Bugs in soil prefer a good spread of food...by Barry Rawlins

Barry and the synchrotron

On my way to work I cycle past a field of grazing cows which is next to a milking parlour. A while ago I stopped to ask the friendly farmer, "Can I have a bit of the soil from your field?”

"Yes”, he replied, “but what do you want it for?"

I almost sighed. I knew if I had to explain thoroughly why I wanted it, my answer would be long and complicated. "Well, I want to do an experiment where I take lumps of your soil to a lead-lined room in Oxfordshire and shine light onto it that is 10 billion times brighter than the sun."

He looked over at his colleague with a confused expression, as if to say, we've got a right one here. But he did let me take some of his soil.

So, why did I need these lumps of soil? I want to understand what might help or prevent bugs (bacteria) in the soil eating their source of food – carbon. When bugs eat the carbon it is released to the air as carbon dioxide, the main greenhouse gas. If we understand this process better, we might be able to store more carbon in the soil and slow down climate change.

 A slice through a soil aggregate showing the
position of organic matter, minerals and pores.

Bugs in soil live in the pore spaces – the air- and water-filled gaps between the solid bits. The food which bugs want to eat – the carbon – is unevenly spread throughout each lump, which we soil scientists call ‘aggregates’. Sometimes carbon occurs as big clumps in only a few places in an aggregate, whilst in other aggregates the carbon occurs more frequently as lots of small pieces. The question I wanted to answer in my experiment was: in which of these two cases do the bugs eat more carbon?

A novel way to answer this was to do three things: first, measure how well the bugs fed on carbon in many soil aggregates in the laboratory; second, stain the carbon in these aggregates using a chemical, and finally put the stained aggregates of soil into a bright light source called a synchrotron. The synchrotron shows us in three dimensions the location of the stained carbon inside the aggregates. It also shows us the location of the pore spaces.

(When I was explaining this to the friendly farmer to make it sound exciting I said, "No one has done this before!" He still didn't look convinced.)

After the experiment I compared the amount of carbon the bugs had eaten with how it was spread out inside each aggregate. I found that the bugs had eaten more of the carbon when it was distributed more frequently, more pieces throughout the aggregate. I need to analyse more aggregates to have greater confidence in this finding, so I will have to visit some more friendly farmers and hope I can convince them to give me some lumps of their soil. I am expecting to see more confused expressions.

The research I describe above was a collaboration of several BGS scientists and other colleagues at the Diamond Light Source campus in Harwell. We wrote this paper and there is a movie on the BGS YouTube channel that shows me talking about the experiment at the synchrotron.