| My research has focused on understanding N and C cycling in soils at small scales. Isotope dilution methods commonly used to estimate gross rates of soil chemical transformations assume homogeneous distribution of label. I explored the effects of diffusion limitations on isotope dilution experiments in soil aggregates using spherical diffusion-reaction models. Equations describing transport and reaction assumed Fickian diffusion, linear-equilibrium adsorption, zero-order production of natural abundance NH4+, and either pseudo-first-order or zero-order consumption of ambient and label NH4+. In the case of pseudo-first-order consumption, rate calculations were sensitive to substrate adsorption coefficient, but not to other transport parameters; however, rates calculated from simulations assuming zero-order consumption underestimated both rates of production and consumption. These simulations reemphasize the need to optimize experimental protocols when performing isotope dilution experiments in structured soils.; I also explored soil microsite heterogeneity of C and N assimilation using time-of-flight secondary ion mass spectrometry (TOF-SIMS). I have shown that TOF-SIMS is capable of detecting N assimilation in bacterial and fungal biomass and both C and N assimilation in individual bacterial cells. I extended this research to explore factors that might affect differential NH4 + and NO3- assimilation by soil microbes. Model systems were created with kaolin clay (labeled with 15NO 3-), straw, and manure. The spatial relationship of N assimilation by fungal hyphae was preserved on Si contact slides. Analysis of the slides showed nearly a 100% change between in 15N content of hyphae that were associated with manure and hyphae that were associated with straw. This was presumably due to a relatively higher 14NH 4+:15NO3- ratio near the manure. This dramatic change in assimilation of N sources occurred over relatively short distances of 40 to 200 mum. Although further experimentation is required to fully understand the mechanisms controlling the bioavailability of NH4+ and NO3- at this scale, these results illustrate the powerful potential of TOF-SIMS to explore microbial activity at the sub-mm scale in soil systems. |
