Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 382

Moreover, unpublished results indicate that even low quality plant residues
decompose readily in the nutrient-depleted soil.
The test of the freezing revealed a difference in susceptibility of the
labelled and unlabelled SMB. Since only a minor fraction of the SMB was
labelled, this does not necessarily have any implications for the results.
However, it suggests that microbes are affected differently by frost depend-
ing on their guild and/or physiological status. Hence, the SMB may have
responded differently to freezing in the rhizosphere than in the bulk soil.
Conclusion
High-resolution microtome slicing, combined with microscale fumigation
extraction, enabled direct measurements of the SMB in the rhizosphere.
Within 2 weeks, the total amount of SMB N was roughly doubled in the
rhizosphere of the two different soils. The effect of root deposition
extended 1-3 mm into the soil, with the largest effect observed in the fertile
soil. Increased decomposition of labile 14 C-labelled soil organic matter in
the rhizosphere, or revitalization of dormant organisms, was indicated by
an increase in the SMB 14 C in the soil fractions nearest the root mat. This
effect was not significant in the nutrient-depleted soil.
Acknowledgements
The authors thank Dr T.S. Gahoonia for his help and advice with the
rhizosphere system.
References
Amato, M. and Ladd, J.N. (1988) Assay for microbial biomass based on
ninhydrin-reactive nitrogen in extracts of fumigated soils. Soil Biology and
Biochemistry 20, 107-114.
Badalucco, L., Kuikman, P.J. and Nannipieri, P. (1996) Protease and deaminase
activities in wheat rhizosphere and their relation to bacterial and protozoan
populations. Biology and Fertility of Soils 23, 99-104.
Brookes, P.C., Landman, A., Pruden, G. and Jenkinson, D.S. (1985) Chloroform
fumigation and the release of soil nitrogen: a rapid direct extraction method to
measure microbial biomass nitrogen in soil. Soil Biology and Biochemistry 17,
837-842.
Carter, M.R. (1991) Ninhydrin-reactive N released by the fumigation-extraction
method as a measure of microbial biomass under field conditions. Soil Biology
and Biochemistry 23, 139-143.
Cheng, W. and Coleman, D.C. (1990) Effect of living roots on soil organic matter
decomposition. Soil Biology and Biochemistry 22, 781-787.