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FURTHER OBSERVATIONS ON HUMUS DECOMPOSITION AND NITRIFICATION

H. F. BIRCH
Plant and Soil
Vol. 11, No. 3 (September 1959), pp. 262-286
Published by: Springer
Stable URL: http://www.jstor.org/stable/42931814
Page Count: 25
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FURTHER OBSERVATIONS ON HUMUS DECOMPOSITION AND NITRIFICATION
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Abstract

Evidence is presented to show that the longer a soil is kept in an air-dry state the greater is the amount of water-soluble organic material that can be extracted from it even though the soil does not become drier. This effect is greatly enhanced with dry soils kept at 100° C. Soils kept air-dry for increasingly long periods also showed increasing magnitudes of carbon and nitrogen mineralisation on moistening which appear from the data so far available to be a linear function of the log of the period that the soil was dried. The organic material going into solution on moistening a dry soil rapidly decomposes, but without concomitant mineralisation of nitrogen. This decomposition partly accounts for the flush of decomposition that follows the moistening of a dry soil. The brief period of enhanced nitrogen mineralisation that simultaneously occurs is considered to involve surface decomposition of organic material. The drying effect is shown largely to involve changes in the organic material rather than in the soil itself, and it is postulated that drying brings about changes in the organic gels that continue during ageing and are enhanced by heat. The changes visualised are fragmentation or increased porosity of the gel on drying and heating, each of these leading to increased surface area. The changes are considered to be reversible on moistening. Such behaviour would account not only for the increased amount of organic material that can be extracted from a dry soil compared with a moist one, but also for the decline in the rate of nitrification, after moistening, consequent upon the swelling and cohesion of the gel with decreased surface area. It is also possible that the high rate of decomposition and nitrogen mineralisation following the moistening of a dry soil, and the rapid decline in this rate, is due to the high microbial activity associated with the youthful phase of a developing microbial population, this activity declining as the microbial population ages. Evidence for such behaviour is presented in experiments with moist soils treated with organic vapours. Successive treatments resulted in successive flushes of decomposition without however involving any enhanced solution of organic material such as occurs after drying. The high activity, which rapidly declines, indicates a decline in microbial activity rather than in the amount of substrate, since this is adequate for the occurrence of several successive flushes of decomposition.

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