Wednesday, May 25, 2011

Overloaded with Nitrogen and approaching peak phosphorus

We have loaded the farming systems with more and more nitrogen and phosphorus. Initially, the soil could take care of it, but now it leaks into the water and the air everywhere. Eutrophication[1] is a substantial problem in all countries where modern, industrial farming, is practised. Use of nitrogen fertilizers increased from 11 million tons 1960 to 85 million tons 2003 (MEA 2005), see figure. At the same time farming lost 95 million tons 1990, i.e. most nitrogen is simply wasted through denitrification, leaking, erosion and volatilization of ammonia. In Rwanda, erosion causes loss of almost 1 million tons of organic matter, some 40,000 tons of nitrogen, 280 tons of phosphorus and 3,000 tons of potassium - more than the total use of chemical fertilisers (Minagri 2009). Through denitrification, nitrogen is lost as nitrogen gas (which in essence is harmless) but some 5-7% is emitted as nitrous gas, which is a potent green house gas. Nitrogen losses will continue, and increase at the same rate as the use of nitrogen fertilizers (Ayres 1998). In the USA, half of the fertilizer used is used to compensate the nutrient losses caused by erosion (Montgomery 2007). These losses are not only representing an economic and agronomic problem but in an even higher extent an environmental problem. The Mississippi, the Columbia, and the Susquehanna rivers together discharge approximately 1 million tons of nitrogen in the form of nitrate per year to coastal waters (H. John Heinz III Center 2008). The production of Nitrogen fertilizers is very energy consuming and in addition, it incurs great emissions of laughing gas, one of the greenhouse gases. Mono-cropping of grain which is rather closely associated to the use of synthetic fertilizers[2] leads to a reduction of carbon in soils and thereby increase the green house gases.

In the high income countries, a lot more nitrogen is used than what is taken out from farming in the form of products. Countries like South Korea and the Netherlands, with very intensive farming systems, used, in the end of the 1990s, more than 250 kg of nitrogen per hectare more than they took out, and these figures are not even including all nitrogen sources (OECD 2001). The International Assessment of Agricultural Knowledge, Science and Technology for Development[3] (IAASTD 2009) assess that the fertilizer uptake efficiency is less than 30 percent for rice production in South and South East Asia. Globally the nitrogen efficiency in grain production has deteriorated drastically and rapidly. Around 1960, each ton of chemical fertilizer resulted in an increase of grain yield of 75 ton, while in the end of 1990 resulted in just 25 ton, a glaring example of decreasing marginal utility, as nitrogen fertilizer use increased tremendously in the same period.

Over and above the use of chemical fertilizers, there is a substantial supply of nitrogen through biological nitrogen fixation. This is partly done by bacteria living in symbiosis with (mainly) leguminous plants and partly by bacteria and algae which fix nitrogen independently. Biological nitrogen fixation represents one third of the nitrogen brought to farming (Vitousek et al 1997). Even if nitrogen in chemical fertilizers and nitrogen in biological nitrogen fixation are both from the same source, the air, one can’t see them as totally equal sources, especially not regarding their effects in the soil. In theory, there could be substantial nitrogen leakage caused by biological nitrogen fixation; in practice, it is difficult to substantiate that. One reason is that the process, as most natural processes, to a large extent is self-regulating; if there is a lot of free nitrogen in the soil, nitrogen fixation from the air ceases, as it is easier (it ”costs” less) for the organisms to take it from the soil than from the atmosphere.  

We have mainly looked at nitrogen, but there are similar problems with phosphorus. A main difference is that phosphorus leaks mainly through erosion. Phosphorus is mined and is thus a limited resource and there are indications that we approach “peak phosphorus”, i.e. the point at which less phosphorus can be produced than previously, because of limited supply. A complication with phosphorus fertilizers is that they often contain cadmium, a highly toxic heavy metal of which the load in our food is already alarmingly high. Rich countries can chose the cleaner phosphates, or even clean contaminated ones, while low income countries are left with the contaminated fertilizers. 
(modified extract from Garden Earth)

[1]       Over-abundance of nutrients leading to algal blooms etc.
[2]       It is nitrogen fertilizers that have enabled farmer to skip sound crop rotations and go for mono-cropping, so it is not so far-fetched to “blame” nitrogen fertilizers for being a major factor for increase in mono-cropping.
[3]       The IAASTD is an intergovernmental agency with representation of UN agencies, the World Bank and international and regional NGOs. Its main report was published in 2008. 

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