The history beneath your feet

I think about dirt a lot more than most people. Probably this is the result of my background in ecosystem science, the study of how nutrients and energy flow throughout the biosphere. The soil represents a huge sink for all major nutrients that sustain life on earth, and an important source of nutrients such as carbon and nitrogen that are naturally recycled to the atmosphere.

But it’s not economic, or really even environmental value that makes soil so fascinating to me. It’s history. In the forests of northeastern Puerto Rico where I’m currently conducting research, the historic record deposited in soil is up to 8, 10 or 15 meters thick and stretches back 300,000 years into the past, to a time when the island itself was shooting up out of the Gulf of Mexico due to rising magmatic rock deep beneath the ocean floor.

If you start looking closely at the composition of soil, you will quickly discover a wealth of information recorded within it. Tiny grains of minerals, produced from thousands to millions of years of water dissolving rock, form the physical matrix on which life has developed. As these clean, crystalline minerals slowly rot, they are chemically transformed into new compounds such as clays. Clays and other secondary minerals add stickiness to the soil, allowing decomposing organic materials to adhere. Slowly, a strange collection of organic materials that were once living and the inorganic ingredients that supported their existence begins to accumulate. As this assortment of the dead and rotting grows, so does the living biomass that it sustains. Most of the soil microfauna is involved in feeding off dead (or other living) organic materials and ultimately recycling nutrients that would otherwise be locked away forever. Embracing death is a way of life in the world’s most biologically diverse ecosystem.

But wait- I was talking about history. Yes, living organisms, dead organic materials, clays and minerals are all important components of the soil, but how do we piece together a history (and of what? The geology that the soil formed over? The forest that once stood atop it? The long-dead animals whose traces still linger within it?) from such a complex and dynamic system?

The answer is not entirely clear. But I am convinced that history is sitting in the dirt, rotting away, waiting patiently for someone to find a way to unwrap the stories contained within.

 

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