Progress 08/15/07 to 08/14/08
Outputs
OUTPUTS: All heterotrophic N-exporting organisms (from microorganisms to mammals) will ultimately utilize organic N as source of carbon and energy, and export excess N. As a result of fractionation during deamination and NH4+/NH3 state change, main steps of the N-dissimilation pathway, these organisms becomes 15N-enriched relative to their diet. This enrichment is to some degree balanced by N-isotope fractionation during N-assimilation and transamination, especially relevant for microorganisms that are involved in N-immobilization. This 15N enrichment is generally observed for, for example, animals, mycorrhizal fungi and even some heterotrophic plants. We showed that this enrichment also occurs in microorganisms, but only when an organic N source is provided, not when grown with inorganic N (Collins et al., 2008). In other words, we can use the N isotope composition of the microbial biomass to probe the balance between N assimilation and dissimilation, driven by the relative C and N availability (physiological condition) and N-mineralization rate (ecological consequence). We are the first research group that has successfully applied the chloroform-fumigation-extraction technique to determine the N-isotope composition of the soil microbial biomass (Dijkstra 2006a). We have shown that the microbial biomass is 15N enriched relative to soil and soluble N pools (Dijkstra et al. 2006a), that this enrichment is related to C availability in response to dung addition (Dijkstra et al. 2006b) and to C:N ratio for sites along an elevation gradient in Arizona, and two substrate age gradients in Arizona and Hawaii (Dijkstra et al. 2008). We also showed that the N isotope composition of the microorganisms is positively correlated with N mineralization (Dijkstra et al. 2008, Coyle et al. submitted). In addition, we have shown that the 15N isotope composition of the soil microbial increases during soil incubation, but only for soils that exhibited significant net N mineralization (Dijkstra et al. 2008; LaViolette et al. in prep). We are currently investigating the relationship between temporal variability in microbial 15N signatures and N mineralization. We have conducted monthly sampling of seven sites along an elevation gradient, analyzed the microbial, extractable and soil total N pools for isotope composition, and measured net N mineralization rates in the field and in the lab. This analysis is currently in its final stage. We have also analyzed the effect of ponderosa pine forest restoration on the N cycling and microbial signatures. Preliminary data show that forest restoration increases net N mineralization rate and the N isotope composition of the microbial biomass, again supporting the hypothesis that the N isotope composition of the microbial biomass reflects the physiological condition related to C and N availability and N mineralization. Finally, we have developed a conceptual model on how N fractionation during N mineralization affects the N isotope composition of other soil N pools. PARTICIPANTS: The following persons worked on this project: Paul Dijkstra, Steve C. Hart, Bruce A. Hungate, Egbert Schwartz (PIs) Jeffrey S. Coyle, Corinne M LaViolette (Master Students) At least three undergraduate students In the course of this project, we have collaborated with 1- Flint Hughes (USFS-Hilo Hawaii) and George Koch (NAU) on determining the effects of an invasive tree on N cycling and microbial isotope signatures 2- Tom Whitham (NAU) and Todd Wojtowicz (NAU graduate student) on the effect of plant traits on N cycling, N isotope composition of the microbial biomass, and food web structure Training and professional development opportunities was given to two master students, several undergraduate students, and post doc in this project. TARGET AUDIENCES: This research is aimed at developing new tools that can be used to manage and observe the effects of management on soil C and N cycling. As such the audience includes farmers and foresters, but also scientists (particular involved with isotope geochemistry and soil scientists) PROJECT MODIFICATIONS: We requested a one-year no-cost extension related to delays in isotope analysis of samples.
Impacts
The microbial breakdown of organic matter and the release of N in inorganic form to plants are central to maintaining a long-term productive and healthy agriculture and forestry. The 15N signature of the soil microbial biomass gives us a new perspective into microbial functioning, which allows us to evaluate more completely how plant traits, climate, management, and other factors affect microbial functioning and soil fertility. Moreover, fractionation during N mineralization, as demonstrated in this study, has consequences for the isotope composition of other N pools, such as inorganic N, plant N and ultimately soil N pools. A better understanding of how N mineralization by microorganisms affects the isotope signatures of these N pools will be crucial in further developing the measurement of the N isotope signatures as an effective tool in ecosystem, soil and agricultural sciences.
Publications
- Collins J, Dijkstra P, Hart SC, Hungate B, Flood N and Schwartz E 2008. Nitrogen source influences natural abundance 15N enrichment of E. coli. FEMS Microbiology Letters 282: 246-250.
- Schwartz E 2007. Characterization of growing microorganisms in soil through stable isotope probing with H218O. Applied and Environmental Microbiology 73, 2541-2546.
- Dijkstra P, Ishizu A, Doucett R, Hart SC, Schwartz E, Menyailo O and Hungate BA 2006. 13C and 15N natural abundance of the soil microbial biomass. Soil Biology and Biochemistry 38, 3257-3266.
- Dijkstra P, Menyailo O, Doucett R, Hart SC, Schwartz E and Hungate BA 2006. C and N-availability affects the 15N-natural abundance of the soil microbial biomass across a cattle manure gradient. European Journal of Soil Science 57, 468-475.
- Dijkstra P, Coyle JS, Doucett RR, Schwartz E, Hart SC and Hungate BA 2008. 15N enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function. Ecology Letters 11:389-397.
- Schwartz E, Blazewicz S, Doucett RR, Hungate BA, Hart SC and Dijkstra P 2007. Natural abundance δ15N and δ13C of DNA extracted from soil. Soil Biology and Biochemistry 39, 3101-3107.
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Progress 08/15/06 to 08/14/07
Outputs
We are developing a new tool using stable N isotope analysis to determine how C and N availability affects metabolism of soil microorganisms and ecosystem properties, specifically the net N mineralization rates. All heterotrophic N exporting organisms (from microorganisms to mammals) are able to utilize organic N as a carbon source, and export excess N. Because deamination and NH4/NH3 state change discriminate against the heavy 15N isotope, organisms become 15N-enriched relative to their N substrates. This is often observed in animal food webs and should be true for soil microorganisms as well. This enrichment is to a variable degree balanced by N-isotope fractionation during N-assimilation and transamination. In other words, the microbial N-isotope composition relative to its substrates, the result of the balance between N assimilation and dissimilation, should reflect the C and N availability (physiological condition) and N-mineralization rate (ecological
consequence). The goal of this project is to see if we can use these isotope signatures to learn more about the regulation of microbial metabolism and utilize this technique as an alternative estimate of net N mineralization rates. We have shown that the microbial biomass is 15N enriched relative to soil and soluble N pools (Dijkstra et al. 2006ab, Coyle et al. in review). We observed that land-use has a major impact on the microbial 15N enrichment. Cattle dung, accumulated over many years, caused a decrease in the microbial 15N enrichment, concomitant with an increase in available C and N (Dijkstra et al. 2006b). Similarly, we are analyzing the effects of forest restoration treatments on microbial and other soil N pool isotope signatures and C and N cycling. In addition, we have sampled soil from five grassland ecosystems along an elevation gradient near Flagstaff in AZ. These grasslands (Great Basin desert, cold desert grassland, interspaces in pinyon-juniper woodland, meadow in
ponderosa pine forest and meadow in the mixed conifer zone) all showed a significantly 15N enriched microbial biomass (Dijkstra et al 2006a). Corinne M. LaViolette, a master student working on this project, has sampled soil from these sites on a monthly basis and determined net N mineralization rates and isotope composition of the various soil N pools. She is focused on the temporal changes in microbial 15N enrichment that are occurring associated with changes in C and N availability across the seasons. Jeff Coyle, a master student employed by a related NSF funded project, found large temporal fluctuations in microbial 15N enrichment across a semiarid substrate age gradient (Coyle 2007). Corinne has found that the microbial isotope signature changed during a 1-month soil incubation, while the signature of the soluble N remained the same or decreased slightly. In her experiments, as well in other experiments conducted up to now, we find that 15N enrichment correlates with C:N ratio and
net N mineralization rates.
Impacts
The measurement of the 15N composition of the soil microbial biomass allows us to observe microbial functioning and its ecological consequences from a completely new angle. The 15N enrichment determined from soil samples indicates how much C and N is processed by the soil microbial biomass, probably integrates aspects of quality, and shows a direct relation to soil N mineralization potential. These measurements will give us a much better understanding of the factors that determine soil N cycling, N mineralization potential and the effectiveness of measures to manage soil fertility and soil organic matter stocks in the long-term for a sustainable agriculture, rangeland and forestry.
Publications
- Dijkstra, P., Menyailo, O.V., Doucett, R.R., Hart, S.C., Schwartz, E., and Hungate, B.A., 2006b. C and N availability affects the 15N natural abundance of the soil microbial biomass across a cattle manure gradient. European Journal of Soil Science 57: 468-475
- Li, J, Johnson, DP, Dijkstra, P, Hinkle, CR and Drake, BG. (2007). Elevated CO2 mitigates the adverse effects of drought on daytime net ecosystem CO2 exchange and photosynthesis in a Florida scrub-oak ecosystem. Photosynthetica 45: 51-58
- Schwartz, E, Blazewicz, S, Doucett, RR, Hungate, BA, Hart, SC and Dijkstra, P. (2007). Natural abundance δ15N and δ13C of DNA extracted from soil. Soil Biology and Biochemistry (in print)
- Coyle, JS, 2007. 15N natural abundance of the soil microbial biomass along a semi-arid soil chronosequence. Master Thesis Northern Arizona University
- Dijkstra, P, Doucett, RR and Hungate, BA (2008). A new gas spike dilution method for analyzing large numbers of highly enriched samples. Rapid Communications in Mass Spectrometry (accepted with changes).
- Dijkstra, P., Ishizu, A., Doucett, R.R., Hart, S.C., Schwartz, A., Menyailo, O. and Hungate, B.A., 2006a. 13C and 15N natural abundance of the soil microbial biomass. Soil Biology and Biochemistry 38: 3257-3266.
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Progress 08/15/05 to 08/15/06
Outputs
AAll heterotrophic N exporting organisms (from microorganisms to mammals) are able to utilize organic N as a carbon source, and export excess N. As a result of fractionation during deamination and NH4/NH3 state change, main steps part of the N-dissimilation pathway, the organism becomes 15N-enriched relative to its diet. This is often observed in animal food webs, and should also be the case for soil microorganisms that are involved in N mineralization. This enrichment is to some degree balanced by N-isotope fractionation during N-assimilation and transamination, especially relevant for microorganisms that are involved in N-immobilization. In other words, the microbial N-isotope composition relative to its diet, the result of the balance between N assimilation and dissimilation, should reflect the C and N availability (physiological condition) and N-mineralization rate (ecological consequence). We are the first research group that has successfully applied the
chloroform fumigation extraction technique to determine the N-isotope composition of the soil microbial biomass (Dijkstra 2006a). We have shown that the microbial biomass is 15N enriched relative to soil and soluble N pools (Dijkstra et al. 2006a), that this enrichment is correlated negatively with C availability in response to dung addition (Dijkstra et al. 2006b). We have sampled soils from 5 grassland ecosystems along an elevation gradient near Flagstaff in AZ. These grasslands (Great Basin desert, cold desert grassland, interspaces in Pinyon-Juniper woodland, meadow in Ponderosa pine forest and meadow in the mixed conifer zone) all showed significant 15N enriched microbial biomass. This enrichment, we have now found, correlates negatively with C/N ratio for a range of grasslands ecosystems located along an elevation gradient, and two other gradients. It also correlates positively with N mineralization for the same gradients (Dijkstra et al. in prep, Coyle et al. in prep). Work is
now in progress to ascertain the temporal variability in microbial isotope composition in relation to that in N mineralization. We have established field sites in the grasslands along the elevation gradient and are conducting monthly determinations of field net N mineralization, combined with potential net Nmin incubations in the lab, and measurements of the 15N-isotope composition of the soil total, soluble and microbial N-pools. We have also established sites in a high elevation aspen and ponderosa pine site and in a forest restoration experiment (control and three levels of restoration intensity) as well as in a grazing experiment (no grazing, traditional and holistic management) and are sampling these sites for the same Nmin and isotope characteristics. Our results so far indicate that the measurement of the microbial (15N and 13C) isotope composition is an important and missing piece of the complex soil N-cycle and integrates physiological aspects of microbial functioning
(dependent on C and N availability) with ecological processes such as C and N cycling, N-mineralization, soil organic matter stabilization, litter decomposition and litter quality via the trophic level 15N enrichment.
Impacts
The measurement of the 15N composition of the soil microbial biomass allows us to take a look at microbial functioning and its ecological consequences from a completely new angle. The 15N enrichment determined from soil samples indicates how much C and N is processed by the microbial biomass, probably integrates aspects its quality, and shows a direct relation to soil N mineralization potential. These measurements will give us a much better understanding of the factors that determine soil N cycling, N mineralization potential and the effectiveness of measures to manage soil fertility and soil organic matter stocks in the long-term for a sustainable agriculture, rangeland and forestry.
Publications
- Li, J, Johnson, DP, Dijkstra, P, Hinkle, CR and Drake, BG. (2007). Elevated CO2 mitigates the adverse effects of drought on daytime net ecosystem CO2 exchange and photosynthesis in a Florida scrub-oak ecosystem. Photosynthetica (in press)
- Dijkstra, P, Doucett, RR and Hungate, BA (2007). A new gas spike dilution method for analyzing large numbers of highly enriched samples. Rapid Communications in Mass Spectrometry (submitted)
- Coyle, JS, Dijkstra, P, Selmants, PC, Doucett, RR, Schwartz, E, Hart, SC and Hungate, BA (2007). 15N natural abundance of the soil microbial biomass associated with ecosystem development along a semi-arid soil chronosequence. Ecology (in prep)
- Dijkstra, P, Coyle, JS, Selmants, PC, Doucett, RR, Schwartz, E, Hart, SC, Vitousek, PM and Hungate, BA. (2007). 15N enrichment of the soil microbial biomass integrates ecosystem C and N dynamics (in prep)
- Dijkstra, P, Coyle, JS, Selmants, PC, Doucett, RR, Schwartz, E, Hart, SC, Vitousek, PM and Hungate, BA. (2007) 13C natural abundance of the soil microbial biomass across two soil age gradients (in prep)
- Menyailo, OV, Dijkstra, P and Hungate, BA. (2007) Separating heterotrophic and autotrophic soil respiration using carbon stable isotopes and a simulated rain event. Global Change Biology (submitted)
- Schwartz, E, Blazewicz, S, Doucett, RR, Hungate, BA, Hart, SC and Dijkstra, P. (2007). Natural abundance δ15N and δ13C of DNA extracted from soil. Soil Biology and Biochemistry (in prep)
- Dijkstra P, Ishizu A, Doucett R, Hart SC, Schwartz E, Menyailo O and Hungate BA, 2006. 13C and 15N natural abundance of the soil microbial biomass. Soil Biology and Biochemistry 38: 3257-3266
- Dijkstra P, Menyailo O, Doucett R, Hart SC, Schwartz E and Hungate BA 2006. C and N-availability affects the 15N-natural abundance of the soil microbial biomass across a cattle manure gradient. European Journal of Soil Science 57: 468-475
- Hungate BA, Johnson DW, Dijkstra P, Hymus G, Stiling PD., Megonigal JP, Pagel AL, Moan JL, Day F, Li J-H, Hinkle CR and Drake BG, 2006. Nitrogen cycling during seven years of atmospheric CO2 enrichment. Ecology 87: 26-40
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