Humus – Guardian of Fertility and Global Carbon Sink

Abstract

Humus is the defining constituent of soil, the mainspring of soil fertility and the outstanding feature of the chernozem. The quality of humus in chernozem is uniquely favourable to agriculture, with a preponderance of humic acids bound up with calcium. Eastwards, across the Ukraine and Russia, chernozem keep their humus but not the same great fertility – because of the increasingly severe continental climate. Quantitatively, the main elements in living organisms are carbon, nitrogen, oxygen, hydrogen, phosphorus and sulphur. The same is true of humus – so decomposition of humus provides plants with all essential nutrients. But, in farming systems, nutrients taken up by the crops are carried away so, without the renewal of humus reserves, farming is not sustainable. Soils are the largest pool of terrestrial organic carbon; estimates range between 1000 and 3500 billion tonnes. Between one-quarter and one-third of the excess carbon dioxide in the atmosphere, which is considered to be driving climatic change, has come from land use change over the last century. Annual carbon emissions from chernozem are of the order of 200 million tonnes, a little over 2% of today’s global emissions, but farmland represents the outstanding opportunity to fix carbon and put it back where it came from – in the soil, where it will be useful! Comparison of today’s humus stocks with those measured at the end of the nineteenth century shows a dramatic decline following the ploughing of the steppe. It might have been expected that the humus content would stabilize at a new equilibrium but this has not happened; over the last 40 years, the humus content has declined by about 0.3%/year. Over the last 100 years, the loss of humus from the 0 to 30 cm layer has been in Moldova 51–71 t/ha (32–40% of the original stock or 0.5–0.7 t/ha/year); in the Kursk and Kharkov regions of Russia and Ukraine 67–79 t/ha (21–36% of the original stock or 0.7–0.8 t/ha/year); and in the Samara region 150–180 t/ha (38–39% of the original stock or 1.5–1.8 t/ha/year). Recent data show equally dramatic decline of humus content below the plough layer to a depth of 120–130 cm, especially under bare fallow. Calculation of data in terms of equivalent soil mass gives less dramatic but probably more realistic values for these losses: for Typical chernozem near Kursk, a loss of 40.4 t/ha of carbon and 3.5 t/ha of nitrogen after a century of arable cropping; 76.9 t/ha of carbon and 4.5 t/ha of nitrogen after 50 years of continuous bare fallow. At the high noon of chemical agriculture the balances for nitrogen, phosphate and potassium were positive. This decreased but did not eliminate the loss of humus. Nutrient balances are again declining sharply which implies a long-term decline in productivity; food security is not assured – and by 2025 Moldova will no longer be a country of chernozem.