Soils, especially those in tropical forests, play an important role in absorbing carbon and keeping it out of the atmosphere; the world’s soil currently holds about twice as much carbon as the air. Tropical forests account for about a third of the carbon sequestered in soils worldwide, and a significant portion of that carbon resides in subsoils, buried 20cm to 30cm deep, where it usually spends thousands of years before returning to the atmosphere.

Deforestation leads to soil erosion, and it can change the chemistry that keeps carbon molecules bound to other minerals in the soil. A new study suggests that people have been damaging carbon reservoirs in tropical forests for much longer than we thought.

The Maya began farming in the lowlands of northern Central America around 4,000 years ago. By about 2,500 years ago, the civilization was at its peak, clearing away jungle to make room for farms and cities. Maya civilization began to decline shortly before Spanish conquest brought about a final, massive population crash 500 years ago. Today, the jungle has reclaimed most of the land once farmed by the Maya, and many of their cities now lie hidden beneath dense foliage. But the soil turns out to have a much longer memory.

“When you go to this area today, much of it looks like dense, old-growth rainforest,” McGill University geochemist Peter Douglas said in a statement. “But when you look at soil carbon storage, it seems the ecosystem was fundamentally changed and never returned to its original state.”

How long does carbon stay buried?

Douglas and his colleagues took sediment core samples from three lakes: Lake Chichancanab in the Yucatan Peninsula and Lake Salpeten and Lake Itzan in northern Guatemala. These were once fed by areas that drained Maya farmland and settlements. The layers of sediment that washed into the lakes over several thousand years helped the researchers reconstruct a timeline of how carbon retention in the region’s soils had changed over time. In particular, that work relied on molecules called plant waxes: alkanoic acids with long chains of carbon, formed in the cuticles of plant leaves.

Plant waxes are a good representative of how long carbon stays in deep soils, according to Douglas and his colleagues. The waxes are usually among the oldest molecules washing into lakes from surrounding landscapes, according to radiocarbon dating in previous studies, and the molecules tend to bind to minerals in the soil, which is one of the most important mechanisms for locking carbon into the deep soil.

“So really we are looking at an indicator of how long the oldest carbon is stored in soils and then assuming that this is representative of changes in average carbon-storage times,” Douglas told Ars Technica. “More work is needed to validate this assumption, but based on available data it appears to be reasonable.”

Douglas and his colleagues compared the radiocarbon age of the plant waxes in each layer of sediment to the age of tiny plant fossils that had washed into the lake at the same time. The plant fossils, they reasoned, probably formed around the same time the soil washed into the lake, while the plant waxes would represent the carbon molecules that had already been stored in the soil. So the difference between the two would tell the scientists how long the lowland soil held on to stored carbon.

The ancient anthropocene strikes again

The researchers started with layers dating back to 7,000 years ago—at least 3,000 years before agriculture, or even settled village life, arose in the Maya Lowlands. Back then, the undisturbed tropical forest soil held stores of carbon sequestered away from the atmosphere for about 1,500 to 1,760 years. But about 3,500 years ago, that began to change.

The transit time for plant waxes in Maya Lowland soils started to decrease around 3,500 years ago, during what Maya historians call the Preclassic period. Around this time, archaeological records and ancient pollen in lake sediments show that the ancient Maya had started cultivating crops in the region and cutting down swaths of forest to make room for their fields. At Lakes Salpeten and Itzan in northern Guatemala, carbon sinks seemed to recover slightly between 2,400 and 1,700 years ago; it’s no coincidence that this period of Maya history was marked by a dip in population and the abandonment of many of the major cities of the time.

When Maya populations recovered at the beginning of what scholars call the Classic period, farming and urbanization returned with more vigor than ever. Carbon transit times at Salpeten dropped and kept dropping until the eve of Spanish conquest, around 500 years ago.

After Spanish conquest, slavery and waves of epidemic disease devastated the Maya population. The forest eventually moved in to reclaim farmland and cities alike, and the soil began to recover from centuries of erosion and chemical changes, albeit very slowly. Carbon transit times still haven’t made it back to pre-Mayan levels at any of the three sites in the study, despite centuries of forest growth and relative stability for the soil. Douglas and his colleagues estimate that Maya deforestation reduced the storage time of carbon in soils by 70 to 90 percent.

“I think these findings do support the hypothesis that human societies were affecting the global carbon cycle well before the Industrial Revolution, which some have termed the ‘early Anthropocene’ hypothesis,” Douglas told Ars. “We see here that humans were strongly impacting soil carbon reservoirs for thousands of years; if this is found in other places, it would probably represent a major disturbance to the carbon cycle.”

Carbon offsets may not be as easy as we think

And that could have implications for the decisions we make today. Over the last century and a half, a new wave of deforestation has threatened Central America’s tropical forests, their delicate and diverse ecosystems, and the vital carbon sinks they protect. Douglas and his colleagues compare it to the initial phase of Maya land clearing, 3,500 to 2,500 years ago.

Planting new trees is a popular form of carbon offset—an effort by a company or country to make up for some of its carbon footprint by doing things to help reduce the amount of carbon going into the atmosphere. But if it turns out that just letting the forest grow back doesn’t restore soil’s ability to sequester carbon for thousands of years, then reforestation may not be as much of a carbon offset as we think. That’s something to take into account when designing offset programs, suggests Douglas.

“In order to really sequester carbon in soils, you want to be sure it will stay there for a long time. This work suggests that this doesn't always happen with reforestation in the tropics,” he said. “This work can’t directly be applied in designing or evaluating offsets, but it does imply that tropical reforestation does not necessarily lead to the long-term sequestration of carbon that would be ideal.”

Nature Geosciences, 2018. DOI: 10.1038/s41561-018-0192-7 (About DOIs).