The oxygen/ethylene cycle is tied up with soil microorganism activity. This influences the rate of decomposition of plant residues, the uptake of nutrients by plants, and the general state of soil health. Glenneis Kriel got the lowdown from Dr Paul Syltie.
Soil organisms play a pivotal role in one of the key processes in soil – the oxygen/ethylene cycle. This cycle regulates soil microorganism activity, but is greatly inhibited by modern agricultural practices. Dr Paul Syltie, renowned microbiologist and author of How Soils Work, explained the cycle to farmers at a recent information day in the Western Cape. He explained that ethylene’s effect on soil microorganisms affects the turnover rate of organic matter, the recycling of plant nutrients and the incidence of soil-borne diseases. Ethylene is also a growth regulator, and the oxygen/ethylene cycle plays a role in mobilising nutrients.
During this cycle, soil organism activity is critical. When microorganisms multiply on the plant root exudates created during photosynthesis, they create small areas where oxygen is depleted. These areas are called “anaerobic microsites”. To restore the balance, ethylene is released from old plant matter. When the ethylene diffuses out, the microorganisms temporarily become inactive. This reduces the demand for oxygen, which then diffuses back into the microsites, inhibiting ethylene production. The microorganisms become active again, and the cycle is repeated.
Nitrate versus ammonium
Alan Smith, principal research scientist for the New South Wales Department of Agriculture, is an authority on the importance of the oxygen/ethylene cycle.
He stresses ethylene is found in most natural ecosystems, from forests to grasslands, but its concentration is low in agricultural soils, indicating a disturbance of the cycle. Smith suggests agricultural soil management practises result in different soil microorganisms being needed for nitrogen conversion. In undisturbed soils, virtually all the nitrogen present is in the form of ammonium, with only a small trace of nitrate. However, in disturbed soils most nitrogen occurs in nitrate form. Different microorganisms convert ammonium and nitrates.
Smith’s studies show that ethylene production is inhibited by anything more than trace amounts of nitrate. Ammonium has no such effect. He therefore advises farmers to apply nitrogen in the form of ammonium in several small applications, rather than one or two heavy ones.
The role of iron
Smith explains that nitrate interferes with the formation of anaerobic microsites. This prevents the mobilisation of ferrous iron, which usually triggers the release of soil ethylene. The iron reacts with a precursor chemical, from which ethylene is produced and which accumulates in dropped leaves and builds up in the soil after their decomposition. In the oxygen/ethylene cycle, iron also plays a huge role in mobilising plant nutrients. Under normal conditions, iron mostly occurs as iron oxide. Plant nutrients such as phosphate, sulphate and trace elements bind to these crystals. When the crystals break down in the anaerobic microsites formed by the cycle, these nutrients are released, and plants can absorb them.
Iron is released at the same time and replaces cations such as copper, magnesium, potassium and calcium on clay particles. This makes these cations available for uptake. When the cycle makes the microsites aerobic, the extra oxygen binds with the iron to produce iron oxide. Nutrients that aren’t taken up rebind with iron oxide instead of leaching away. The oxygen/ethylene cycle relies on having enough organic residue in the soil, which serves as the ethylene precursor, and contains essential plant nutrients for recycling, to stimulate microbial activity and restrict the rate of nitrification. A healthy microbial population plays an important role in decomposing plant material and rendering microsites anaerobic. Contact Dr Paul Syltie on (012) 333 4222. |fw