Humus

by Bill Schlesinger

    Among my peers I am perhaps best known for developing one of the first estimates of the amount of carbon in the world’s soils.  Soils contain organic matter, also known as humus, that has about 50% carbon content by weight.  Added up for all the soils of the world, humus contains a lot of carbon—about 1500 billion metric tons in the top 1 meter of soil.  Carbon is also contained in carbonate minerals in desert soils.  The total carbon in soils is about 3 to 4X as much carbon as found in Earth’s atmosphere, where rising levels of carbon dioxide are the source of our concern about climate change.

    It is not so important how much carbon there is in soils as how it is changing.  When soils are brought into cultivation, organic matter is exposed to the work of microbes, which convert it to carbon dioxide that escapes to the atmosphere.  A smaller amount of organic matter is lost in erosion of soils to the sea.  All told, about 116 billion metric tons of carbon, or about 8% of the organic matter in soils has been lost to the atmosphere since the earliest times of organized agriculture.

    We can expect even more carbon to be lost to the atmosphere as the Earth’s climate warms.  At northern latitudes, a vast area of soils has been accumulating carbon since the last continental glaciations.  Here, dead plant materials are stored in permafrost, which prevents microbial attack.  As permafrost melts, the organic matter will be oxidized, contributing carbon dioxide to the atmosphere.

    One might ask if there are any ways to enhance the storage of organic matter in soils, to do something proactive about the problem of rising atmospheric carbon dioxide and climate change.  One group has suggested that increasing soil organic matter by as little as 0.4%/yr in all the soils of the world would soak up all the carbon dioxide emissions from fossil fuels. That is easier said than done because a lot of land is not under active management. In natural lands, a small amount of carbon may be stored in soils as a result of greater growth of plants under high carbon dioxide.  In other areas, a large amount of organic matter may be lost as the soils respond to a warmer climate.

    If we are to store carbon in soils, better management of agricultural lands might be one avenue to pursue—albeit confined to about 10% of the Earth’s land surface that is cultivated.  A recent paper suggests that better soil management might have the technical potential to soak up 2.45 x 1015 gC/yr, or about 20% of the current flux of CO2 to the atmosphere from fossil fuel combustion.  A separate analysis by scientists organized by the Nature Conservancy suggests that better soil management might store 0.41 x 1015 gC /yr, about 16% of the technical potential and only a tiny fraction of our fossil fuel emissions.

    Listen closely to what agriculturalists say about storing carbon in soils. Soils show rapid losses of organic matter when brought under cultivation.  Unfortunately, restoring the pool of humus in soils takes millennia.

References:

Griscom, B.W. J. Adams, and 30 others.  2017.  Natural pathways to climate mitigation.  Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1710465114

Jastrow, J.D. and 6 others. 2005.  Elevated atmospheric carbon dioxide increases soil carbon.  Global Change Biology 11: 2057-2064.

Kopittke, P.M., R.C. Dalal, D. Finn and N.W. Menzies.  2017.  Global changes in soil stocks of carbon, nitrogen, phosphorus, and sulphur as influenced by long-term agricultural production.  Global Change Biology 23: 2509-2519.

Lal, R. 2018.  Digging deeper: A holistic perspective of factors affecting soil organic carbon sequestration in agroecosystems.  Global Change Biology doi: 10/1111/gcb/14054

Lichter, J., S.A. Billings, S.E. Ziegler, D. Gaindh, R. Ryals, A.C. Finzi, R.B. Jackson, E.A. Stemmler, and W.H. Schlesinger, 2008.  Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment.  Global Change Biology 14:2910-2922.

Minasny, B. and 33 others. 2017.  Soil carbon 4 per mille.  Geoderma 292: 59-86.

Poulton, P., J. Johnston, A. MacDonald, R. White and D. Powlson.  2018.  Major limitations to achieving “4 per 1000” increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, UK.  Global Change Biology doi: 10.1111/gcb.14066

Sanderman, J., T. Hengl and G.J. Fiske. 2017.  Soil carbon debt of 12,000 years of human land use.  Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1706103114.

(This article appears on Translational Ecology, Citizen Scientist,

 http://blogs.nicholas.duke.edu/citizenscientist/billschlesinger/ Reprinted with permission).