Acidification caused by nitrogen deposition

I’m going to restart the blog after the Christmas break by discussing acidification, another environmental issue caused by human interference in the nitrogen cycle. Soil acidification is one probable mechanism linking nitrogen deposition to plant diversity loss, a topic already blogged on earlier this month. I’m hoping this post should go some way to explaining the processes occurring to produce the acidification effect and the damage that is currently being caused as a result.

Atmospheric nitrogen deposition can result in soil acidification directly as a result of acid deposition (nitric acid) and indirectly through processes and reaction in soil and water (Stevens et al., 2010). This indirect acidification is produced by nitrogen deposition from airborne sources, such as nitrogen oxides (produced by fossil fuel burning for power and transport), or ammonia (produced as a result evaporation from manure used in agricultural processes).  Some statistics indicate that the quantity of these compounds being emitted into the atmosphere has in fact been falling over the last few decades, land based emissions of nitrogen oxides in Europe have fallen by over 30% from 1980-2000, but this has been largely offset by increased emissions at sea.  European ammonia emissions from agriculture did fall by 25% between 1990 and 2000, possibly due to changes in handling of manure (statistics are from the Air Pollution and Climate Secretariat).

There are several papers indicating the damage caused to vegetation as a result of acidification.  One such paper is from Horswill et al. (2008), whose experiments in highly nitrogen and acid rain polluted regions of the UK found that Nitrogen deposition treatments caused grassland soils to lose 23 to 35% of their total available bases (Ca, Mg, K, and Na) and treated areas became acidified by 0.2 to 0.4 pH units.  This study provided the first definitive evidence that nitrogen deposition depletes base cat ions from grassland soils.

The problem of acidifying pollutants is a serious one, and it is necessary to take a broad geographical approach to understand the solutions and dangers. Nitrogen and sulphur oxides can be transported thousands of kilometres by the air, this means that some countries are net exporters of these pollutants and others net importers.   

There are two main factors that determine which areas are affected by acidification: 1) the amount of acid deposition and 2) the resistance of the soil to pH change.  The limits to what a particular area can tolerate in terms of acidifying pollutants are called critical loads, in order to make sure that critical loads in Europe are not exceeded it is necessary to reduce emissions of sulphur dioxide and nitrogen compounds in parts of Europe by 80-90% compared with 1990 levels (Air Pollution and Climate Secretariat).

Fig 1. 'Map indicating the deposition of hydrogen ions that sensitive ecosystems across Europe can tolerate without being acidified.' (Air Pollution and Climate Secretariat)

It seems that although progress has been made on reducing the production of the nitrogen and sulphur pollutants that cause acidification, significantly more action is required if we are to prevent continuing damage to the vulnerable ecosystems shown above.  Reducing the human input of nitrogen compounds to the atmosphere is a key part of this, requiring changes in the way we use and generate energy for our homes, industries and transport.

Climate-smart agriculture

After considering the various detrimental impacts of the anthropogenic input of nitrogen into the biosphere, which have been and will continue to be discussed on the blog, it is also important to look at solutions to this environmental problem.  Below is a youtube clip on 'climate smart agriculture', it is mainly focused on the carbon implications of agriculture, however the same concept of finding ways to increase yields without the excessive use of resources will reduce the anthropogenic impact on both nitrogen and carbon cycles.



I found the video whilst reading another blog, 'big picture agriculture' , it was originally produced to introduce the Learning Events section of Agriculture and Rural Development Day at the recent United Nations COP17 climate change conference in Durban, South Africa, Dec 2011. Information on the Cimate Change, Agriculture and Food Security research program can be found at www.ccafs.cgiar.org

On a related note, there is also an interesting article from outreach magazine on 'sustainable intensification' , published for day 9 of the COP17 conference.

Can reducing inputs of reactive nitrogen really reduce eutrophication?

I’ve come across an interesting debate in the literature on the benefit of nitrogen management with respect to reducing eutrophication, a widely known consequence of anthropogenic nutrient inputs to aquatic ecosystems.    

Schindler et al. (2008) carried out a 37 year experiment in a small lake designed to test the assertion that controlling nitrogen inputs could be effective in controlling eutrophication.  Over the 37 year period, the annual addition of phosphorous remained constant, whereas the amount of nitrogen added was gradually reduced each year.  For the final 16 years of the experiment only phosphorous was added to the lake.  The results showed that reducing nitrogen inputs increasingly favoured nitrogen-fixing cyanobacteria as a response by the phytoplankton community to extreme seasonal nitrogen limitation.   The additional nitrogen fixation that resulted allowed biomass to continue to be produced in proportion to phosphorus, and the lake remained highly eutrophic, despite showing indications of extreme nitrogen limitation seasonally.  These results led Schindler et al. to conclude that management of eutrophication is dependent only on the reduction of phosphorous inputs. 

The trends in nitrogen/phosphorous ratio and phytoplankton biomass occurring during the same experiment were later investigated by Scott and McCarthy (2010) and this resulted in an alternate conclusion.  Scott and McCarthy noted that after 1990 (the date when nitrogen addition to the lake ceased) the total N concentration decreased, which resulted in a decrease in the ratio of total N to total P and suggested increasing N deficiency.  There was also a significant decrease in phytoplankton biomass between 1997 and 2005.  These observations led to the conclusion that the lake had become increasingly N-limited since the input of nitrogen was stopped and that natural nitrogen fixation by cyanobacteria wasn’t enough to make up the shortfall in Nr concentration.  It was also found that phytoplankton biomass decreased in response to the decreased N availability, leading Scott and McCarthy to conclude that the degree of eutrophication can in fact be controlled by managing N inputs together with phosphorous.   

Has the anthropogenic production of reactive nitrogen impacted more remote ecosystems…and for how long?

There was report published yesterday from Holtgrieve, et al. in Science which I feel is highly relevant to the blog and is one of a steady number of papers being published in recent times on the issue of reactive nitrogen.

The report details the analysis of the stable nitrogen isotope ratios (¹⁵N:¹⁴N) found in dated sediment cores taken from 25 pristine northern hemisphere lakes.  Looking into the sediment record in this way enables scientists to infer the effect humans may have had on the nitrogen cycle in the past.  This is possible because changes in the N isotopic composition of the reactive nitrogen (Nr) found in the atmosphere can indicate historic variation in the contributions from human-derived sources.  The Nr produced through human activity (e.g. fossil fuel burning and the production and use of fertilizers) has depleted ratios when compared with catchment and preindustrial atmospheric N sources.

The analysis found that changes to the N isotope ratio and the increased presence of isotopically lighter nitrogen (often from anthropogenic sources) started during the late 19^th century, a period before artificial fertilizers, a major source of Nr, were widely used.  However, this period does coincide with a large global increase in fossil fuel burning as the world became more industrialised.  There is a second major shift in ratios noted across the range of sediment cores at around 1970, a time when the production of artificial nitrogen fertiliser was rapidly increasing.  The effects of these two anthropogenic influences were shown in the sediment record to be immediate and reached across all the core sites.  This challenges the perception of nitrogen deposition as a problem with impacts mainly limited to the area surrounding the source.   

A nitrogen footprint warning

This article from April this year is posted on the Guardian environment section; it highlights the key headline statements made in the European Nitrogen Assessment (ENA), a publication discussed previously on the blog.

The main claim selected by the article is that the damage to water, health, wildlife and climate caused by nitrogen pollution has a financial cost of £650 to every person in Europe.  It is later explained that the annual Europe-wide cost is estimated at €70bn-€320bn (£62bn-£282bn), working out at between a minimum of £130 and a maximum of £650 per person.  The financial argument is weakened slightly when the €25bn to €130bn (£22bn-£115bn) benefits to agriculture that artificial nitrogen fertilisers deliver are considered, this is a very similar figure to the estimated environmental cost of €25bn to €145bn (£22bn-£128bn). However, the additional cost to our health and well being is significant, it is claimed that nitrogen pollution in the atmosphere reduces life expectancy over much of Europe by up to six months.  This and the damage to fish stocks and enhancement of global warming have far more than simply a financial cost to the people of Europe.

The article tells us the ENA calculates that up to 60% of nitrogen damage occurs from fossil fuel burning for energy and transport, so more efficient energy use in the home and in our transport might be an important part of mitigating the problem.

Furthermore, a highly significant portion of agricultural nitrogen pollution is due to meat and dairy farming, because livestock require vast quantities of food grown using fertilisers.  This point was made in comment from Dr Mark Sutton, of the UK's Centre for Ecology and Hydrology, who stated that the numbers of livestock we keep are important in determining the scale of environmental impacts.  The diet of most Europeans contains 70% more meat and dairy than is necessary, so it seems that there is room to reduce the problem, although this not an easy sacrifice to convince many to make!  

 

Calculate your personal nitrogen footprint!

I have come across an interesting online tool that allows you to calculate your personal ‘nitrogen footprint’.  It seems like a fairly good way to engage the public with the problem of anthropogenic nitrogen deposition and, like the carbon footprint concept, it promotes taking some personal responsibility in solving the problem.   Unfortunately, because the tool has only been recently developed, there is not yet a version of the calculator designed for UK residents (it is currently in development), but if you happen to live in the US, the Netherlands or Germany it is ready to use! 

An explanation of the methodology used by the creators (James Galloway, Allison Leach and colleagues in the US and Netherlands) to develop the N-footprint calculator is due to be published in January 2012, a draft copy of the paper (Leach et al.) has been made available online.  At a basic level, the calculator works by scaling the average per capita data on reactive nitrogen loss to the environment based on how an individual answers questions on resource consumption.  The ‘N-Calculator’ is a an attempt to improve communication to help consumers minimise their disruption of the nitrogen cycle, it is planned that the tool will sit alongside other tools that calculate the footprints of producers and another that calculates the effect of policy decisions on nitrogen footprints.  These tools are part of an overall system called ‘N-Print’.  An article was published back in February on ScienceDaily describing the project.  Included is a quote from Allison Leach stressing the importance of wider awareness of the issue:

"Solving the nitrogen dilemma is a major challenge of our time…..By calculating our individual impact, and taking small steps to reduce it, we can all play a part -- and send a strong message to our nation's leaders that we want this issue taken seriously." (A Leach, ScienceDaily 22/2/11)

Diversity Loss due to Nitrogen Deposition

In this post I’m going to look at another of the important environmental impacts of the anthropogenic alteration of the global nitrogen cycle: loss of diversity.  A key recent paper in this area has been written by Bobbink et al. (2010), who give a synthesis of the global changes in terrestrial plant diversity caused by nitrogen deposition. 

Bobbink et al. state that nitrogen accumulation is the main driver of changes to species composition across the whole range of global ecosystem types.  This is because N accumulation drives competitive interactions that lead to composition change and can also produce conditions that are unfavourable for some species. There are also other effects playing a supporting role, such as the toxic nature of N gasses, the long-term negative effects of increased ammonium and ammonium availability, and acidification, in addition to more localised secondary stresses.  The main impacts of increased nitrogen deposition on terrestrial ecosystems are summarised in the diagram below: (Bobbink et al., 2010)

 

(positive (+) and negative (-) feedbacks are shown in brackets, ↑ denotes productivity increase ↓ productivity decrease)  

Bobbink et al summarise the impacts of N deposition in every ecosystem type and the mechanisms by which they occur.  They have also created a new method to identify the ecosystems of high conservation value that are most threatened by the trend of increasing nitrogen deposition. This has been done by overlaying modelled nitrogen deposition with WWF G200 ecoregions, ensuring that both diversity hot spots and regions with their typical ecosystems are covered. Importantly, the ecoregions relate to ecosystem types whose response at different locations to N deposition can be compared and contrasted. The model show the areas of greatest deposition to be Europe, N. America, southern China and parts of southern and SE Asia, by 2030 Latin America and Africa are also predicted to be more significantly affected (see below).  

The total N deposition rates for (a) the year 2000 and (b) the year 2030 within the G200 ecoregions (deposition outside these areas is not shown.) (Bobbink et al., 2010)

Bobbink et al. state that it could be later than we think in the fight against biodiversity loss due to N deposition, as the problem in boreal forests, mediterranean systems, and some tropical savannas and montane forests could be more serious than thought previously.  They say that there are many questions about the impacts of N deposition on biodiversity still open, particularly in more remote regions.

European Nitrogen Assessment: Launch Video

One of the few quality videos that I've been able to find on this topic so far, created for the launch of the European Nitrogen Assessment back in April. Definitely the best attempt to highlight the problem I've seen so far.



Caption: In a five year project funded by the European Science Foundation programme "Nitrogen in Europe", 200 scientists/experts in the field produced "The European Nitrogen Assessment", which explains the state of the threats to water, air and soil quality and the impacts on biodiversity and climate change in Europe and highlights the possible solutions.

Management of Nitrogen: Recent and Current Research and Working groups

The importance of the global nitrogen cycle with respect to its impact on climate has only recently moved towards the top of the scientific agenda.  In November 2009, at the COP15 UN Climate Change Conference in Copenhagen, a side event was hosted entitled ‘Options for Including Nitrogen Management in Climate Policy Development’, intending to highlight the need for a new assessment of nitrogen-climate links and possible nitrogen management strategies that may reduce the extent of climate change. A press release on the event by the International Nitrogen Initiative (INI) can be found here. 

The most recent INI event, a Workshop on Nitrogen and Climate, took place this month in the Netherlands, unfortunately, it looks as if the INI website has seen little updating recently…hopefully some details and findings will be published there soon!  

A key recent publication on the subject of nitrogen management is the European Nitrogen Assessment: sources, effects and policy perspectives, a report launched this April.  It is the first comprehensive scientific assessment for European policymakers on the problem of excess nitrogen, it also estimates economic cost of the damage caused, and defines the geographical areas at greatest risk.

An article from the Centre for Ecology and Hydrology noted the key messages from the assessment:

• At least ten million people in Europe are potentially exposed to drinking water with nitrate concentrations above recommended levels.
• Nitrates cause toxic algal blooms and dead zones in the sea, especially in the North, Adriatic and Baltic seas and along the coast of Brittany. 
• Nitrogen-based air pollution from agriculture, industry and traffic in urban areas contributes to particulate matter air pollution, which is reducing life expectancy by several months across much of central Europe.
• In the forests atmospheric nitrogen deposition has caused at least 10% loss of plant diversity over two-thirds of Europe.

The report has been made available to download for free on the Nitrogen in Europe website.

Can management of the nitrogen cycle mitigate climate change?

There are some interesting ideas around using nitrogen as a tool to combat climate change, which have attracted attention from scientists and policymakers.  However, it seems as though we must be cautious about such potential solutions, due to the complexities of the global nitrogen cycle and its many interactions already touched upon in this blog.  

One example of research in this area was summarised nicely in the Scientific American magazine in 2009.  The article discusses the experiments of Pregitzer et al. (2008) in forested environments in northern Michigan USA.  Pregitzer et al have applied increased concentrations of reactive nitrogen to soils in four different areas of forest (simulating the expected quantity of available nitrogen in 100 years time).  They found, expectedly, that tree growth and carbon sequestration increased, but more interestingly rates of decomposition of organic material on the forest floor slowed. This was because the microbial community in the soil had been altered; meaning that lignin (a strong substance in plants that is effective at carbon storage) was less easily broken down by bacteria. The overall result is that nitrogen addition directly alters the ability of the soil to store carbon, and sequestration in forest trees and soils increased considerably.  As the soil organic matter is able to store similar quantities of carbon to the forest trees, this is a very significant factor.

Despite these findings, Pregitzer warns that the many negative impacts of excess nitrogen (e.g. acidification, biodiversity loss, smog etc.) outweigh this positive….."One thing that would be a mistake would be to give the implication that nitrogen deposition is a good thing..".  There are also major problems with forests becoming saturated with nitrogen, resulting in the leaching of nitrogen into drinking water sources and nitrous oxide emissions (a major greenhouse gas) counteracting the forests benefit in mitigating climate change.