Changes in N-15 abundance and amounts of biologically active soil nitrogen

Estimation of the capacity of soils to supply N for crop growth requires es timates of the complex interactions among organic and inorganic N component s as a function of soil properties. Identification and measurement of activ e soil N forms could help to quantify estimates of N supply to crops. Isoto pic dilution during incubation of soils with added (NH4+)-N-15 compounds co uld identify active N components. Dilution of N-15 in KCI extracts of miner al and total N, non-exchangeable NH4(4)(+), and N in K2SO4 extracts of fumi gated and non-fumigated soil was measured during 7-week incubation. Samples from four soils varying in clay content from 60 to 710 g kg(-1) were used. A constant level of N-15 enrichment within KCl and K2SO4 extracted compone nts was found at the end of the incubation period. Total N, microbial bioma ss C and non-exchangeable NH4+ contents of the soils were positively relate d to the clay contents. The mineralized N was positively related to the sil t plus clay contents. The active soil N (ASN) contained 28-36% mineral N, 2 9-44% microbial biomass N, 0.3-5% non-exchangeable NH4+ with approximately one third of the ASN unidentified. Assuming that absolute amounts of active N are related to N availability, increasing clay content was related to in creased N reserve for crop production but a slower turnover.