MEASUREMENT OF THE BIOLOGICALLY-ACTIVE SOIL-NITROGEN FRACTION BY A N-15 TECHNIQUE

An isotope dilution method for measuring biologically active soil nitrogen (ASN) was developed. It involved insertion of N-15 into the soil N cycle and stimulation of microbial activity until added N-15 was mixed with the ASN pool. Both glucose addition (GLU) and glucose addition-CHCI3 fumigation (GFU) were used to promote mixing. Experimental tests with model systems provided evidence that isotopic mixing can be achieved. Because the concept of a discrete pool is an approximation, the ASN pool was operationally defined as that measured after a 40-day incubation at 22-degrees-C. The GLU treatment gave lower ASN values than the GFU treatment, probably because fumigation killed microorganisms not responding to glucose addition. Microbial biomass was estimated to contribute 23-48% and 34-77% of the N in the ASN pool in a wheat field and a long-term pasture, respectively, depending upon the method of calculation of microbial biomass N. ASN values in these same fields showed depth patterns consistent with tillage and cropping histories. About 40% of fertilizer-derived organic N-15 remaining in soil following growth of a wheat crop was determined to be in the ASN pool, which is similar to potentially mineralizable values found in other N-15 studies.