The Dutch ammonia policy is effective, but not as effective as many Dutch researchers have calculated it to be over the years, says Mark Sutton, professor at the British Centre for Ecology and Hydrology. Since 1993 ammonia emissions in the Netherlands have gone down not by 70 percent but by 50 percent, reckons Sutton. But even that is a unique achievement in the world, says Europe’s leading ammonia expert. The reduction is due to the use of slurry injection in the Netherlands. ‘That approach is effective, without a doubt.’
Sutton subjected the Dutch ammonia policy to close scrutiny this summer, as chair of an international review panel. The impetus for the study came from ongoing discussions as to whether the measuring method and calculation model for ammonia emissions were sound. The model, developed by the Central Bureau for Statistics (CBS) using research data from Wageningen UR and the RIVM, does not accord with another RIVM model for determining ammonia concentrations in the air. While the first model showed a steep drop in ammonia emissions in agriculture, the drop in ammonia concentrations in the air was much smaller, according to the second model. So there was an ‘ammonia gap’. This was fuel to the fi re of persistent resistance to slurry injection from a number of livestock farmers. The Sutton panel’s task was to find out how the ammonia gap came about and who was right about it. Tan factor Ammonia is a complex and rather elusive compound. It makes its first appearance as nitrogen in livestock feeds or grass, changes into ammonium in the manure and then partially evaporates to form ammonia gas. Most ammonia comes from livestock, which is why Wageningen researchers such as Jan Huijsmans (Plant Research International) work on ammonia measurements from manure and Gerard Velthof (Alterra)
works on modelling for ammonia emissions. Velthof also coordinates the committee which developed the emissions model.
How do the scientists calculate ammonia emissions? To get an idea, here’s a crash course on ammonia and the NEMA model. You take the number of cows in the Netherlands and the annual quantity of feed per cow. That feed gets converted into ammonium in the manure: the ‘TAN factor’. The number of cows multiplied by the TAN factor gives the amount of ammonium coming from dairy farms.
For part of the year the cows stay in the cowshed. There, depending on the type of shed, some of the ammonium evaporates to form ammonia. The rest of the ammonium stays in the manure. That manure is stored in the manure cellar, where it gives off ammonia, and then it is spread on the land. During spreading a bit more of the nitrogen evaporates in the form of ammonia. Also, some of the ammonium is absorbed by the crop and some of the manure is exported or processes. The same steps can be followed for pastured cows and for all the pigs, chickens and beef calves in the Netherlands.
There are emission factors for each of these steps: the percentage of ammonium that goes into the air in the form of ammonia. To determine these emission factors, the researchers take measurements. For each shed type, for example, they look at the amount of ammonia that ends up in the environment. They also find out how much ammonia an air filter gets rid of. Then they can calculate the ammonia emissions per animal for each shed type, with and without an air filter. The researchers have calculated the emission factors for manure-spreading too. When the manure is spread on the surface on grassland, the emissions factor is 74 percent (74 percent of the ammonium goes into the air, 26 percent is absorbed by the crop). When slurry injection is used, the emission factor is only 16 percent, research has shown. This method was made compulsory in the Netherlands in 1993.
Measurement towers
Thanks to manure injection methods, the emission model shows, ammonia emissions have gone down dramatically since 1993. But that dramatic drop in ammonia emissions does not tie in with the ammonia concentrations measured in the air every year by the RIVM, using measurement towers in the Netherlands. These measurements only show a limited reduction in the ammonia concentration in the air, and in the last few years it has even been going up.
The drop in ammonia emissions is smaller in reality
So something is wrong. Is the calculation model sound? Are all the assumptions or data the model is based correct? And have the data from the ammonia measurement towers been upscaled accurately? These questions were put to Sutton. Before the summer vacation, the British ammonia expert invited the Wageningen UR and RIVM researchers and several other Dutch ammonia experts to Utrecht to look for the reason for the ammonia gap. In his review, which came out on 8 September, he sketches the most likely reasons for the gap between the lower emissions and the higher concentrations.
According to Sutton, the drop in ammonia emissions since 1990 is probably smaller than has been claimed. In 1990, cows excreted an awful lot of ammonium. They ingested a lot of nitrogen because of the heavy use of fertilizer in those days, all of it spread on the surface. According to the emission model, that meant high ammonia emissions per cow. But this calculation was based on an emission factor of 74 percent, whereas the actual emission factor must have been much lower, as some of the manure was spread in the winter, and less ammonium evaporates at lower temperatures. So ammonia emissions in the early nineteen nineties were overestimated, Sutton believes. By how much is not clear. That is something Jan Huijsmans (PRI) is now trying to estimate, so that the emission model can be adjusted. Along with cold weather, rain reduces ammonia emissions too. So the task is to include weather conditions in the emission model.
The Dutch have lost their headstart
A second reason for emissions not having gone down as fast as the Wageningen models indicate lies in the behaviour of farmers. Until recently the emission model worked on the assumption that an air filter in the shed reduced emissions considerably. But what if the filter is not switched on? A study in North Brabant in 2009 showed that some livestock farmers do not switch on the air filter at the right time, perhaps to save on energy. This factor is now included in the model. A similar correction is needed for the spreading of manure on the land, concludes Sutton. In their responses to the Dutch agricultural survey, nearly all the farmers say they use a slurry injector, while in practice many farmers and companies use a ‘trailing shoe applicator’, especially on peat soils. The emission factor of a slurry injector, which puts the manure 5 centimetres into the ground, is lower than that of the trailing shoe applicator, which lays the manure in strips on the surface. On that point too, says Sutton, the data fed into the emission model needs correcting.
Extra government funding
By contrast, the ammonia concentration in the air is overestimated in the RIVM’s deposition model, says Sutton. Here he points out the interaction in the atmosphere between ammonia and the gases SO2 and NOx. Because atmospheric levels of these pollutants have gone down in the last 20 years, ammonia stays in the air longer. The RIVM deposition model should be corrected for that effect, says Sutton. And he thinks the margins of error in this model are too big. That is partly because the RIVM only has six measurement towers, so the measurements are not representative of the farming sector. ‘The OPS model is not state-of-the-art; the RIVM has to improve it,’ concludes the panel chair.
Other countries are starting to invest now
When all is said and done, the Dutch policy has been successful in cutting ammonia emissions, concludes Sutton. In fact the Netherlands is a global frontrunner in this field. But he also notes that the Dutch government is now neglecting ammonia research. If Dutch ammonia policy wants stay in the lead, there will have to be some serious investment in research, asserts Sutton. In the nineteen nineties the Dutch government invested in measurements and model development, but then it cut back on spending. It has therefore lost its headstart, says Sutton, just when other countries want to start investing in measures to cut ammonia emissions. And Sutton knows what he is talking about, as chair of an international nitrogen panel and coordinator of a European research programme on ammonia.
In his report, Sutton describes his panel as ‘very concerned’ about the closing of two RIVM measurement towers for measuring ammonia concentrations, and says he is ‘very surprised’ that RIVM researchers are not doing any studies of the atmospheric interactions with ammonia. He notes uncertainties in the Wageningen emission model which could easily be solved with further research. He mentions for example that the Integrated Horizontal Flux method for determining ammonia emissions, used by Wageningen UR, overestimates emissions by 0-20 percent and that new research data is needed in several areas. ‘In the nineteen nineties the Netherlands invested millions of euros a year in research; those were the days,’ says Sutton. ‘If you want to regain your leading role, it will take new investments on that kind of scale.’
State Secretary Dijksma of Economic Affairs, for whom the advice is intended, will now consult her colleague at the ministry of Infrastructure and Environment as to which of the panel’s recommendations she is going to adopt. The crucial question is likely to be: who is going to pay for that? Her letter to the Lower House of Parliament lacks any sense of urgency and drive. She says she is happy about the panel’s ‘positive evaluation’ of the ammonia model AERIUS used by the government in the implementation of ammonia legislation on livestock farms. This is a very rose-tinted interpretation of Sutton’s report, which actually says the model is OK ‘as far as the panel can see,’ but adds, ‘However, insufficient information was provided on the technical basis of the AERIUS model, which should be subjected to ongoing review.’ In other words: we don’t know because the ministry hasn’t provided enough data. More research needed. And why is that research needed? Sutton: ‘The Dutch government needs to restore confidence.’
SLURRY INJECTION WORKS
Low-emission manure application is a good way of lowering ammonia emissions, claims Mark Sutton on the basis of his Review on the scientifi c underpinning of calculations of ammonia emission and deposition in the Netherlands. He points to a recent publication by Wageningen researchers in Soil Use and Management. They show that slurry injection not only considerably reduces ammonia emissions but also improves nitrogen absorption by grass, compared with spreading the manure on the surface. Moreover, they write, slurry injectors do not necessarily cause soil compaction, as some farmers have feared.
Slurry injection is often seen as an extra cost for farmers for the sake of the environment, says Sutton, but it is also a way of dealing with manure effi ciently. By using slurry injection you save on artifi cial fertilizer. He knows an English livestock farmer who has bought a ‘Dutch’ slurry injector, without any pressure from British legislation. It is economically viable because he won’t have to buy any more artifi cial fertilizer, says Sutton.
Photo: Fotobureau FPS