Carbon

Carbon Loss

Just in UK, there was a paper a few years back Nature by P.Bellamy et al, UK soils have lost 12-15% carbon of their total carbon. They used the National Soils Inventory which has been in operation since the early 1960's. the implication was that this may be due to global warming. But the finding of Bellamy was NOT confirmed by Countryside Survey Executive (Summary & Chap 2). It did not find any decrease in most soil of the ten habitats – like woodland, bracken, fen & pasture. However there were significant losses from arable soils - those growing crops and weeds.

That 'arable' land (Crops and weeds) is land which is usually (or used to be) ploughed. There was a statistically significant decrease in soil carbon over that last ten years - and last 30 years (counting from the 2007 survey). The authors clearly did not think global warming was the cause – with only one habitat with reduced carbon.  The report spelled out their concerns as being to 'management practices:

Our cropping land has lost 17% of its carbon in last 30 years. That sounds serious to me. This is across 15 million acres – about 1/34 of Britain’s land. So I did the calculations, and if this loss translated to carbon dioxide emissions, then that would represent a contribution of about 1% of all UKs' contributions.[24]

Where has all the carbon gone from the soil? The likely four candidates are ploughing, weedkillers, yields and mixed loss of farming.

Certainly, whenever you plough, carbon dioxide is released.[25]  This is why 'no-tillage' systems are popular in organic systems. A law ( Environmental Impact Assessment (Agriculture)(England)(2) Regulations 2006) was introduced to control ploughing up pastures. So why has there been a significant decrease in carbon over the last 20 or so years (we could do with more recent samples as the last were ten years ago). Has there been an increase in ploughing or deeper ploughing in more conventional arable systems??

So I asked some farmworker friends who are out in these fields all day. They said:“with minimum tillage and strip till the decrease in ploughing has been immense, many farmers have sold their ploughs, those that do still plough only do so rotationally or behind grass.” With blackgrass coming some may move back to ploughing.

“ploughing has probably become less frequent in recent years. We run a plough based system for both cereals and forage maize establishment but many others locally are using either non inversion or direct drilling. Non inversion involves surface cultivation often combined with deeper sub soil loosening. Typically at least 1 pass to create a stale seed bed which is then sprayed to kill volunteers/weeds before drilling.”

Direct drilling typically involves no surface cultivation and seed is planted straight into the previous years stubble. From the technical press it is clear that ploughing is seen as both slower and more expensive by many farmers. The trend towards larger and larger arable units together with a predominance of winter established crops has reduced ploughing, although grass weed pressure and spray resistance has meant that some are returning to ploughing as part of a rotation. But overall it is hard to see how poughing is the problem.

Is is quite possible even likely, that the use of weedkillers (herbicides) - as part of the no-till regime - may be a major part of the problem. Quite simply, killing off weeds - i.e. plants, kills of the herbage that would otherwise go back to the soil for the soil animals to work on - and hold the carbon. Each plant killed is a small carbon capture and storage unit lost.

So what is it? With bigger and bigger yields, lots more food taken off the land, and less left behind? Has anybody measured this? Since the early 1990s, many farmers did away with  mixed farming, encouraged by EU funds. Instead of having farms producing both crops and animals, with the animals often put on to fallow  land (after harvest) to eat stubble and fertilise land. England used to be a patchwork of mixed farms, but now most farms in England are either pasture or arable. The moves causing carbon loss  would fit the stats and timing, but there is no evidence as the effects were not examined.

Multiply these carbon losses and emission from arable land and we have a major contributor to global warming, which is rarely considered.

[25] https://link.springer.com/article/10.1023/A:1009766510274

[24] Calculating Carbon Loss

Method 1

Using figures which are in g C kg−1? (carbon grams per kilogram soil)

1. How many arable acres? 15.3 million acres are arable in UK.

2. How much does 8cm topsoil weigh? One old furrow 6-7 cm deep)weighs 2,000,000 lbs = 980kgm

3. How much lost throughout UK?

45.6g/kg (from Table 2 above) X 980 kgm 44688 goes to 38.0 X980 kg 37240 per acre ie loss of 7448 kg times 15 million acres = ca 112,000 million kg (112 million metric tons or tonnes) carbon lost from arable fields in last 30 years

4. Carbon dioxide (given off from soil) weighs 3.6667 times more than the atomic weight of the carbon lost from the soil. Call that 410 million tonnes of carbon dioxide come off arable soil..

5. So over last 30 years, 410/30 = 13.5 m tonnes /yr contribution

6. Current (2008) UK emissions of carbon dioxide are about 530 million tonnes per year (Forest commission) So 13.5 m tonnes /yr is over 2.5% of total…

This amounts to ca 2.25% of ALL our total GHG emissions.

Method 2

Calculating carbon dioxide emitted from soil as a result of loss of carbon based on 0-15cm figures

1.4.3 Bulk density and soil (0-15cm) carbon stock The mean bulk density of soils (0-15cm) in the Arable and Horticulture Broad Habitat in England in 2007 was 1.25 g/cm3 which when combined with soil (0-15cm) carbon concentration gave a soil (0-15cm) carbon stock estimate of approximately 43tC/ha . This was the lowest carbon stock estimate of all Broad Habitats  CS Reports 2007 Chapter 3 Enclosed Farmland

Using the earlier figures to get percentage fall. This translates to 51.6 in 1979, and would amount

51.6 – 43 t/ha (8.6) difference & 15 million acres (= 6 m hectares) = 8.6 X 6070000 = 52mt lost

Or 52 X 3.66 carbon dioxide emitted = 191 mt CO2

That second calculation is about half the first estimate. That would be due to the depth of soil – about twice as deep in second calculation. I went for 8cm in first calculation, because it was the only depth I could get a weight for and because thought that any carbon loss would be mainly from that top 8cm.

We hear nothing about this carbon loss. Where is the discussion about how we are going to put this back?