A Blog Summary

I'm afraid that my time blogging on the anthropogenic disturbance of the global nitrogen cycle is drawing to a close..... In order to make the most of the information posted, and for me to have a personal refresher on a few things that I may have forgotten, I thought that I would try and produce a short summary of the key messages from the blog.

To start with, I'm conscious that I have not yet explained my blog title, I chose the 'Destroying life support' bit because that it exactly what the nitrogen cycle is - life support.  Without the creation and transportation of reactive nitrogen in various forms, the earth could not support the levels of primary production that currently occur on the planet. The unfortunate situation is that through the advancement of the technologies (most notably the Haber-Bosch process and fossil fuel use in energy and transport) that have allowed the Earth to support so many additional lives, we are adding to the cycle in a way that is very damaging environmentally.

The anthropogenic disturbance on the global nitrogen cycle is a dramatic one. The cycle is changing faster than any other, due to a level of man-made influence which far outstrips our impact on the carbon cycle  (Nr creation has increased by 120% since 1970 (Galloway et al., 2008). The environmental problems caused by this have been discussed and shown in several posts e.g. nitrogen's dark side, the nitrogen cascade and posts on eutrophication, acidification, diversity loss and impacts on the atmosphere and ocean.

Just as important, and more interesting to me personally, were the interactions between various different environmental processes arising from the disturbance of the N cycle.  This was discussed early on in the post on Gruber and Galloway's earth-system approach and continued as a theme throughout in posts describing the impact of acidification on the ocean nitrogen cycle and the impact of oceanic nitrogen pollution on the atmosphere.

There was also some research thrown up which I found particularly unusual or surprising.  For example, the work of Pregitzer et al. in Michigan where they found that inputs of Nr in forests increased the carbon sequestration of not only the plants, but also soils due to alteration of the forest microbial community.  An excellent insight into past anthropogenic influence on Nr was described in the post on Holtgrieve et al. (2011), whose work analysing lake sediment records showed that anthropogenic disturbance has been happening since the late 19th century, a time earlier than thought previously.  I feel that this was the most significant paper to be published during my time writing the blog.

Finally, I think possibly the most important topic for future management of the N cycle was the discussion of the nitrogen footprint calculator devised by Leach et al. in to bring the problem to the public's attention and emphasise the role that individual choices can play.  Whilst writing the blog it quickly became apparent that the issues of excess Nr were appearing fairly frequently in scientific literature, but it was a challenge to find anything at all in the general media or even something as simple as a youtube video that might engage a wider audience.  Things became easier as I found my way around the subject, but it appears that this issue is largely hidden from view.  I believe this has a massive impact on the potential for managing the Nr problem, as public awareness is essential, particularly when our lifestyles and consumer choices (e.g. cutting back on overconsumption of meat and dairy) could do so much towards solving this major environmental problem.

I hope that the blog has given an informative and accessible insight into the disturbance of the global nitrogen cycle and has perhaps encouraged some people to consider the impact that their daily activities can have.

Thanks for reading!

Ed 

Oceanic Dead Zones: Is agriculture destroying marine habitats?

Continuing on the theme of anthropogenic nitrogen’s impact on the ocean, it is time I did a post dedicated to a major nitrogen-related environmental problem: Oceanic dead zones.   ‘Dead Zones’ are areas of the Ocean where the bottom water has become anoxic (i.e. has low or zero dissolved oxygen concentration), very few organisms are able to survive in such low oxygen conditions.  Dead zones occur along large sections of the coastline of major continents and are continuing to spread over the sea floor, destroying the habitat of many organisms.

Fig. 1. ‘Global distribution of 400-plus systems that have scientifically reported accounts of being eutrophication-associated dead zones.’ (Diaz and Rosenberg, 2008)

These dead zones are created when the organic matter produced by phytoplankton at the surface of the ocean (in the euphotic zone) sinks to the bottom (the benthic zone), where it is broken down by the action of bacteria, a process known as bacterial respiration.  This is problematic because while phytoplankton use carbon dioxide and produce oxygen during photosynthesis, bacteria use oxygen and give off carbon dioxide during respiration. The bacteria use up the oxygen dissolved in the water which is essential to all of the other oxygen-respiring organisms on the bottom of the ocean, such as crabs, clams and shrimp, and also those swimming in the water, such as fish and zooplankton.  The overall impact is to make large parts of the ocean uninhabitable for the majority of organisms. For further explanation there is an excellent review of dead zones in the ‘Science Focus’ section of the NASA website, which provided much of the core information for this post.

An influential paper on dead zones is from Diaz and Rosenberg, published in Science in 2008, they state that oceanic dead zones have spread exponentially since the 1960s and that this formation of dead zones is exacerbated by anthropogenic influences on nitrogen entering the ocean due to riverine runoff of fertilizers and the burning of fossil fuels.  This extra nutrient input fuels coastal eutrophication and the accumulation of particulate organic matter, which encourages microbial activity and the consumption of dissolved oxygen in bottom waters. The resulting lack of oxygen causes fish to migrate away from affected waters and the death of large numbers of less mobile organisms.

The issue is also discussed in The Scientific American magazine, the article includes this quote from Robert Diaz (co-author of the paper discussed above) highlighting the main cause of the problem  

"The primary culprit in marine environments is nitrogen and, nowadays, the biggest contributor of nitrogen to marine systems is agriculture. It's the same scenario all over the world.." (R. Diaz, Marine Biologist, The College of William and Mary)

The article also investigates various possible measures that could be used to prevent coastal eutrophication, but preventing damage caused by agriculture is highly problematic.  Suggested solutions range from engineering crops to overexpress a gene causing roots to absorb more nitrogen, to large-scale geoengineering projects, such as attempting to artificially aerate the ocean. The latter solution sounds unlikely in my opinion, as trying to directly influence an open system on such a massive scale is sure to be very challenging, even though experiments on a much smaller scale have shown success.  It seems that dead zones are another anthropogenic nitrogen related environmental problem that may require more drastic change to slove!

Nitrogen pollutants in the oceans: The impact on the atmosphere

As my research for the blog continues, I’m finding that the role of nitrogen in the world’s oceans is more important than I had imagined, and anthropogenic changes to the ocean’s nitrogen cycle can have far reaching effects, many of which are uncertain but potentially significant.

The paper that I want to draw attention to today is by Duce et al. and was published in Science in 2008.  The paper is an interesting one as it highlights the impact that atmospheric anthropogenic reactive nitrogen being dissolved in the open ocean has on the ability of the ocean to act as both a source and a sink for greenhouse gasses.

According to the research, this anthropogenic disturbance could cause around 3% of annual increase in new ocean biological production.  This causes a removal of CO₂ from the atmosphere as the marine carbon that is used to create new life must remain in constant equilibrium with the carbon in the atmosphere.  However, this greenhouse gas reduction is less significant when the possible increase in nitrous oxide is considered.  Duce et al. state that as much as an extra 1.6 teragrams of nitrous oxide (N₂O) could be produced as a result of the increased reactive nitrogen availability, accounting for around 2/3 of the reduction in radiative forcing achieved by the extra carbon sequestration.

The (complex!) impacts of ocean acidification on nitrogen cycling

Sticking to the topic of acidification…I’ve encountered some research on acidification of the world’s oceans and how this may be impacting on the way nitrogen is cycled within them.

An article appearing in Proceedings of the National Academy of Sciences (PNAS) by Beman et al. (2010) details how the quantity of anthropogenic carbon dioxide dissolved in the oceans, and the acidification that occurs as a consequence, has impacted the microbially mediated biogeochemical processes that are so vital to the response of the earth system to environmental change.  

Beman et al. state that microbial nitrification (a process where ammonia is oxidised to nitrite and then nitrates by two different groups of microbes) decreased at all of the 6 sites analysed, in both the Pacific and Atlantic oceans, when ocean pH was experimentally reduced.  Based on their experimental data, Beman et al. believe that that ocean acidification could reduce nitrification rates by 3–44% within the next few decades; they say that this will affect oceanic nitrous oxide production, reduce supplies of oxidised nitrogen in the upper layers of the ocean, and fundamentally altering nitrogen cycling in the sea.

The point about reduced production of nitrous oxide is an interesting one, as it is a potent greenhouse gas (298 times more impact 'per unit weight' than CO₂ over a 100 yr period, according to the IPCC) and the ocean is already a significant emitter to the atmosphere.  However, there is a chance that this may be offset by other changes, such as increased contributions on nitrogen to the ocean.  

Marine food webs can also be affected. Acidification could produce a competitive shift away from ammonia-oxidising organisms, this would result in less nitrate being produced and ammonia would instead be converted into other forms such as regenerated ammonium.  An increase in ammonium based primary production would cause a cascade of complex effects throughout marine food webs.  

The impacts of current ocean acidification are too complicated to properly understood without further experimentation such as this, and it may never be possible to predict all possible consequences. 

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.