Agriculture 2 - Nitrogen: too much of a good thing?

Last time on Standing Room Only....

• Due to its lack of adequate arable land Hong Kong relies upon imports from China for 90% of its fresh food.
• Annually increasing applications of nitrogen chemical fertilisers (CF-N) allow China to supply not only its world leading population but export the excess to Hong Kong and numerous other countries.
• Of late China accounts for 30% of the worlds yearly total Nitrogen fertilisers (CF-N's) used.

and now Part 2...

Clearly using larger and larger amounts of CF-N can result in higher yields being produced more quickly and therefore enable multiple crop cycles within a time frame that previously would have only allowed a single harvest. 

The downside is that the efficiency of the fertilisers decreases substantially when greater volumes are applied: physical factors such as the proportion of nitrogen that a soil can retain and the methods used to apply the fertiliser place hard limits on how much nitrogen (N) is actually utilised by the plants. Perhaps it's a little easier to explain this by thinking in terms of a 'nitrogen balance' within a soil:

Known input of CF-N = N present within harvested crop + Unaccounted N. 

This unaccounted N is 'lost' through the crop process in a variety of manners,  Zhu and Chen, 2002 provide a great quantitative assessment of how this 'Unaccounted N' is distributed, although it's a little dry if you're not into numerous statistical comparisons etc. so here's my take on it:

• Up to a third of the applied N is retained in the soil. This proportion is dependent upon crop type, irrigation and the history of N application (longer periods of fertiliser use result in greater accumulation of N). Potentially this retained nitrogen can be used up by the next crop cycle but only if the N remains in the root zone.
• Potentially a fifth of the CF-N can leach out of the root-zone (again this is highly dependent upon soil type, depths, irrigation etc). Predominantly this leaching occurs during the crops' initial growing process when there is a lot of water flowing through the ground but the plants have not developed sufficiently to use the majority of the fertiliser. 
• A small amount of the excess nitrogen can be lost due to above ground run-off. As with leaching this generally occurs early on in the crop cycle, flooding or excess irrigation carries nitrogen rich silts away from the crop-land and into local watercourses.
• The primary cause of nitrogen loss is denitrification and may account for up to a massive 50% of unaccounted nitrogen. Dentrification refers to the the microbial reduction of CF-N producing gaseous emissions which are subsequently lost to the atmosphere.

Visual representation of the nitrogen balance courtesy of the FAO.

So, we know that the application of increasing quantities of CF-N can lead to larger quantities of N escaping the agricultural system and reaching the natural environment. Now we need to consider what are the effects of dumping an excess of nitrogen into a carefully balanced ecosystem?

We've seen that the unaccounted N is basically transported in two ways: hydrologically (leaching and run-off) and through gaseous emission (denitrification). 

Hydrologically lost CF-N have an incredibly high chance of reaching local water systems where they can cause large scale contamination.  This form of contamination is so common and wide spread that the World Health Organisation denotes agricultural application of nitrogen as a primary cause for nitrate water pollution (WHO, 2011). They go on to state that drinking water sourced from agricultural areas often exceeds 50mg/l (that's 5 times the level generally observed from drinking water derived from surface water in non agricultural areas). 

Nitrogen that finds its way into lake systems also leads to a substantial increase in 'Algae Blooms'. The eutrophication of algae has a hugely destructive effect on lake ecology and is potentially irreversible in some cases (Carpenter et al, 1999).

Chemical fertilisers (including CF-N) causing eutrophication and its resultant impact on lake ecology. Courtesy of sciencebitz.

The major emissions resulting from microbial reduction of CF-N (denitrification) are nitric oxide (NO), nitrous oxide (N2O) and di-nitrogen (N2) (Zhang et al, 2009).  Perhaps the most important of these to consider within the scope of this blog is nitrous oxide. Emissions of N2O are a major contribution to the greenhouse effect (which is widely documented so let's not dive into its intricacies at the moment) Wikipedia. Although agriculturally derived quantities of nitrous oxide are currently considered to be relatively low (around 0.2kg N/ha./year) the amount is increasing each year (Kim et al, 2007). More importantly the IPCC has predicted that N2O has nearly 300 times the global warming potential of CO2 over the next 100 years!

OK, I feel that we've identified that we have the ability to drastically enhance a soil's naturally sustainable output to a level that can cope with the demands of an ever increasing population. We've also determined that this prolific productivity comes with a price: two significant impacts upon the environment. Let's put a pin in agriculture for now so that next time we can explore some other environmental impacts caused by an increasing population.

Even Paul Walker and Vin Diesel are obsessed with NOS (Nitrous oxide for the uninitiated)!

P.S....

If you fancy a bit of further reading Gruber and Galloway, 2008 have an informative and accessible article in 'Nature' that explains some more of the nitrogen cycle. I strongly recommend checking it out.