Atmospheric pollution, grazing numbers and soil compaction

Three things have happened to me during the last week which have really got me thinking.

Firstly, last week I gave a talk at the Fingle Bridge Inn – ‘The Elephant in the Uplands and a tale of two narratives’ – a story about the impact of atmospheric pollution on Dartmoor – see here. During the Q & A session after the talk we were discussing purple moor grass (Molinia caerulea) and I suggested that Molinia needed hard summer grazing from cattle to reduce its abundance. A member of the audience who was from an agricultural background then asked whether it would be a good idea therefore to lime the Commons ….. (i.e. the addition of calcium rich minerals to the soil to reduce the acidity).

To be honest I was a bit stumped. I thought that such an intervention on a Site of Special Scientific Interest would be a ‘potentially damaging operation’ and as a result Natural England would not permit it. More to follow later on ….

Secondly, my Twitter feed is full of discussions around soil compaction on Dartmoor.

 

 

 

 

 

 

 

Thirdly, I received an email from Kevin Cox, owner of part of Holne Moor Common which discussed the previous two issues. Kevin, like myself has heard Commoners recently advocating the use of lime, a practice that was common in the past even on the Commons of Dartmoor. He also sent me a paper (McCallum et al 2016) which suggested that the liming of upland pastures could be justified from a conservation perspective as it raised the pH which led to a significant increase in the earthworm population and thus provided an enhanced food supply for breeding lapwings.

Might the issues of atmospheric pollution, liming and soil compaction actually all be part of the same story?

The atmospheric pollution narrative
‘The Elephant in the Uplands and a tale of two narratives’ argues that the evidence that atmospheric pollution has detrimentally impacted on the vegetation of the uplands is compelling. This narrative explains the rise in extent and vigour of purple moor grass at the expense of heather especially in the light that heather beetle outbreaks have been and are widespread and destructive. Additionally, even in an era of reduced sheep numbers, heather shoots are vulnerable to selective grazing by sheep as nitrogen deposition has enhanced their nutritional value. I conclude that atmospheric pollution is a driver of change and not just an inconvenience.

However, although I referred to it on my extended blog on atmospheric nitrogen pollution (here), I had failed to make the link to one of the other impacts of nitrogen deposition – it also acidifies the soil.

The role of lime?
I was keen to understand better what my lime questioner had meant by his query, so I emailed him (Fairfax Luxmore). He replied that he had heard from a farmer in the Peak District that liming had increased the grazing output ten-fold and that liming also releases phosphate if the pH is low. He also stated that on Dartmoor where liming had traditionally occurred in the past it had encouraged the growth of ‘sweet grasses’ (which need phosphate) which the stock favoured. My previous literature review had identified that atmospheric nitrogen pollution on Dartmoor had changed the environment from being nitrogen limited to a phosphate limited one and that this change had favoured the growth of Molinia at the expense of heather.

The McCallum lime, earthworms and lapwing paper referred to above also outlines that lime sales for agricultural purposes peaked in the 1960s and has subsequently tailed off dramatically. So, at the very time that the deposition of atmospheric nitrogen pollution was sill increasing (and causing soil acidification), the application of lime which counter-acted it was decreasing. I have no data for the soil fauna and earthworms on Dartmoor but it is perhaps not unreasonable to suggest that the acidification of Dartmoor’s peaty soils would also have led to a decrease in earthworms and other soil fauna biomass.

Soil compaction on Dartmoor
It is uncontestable that the patter of cloven and uncloven hooves over the decades has led to the compaction of soils on the Commons of Dartmoor, especially as Kevin Cox explained in his email to me that the ungrazed soils on the other side of the Common fence were ‘friable and free-draining’.

Richard Smith & David Hogan

Compacted soil

Fence line on Buckfastleigh Moor edge

Free draining soil from over fence line

The question for me though is, has this compaction occurred as a result of grazing pressures post World War Two or is it the result of grazing pressures from the 12th century onwards? I ask this question as there appears to have been eras in the past when the Commons were grazed as heavily if not more heavily than the post war era without detrimental impact.

I have written before about Charles Vancouver and his claims that there were 14,000 sheep and the ‘usual proportion of cattle’ on Widecombe and Buckland in the Moor Commons in 1807, that the grass was knee high in May and barely half consumed in the beginning of November (see here). If Vancouver’s figures are correct I have calculated that the grazing pressure in 1807 was six times the figure considered acceptable today if the moorland vegetation is not to be damaged from overgrazing. Vancouver’s sheep figures are perplexing and many are sceptical about their validity, but perhaps there is an explanation.

A speculative synthesis
Atmospheric pollution levels in Britain rose following the beginning of the Industrial Revolution (a transitional process which started in 1760 and was in full swing by 1840), atmospheric nitrogen emissions peaked in the late 1980s but deposition levels have remained high especially in upland and urban areas. These compounds of nitrogen are driving vegetational change and have reduced the acidity of the soil in the uplands.

As a result, plant communities have become phosphate limited and nitrogen rich which has favoured grasses such as Molinia and the peaty soils have a much reduced soil fauna. The loss of the soil fauna (especially earthworms) has meant that soils are unable to recover from animal trampling and have become compacted.

In 1807, when Vancouver visited Dartmoor, the Industrial Revolution had barely started and atmospheric nitrogen pollution was very low. Phosphate was not a limiting factor and as a result ‘sweet grasses’ flourished and the soil was healthy and not acidified, earthworms and other soil fauna were abundant and were able to counter the compaction of the trampling of herbivores.

Conclusions
The evidence suggests that the impact of atmospheric nitrogen pollution is considerable, both to vegetation and the soil. It is also clear that levels of atmospheric nitrogen pollution will remain high for decades to come (Stevens 2016).

So, what of liming? There is some evidence to suggest that the liming of acid grassland does not reduce species richness in plant communities as long as manures or fertilisers are not also added (Kirkham et al 2008). However even if it were considered desirable (I think more work would be needed if SSSIs / SACs were to be limed) it is difficult to see how it would be practical across the 35,882 ha of Dartmoor’s Commons.

Photos courtesy of Kevin Cox.

References
Kirkmam F.W., Tallowin J.R.B., Sanderson R.A., Bhogal A., Chambers B.J. & Stevens D.P. (2008). The impact of organic and inorganic fertilizers and lime on the species richness and plant functional characteristics of hay meadow communities. Biological Conservation 141: 1411-1427.

McCallum H.M., Wilson J.D., Beaumont D., Sheldon R., O’Brien M.G. & Park K.J. (2016) A role for liming as a conservation intervention? Earthworm abundance is associated with higher soil pH and foraging activity of a threatened shorebird in upland grasslands. Agriculture, Ecosystems and Environment 223:182-189.

Stevens C.J., (2016). How long do ecosystems take to recover from atmospheric nitrogen deposition? Biological Conservation, Vol. 200: 160-167

One thought on “Atmospheric pollution, grazing numbers and soil compaction

  1. Pingback: Soil compaction on Dartmoor – A Dartmoor blog

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