Sunday 28 August 2016

Putting Up Roots, Laying Down Soil

The stems of marram grass anchor coastal dunes to a great depth
Smooth white sand is the look of an ideal beach holiday. Yet such perfect stretches bordering coastal dunes are restricted to a narrow band practically on the water line.

Move just a little further in-land and the sand becomes hummocky and dotted with thin stems of marram grass - a dune environment.

As Dunes are continually in flux, the grasses are often buried. They grow back rapidly towards the light and flourish until the next deluge. Yet tug on one of these grasses and you may end up pulling several metres of buried stem from the dune. Their presence plays a key role in stabilizing the dunes. In most environments roots anchor soils together, in coastal dunes it is the long stems of grasses. 

Recent evidence, however, has shown that a similar process may have also played a role in the geoengineering of the Earth during the invasion of the land by plants. Around 390 years ago, the first trees with long, deep-reaching roots evolved. These roots enhanced the weathering of rock which in turn promoted the formation of thick, nutrient rich soils. The soils themselves were then bound together by the roots of other, smaller plants.

This was the end of the geoengineering process, occurring tens of millions of years after plants made the move onto land. The earliest forms did not possess roots and so had no geoengineering impact. The intermediate stages are less well documented, but a recent discovery has shed light on the role stems played as well as the roots.

The 410 million year old rhizomes in floodplain sediments, with modern
plant roots hanging from the cliff face and breaking through the rock
Researchers, led by Jinzhuang Xue from Peking University, examined early Devonian sediments near Qujing City in Yunnan Province, China. Cliff exposures in one formation showed that the original floodplain sands and silts were interspersed with fossil rhizomes - stems - of the plant Drepanophycus.

These had not grown down through the sediment, but upwards to counter their burial by floodplain sediments, leaving behind an incredible 15 metres of rhizomes. Only the surface plant would have been alive, yet the dead rhizomes would have played a crucial role in stabilising the floodplain sediments into a primitive type of soil. This soil was not of the kinds which exist today.

'It’s a spectacular sequence of sediments,' said Paul Kenrick from the Natural History Museum. London. 'It’s not creating a structured soil profile, more of a stabilised sediment. It's important in that respect.' Soils form the basis of virtually all terrestrial ecosystems, allowing a wide variety of plants to develop and in turn support more complex ecosystems. Understanding their origin is therefore a key step in understanding the colonization of the land, as well as giving a different viewpoint on the problems facing soils today, including leaching and deforestation.