nitrate attenuation in groundwater: a review of biogeochemical controlling processes.

The influence of hydrology andpatterns of supply of electron donors and acceptors onsubsurface denitrification was studied in a forestriparian zone along the Boyne River in southernOntario that received high nitrogen inputs from a sandaquifer. Two hypotheses were tested: (1) subsurfacedenitrification is restricted to localized zones ofhigh activity; (2) denitrification zones occur atsites where groundwater flow paths transportNO3
− to supplies of available organiccarbon. A plume of nitrate-rich groundwater withconcentrations of 10–30 mg N L−1 flowed laterallyat depths of 1.5–5 m in sands beneath peat for ahorizontal distance of 100–140 m across the riparianzone to within 30–50 m of the river. In situ acetyleneinjections to piezometers revealed that significantdenitrification was restricted to a narrow zone ofsteep NO3
− and N2O decline at theplume margins. The location of these denitrificationsites in areas with steep gradients of groundwater DOCincrease supported hypothesis 2. Many of thesedenitrification “hotspots” occurred near interfacesbetween sands and either peats or buried river channeldeposits. Field experiments involving in situadditions of either glucose or NO3
− topiezometers indicated that denitrification wasC-limited in a large subsurface area of the riparianzone, and became N-limited beyond the narrow zone ofNO3
− consumption. These data suggest thatdenitrification may not effectively removeNO3
− from groundwater transported at depththrough permeable riparian sediments unlessinteraction occurs with localized supplies of organicmatter.

Subsurface denitrification in a forest riparian zone: interactions between hydrology and supplies of nitrate and organic carbon.

K. Devito,1* I. Creed,2 T. Gan,3 C. Mendoza,4 R. Petrone,5 U. Silins6 and B. Smerdon4 1 University of Alberta, Department of Biological Sciences, Edmonton, Alberta T6G 2E9, Canada 2 University of Western Ontario, Department of Biology, London, Ontario, Canada 3 University of Alberta, Civil and Environmental Engineering, Edmonton, Alberta T6G 2W2, Canada 4 University of Alberta, Department of Earth & Atmospheric Sciences, Edmonton, Alberta T6G 2E3, Canada 5 Wilfrid Laurier University, Department of Geography and Environmental Studies, Waterloo, Ontario, Canada 6 University of Alberta, Department of Renewable Resources, Edmonton, Alberta T6G 2H1, Canada

/pdf/a-framework-for-broad-scale-classification-of-hydrologic-41ym7kwm9g.pdf

A framework for broad‐scale classification of hydrologic response units on the Boreal Plain: is topography the last thing to consider?

The efficiency with which riparian zones remove nitrate (NO 3 ) from contaminated ground water can vary with landscape setting. This study was conducted to determine the influence of flood plain geometry, lithology, hydrologic flow path, and nitrate transport on mechanisms of nitrate depletion of contaminated ground water. Patterns of NO - 3 -N, chloride, and dissolved organic carbon (DOC) concentrations and δ 15 N-NO - 3 and δ 18 O-NO 3 values in combination with detailed piezometric head measurements were investigated in a river floodplain connected to a large upland sand aquifer in an agricultural region near Alliston, Ontario, Canada. Ground water discharging to the forested floodplain from the sand aquifer exhibited large spatial variability in NO 3 -N concentrations (10-50 mg/L). The transport and depletion of NO 3 was strongly influenced by floodplain geometry and lithology. Little ground water flow occurred through the low-conductivity matrix of peat in the floodplain. Plumes of NO 3 -rich ground water passed beneath the riparian wetland peat and flowed laterally in a 2- to 4-m-thick zone of permeable sands across the floodplain to the river. Analyses of the distribution of the NO + 3 -N concentrations, isotopes, and DOC within the floodplain indicate that denitrification occurred within the sand aquifer near the river where nitrate-rich ground water interacted with buried channel sediments and surface water recharged from peat to the deeper sands. This study shows that the depth of permeable riparian sediments, ground water flow path, and the location of organic-rich subsurface deposits may be more important than the width of vegetated strips in influencing the ability of riparian zones to remove nitrate.

Kevin Devito

Papers

Subsurface denitrification in a forest riparian zone: interactions between hydrology and supplies of nitrate and organic carbon.

A framework for broad‐scale classification of hydrologic response units on the Boreal Plain: is topography the last thing to consider?

Nitrate Dynamics in Relation to Lithology and Hydrologic Flow Path in a River Riparian Zone

Groundwater-surface water interactions in headwater forested wetlands of the Canadian Shield

Shallow groundwater–surface water interactions in pond–peatland complexes along a Boreal Plains topographic gradient