Simultaneous measurement of soil microbial nitrogen, carbon, and carbon isotope ratio

Methods used for analysis of soil microbial biomass N and C are limited in precision and are time consuming. A method involving direct combustion of evaporated soil extracts was developed for use with an automated nitrogen and carbon analyzer (ANCA). The objectives of this study were to: (i) evaluate the usefulness of ANCA determination of microbial C and N, and (ii) ascertain whether the microbial C isotope ratios determined by the ANCA method would reflect C contributions from actively growing crops of wheat (Triticum aestivum L.) and maize (Zea mays L.) differing in C isotope ratio. Cropped and fallow soils were sampled at planting, anthesis, and maturity. Field-moist soils were fumigated with chloroform and extracted with 0.05 M K2SO4 for the ANCA method and with 0.5 M K2SO4 for Kjeldahl analysis. Small volumes of the 0.05 M K2SO4 extracts were evaporated on cone-shaped tin disks that were analyzed using combustion at 1700 degrees C and mass spectrometry. Reproducible values for C, N, and delta(13)C were obtained with less effort than that typically expended for Kjeldahl N analysis of similar extracts. Temporal changes in microbial delta(13)C occurred in both cropped and fallow soils during a single growing season. At maturity of the maize crop, an estimated 23% of the soil microbial C was derived from the plant material of the current maize crop. Carbon contribution from the wheat crop to the soil microbial biomass was not detected. Further refinements in the ANCA method may make the procedure routinely usable for enhancing the understanding of chloroform labile C and N as indicators of microbial dynamics in the soil.