Carbon and humus

Where the cycle breaks in ordinary land use

The cycle runs as long as two conditions hold: a steady supply of plant matter, and a soil that stays covered, shaded and largely undisturbed. Ordinary land use takes away one or both, and carbon leaks out at several points.

Ploughing and bare soil. Turning the soil breaks crumbs open and exposes protected organic matter to air, and soil life then consumes it quickly. Bare ground in the sun heats up and loses its fungal networks. In a hot climate this speeds up the return of carbon to the air while the supply of fresh material stops.

Erosion. The dark, humus-rich topsoil is the lightest layer and the first to go. Every heavy rain on open ground carries organic matter downhill into ditches and rivers.

Burning and clearing. When prunings are burned at the edge of the field, or scrub is cleared and burned, carbon that could have fed the soil goes straight back into the air. In many traditional olive groves in southern Italy, pruning residues are still burned on the spot.

Removal without return. Harvest, hay and wood leave the land every year. When nothing comes back, the supply side of the cycle shrinks.

The accumulated result can be estimated. A global modelling study put the loss of soil carbon caused by agriculture at well over 100 billion tonnes over the last 12,000 years, counted over the top two metres of soil. In southern Europe the deficit shows on the map: on about three quarters of the land area, all land uses together, modelled topsoil organic carbon is at most two percent. Dry summers, a long history of grazing, fire and tillage, and erosion on slopes have left many Mediterranean soils thin and pale.

Techniques that close the cycle

Closing the carbon cycle means two things: bringing more carbon into the soil, and losing less of it on the way.

More leaves, for more of the year. A food forest or an agroforestry system stacks several layers of foliage over the same ground, so more sunlight becomes plant matter on every square metre. Polyculture keeps something green in every season. On damaged land, pioneer plants and nurse trees start the pump where nothing else would grow, and regreening carries this to whole landscapes.

Wood as a long-term store. Trees hold carbon in trunk and roots for decades and centuries. Old trees such as millennial olive trees, the holm oak and the cork oak are stores that cannot be replaced quickly, which is why protecting them comes before planting new ones. Silvopasture brings trees back onto grazing land.

Litter back onto the ground. Mulch and sheet mulching return cut material to the surface instead of burning or carting it away. Compost gathers kitchen and garden residues in one place; part of their carbon escapes as carbon dioxide while they rot, and the rest reaches the soil as a crumbly material full of life. Biomass plants are grown precisely to be cut and laid down. Hügelkultur buries wood that breaks down over years.

Living roots all year. Cover crops, living mulch and green manure keep roots in the ground between trees and outside the main season, delivering carbon underground where it is most likely to stay. Nitrogen-fixing plants supply the nitrogen that soil life needs to turn carbon into lasting organic matter. Deep-rooted grasses such as vetiver carry carbon into depth and hold slopes in place.

Feeding the partners underground. Mycorrhizal fungi carry plant sugar far into the soil and help glue crumbs together. The whole soil food web depends on ground that is not turned over again and again. On bare dry land, a biological soil crust is often the first living skin that takes up carbon at all.

Protecting the stock. A good soil structure keeps organic matter shielded inside crumbs. Biochar is the most durable form of carbon a soil can receive: in a meta-analysis of isotope studies, about 97 percent of biochar carbon belonged to a pool with an estimated mean residence time of around 556 years. The terra preta soils of the Amazon show that charred material can keep land fertile for centuries.

Slowing the return to the air. Ground that stays covered and shaded stays cooler, and soil respiration slows. Mulch, living cover and shade do more here than any additive. Where organic material would blow away or burn, rock mulch at least keeps the surface cool.

Measuring instead of believing. Only a soil analysis, repeated over the years at the same spots, shows whether carbon is really accumulating. In between, the spade test shows crumbs, roots and colour in the field.

In Mediterranean olive groves, a meta-analysis found that cover crops raised soil carbon by about 1.1 tonnes per hectare and year compared with conventional management, and organic amendments, above all olive mill pomace, by about 5.3 tonnes. These are rates from the first years after a change, and they do not continue indefinitely. Soil carbon approaches a new equilibrium and then stops rising; the capacity of a soil is finite; and the gain is reversible, because ploughing and bare ground send the carbon back into the air. For Terra Humana, soil carbon is therefore first a matter of fertility, water and life. We make no offset promises with it.

Carbon at Sa Medada

Sa Medada is a slope at Jerzu in Ogliastra, on the east side of Sardinia, with a dry summer from about May to September and mild winters. Above ground, the carbon of the land is held above all in the old olive grove and in the macchia on the steeper slopes.

How much carbon the soil holds is not known. There is no laboratory analysis of the soil yet, and so no starting value against which a change could be measured; for that reason we make no statement about it. The LiDAR wetness map shows a comparatively dry main area and moister channels along the edges. On land like this, the supply side of the cycle is the bottleneck: in the long dry season, little new plant matter grows on open ground.

Project Eden therefore begins at the stations where the cycle starts. Pioneer plants bring leaves onto open ground, a soil cover keeps the surface cool and feeds soil life, and water held back on the slope is meant to keep plants green further into the summer. Nitrogen-fixing species grow in the ground, so far without inoculation: tephrosia, the coral tree, Inga edulis, pacay, Bambara groundnut, fava bean and leucaena. They are meant to supply the nitrogen that soil life needs to build organic matter. Whether all this raises the carbon in the soil will only be shown by measurements over years.