Effect of sampling date on chemical indices of the mineralizable nitrogen in soil
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Abstract A study of the changes in total N, fixed ammonium N, and hydrolyzable and nonhydrolyzable forms of N that occurred when soils were incubated under conditions which promoted mineralization of soil N showed that prolonged incubation led to a marked decrease in all forms of N determined excepting fixed ammonium N and indicated that native fixed ammonium is practically unavailable to soil microorganisms. It also showed that there are marked differences among soils with regard to the nature of the N that is most readily mineralized and made available for plant growth and indicated that any chemical method of obtaining an index of soil N availability based solely on determination of a hydrolyzable or nonhydrolyzable form of soil N will prove unsatisfactory.
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This chapter evaluates methods of measuring biomass nitrogen (N) and active organic N pools, and assesses the utility of biomass N and active soil organic N measurements as indices of soil quality and as predictors of soil N availability to crops. It describes and discusses relationships between the various measurements. All three parameters, microbial biomass N (MBN), potentially mineralizable N (PMN), and active soil organic N, show promise as a measure of soil quality, although more research is needed to better define how they should be used and the limits of their use. These measurements also may ultimately prove valuable as predictors of soil N supply. Agreement on standardized procedures for measuring MBN and PMN would greatly help efforts to understand relationships between different active soil N measurements and to develop general principles of soil science in this area.
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A simple, convenient method was developed to control precisely moisture content in soil samples during aerobic incubation for determination of mineralizable N. The sample is contained in a polystyrene filter unit with a filter membrane having a bubble point pressure well above the 33-kPa suction tension normally used for equilibration. Equilibration at seven moisture tensions (5-67 kPa) gave soil water contents that were very similar to those obtained by conventional pressure plate measurements. Nitrogen mineralization values obtained at different tensions varied as expected, but reproducibility of three determinations on four Chernozemic soils was good (C.V. = 3.9%) at all tensions used.
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