Nutrient Cycles Nutrient requirements Biogeochemical cycles

Nutrient Cycles Nutrient requirements Biogeochemical cycles
Rates of decomposition Plant adaptations in low nutrient conditions There is a whole science behind the area of biogeochemical cycles and nutrient cycling – Here I want to focus on a few of them from an ecosystem perspective

Nutrient Requirements for Plant Growth
Taken up in gaseous form, Oxygen (O2), Carbon CO2, and from roots - Water (H2O). Derived from water and carbon dioxide Rest are taken up from soil solutions Macro-nutrients –Nitrogen (N), Phosphorous (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S) Micro-nutrients – Boron (B), Copper (Cu), Iron (Fe), Manganese (Mn), Molybdenum (Mo), Zinc (Zn) Plants require certain elements for growth · Basic elements that are used in photosynthesis and respiration (Carbon, Oxygen) or are extracted directly from the atmosphere via breakdown of water (Hydrogen) · There are the macro-nutrients – needed in significant amounts · There are micro-needed in trace amounts · Note a variety of other elements can be found in plants, but none are actually required for growth

Biogeochemical Cycling
The cycling of nutrients through ecosystems via food chains and food webs, including the exchange of nutrients between the biosphere and the hydrosphere, atmosphere and geosphere (e.g., soils and sediments) This is what we are going to explore more closely in this lecture

Atmospheric carbon is rarely limiting to plant growth
Ecosystems produce and process energy primarily through the production and exchange of carbohydrates which depends on the carbon cycle. Once energy is used, it is lost to the ecosystem through generation of heat Carbon is passed through the food chain through herbivory, predation, and decomposition, it is eventually lost to the atmosphere through decomposition in the form of CO2 and CH4 . It is then re-introduced into the ecosystem via photosynthesis. However, the amount of carbon present in a system is not only related to the amount of primary production, as well herbivory and predation (e.g., secondary production), it is also driven by the rates of decomposition by micro-organisms Atmospheric carbon is rarely limiting to plant growth Up to now, we have discussed how ecosystems produce and process energy, primarily through the production and exchange of carbohydrates This focus has also provided some insights with respect to the carbon cycle as well The energy cycle itself is a one way street, e.g., once energy is used, it is lost to the ecosystem through generation of heat Carbon is somewhat unique in terms of its cycle, because while carbon is exchanged through herbivory, predation, and decomposition, it is eventually lost to the atmosphere through decomposition in the form of CO2 and CH4 It is then re-introduced into the ecosystem via photosynthesis However, the amount of carbon present in a system is not only related to the amount of primary production, as well herbivory and predation (e.g., secondary production), it is ultimately driven by the rates of decomposition by micro-organisms Atmospheric carbon is rarely limiting to plant growth

Algae remove dissolved phosphorous from the water
When we look at other nutrients, a somewhat different picture emerges than with the energy cycle – e.g., phosphorous in a food chain within a small pond. Algae remove dissolved phosphorous from the water The phosphorous is then passed through different trophic levels through herbivory and predation. At each level there is some mortality, and then the phosphorous is passed to decomposers These organisms release phosphorous into the water where it is again taken up by primary producers and the whole cycle starts up again When we look at other nutrients, a somewhat different picture emerges than with the energy cycle Here we have an example of the pathways for exchange of phosphorous in a food chain within a small pond Algae remove dissolved phosphorous from the water The phosphorous is then passed through different trophic levels through herbivory and predation At each level there is some mortality, and then the phosphorous is passed to decomposers These organisms release phosphorous into the water Where it is again taken up by primary producers And the whole cycle starts up again Point – with most essential nutrients, there is a continuous recycling of nutrients

Key Elements of Biogeochemical Cycles
There are many different processes involved exchanges of the nutrients that are eventually used by the biotic component of our earth Discuss key elements From an ecosystem/community standpoint, we study nutrient cycling via understanding the exchange of materials and energy via food chains and food webs – e.g., the terrestrial or aquatic food webs However, there are many different factors involved in nutrient cycling outside of the community itself From an ecosystem/food web perspective, it is important to understand Where do the nutrients that ecosystems use come from What happens to the nutrients within the ecosystem itself What happens to the nutrients once they leave the ecosystem? Once nutrients are cycled through an ecosystem, how do they get back? What are the rates of exchange of nutrients between the different pools where they are found Where do the nutrients that ecosystems use come from? What happens to the nutrients within the ecosystem itself? What happens to the nutrients once they leave the ecosystem? Once nutrients are cycled through an ecosystem, how do they get back? What are the rates of exchange of nutrients between the different pools?

Nutrient Pools and Nutrient Flux
Nutrient pool – a specific component or compartment where a nutrient resides Can be a single organism, a population, a community, a trophic level, and an abiotic feature (e.g., lake, soil, atmosphere, etc.) Nutrient flux – the rate of exchange (e.g., unit of material per unit time) of nutrients between pools For example, in the previous examples, the boxes represent different pools, while the arrows represent fluxes

Example of changes in the amounts of tracer phosphorous being exchanged within an aquatic food web
The values themselves represent changes in the pool levels, where each one of the lines represents a different pool Understanding the feeding relationship allows us to build a nutrient cycle model for this ecosystem

This system is not closed – inputs, probably from run-off from land.
Model of phosphorous cycle for an aquatic ecosystem – flux rates per day shown. This system is not closed – inputs, probably from run-off from land. Exports include  herbivores moving outside of system and dead plant/animal material moving out of system, probably through sedimentation. Rate of uptake by plants is directly proportional to net primary production. Exchange of nutrients by higher trophic levels is controlled by processes regulating secondary production. Rates of inputs and outputs of nutrients from an ecosystem are driven by both biotic and abiotic factors. Here we have a simple phosphorous cycle model for an aquatic ecosystem – These are flux rates per day Model shows that This system is not closed – have inputs, probably from run-off from land Exports include  herbivores moving outside of system and dead plant/animal material moving out of system, probably through sedimentation Important things to remember about nutrient cycling within a system Rate of uptake by plants is directly proportional to net primary production Exchange of nutrients by higher trophic levels is controlled by processes regulating secondary production Rates of inputs and outputs of nutrients from an ecosystem are driven by both biotic and abiotic factors

Types of Biogeochemical Cycles
Three major categories of biogeochemical cycles based on slowest-changing pool(=reservoir): Gaseous cycles of C, O, H20 Gaseous cycle of N, (S) Sedimentary cycles of the remaining nutrients In terms of understanding nutrient cycles, it is useful to divide the nutrient cycles into two broad categories Gaseous cycles – those nutrients whose primary source is from the atmosphere Sedimentary cycles – Those nutrients whose primary source is from the weathering of sediments and rocks Note the difference in scale – Because of the constant circulation of the atmosphere at relatively rapid rates, the overall availability of nutrients at a local scale is actually controlled by larger scale processes Availability of nutrients in sedimentary cycles is driven by local scale processes Special case of Gaseous cycle are N and S  while atmosphere provides ultimate reservoir for these essential nutrients, they are actually made available to plants within the sedimentary cycle With the other gases, they are taken up directly from the atmosphere for primary producers Global scale Local scale

Sedimentary Cycles Gaseous Cycles
Going back to the major classes of biogeochemical cycles, here is an illustration of the key components of the Sedimentary and Atmospheric Gas cycles · Each cycle has the same major elements, with the Gaseous cycles having one additional cycle  interactions with the atmosphere · Note that the interactions with the atmosphere occur with different components of the terrestrial biome o Plants remove and add carbon, o Microbes and animals all add carbon, remove oxygen Gaseous Cycles

Major Components of Nitrogen Cycle
Nitrogen cycle is unique in that nitrogen is removed from the atmosphere by microbes, and then taken up by plants in solution when released by the microbes

Biological Nitrogen Fixers
Cyanobacteria – blue-green algae Free living soil bacteria Mycorrhizae Symbiotic bacteria living in root nodules There are two general types of nitrogen fixers Cyanobacteria – primary producers, e.g., blue/green algae – fix their own carbohydrates Bacteria that require a source of carbohydrates Get it from dead organic matter in soil Get it directly from plants in a symbiotic relationship - mychorhizae

Root nodules on ? Cassia fasciculata

Nitrogen is available in three soluble forms in soils
· Nitrification is a series of processes associated with decomposition, fixation and transformation of N in the soils Nitrates are the form of nitrogen used by plants But nitrogen also exists in soil solutions as ammonia, and nitrite Nitrogen compounds are extremely volatile, breaking down very quickly

Chemical reactions occur such that Ammonia, Ammonium and Nitrite are transformed into Nitrates, which are then taken up by plants

Now, nitrogen fixation by biota (blue/green algae and bacteria) represent the major source of nitrogen for biological systems However, there are other sources Lightning produces nitrates (see next slide) Humans produce nitrogen Through fertilizers Through burning of fossil fuels A huge amount of nitrogen is cycled within ecosystems 10 times as much being recycled as being fixed There are a series of organisms that fix nitrogen from the atmosphere

NO from lightning Lightning + N2 + O2  NO + O2  Nitrate (NO3)

Phosphorous Cycle Phosphate – PO4-3
Phosphorous is an extremely important nutrient as well that is limiting to plant growth · There is lots of phosphorous in mineral form Getting it in phosphate, the form taken up by plants, is the big issue in terms of nutrient cycling Microbes again mobilize Phosphorous, i.e. put it in a form that can be used by plants

Some phosphorous is cycled to the atmosphere, but most of it comes from weathering of soils
One of the unique aspects of the phosphorous cycle is the fact that much P is lost through sedimentation, e.g., the loss of P through run-off or the raining of dead plant and animal material in the oceans However, throughout geologic time, much sedimentary rock that is rich in phosphorous exists The weathering of this sedimentary rock represents an important source of phosphorous Again note – through use of fertilizers, humans are having a major impact on the phosphorous cycle, especially in aquatic systems

Potassium The final type of macro-nutrient is represented by potassium, which is available as a cation – positively charged ion In some regions, this nutrient limits plant growth Most potassium comes from weathering of local minerals

Sources of Nutrients Atmosphere Parent Material Run-off, Ground water
A key question facing ecologists is to understand the ultimate sources of the nutrients being used by In understanding nutrient cycling and nutrient availability in ecosystems, it is important to remember that the characteristics of the soils are very important The overall age of the soils is extremely important Nutrients such as phosphorous, sodium, magnesium, calcium, and potassium are all derived through weathering of soils The older the soils, the longer the weathering process has undergone, and the lower nutrient content of the soils For example, the soils of Alaska are relatively young, originating from the grinding up of bedrock by glaciers They are re-deposited by water and wind processes These soils have high nutrients In many areas, the soils are very, very old (on the order of hundreds of millions of years) and can be very low in nutrients Such is the case of soils in Australia Floods

Simple Model of Soil Decomposition/ microbial respiration
H2O, O2 CO2 or CH4 Litter Energy Microbial Population Organic Soil Nutrients Dissolved Nutrients

Factors Controlling Microbial Respiration
Availability of oxygen CO2 versus CH4 production Temperature Moisture Quality of material comprising dead organic matter There are four things that directly control patterns of decomposition First of these is availability of oxygen  required for respiration In anaerobic conditions, some bacteria are able to respire w/o oxygen, and produce methane Note – areas with aerobic conditions have very slow decomposition rates Rates aerobic decomposition >>>> rates of anaerobic decomposition For the most part, studies have focused on the last 3 topics

Here we have an example of the effects of temperature on soil respiration (production of CO2) in burned black spruce forests of Alaska As temperature increases, so does rates of soil respiration (decomposition) Temperature is known to be a strong controller of respiration The relationship between rates of decomposition and temperature is well established in all different types of ecosystems

Studies have shown that in areas with similar vegetation, rates of decomposition are related to levels of soil moisture Here we have an example of two chaparral ecosystems in Spain where precipitation varied by a factor of 3 Very similar qualities in the types of vegetation being decomposed Area with higher precipitation  higher soil moisture  higher rates of decomposition Again, numerous studies have shown that in areas with moisture condition (but will aerobic conditions) have higher rates of decomposition

Moisture is limiting up to a certain point, at higher soil moistures, decomposition can actually drop

One way to combine the effects of temperature and moisture on total decomposition is to look at rates of actual evapotranspiration This graph shows that in general, areas with higher soil temperatures and moisture have highest rates of biomass loss from decomposition

Simple Model of Simple Model of Soil Decomposition/ microbial respiration
H2O, O2 CO2 or CH4 Litter Energy Microbial Population Organic Soil Nutrients When we look at microbes that carry out decomposition, one must realize that they require essential nutrients just like any other organism Plants that have a high lignin content and low nutrient contents do not support decomposing organisms very well Dissolved Nutrients

Two factors to consider in terms of the composition or quality of material being decomposed
Amount of nitrogen Overall digestibility For example, if one looks at the overall toughness of a leaf (which is an indicator of lignin content), one finds that tough leaves with low nitrogen support less decomposition that soft leaves with high nitrogen

Decomposition as a Function of Lignin Content
k is the fraction of a material that decomposes in a given year Here we have data from different forest types in interior Alaska Shows that as lignin content increases, then decomposition rate decreases Decomposition as a Function of Lignin Content

Lignin is a measure of digestibility of material to microbes
Low lignin and high N  higher rates of decomposition Lower decomposition in N. Hampshire do to lower average temperatures

Residence Time Residence time is the length of time it takes for biomass or a nutrient to be completely decomposed or recycled from the forest floor Turnover times are much shorter in tropical forests than in boreal forest · Nutrients are recycled much more quickly in these ecosystems

Residence times Residence time is another way in which decomposition is measured · The longer the residence time, the slower nutrients are turned over for re-use · Again, deciduous have shorter turn over times than coniferous (quality of DOM) Temperature have shorter turn over times than boreal (temperature dependence) Coniferous forests have longer residence times than deciduous  C/N control Boreal forests have longer residence times than temperate forests  temperature control

Tree Nutrient Content % N % P % K Temperate Conifers 0.147 0.043 0.100
Deciduous 0.289 0.025 0.178 Eucalyptus 0.194 0.008 0.127 When you compare the amount of nutrients in Eucalyptus trees in Australia, you can see that they have about 20 to 33% the amount of phosphorous in their tissues than trees grown in soils with more nutrients However, in other nutrients, they fall within the range of trees growing on nutrient poor sites

Translocation of Nutrients
Prior to shedding leaves in the fall, translocation of nutrients often takes place in trees This allows tree to retain essential nutrients that are hard to come by Spruce trees remove more nutrients than other coniferous trees An adaptation to poor nutrient sites Translocation of nutrients · A unique adaptation of coniferous boreal species to poor nutrient conditions

Question – do plants growing on sites with low soil nutrients have low nutrient contents as well?
The answer is no – Plants on sites with low nutrients tend to have higher nutrient contents They have a higher nutrient use efficiency

Nutrient Use Efficiency (NUE)
Some plants are more efficient at using nutrients because it gives them selective advantages in low nutrient conditions NUE = A / L A – the nutrient productivity (dry matter production per unit nutrient in the plant) L – nutrient requirements per unit of plant biomass Thus, plants that live in low nutrient conditions have a higher NUE

A common pattern found in ecosystem productivity is saturation curve.
Productivity increases linearly with N availability, up to a certain point, when other resources become limiting (e.g., light, water, temperature, other nutrients) A common pattern found in ecosystem productivity is presented in this graph. Vitousek found that productivity increases linearly with N availability, up to a certain point At some point, other resources become limiting Can be light, water, temperature, other nutrients

Three types of relationships with respect to limitations of nutrients:
Production is independent of resource availability Production is a linear function of resource availability At some point, another resource becomes limiting Shows three types of relationships with respect to limitations of nutrients Production is independent of resource availability Production is a linear function of resource availability At some point, another resource becomes limiting

Factors Influencing Nutrient Availability
Presence of nitrogen fixers Microbial activity Fire Precipitation patterns Soil drainage Soil temperature, moisture In looking at the entire nutrient budget of a single ecosystem, one must consider a wide range of factors – we have discussed a few of these, e.g., nitrogen fixers, microbial activity, soil temperature, moisture/drainage There are others as well

H2O - Precipitation CO2 Fire N2, O2 Litterfall Leaching, run off GHG
Photosynthesis Aeolian, Atmospheric Deposition Internal translocation N2, O2 Litterfall nutrients N fixers CH4, CO2 Organic soil Dissolved nutrients Through-fall nutrients Nutrients As an example of the complexity of this situation, here we have all the processes that can influence nutrient cycling in the boreal forest This figure presents a simplified diagram of factors and processes that influence nutrient cycling within a forest ecosystem of Alaska · Obviously a very complicated process · It is important to recognize that the ecosystem concept is very important in nutrient cycling o Without microbes  nutrient recycling does not occur o Without plants  microbes have no energy source · Ecosystem productivity to a large extent is dependent on the ability to both capture nutrients as well as recycle these nutrients · Note that many of these processes not only indirectly influence a plant via nutrient availability, but also can directly and indirectly influence a plant in other ways Example – Fire Direct – kills plant Indirect – alters thermal, moisture characteristics of the soils  directly influence plant growth A few pathways not discussed include fire – direct release of nutrients to the ground Also, precipitation and through fall – nutrients are washed off of plants – this can represent a significant amount of nutrient input Soil drainage is extremely important Energy, Nutrients Upper mineral soil Microbes Lower mineral soil Leaching, run off

One of the striking thing one notes when comparing the amounts of organic matter present in ecosystems (top table). While on average, there is more organic matter present in temperate systems than in boreal systems, the percent differences between living biomass and dead biomass are quite striking Reason why a much higher percentage is present in dead organic matter in boreal regions is the fact that decomposition rates are very low

Boreal forest has the largest available nutrient pool in soil, but lowest rates of production, where as tropical forest has lowest soil pool, and highest production. Forest Type Living Biomass Pool Primary Production Rates Soil Carbon/ Nutrient Pool Decomposition Rates Tropical Highest Lowest Temperate Middle Boreal It is interesting to note that the boreal forest has the largest available nutrient pool in the soils, but lowest rates of production, where as tropical forest has lowest soil pool, and highest production Clearly, amount of soil nutrients does not limit primary production What is happening in Tropical regions As a community, tropical forests are very efficient users of nutrients Low soil nutrients are due to the fact that a. Tropical forests have extremely high decomposition rates  nutrients are recycled very quickly b. Plants are very efficient at removing soil nutrients  high decomposition + high production  low nutrient pool Conversely, in boreal systems High soil nutrient/carbon pools are due to  cold temperatures  slow decomposition  build up of organic matter

Role of Disturbances in Nutrient Cycling
Type of disturbance important Fire versus logging versus large-scale mortality Disturbances directly alter biotic and abiotic controls on nutrient cycling Rates of primary production Controls on evapotranspiration Influences on surface runoff Soil temperature/moisture  decomposition rates Linkages between terrestrial/aquatic systems

Hubbard Brook watershed, upstate New Hampshire.
One of the classic studies on the effects of disturbances was carried out at the Hubbard Brook watershed in upstate New Hampshire In essence, this was a study of an entire watershed, that consisted of a number of different forest communities, as well as aquatic (Stream) communities Nitrogen cycling within the watersheds was very efficient They lose less than 1 mg/L of N in stream outflow – Studies show that N inputs to the system roughly equal outputs Large-scale deforestation completely changed the nutrient dynamics Resulted in large-scale exports of nutrients do to: 1. Increased decomposition 2. Direct runoff of sediments into streams 3. Increased rates of nutrient leaching out of soils into ground water  goes into streams 4. Increased detritus into streams

Upland White Spruce Succession
Nutrient cycling is not a static process, but changes throughout succession as the ecosystem changes

Nutrient Cycling in Upland White Spruce
Show Figure 5 from Article – Shows nutrient cycling from upland sites · Five major stages as well, very similar to floodplain succession, but there are major differences o Different from floodplain because you have a recurrent pattern of succession after fire o While fire kills mature plants, it does not remove propagules (e.g., seeds, roots, and other sources for vegetative reproduction) The presence of live roots and stems provide a nutrient and water source for new plants on site  not present in floodplain sites

Nutrient Cycles Nutrient requirements Biogeochemical cycles

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Nutrient Cycles Nutrient requirements Biogeochemical cycles

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