Effect of barley plants on the decomposition of ¹⁴C‐labelled soil organic matter

Abstract

The effect of barley plants on the rate of decomposition of soil organic matter over a 6-week period was studied using soil that had been previously labelled by incubation with IF-labelled ryegrass for 1 year. The plants reduced the loss of ' 4 2 0 , from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little '42 label being associated with the shoot.Chemical fractionation of the root showed that the IF was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of WO, being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.

Nitrogen mineralization in a green manure-amended soil as influenced by cropping history and subsequent crop

“…Conversely, the retention of green manure-derived N in organic form was 25% higher in cropped than in uncropped treatments. These observations parallel those of studies examining C dynamics in the rhizosphere which reported suppression of the decomposition of ~4C-labelled organic substrate by the presence of a crop (Martin, 1987;Reid and Goss, 1982;Sparling et al, 1982). Our results differ, however, from those of Haider et al (1987) who observed higher mineralization of organic N in planted than in unplanted soils.…”

Effects of starch additions on N turnover in Sitka spruce forest floor

“…It could reduce the supply of nutrients to plants by two methods: first, increasing C availability causes net immobilization, reducing the concentration of plant-available N, and second, it could also reduce the rate N flow into the NH 4 pool, reducing the NH 4 supply . Root exudation could also reduce SOM decomposition in the rhizosphere, as has been observed in several studies Goss, 1982, 1983;Sparling et al, 1982).…”

Measurement of rhizosphere respiration and organic matter decomposition using natural 13C

“…Decomposition of labeled plant material was markedly lowered in the presence of cultivated plant cover or in natural grasslands when compared to bare soil controls (Shields and Paul, 1973;Jenkinson, 1977). When 14C-labeled plant material was decomposed in soil planted with maize, ryegrass, wheat or barley, 14CO2 release from the soil was reduced compared to bare soil controls during experiments under controlled conditions (Reid and Goss, 1982;1983;Sparling et al, 1982). It was generally suggested that this negative effect of living roots on soil organic matter decomposition was probably due to the competition between the living roots and the rhizosphere microflora for substrates and due to the changed physical and chemical conditions by roots.…”