Lecture 9 : Summary

• Nitrogen cascade

• Hubbard Brook Experimental Forest (New Hampshire)
o 1960s to present
o Goal was to quantify inputs and output fluxes of elements (N, S, Ca, K, Mg) in a forest ecosystem, and compare those to fluxes of same elements cycled internally.
o Internal cycling quantified (elements taken up by vegetation each year, cycled back to soil via litterfall)
o Measuring input : Rain gauges are used to measure solute inputs in precipitation and “throughfall”
o Measuring output : At the bottom of the watershed, water outflow is measured with a gauge and samples are taken to measure solute fluxes out
• Example, “small watershed ecosystem”
• Watershed boundaries used for hydrologic convenience:
o Inputs quantified easily (elements in deposition falling on area)
o Outputs quantified easily (elements in stream flow at that point)
o Assumption of nearly watertight bedrock, no losses down into groundwater
• Boundary somewhat arbitrary, but an important theoretical tool because it allowed development of this type of environmental analysis
• Allowed quantification of inputs, outputs, and comparison against internal cycling, under various conditions, various vegetation types or soil types
• Nutrient flows terrestrial to aquatic
o Can think of this as nutrient cycling in two different types of ecosystems that were much more isolated from one another historically …
o But are now more closely linked through this important transfer of nutrients.
o This is a major aspect of global change
o These produce large-scale interconnections across terrestrial and aquatic environments, industries, states, institutional concerns, cultures … very hard to solve

• A rapid rise in “reactive N” : part of the “green revolution” that began ca. 1960
• Use of industrial N fertilizer: One of the most rapid facets of global environmental change

• Wetlands often occur at the transition between terrestrial and aquatic systems
o at the bases of small watersheds
o in the floodplains of streams and rivers
o at the mouths of rivers
o in embayments
o generally in low-lying areas that border terrestrial and aquatic systems

• Riparian zones
o High water table because of proximity to an aquatic ecosystem
o Ecotone between aquatic and upland terrestrial ecosystems
o Exposed to lateral water flow (i.e. rivers and streams); or more simply, in or near river channels and directly influenced by river-related processes
o May be exposed to intermittent flooding
o Supports riparian vegetation (plants that do well with a high water table)
o (Some, but not all, definitions include edges of lakes and ponds)
• Riparian buffers: example of BMPs (Best Management Practices)
o Three zones : Run off control (farthest from the water body), managed forests, undisturbed forests (closest to aquatic system)

• Inner coastal plains
o High removal of nitrate from ground water
o Moderate removal of sediments and sediment-borne pollutants
o Low removal of dissolved phosphorus

• Nitrogen transformations
Conversion of one type of N molecule to another, which can either consume or release energy depending on the transformation

• A little synthesis and review to help keep this clear …
o Acid rain includes H+ and NO3- (nitrate anion)
This can acidify soils and strip nutrient cations from soils.
As the NO3- anion moves through soils, the H+ can be retained on the soil exchange complex, while Ca2+, K+, Mg2+ can be stripped to accompany the NO3- anion on its way to streams
o NH4+ can be converted to NO3- in soils through the process of nitrification
This is also a soil acidifying process, because it produces H+ that can be retained in the soil
NH4+ is not mobile in soils, but NO3- anion is mobile in soils, so nitrification converts inorganic N to a mobile form in soils – which then leaches to groundwater and surface water

• Nitrogen undergoes a large array of redox reactions that are biogeochemically and ecologically important
• Nitrification : Ammonia to nitrite. Nitrite to nitrate. 2 moles of oxygen per mole of ammonia
• Denitrifitcation : Nitrate or nitrate used as electron acceptors by anaerobic bacteria during metabolism of organic carbon (carbohydrates). Produces N2 gas.
• Nitrification is aerobic and produces NO3- as an end product
• Denitrification is anaerobic and uses NO3- as an input, and consumes it, producing gaseous forms of N as an output
• Where N transformations occur
o Wet soil : Anaerobic. More denitrification. More ground water losses.
• Nitrification
o In soils, sediments (bottoms of streams, rivers, lakes), wetlands, but only if aerobic
o In the water column of streams, rivers, lakes, but only if aerobic
• Denitrification
o In soils, sediments, wetlands if anaerobic
o In the bottom of the water column, if anearobic
o Also … both occur in water treatment plants

• Formation of ‘dead zones’ in estuaries and coastal ocean
o Gulf of Mexico hypoxia due to nutrient run off from the Mississippi River Basin
• Great Lakes coastal freshwater marshes as a case study
o Coastal wetlands near Cheboygan, MI receive N from rivers/streams, precipitation, groundwater flow
• Changes in Great Lakes coastal marshes
o Historically nutrient poor (oligotrophic)
o Now being exposed to greater nutrient inputs
o Invasion of exotic plants may be getting aided by elevated nutrient inputs

• Even though the N cycling increases through a positive feedback as a result of increased N inputs, the retention of N flowing into the system is still only partial
o Most of the N entering the wetland passes through and enters the receiving body of water

• The power of mass balance
o The gross fluxes into a system (or pool) minus the gross fluxes out of a system (or pool), over a time interval, must equal the change in stock in the system (or pool).
o Mass balance is a very powerful tool in ecosystem science and biogeochemistry, and the only math it requires is simple arithmetic and the manipulation of units.
o One of the reasons this is powerful is because things that are very difficult to measure at large scales can be quantified by difference
• The power of mass balance
o Example: loss of calcium from a soil over a time period
(calcium is an important component of soil fertility and acid buffering capacity)
If all input-output fluxes are quantified, then change in pool size can be calculated by difference
This change in soil Ca is difficult to measure directly at the watershed scale

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