Desert decomposition conundrum

The planet is a giant nutrient recycling machine: energy from the sun, captured by photosynthesis, feeds a food chain of consumers, which, when they die, are recycled by an army of decomposers that release the nutrients, and embodied energy, back into the environment for reuse. And hitherto, much of the attention around these decomposers has, quite appropriately, focused on microbes - bacteria and fungi. These thrive in warm, wet environments. So, unsurprisingly, the wetter it is the faster things tend to be broken down. But there’s a catch: in extremely dry places, where the normal assemblage of microbial recyclers ought to struggle, decomposition nevertheless proceeds at the same expected rate. So something must be stepping in to fill the gap, but no one knew what. Now, as he explains to Chris Smith, Nevo Sagi, working originally at the Hebrew University in Jerusalem and now at the University of Texas at Austin, thinks he may have solved what’s been dubbed the “desert decomposition conundrum”...

Nevo - This is a question that exists for about 50 years already. What we know is that decomposition or the breakdown of plant litter is determined by the activity of microorganisms such as bacteria or fungi, and they are very much moisture-dependent. So, we expect that the composition should increase with moisture availability. But the problem is that, in dry lands, which is like the dry part of the world, it is more than 40% of the global land area. This model doesn't work. Decomposition doesn't increase with annual precipitation.

Chris - So, if I drew a graph of the rate of decomposition on the Y axis and the amount of moisture in the soil on the X axis, what you're saying is we should see an upward sloping line. As it gets wetter, the decomposition should go faster, but what we actually see is almost a straight line between the dry and the wet. So it doesn't follow the prediction of wetter equals faster breakdown.

Nevo - Yeah, I would just comment here that this is the case in dry lands. So basically most of the research is done in the most wetter ecosystems like temperate ecosystems, but dry lands, which are less studied, just show very different trends.

Chris - So, something is changing when it's dry, something must be stepping in to compensate to keep up the rate of decomposition because the normal mechanismis not doing what it would normally do. There has to be something filling the gap.

Nevo - Exactly. And this is what was termed the dry land decomposition conundrum. And it stimulated much research that basically what what people asked: is what is different between dry lands and this other weather ecosystems? And there are several mechanisms that were tested, but all of them were mostly focused on the role of microbes or the other things that facilitate the activity of microbes. And what we offer here is a different hypothesis. Basically some other organisms, larger organisms are more adapted or activity under very dry conditions, and maybe they can kind of compensate for the lack of microbial decomposition. And this is why we see same decomposition rate across different precipitation levels.

Chris - How did you test that hypothesis then?

Nevo - What we did is a multi-site decomposition experiment. We went to seven different sites across the gradient of precipitation. We did this litter box experiment. We have this box - or cage - where we keep plant litter and we allow different sizes of organism to access this litter. So we have three different treatments. One was just allowing microorganisms to access the litter. The other allowed also mesofauna. And the third treatment allowed the whole decomposer community, which is also includes the macrofauna, which we believe has the more important effect in drier sites.

Chris - Right. So, basically what, what you're able to say is, well, if we allow different categories of decomposer in - and they vary by size - if they are filling the gap in the dry place and we exclude them, we should see that desert decomposition gap come back if we exclude the big guys and they're responsible.

Nevo - Exactly. Exactly. You are just right to look if the mechanism we suggest is right. We also use default traps just to capture ground active insects. And we use this to really characterise the macro decomposer community or assemblage in these different sites. And we repeated this experiment in the winter, which is cool and wet then in the summer, which is a hot and very dry.

Chris - What did it show? Have you got to the bottom of the desert decomposition conundrum? Do we think what you have done shows that it is the bigger consumers that move in and fill the gap?

Nevo - Yes. So, what we have found is very beautiful because in the winter we saw that microbes dominated decomposition in general, and that decomposition increased with precipitation. But in the summer, what we saw is that macro decomposers were the dominant decomposers, and the decomposition in general peaked in the, we call it the arid sites. And when you look at the two seasons combined ,and the total decomposition, you really see comparable decomposition rates across the gradient. Actually, for one of the arid sites total decomposition was higher than the wetter sites.

Chris - Well, congratulations on solving a 50-year-old mystery. And apart from being extremely satisfying academically, why does this matter?

Nevo - Dry lands cover more than 40% of the global land area. So, this is kind of a big deal. It means that we should look at different sizes of decomposers and treat them separately when we do, for example, carbon cycle models, when we want to understand how trajectories of carbon cycling and then of ecosystems in the world in general are gonna change in the future. Because we do see desertification, we see these trends, and therefore I think this is very crucial.