Progress 12/15/00 to 12/31/04
Outputs
Nitrate, a mobile ion in the soil solution, can be readily lost by leaching and denitrification, causing environmental degradation due to contamination of groundwater and to nitrous oxide production, which results in stratospheric ozone depletion. The role of microbes and plants in nitrate production, consumption, and loss in conventional and organic production systems were studied. Although the two systems have different organic matter inputs, their soil nitrate and ammonium concentrations were similar. Higher microbial biomass and net mineralizable N occurred in the organic system. Gross mineralization rates and nitrification rates, measured by 15N-isotope pool dilution, were higher in the organic soil, and microbes immobilized more of the nitrate that was produced. In the organic system, highest nitrous oxide emissions occurred after spring incorporation of cover crops and composted manure, whereas rates were highest in the conventional system in the fall after
incorporation of crop residues and ammonium fertilizer. Wet/dry cycles caused rapid changes in microbial C and N transformations that coincided with changes in community composition as determined by phospholipid fatty acid analysis. A quantitative real-time PCR assay was developed for the ammonia-monooxygenase (amoA) gene to estimate ammonia oxidizer bacteria (AOB) population size in soil. Changes in AOB population sizes were greater in ammonium -fertilized soils, and were related to the production of nitrate in controlled microcosm studies. New evidence of a complex role in plant-microbial N cycling was shown in a study of 15N-nitrate fates in the presence and absence of roots in soil from the organic system. After one day, more 15N-ammonium was found in the presence of roots, indicating re-cycling of 15-nitrate. These are novel results that suggest that either root/food web interactions, e.g., remineralization of N, dissimilatory nitrate reduction, or plant efflux of ammonium is
occurring at much more rapid rates than previously thought.
Impacts
These results on the rapidity and magnitude of soil N transformations emphasize the benefits of continuous plant cover in retaining N in non-mobile forms to prevent losses. Under organic production, microbes take up substantial amounts of nitrate from nitrate-rich soils, a factor which increases N retention. Also, in addition to plant N uptake, plant roots affect rhizosphere soil by increasing the recycling of nitrate to ammonium, and thus make nitrate less vulnerable to loss. After irrigation events, nitrous oxide loss can be very high from these soils at key times, e.g., following fertilizer addition in the fall (conventional production) or when substantial available carbon is present (organic production). These results are being presented to grower audiences and at scientific meetings, with an emphasis on the importance of maintaining plant cover to retain N in agricultural systems.
Publications
- Burger, M. and Jackson, L. E. 2004 Plant and microbial nitrogen use and turnover: rapid conversion of nitrate to ammonium in soil with roots. In press, Plant and Soil.
- Jackson, L.E. 2005. Soil biology: root architecture and growth. Encyclopedia of Soils in the Environment. Elsevier Ltd. Pp. 411-421.
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Progress 01/01/03 to 12/31/03
Outputs
Soil nitrate can be readily lost, causing contamination of groundwater and nitrous oxide production, which results in stratospheric ozone depletion. The role of microbes and plants in nitrate production, consumption, and loss in conventional and organic production systems for tomato is the focus of this research. The two systems have different organic matter inputs, but their soil nitrate and ammonium concentrations are similar. Actual, i.e. gross mineralization rates and nitrification rates, measured by 15N isotope pool dilution, were higher in the organic soil as well, and microbes immobilized more of the nitrate that was produced, probably because ammonium was in low supply. Peak nitrous oxide emissions differed seasonally in the two systems and can be related to C and N inputs prior to irrigation events. Wet/dry cycles also caused rapid changes in microbial C and N transformations that coincided with changes in microbial community composition as determined by
phospholipid fatty acid (PLFA) analysis. Plants play a more complex role in N cycling than previously realized, based on a study of 15N-nitrate fates in the presence and absence of roots in soil from the organic system. After one day, >30 times more 15N-ammonium was found in the presence of roots, indicating re-cycling of 15N-nitrate. Either root/food web interactions, e.g., remineralization of N, or plant efflux of ammonium is occurring at much more rapid rates than previously thought. Arbuscular mycorrhizae in tomato increased N uptake, based on experiments using the fresh-market tomato rmc (reduced mycorrhizal colonization) mutant. A useful new tool is our collaborative development of a real-time PCR method for quantifying for soil ammonia-oxidizing bacteria in, which is now being expanded for a broader range of taxa for use in field and controlled studies.
Impacts
Important findings of this research are that plant roots and soil microbes are re-cycling inorganic N much more rapidly than previously thought, and that microbes take up substantial amounts of nitrate from nitrate-rich soils, a factor which increases N retention. Our measurements of nitrous oxide emissions indicate that very high rates can occur at key times, e.g., following fertilizer addition in the fall when substantial available carbon is present. These results emphasize the benefits of continuous plant cover in retaining nutrients in non-mobile forms to prevent losses.
Publications
- Okano, Y., K.R. Hristova, C.M. Leutenegger, L.E. Jackson, R.F. Denison, B. Gebreyesus, D. LeBauer, and K.M. Scow 2003. Effects of ammonium on the population size of ammonia-oxidizing bacteria in soil application of real-time PCR. Applied and Environmental Microbiology, in press.
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Progress 01/01/02 to 12/31/02
Outputs
Immobilization of ammonium and nitrate by plants and microbes is being tested for its role in controlling ecosystem nitrogen (N) retention. A comparison is being made between soils managed with high organic matter inputs (i.e., organic production) vs. inorganic fertilizers (conventional production). Agricultural systems that receive high or low organic matter inputs would be expected to differ in soil N transformation rates and fates of ammonium and nitrate. The specific objectives are to measure 1) rates of microbial nitrogen transformations, fates of ammonium, and rates of nitrate loss by gaseous emission and leaching, 2) microbial community structure based on phospholipid fatty acids in the soil, 3) ammonium and nitrate uptake by tomato roots in the soil, and 4) factors affecting tomato uptake such as severity of drying of the surface soil between irrigation events, and susceptibility to mycorrhizal symbiosis. Three projects are now complete. Field data on soil N
dynamics, gaseous emissions, and a microcosm study examined responses of N transformations and microbial community structure to irrigation events in organic and conventionally managed soils. 15N tracers and isotopic pool dilution experiments measured gross mineralization and nitrification rates after rewetting dry soil and showed the importance of microbial nitrate immobilization in both soils. The fate of ammonium in the presence and absence of tomato roots was measured in soil microcosms and showed that N transformations differed strongly in the presence of roots, possibly because of remineralization of microbial N, dissimilatory nitrate reduction, or plant N efflux.
Impacts
Important findings of this research are that soil microbes take up substantial amounts of nitrate from nitrate-rich soils, a factor which increases N retention. Our methods to refine the 15N pool dilution method provided this information. We also demonstrated that plant roots may stimulate microbial communities in the rhizoplane that have rapid turnover of inorganic and organic N, which may facilitate N retention. A better understanding of biological and environmental factors that control soil microbial N transformations is necessary for developing practices that limit nitrous oxide (N2O) emissions to the atmosphere. Results will be pertinent to improving nitrogen management on organic and conventional farmers' fields.
Publications
- Burger, M. and L.E. Jackson. 2002. Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems. In press, Soil Biology and Biochemistry.
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Progress 01/01/01 to 12/31/01
Outputs
Several projects are underway to measure soil nitrogen transformations in relation to water availability in organic and conventional production systems for tomatoes. A molecular technique is now being used to target the gene that codes for ammonia monooxygenase, in order to study changes in ammonia oxidizer population size in both cropping systems. These changes will be related to nitrification rates under different environmental conditions. Methods have been compared to improve the resolution of 15N pool dilution for measurement of gross rates of ammonification and nitrification. An experiment to compare rates of nitrification in the presence of plants with rates of without plants has shown that rapid re-mineralization of inorganic N is occurring in the presence of roots, such that roots greatly enhancing rates of soil nitrogen release in their immediate vicinity. Efforts have started to assess mycorrhizal colonization in isogenic lines of tomato that differ in
mycorrhizal susceptibility.
Impacts
42 Study of soil N transformations during irrigation events is important in developing management strategies to reduce nitrate production by soil microbes as it leads to leaching and gaseous emission of nitrous oxide and contributes to decreased soil, water, and atmospheric quality.
Publications
- No publications reported this period
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