6 Aug 2012

Initially, Mrs Orson and I were disappointed to get tickets for only one of ouJess Ennisr three carefully chosen athletics sessions at the Olympics. However, it soon became clear that we were lucky to get any at all. This luck was compounded by the fact that the tickets were for the night that Team GB won three athletic gold medals. It was an extremely noisy, dramatic and unforgettable experience. The media described it as historic. That’s perhaps a bit strong but it was truly exceptional. How lucky we were!

Also exceptional was the rainfall for the three months from mid-April to mid-July. When I started to think about its implications on the nitrogen status of crops and soil, I asked Rothamsted Research for a view. I think I’ve said before that the behaviour of nitrogen in the soil is fiendishly difficult, even impossible, to grasp but Rothamsted has been researching this subject since 1843 and know more than anyone else.

The (very rapid) response is intriguing and I should like to thank Keith Goulding and Andy Whitmore for their help. I should also point out that neither has been at Rothamsted since 1843, but Keith has been there since 1974!

The possible scenarios I had in mind were that excessive rain may have caused leaching of applied nitrogen; the rain-induced lush growth may have increased potential nitrogen uptake; the constant moist state of the soil may have resulted in more nitrogen being released (increased mineralisation); or that waterlogging may have caused losses through denitrification.

In today’s context, the latter has serious connotations. In anaerobic (e.g. waterlogged) conditions, bacteria convert nitrates and nitrites in the soil to nitrous oxide gas. During this process the main gas produced is nitrogen, which is inert, but a lot of nitrous oxide is also produced. This is a greenhouse gas 300 times more potent than carbon dioxide. So not only do we lose the major plant nutrient through this process but also the greenhouse gas output from cropping increases significantly.

The Rothamsted team kindly ran one of their models to see what the implications of the wet weather could be in the context of their own rainfall data and for winter wheat grown on their flinty silty clay loam. The results may surprise some. Leaching losses were only marginally higher this year at Rothamsted.

This sounds surprising but please remember that when it started to rain in mid-April the soil under winter cereals was very dry to depth. There needed to be a lot of rainfall in excess of transpiration losses before the drains actually ran. Even then, the applied nitrogen had to move a long way before it got into the drains.

The model also shows that crops had a reduced nitrogen status this year. Personally, I was surprised that I didn’t see winter cereal crops looking very short of nitrogen but on the other hand, they were not quite so dark green this year.

The main cause of any nitrogen loss since 1st March on a clay loam soil at Rothamsted was calculated to be denitrification; up to around 40 kg/ha of nitrogen could have been lost through this process. To put this into context, the same model suggests that the loss due to denitrification for the same time period last year was just 0.1 kg/ha of nitrogen.

Perhaps this exceptional spring represented the worst case scenario for this process and should be treated as a ‘one-off’? Soils were wet at a time when all the bag nitrogen had just been applied and also they were warm(ish), so bacterial activity was high. Logically, denitrification losses may have been lower on better natural draining soils and even higher on very heavy clays. Conversely, leaching losses may have been more significant on better natural draining soils.

This Rothamsted model also suggests that the soils were so waterlogged that there was insufficient oxygen for root growth. Perhaps this wasn’t so critical this year as the crops didn’t suffer any subsequent drought stress but it may have meant that the ability of the crop to scavenge nitrogen at depth was inhibited.

My next question to Rothamsted was what does this all mean for soil nitrogen levels this autumn?

The answer was that there are too many ‘ifs’ and ‘buts’ to be definitive two months in advance. However, it seems logical to me that there will not be a lot of nitrogen kicking around.

The problem with nitrogen is that logic does not always apply. To return to simpler subjects: it is certain that in two months time our ears will still be ringing from the crowd noise during Mo Farah’s last and glorious lap of the Olympic 10,000 metres final. Truly exceptional!