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On top of a hill—soil bulk density and burrowing animals

Updated: Oct 18, 2021

On top of a hill, in a wheat paddock, beneath a hot, hot sun, and without any wind, three of us were digging a two metre hole in the ground. We were carefully placing a steel ring into the undisturbed ground every 20 to 30 centimetres. The soil looked like a small uncooked cake in a tin.

Our bulk density sampling site

It was hot, hard, and painstaking work but we got what we were after—bulk density measurements. We found the soil bulk density in the hole to be between 1.3 and 1.7 g cm-3. This was interesting because bulk densities above 1.4 g cm-1 in clays make it difficult for plant roots to penetrate the soil. Access to moisture and nutrients in the soil is restricted when bulk density gets this high and crop productivity therefore drops. So why is the soil here so dense —and how does one fix it?

Well. There are many ways that question could be answered. We could look at the chemical structure of the clays in the soil that allow them to become so compact, or the mixture of sands, silts, and clays. It is also known that driving heavy machinery across paddocks can also increase a soil's bulk density to negatively high levels.

But one of the things that struck me while doing this field work was the biological processes that didn’t appear to be happening: those driven by plants and animals.

Thinking of the bettongs in Mulligans Flat Reserve, I looked around at the hill and imagined what this landscape would have looked like 200 years ago with them around. Would the soil be less dense, hold more water, cycle nutrients better? Thinking even further back, when Wiradjuri folk in this area shared the land with the Australian megafauna for a brief period of geological time, would diprotodons have also dug through these soils, turning them over and decreasing the bulk density?

Ecologists use the term ecosystem engineers to refer to animals which influence resource availability in some way. This could be by modifying a habitat, creating a habitat, or maintaining one. Ecosystem engineers not only modify the physical functioning of a landscape—the infiltration of water and nutrients into soils—but the biological functioning also.

The bettong is an ecosystem engineer and were once one of the most widely distributed animals in Australia. Some bettongs dig between 20 and 100 small pits into the soil every night, excavating more than three tonnes of soil annually. When then do this, they mix organic matter into the soil, which decreases soil hydrophobicity (water repellence), and helps to plant seeds. Importantly, this action decreases soil bulk density and allows plant roots to penetrate soils. Thus they engineer the environment in a way which makes it more conducive for other species to thrive.

Once a bain to many a farmer (they were known to be lovers of potatoes) bettongs are now restricted to a few predator free islands and mainland islands within predator-proof fences, such as Mulligans Flat.

But the bettong was not the only engineering species in Australia.

Twenty-nine of the 160 Australian marsupial and monotreme mammals are diggers. Prior to European settlement, bilbies, wombats, numbats, bandicoots, potoroos, and even marsupial moles were burrowing through the hard soils, sticking their noses into leaf litter and excavating pits around throughout the grassy woodlands of south-eastern New South Wales.

These activities had substantial benefits for ecosystems. The pits left behind after echidnas have a feed of ants or termites can have twice as much leaf litter (organic matter) as surrounding soils. Microbial respiration (an indication of the work microbes are doing) can be 30% higher, and nitrogen and carbon concentrations many times higher. The water which gathers in these pits can infiltrate deeper into soils than the surrounding soil, especially when such soils are hydrophobic.

Different species have different effects on soil. The northern hairy-nosed wombat dig burrows deep into the soil and can turn over more than 14 tonnes of soil every year. The soil which piles around these burrows provide fresh soil for plants to establish. The soil also enters streams and rivers and cause sedimentation, although this occurs at a much higher rate when wombats co-exist with cattle.

So what does this mean for bulk density?

The landscapes of south-eastern Australia are the result of tens of thousands of years of landscape management, with this detailed in the most excellent book Dark Emu by Bruce Pascoe. Grasslands of highly palatable native crops such as kangaroo grass and native millet were maintained within highly managed woodlands. They were systematically burned to keep the scrub down. The management of this land, literally over geological time, stopped around 200 years ago when colonisation destroyed the relationship between the land and the aboriginal Australians who managed it. It also destroyed the relationship between animals and plants, and the soil too.

Today the landscape is almost entirely absent of any digging creatures whatsoever, with seventy percent of the digging and burrowing species in Australia now on the conservation listing. Only the short-beaked echidna (its long-beaked cousin is now extinct) has the same range it had prior to European settlement.

We currently face an ethical conundrum, where increasing populations are critically dependent upon the crops we produce. Poverty greatly exacerbates environmental impacts, but the cropping which reduces this poverty causes impacts of its own—the loss of animals that make the soil perfect for plants.

I dream of a time when agroecosystems are restorative. A time when the grassy woodlands of Australia's past are once again places where people, plants, soil, and animals give and take from their environment. A time when some of the critical problems facing agriculture—bulk density, soil acidity, and climate change, have been addressed using all types of knowledge, plants and animals, and where the digging, hopping, burrowing creatures of the past can once again be seen throughout the landscape – helping to make the right soil to grow those crops.

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