Nitrogen cycling, fertilizer and pollution Flashcards
Nitrogen
Typically 1-5% of shoot dry matter
Required as a constituent of proteins, amino acids, nucleic acids, some membrane lipids
The largest requirement:
Ribulose Bisphosphate Carboxylase -
The most abundant enzyme in the world -
Allows plants to fix CO2
Fundamental relationship between
Field and Mooney 1986
shoot N concentrations, plant functional types, and rates of photosynthesis
Nitrogen (N)
The mineral nutrient most in demand by plantsThe 4th most abundant element in plants
(on a mass basis) after C, H, and O
The N content of most rocks and minerals has been considered to be exceedingly low.
Plants have high N requirements relative to supply.
N- most often limits plant growth in natureN- the main constituent of artificial fertilizer
Plants have much more N and P than surrounding crustal rock
Plants extract and retain Silica, P and Iron from earth materials
19 to 31 teragrams of nitrogen are mobilised from near-surface rocks
annually. Bedrock is a nitrogen source that rivals atmospheric nitrogen inputs.
93.5% of nitrogen is inorganic
of that, 96% is dead organic matter and 4% biomass, of this 4%, 94% is plants
The N cycle: processes and pathways
- Microbial depolymerisation and assimilation
- Microbial organic N release
- Mineralization (ammonification)
- Nitrification
- Denitrification
- Microbial immobilisation
- Humification
- Microbial N fixation
Nitrogen is mainly cycled through
organic molecules in litter inputs
Soil profile-distribution of N
Nitrogen is mainly stored at the surface, as is S and P
Rooting responses to localised nitrogen supply
Proliferation of secondary laterals in zones with more nitrate
The N cycle is the biogeochemical cycle most altered by humans - far more than the C cycle.
Globally
Natural N fixation
120 M tonnes y-1 by organisms naturally
10 M tonnes y-1 from lightning
= 130 M tonnes y-1
Anthropogenic
110 M tonnes y-1 fertilizer factories (International Fertilizer Association 2017)
~40 M tonnes y-1 Biological N fixation (legume crops (clover, beans etc)
= ~150 M tonnes y-1
Global food production is currently strongly dependent on nitrogen fertilizer inputs
(Hafner, 2003) Nature 422, 397-8
Commercial fertilizer is responsible for 40 to
60% of the world’s food production.
Increasing care with N fertilizer-
Cost
Precision agriculture
Pollution
Nitrate-vulnerable zone restrictions
Precision agriculture- sensors on the tractor detect the “greeness” of the crop and adjust fertilizer dose to match requirements- and can be linked to historical yield maps of a field via GPS.
Annual excess of 90 Kg ha-1 =
9000 Kg ha-1 per century
Farmers are driven by short-term economics
employment is low economic contribution
Nitrogen use efficiency - Low rates of N fertilizer use in Africa give high efficiency of use, but suboptimal crop yields
Globally more that 50% of N fertilizer is not used by crops
Increasing rates of fertilizer use in China to feed the population, 80% inefficient use
EU Water framework Directive gives 60% efficiency in UK
More than half of the environmental impact of producing a loaf of bread arises directly from wheat cultivation
with the use of ammonium nitrate fertiliser alone accounting for around 40%
Synthetic nitrogen fertilisers deplete soil nitrogen
synthetic N applied as cost-effective insurance against low yields.
Ammonium sources:
ammonification
clay weathering
Sinks of ammonium:
plant uptake
clay fixation
microbial assimilation
What nitrifying bacteria do: NH4+ -> NO3-
If the nitrate is taken up by plants or micro-organisms the acidification is neutralised.
If the nitrate is washed out of soil net acidification results.
In acid soils (pH <4.5) bacterial nitrification is inhibited.
Denitrification- a process of poorly aerated soil
Reductive nature of denitrification. Organisms take an oxygen from nitrate and leave the nitrite ion which in turn loses and oxygen and produces nitric oxide
Nitrification is more temperature sensitive than ammonification.
Nitrification and denitrification can occur simultaneously in different microsites
N2O is a major greenhouse gas with tropospheric lifetime of 120 years.
Its global warming potential is 296 X greater than an equal mass of CO2.
Its major source is soils. Of the atmospheric increase, 89% is due to emissions from cultivated soils, especially those high in nitrogen.
Rice paddy fields are contributing
10 to 25% of global methane emissions, and have made increasing contributions to N2O fluxes too. Increasing rice productivity is associated with increasing N2O release
(Zou et al. 2009)
Agriculture in the UK is a modest source of nitrogen oxide emissions
DEFRA 2016
Livestock farming is the major source of ammonia emissions in the UK
BAD Practice- An open slurry pit not apparently sealed, unfenced – a likely major source of greenhouse gas and ammonia pollution and health hazard.
Illegal in Europe and probably illegal in many US states.
Surface application of slurry results in evaporation/ volatilization
High risks of run-off and wash-off by rain- especially when the soil is compacted (note the wetness in the wheel tracks).
Winter applications with no vegetation present to absorb the nutrients risks contamination of surface water by runoff and groundwater by nitrate leaching.
“splash plate slurry application offers only a 20% nutrient efficiency,
Slurry injection/
but the slit Injector delivers slurry down to the ground and to the root zone, so it gives an estimated 85% nutrient efficiency.
slurry injectors- directly supplied from a tractor-towed tanker and pump unit can cause serious soil compaction- typical tanks hold upwards of 15,000 litres- i.e. over 15 tonnes.
Umbilical slurry injectors- fed by a tanker / pump unit parked off the field to avoid soil compaction. This is the state-of-the-art. Wide tires of low air-pressure, and very wide injector boom reduces the number of times the tractor has to drive across the field.
Nitrogen deposition: 23% species reduction
compared with grasslands receiving the lowest levels of nitrogen deposition
Nitrogen is a critical nutrient that limits growth of plants in many ecosystems.
Nitrogen cycling depends upon a complex series of microbial transformations of atmospheric N2 and recycling of organic N.
These processes are highly dependent on environmental conditions: temperature, water, oxygen
Adaptations to conserve N in ecosystems can result in strong accumulation of soil carbon in humus.
The N cycle has been highly disrupted by humans
