Progress 08/01/08 to 07/31/13
Outputs
Target Audience: The primary target audience during the last year of the project were commercial and Native Hawaiian taro farmers located on the islands of Oahu and Kaua`i and University of Hawai`i Extension Faculty. Efforts included farm visits, a field day for taro farmers in Hanalei, and a 2-day fertility workshop for Extension Faculty held in Hilo, Hawai`i. Changes/Problems: In response to the lack of a significant measurement of anammox activity in the taro sediments, PIs decided to modify Objective 2 to include an additional set of objectives investigating rhizosphere coupling of nitrification-denitrification. Additionally, activities associated with anammox measurements were removed from the field experiment, and replaced with a focus on measuring soil and water N. What opportunities for training and professional development has the project provided? The project employed one Post-Doctoral Fellow and a Junior Researcher who both interacted significantly with the Project Director and collaborating Principal Investigators. The Post-Doctoral Fellow received mentorship and training from PIs in working at the Center for Microbial Ecology at Michigan State and the School of Ocean Sciences and Technology and College of Tropical Agriculture and Human Resources at the University of Hawaii. The Junior Researcher interacted primarily with PIs at the University of Hawaii during the isotope pairing experimental phase. The Post-Doctoral Fellow supervised undergraduate research assistants at the Center for Microbial Ecology providing training in quantitative PCR laboratory techniques. The PD supervised undergraduate research assistants providing hands-on training and experience in agricultural field work. The Post-Doctoral Fellow participated in three professional meetings in the continental United States and numerous less formal farmer meetings and field experiences in Hawaii. The Post-Doctoral Fellow was invited to the University of Hawaii Water Resources Research Center’s seminar series to present a summary of research findings in June 2013. How have the results been disseminated to communities of interest? Dissemination of project results and progress occurred annually for the duration of the project period. The primary targets for dissemination were taro growers in the state of Hawaii, University of Hawaii extension faculty, and new farmer groups and Master Gardener Programs across Hawaii. The PD conducted semi-annual taro farmer group meetings on Oahu and Kauai where participating farmers were kept informed of project activities and project results. The most important of these meetings were the bi-annual meetings with the Kauai Taro Growers Association (the largest association of taro growers in the State of Hawaii. The PD conducted two field days for Kauai taro growers during the span of the field experiment. In conjunction with the PD’s extension duties, knowledge generated from the project were presented at annual soil fertility workshops for extension faculty in 2010, 2011, and 2013. The PD conducts numerous training events on nutrient management for new farmer groups and the statewide Master Gardner Program. All training events nutrient management in wetland taro production where PD provided updated information generated from the project. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
Anammox bacteria has been found in a diverse range of terrestrial and marine environments. In the ocean, anammox has been shown to explain as much as 70% of N loss. Although anammox bacteria was found in a range of flooded taro sediments including conventionally and organically managed fields, experimental results found negligible anammox activity. Anammox contributed to just 5% of N loss with denitrification showing the largest contribution to N loss. Quantitative PCR analysis found high abundance of functional genes involved in nitrification and denitrification evenly distributed throughout the surface and sub-soil. The even distribution of these bacteria coupled with pore water profiles indicating an abundance of NO3 peaks, provided evidence that rhizosphere coupling of nitrification and denitrification may be significant and contributing to N loss. The developmentof a modified whole-core 15NH4 experiment enabled the quantification of the fate of labeled 15NH4 fertilizer in a flooded sediment with a growing 4-month old taro plant. Measured diurnal O2 transport into the sub-surface stimulated nitrification-denitrification in the rhizosphere resulting in the loss of 35% of added ammonium fertilizer. These results suggested that the application of soluble ammonium N fertilizers to a flooded taro environment poised for denitrification would result in large losses of N. A field experiment compared the use of a controlled release urea fertilizer, an organic N fertilizer, and conventional urea fertilizer on N dynamics at three taro farms. The polymer coated urea (PCU) controlled release fertilizer showed the highest average yield and lowest amount of variation. The organicfish bone meal (FBM) and PCU fertilizers provided a long term reservoir of plant available N in the root zone, and reduced N export to the river system. The current farmer practice did not store applied N in the soil, but rather showed an increased export of N to the river system with potential to contaminate fragile downstream freshwater and marine ecosystems. Results from a partial cost benefit analysis showed that the PCU fertilizer treatment showed the highest mean return with the lowest variability across the three farm sites. Overall, results from this project have improved our understanding of the important role denitrification plays in a flooded agroecosystem. The field experiment suggests that N use efficiency is improved by substituting conventional urea fertilizer with a controlled release product or an organic N fertilizer. The first objective aimed to determine the distribution of anammox in flooded taro soils and evaluate its contribution to N2 production. Quantitative PCR targeting anammox and N cycling bacteria nosZ, nirS, amoA, and the 16S rRNA gene was performed on cores from taro farms under conventional and organic management. Anammox and potential denitrification activities were assessed using a modified isotope pairing technique (IPT) and estimates of potential nitrification were developed using a shaken soil-slurry method. Results from quantitative PCR analysis showed the presence of significant numbers of anammox 16S rRNA genes (~105 copies g-1 wet sediment) in the surface increment, and significant numbers of N cycling genes distributed throughout the entire depth. The IPT experiments showed very low to no anammox activity in all sediments, but found high denitrification potentials especially in conventionally managed sediments. Significant numbers of N cycling functional genes distributed throughout the soil profile suggested that there was a sub-surface nitrification and denitrification potential. A modified non-destructive whole-core 15NH4 perfusion technique was developed with 4-month old taro plants to address the following objectives: evaluate O2 flux from the root rhizosphere as a mediator of sub-surface nitrification; assess the relative importance of sub-surface N cycling processes to overall NH4 losses in vegetated and non-vegetated cores; investigate N functional gene abundances in the sediment sub-surface; and assess the overall N balance in intact cores over a ten-day incubation period. The cores were fitted with equilibrators enabling the extraction of pore water and the analysis of 14+15N-NO3, NH4, and N2. Root O2 flux was driven by photosynthesis during the light period stimulating nitrification-denitrification in the extensive root rhizosphere. Sub-surface coupling of nitrification and denitrification accounted for as much as 35% loss of added 15N label. The N cycling functional genes nosZ and amoA were found at high abundances throughout the sub-surface with nirS dominating nitrite reduction. The experiment demonstrated the important contribution of rhizosphere coupling of nitrification-denitrification to fertilizer N loss. The field experiment addressing objective 2 was modified from a focus on anammox activity to a focus on fertilizer effect on taro yield, soil and plant N, and water N. Two fertilizers consisting of a controlled release polymer coated urea (PCU) and an organic fish bone meal (FBM) were compared to the farmer practice of monthly applications of conventional urea. Each treatment was established at four commercial taro farms in Hanalei Valley, Kauai. Surface soil cores were collected from each experimental plot at monthly intervals throughout the 12-month experiment and analyzed for inorganic N. Water samples were collected every two weeks for the first three months and monthly thereafter and analyzed for inorganic N and total N. At harvest total corm weight was recorded and five whole taro plants from each plot were analyzed for N uptake. Fertilizer treatment had no significant effect on mean taro yield, but PCU fertilizer showed the highest average yield and lowest amount of variation. The organic FBM fertilizer and PCU fertilizers accumulated soil NH4 during the first 6 months of the experiment with a significant reduction of N export to the river system. The current farmer practice did not store applied N in the soil, but rather showed increased N export to the river system. Results from a partial cost benefit analysis showed that the PCU fertilizer was the most cost-effective option because it showed consistently high returns to the farmer with the added benefit that it could reduce N contamination of fragile downstream aquatic resources. While the results do not show significant taro yield benefits to the farmers, farmers do not incur an economic penalty for the added environmental benefits. Overall, knowledge of N cycling in flooded taro was improved. The presence of anammox bacteria in taro sediments did not equate to anammox activity and a direct impact on N loss. The whole core experiment showed the importance of subsurface rhizosphere nitrification-denitrification as a N loss mechanism. Results from the on-farm field experiment engaged scientists, extension agents and farmers increasing participant knowledge of N cycling in flooded agroecosystems. There was an improved understanding of N cycling in taro systems and a change in attitude toward alternative N fertilization strategies among the farming community and university extension faculty. The two key changes were, first, the field experiment showed that current N application rates in flooded taro (400 – 600 lbs N per acre) are high and applications rates can be reduced by substituting conventional urea with PCU or FBM alternatives. Second, the farming community was presented with information showing that although PCU and FBM fertilizers are more expensive than conventional urea fertilizer, both fertilizers improve water quality and the PCU fertilizer showed a high potential to increase economic benefit to the farmer. With this increased awareness, farmers attitudes have changed regarding adoption alternate fertilizers on their farms.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2013
Citation:
Penton, C.R., Deenik, J.L., Popp, B.N., Bruland, G.L., Engstrom, P., St Louis, D., Brown, G.A., and Tiedje, J. 2013. Importance of sub-surface rhizosphere-mediated coupled nitrification-denitrification in a flooded agroecosystem in Hawaii. Soil Biology and Biochemistry 57:362-373.
- Type:
Journal Articles
Status:
Under Review
Year Published:
2014
Citation:
Penton, C.R., Deenik, J.L., Popp, B.N., Bruland, G.L., Engstrom, P., Mueller, J., Worden, A., and Tiedje, J. Assessing Nitrogen Transformations in a Flooded Agroecosystem using the Isotope Pairing Technique and Nitrogen Functional Gene Abundances. Soil Science
- Type:
Other
Status:
Published
Year Published:
2013
Citation:
Deenik, J.L., Penton, C.R., and Bruland, G. 2013. Nitrogen cycling in flooded taro agriculture. College of Tropical Agriculture and Human Resources, Cooperative Extension Service Publication, CTAHR SCM-31, pp. 6.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2013
Citation:
Deenik, J.L., Penton, C.R., Popp, B.L., Bruland, G.L., Engstrom, P., Mueller, J., and Tiedje, J. Microbially mediated nitrogen transformations in flooded agroecosystems. Invited Oral Presentation Royal Golden Jubilee PhD Congress XIV, Pattaya, Thailand April 5 7, 2013.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2013
Citation:
Deenik, J.L., Penton, C.R., Popp, B.L., Bruland, G.L., Engstrom, P., Mueller, J., and Tiedje, J. Nitrogen transformations in flooded agroecosystems: a case study with taro. Oral presentation, Western Region Nutrient Management Conference, Reno, NV, March 7 8, 2013.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2011
Citation:
Penton, C.R., Deenik, J., Popp, B.L., Engstrom, P., Bruland, G.L., Worden, A., Brown, G., and Tiedje, J. (2011) Use of novel whole-core incubations to measure the fate of fertilizer N in a flooded agricultural system. American Society for Microbiology. New Orleans, LA.
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Progress 08/01/11 to 07/31/12
Outputs
OUTPUTS: Two manuscripts were written during 2012 and submitted for review. The first manuscript describing the whole core experiment entitled "Importance of Sub-Surface Rhizosphere-Mediated Coupled Nitrification-Denitrification in a Flooded Agroecosystem in Hawaii" was submitted to Soil Biology and Biochemistry and accepted for publication in October. It is currently in press. The second manuscript entitled "Assessing Nitrogen Transformations in a Wetland Agroecosystem using the Isotope Pairing Technique and Nitrogen Functional Gene Abundances" has been submitted to Wetlands and is currently in the review process. The Hanalei field experiment was harvested in April, 2012. We are in the final stages of data analysis and preparation of report for participating farmers. In brief the following conclusions have been reached: 1) at 4 months after planting the fish bonemeal and slow release N treatments showed significantly higher soil NH4 concentrations than the farmer practice and 0 N plots; 2) tissue N concentrations in the fish bonemeal and slow release N were also significantly higher than the farmer practice and 0 N treatments at four months indicating higher soil N availability; 3) soil NH4 data for the entire growing cycle (0 to 14 months) showed the fish bonemeal and slow release N fertilizer materials reached peak NH4 concentrations within the first 3 months followed by gradual decline to the same level as the 0 N treatment by month 9; 4) soil NH4 concentrations in the farmer practice treatment where urea was applied to surface waters monthly for the first 6 month showed no increases in concentration compared with the 0 N treatment; 5) there were no significant fertilizer treatment effects on fresh taro corm yields at harvest (yields were as follows (0N=22,879 kg/ha; farmer practice=24,774 kg/ha; fish bonemeal=22,351 kg/ha; slow release N=27,875 kg/ha; 6) the slow release N fertilizer treatment showed the least amount of variability in yield across the three sites with standard error of the mean = 547 kg/ha whereas the fish bonemeal treatment showed high variability in yields across sites with standard error of 6,354 kg/ha; 7) N uptake by taro plants was generally low for all treatments with 0N=29.0 kg N/ha, farmer practice=34.2 kg N/ha, slow release N=54.7 kg N/ha, and fish bonemeal=64.3 kg N/ha; N uptake improved marginal in the fish bonemeal treatment compared with the farmer practice (P=0.073); the slow release N treatment showed no significant improvement in N uptake despite increased soil NH4 throughout the first 9 months of the growing season. Soil samples collected at 6 months (and kept frozen at 80 C) are currently being analyzed for N functional genes to assess fertilizer treatment effects on microbial populations. Overall, the added cost of applying the fish bonemeal or slow release fertilizer compared with costs for granular urea (46-0-0) were not compensated by hypothesized increase yields due to improved N use efficiency. A farmer meeting was held in July 2012, to relay initial findings of the field experiment. PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: The field experiment provided an excellent opportunity for investigators to interact directly with farmer stakeholders. The interaction provided many opportunities for knowledge transfer. Farmers were appreciative of the educational aspect of the interaction. They have all expressed that the on-farm experiments gave them a practical way to learn the complexities of the N cycle in a flooded system. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Results from the whole core experiment can be used to interpret findings from field experiment. The whole core experiment showed that significant quantities of applied urea in the subsoil are lost to denitrification as a result of coupled nitrification and denitirification in the rhizosphere. The fish bonemeal and slow release fertilizers supplied large quantities of N as NH4 to the soil in the first six months of the field experiment, but soil NH4 concentrations returned to baseline concentrations by month 9. The low uptake by taro plants indicated that an alternate loss pathway must have been at work. Results from the whole core experiment suggest that as much as a third of the added NH4 could have been lost to denitrification in the rhizosphere. The lack of a significant effect of fish bonemeal and slow release N fertilizer on taro yield was discussed. Farmers shared previous experiences with slow release fertilizers where taro yields were not significantly improved. The additional costs associated for both the organic fertilizer and the slow release fertilizer make them unappealing fertilizer alternatives especially if they do not improve fresh taro yields.
Publications
- Penton, C., Deenik, J.L., Popp, B.N., Bruland, G.L., Engstrom, P., St. Louis, D., Brown, G.A., and Tiedje, J. 2012. Importance of Sub-Surface Rhizosphere-Mediated Coupled Nitrification-Denitrification in a Flooded Agroecosystem in Hawaii. Soil Biology and Biochemistry (in press).
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Progress 08/01/10 to 07/31/11
Outputs
OUTPUTS: Since December 2010, whole core incubation N data has been re-analyzed incorporating the refined IPT protocol described by Trimmer and Nicholls (2009) where the isotopic composition of NO3- at each depth is used to correct 29+30N2 rates to total denitrification (d14+15) from both the labeled and unlabeled NO3- pools. Additionally, quantitative PCR was carried out in 1 cm depth increments down to -6 cm on all cores for five functional genes involved in nitrification and denitrification as well as the 16S rRNA gene. The five genes were nosZ (nitrous oxide reductase), nirS and nirK (nitrite reductases), and bacterial and Archaeal amoA (ammonia oxidation). Two meetings with the Kauai Taro Grower's Association were held in September and November, 2010 to design and plan for the field experiments addressing Objective 2. The experimental design was reached through collaborative discussions among the research team (PIs Deenik and Bruland) and four taro farmers in Hanalei Valley (the most important taro growing region in Hawaii). The experimental design was modified from the design presented in the original proposal to accommodate the needs and limitations of the cooperating farmers. The experiment consists of four treatments: 1) check (no amendments), 2) farmer practice (monthly applications of 46-0-0 fertilizer broadcast into the floodwaters, 3) pre-plant application of a polymer coated 43.5-0-0 slow release fertilizer, and 4) pre-plant application of a locally sourced fish/bone meal organic fertilizer. One taro patch at each of the farms was selected for the implementation of the treatments. Each patch was divided into four sections with treated wood separators to minimize water flow between treatment plots and each patch was engineered to have its own water inflow and outflow pipes. All treatments received the same amount of N set at 537 kg N per ha. For treatment 2, the 46-0-0 was applied monthly to reach a total of 537 kg N per ha whereas treatments 3 and 4 received the entire N amount in one pre plant application. The experiment implementation was completed in the first half of February 2011 and taro was planted by the third week of February. Water samples were collected from the outflow of each experimental plots every two weeks through month four (March-June) and analyzed for total N, ammonium, and nitrate. Soil cores (0-15 cm) were collected monthly and analyzed for KCl extractable ammonium and nitrate. Tissues samples were collected from all plots at 4 months after planting and analyzed for elemental composition. During the 5th month soil sampling included the collection of 1 cm slices down to 7 cm from 3 cores from each plot to model ammonium fluxes. In month six, the top 3 cm depth from 3 samples from each plot were collected and sent to MSU for N functional gene abundance. Results from the isotope pairing experiments and the whole core experiment addressing Objective 1 were presented at the American Geophysical Union Annual Meeting in December, 2010. Four farmer meetings have been held in Hanalei to discuss results and plan for the field experiment (October, 2010; January 2011; April, 2011; July 2011) PARTICIPANTS: Ryan Penton continues as Post-Doc on the project. TARGET AUDIENCES: Taro farmers active in the Kauai Taro Farmers Association have been the primary target. We have conducted 4 meetings with the Association to share findings of the research. The on-farm experiments are being conducted in collaboration with 4 farmers from the Association. PROJECT MODIFICATIONS: The field experiment has been modified due to the insignificant contribution of anammox as determined by IPT experiments. The choice to perform the experiment at four farms was made due to logistical complications with replicating all treatments at one farm. The direct involvement of four different farmers increases opportunities for knowledge transfer and potential changes in farmer N management.
Impacts
Re-analysis of the whole core N data showed that wind application significantly increased subsurface denitrification (integrated through the whole column) 7-fold over a no wind treatment. Subsurface-coupled nitrification/denitrification was significantly higher in the vegetated cores versus the control (non-vegetated) cores. There was no difference in surface denitrification as a function of vegetation. However, wind treatment did increase surface rates significantly over both the no-wind and control cores. Total subsurface denitrification rates were higher than surface rates in the vegetated cores with no difference among the control cores. Time of day at which sampling occurred was found to significantly affect in-situ denitrification rates with an approximate 6 h lag after initiation of the day cycle in the incubation chamber. This lag was most pronounced in the wind treatment while the control cores demonstrated a more variable response with smaller magnitude increases. This 6 h lag was also found when measuring O2 flux from root tips. A large increase in O2 concentration was found with wind application after 6 h, followed by a gradual decrease to background levels after the day cycle ended. This response was not observed when there was no wind applied to the leaf surfaces. Taro aerenchyma was a significant conduit of N2 loss from the subsurface with high concentrations of 29+30N2 detected in both the wind and no wind treatments. We are currently reviewing the literature in order to model N2 flux through the aerenchyma for the final N budget. Results from Q-PCR analysis of whole core sediments showed that neither Archaea nor bacteria dominated the ammonia-oxidizing community through the soil profile. Iron-dependent cd1 nitrite reductases (nirS) were as much as 10-fold higher in abundance than nirK. The nitrous oxide reductase gene was significantly more abundant than all other genes, accounting for up to 12% of the total bacterial population, based on the 16S rRNA gene. All nitrogen functional genes showed subsurface abundance peaks, illustrating a functional relationship with the root rhizosphere. Both nosZ and nirS abundances were significantly lower in the control core when compared to the wind treatment. In the field experiment, background water N concentrations were low (<1.5 ppm). Farmer practice plots showed spikes in ammonium (up to 13 ppm) and total N associated with urea fertilization events. The fish/bone meal and slow release N treatments showed elevated ammonium (3-9 ppm) in surface waters during the first month with a return to low concentrations from the second month on. Results from soil sample analysis show that plots that received the slow release N fertilizer had a peak ammonium concentration (mean = 620 kg per ha) at the first sampling date 30 days after fertilizer application. The data showed that the fertilizer released all the N within the first month. For the fish/bone meal plots, ammonium release rate peaked by month 3. The farmer practice plots showed ammonium concentrations that fluctuated between a maximum of 90 and minimum of 10 kg per ha across the 4 sites.
Publications
- Deenik, J.L., Penton, C.R., Bruland, G.L., Popp, B.N., Engstrom, P., Mueller, J., Tiedje, J. 2010. Quantifying nitrogen loss from Hawaiian taro fields. Presented at 2010 Fall Meeting, American Geophysical Union, San Francisco, CA Dec. 13-17 (p. 460)
- Penton, C.R., Bruland, G.L., Popp, B.N., Engstrom, P., Tiedje, J., Brown, G.A., Deenik, J.L. 2010. Use of Novel Whole Core Incubations to Measure the Fate of Fertilizer N in a Flooded Agricultural System. Presented at 2010 Fall Meeting, American Geophysical Union, San Francisco, CA Dec. 13-17 (p. 460).
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Progress 08/01/09 to 07/31/10
Outputs
OUTPUTS: 15N slurry incubations, porewater (ammonium, nitrite and nitrate) and oxygen profiles, and nitrification potential measurements have been completed on 12 intact soil cores (0-20 cm)collected from five taro farms (two on Oahu and three on Kauai). To quantify N loss through denitrification and anaerobic ammonium oxidation (anammox) pathways in these taro systems we utilized a slurry-based isotope pairing technique (IPT). Measured nitrification rates and porewater N profiles were also used to model ammonium and nitrate fluxes through the top 10 cm of soil. Quantitative PCR of nitrogen cycling functional genes (NosZ, NirK, NirS, AmoA, Archael AmoA, and the 16S rRNA) was used to correlate porewater N dynamics with potential microbial activity. We developed a new whole-core perfusion technique for tracking the fate of 15NH4+ added to intact vegetated cores. Taro plants (Colocasia esculenta) were field-grown in (20 cm diameter) cores for three months, which allowed exchange with natural porewater, then harvested. Following core extraction, surface and porewater were removed and 15NH4+ labeled porewater was slowly re-introduced to the core through a perfusion cap in the laboratory. Mini porewater equilibrators were placed in 1 cm increments through the sediment profile for porewater extraction during incubation. We also independently tested the ability of taro roots to oxygenate the subsurface by growing plants in nutrient agar and measuring O2 flux with a microelectrode. In the agar experiment, diurnal O2 transport was monitored and the application of wind across the taro leaves was found necessary to develop an oxygenated zone at the root tips. Using this information, the harvested taro were incubated in growth chambers after perfusion using three treatments: Vegetated without wind, vegetated with wind, and a non-vegetated control. Porewater was analyzed for 29+30N2, 15NH4+, 15NO3-, and unlabeled nitrate and ammonium species. Plant uptake of 15NH4+ was also determined. Quantitative PCR was performed on the sediment profiles of functional genes involved in nitrogen cycling for correlation to N transformations. The PI has conducted two meetings with the Kauai Taro Growers Association (May and August 2010) to present results of the laboratory experiments and plan for the field experiment. PARTICIPANTS: Dr. Ryan Penton conducted pore water modeling efforts in conjunction with collaborating scientist Dr. Pia Engstrom (University of Gothenburg, designed and supevised the whole core experiment, and overseen the microbial analysis at MSU. Dr. Gegory Bruland assisted with the design and implementation of the whole core experiment. Dr. Popp has overseen 15N isotope analysis in his laboratory. Dr. Tiedje has supervised Dr. Penton at MSU. Mr. Garvin Brown has been a laboratory technician assisting with porewater analysis and the implementation and running of the whole core experiment. The project is providing post-doctoral training for Dr. Penton. The Kauai Taro Growers Association has been a primary collaborator. TARGET AUDIENCES: Taro growers in the State of Hawaii are the primary audience. About 25% of whom are of native Hawaiian ancestry. PROJECT MODIFICATIONS: Following results from the 15N slurry incubations, which showed that anammox activity was negligible in all the cores, a critical research question arose regarding the possibility that the coupling of nitrification and denitrification in the oxic rhizosphere as an important pathway for N loss in the taro system. To test the hypothesis that the coupling of nitrification and denitrification in the rhizosphere was an important pathway of N loss in the system we designed the whole core experiment described above.
Impacts
Rates of denitrification calculated using porewater profiles were compared to those obtained using the slurry method. Potential denitrification rates of surficial sediments obtained with the slurry method were found to drastically overestimate the calculated in-situ rates. The largest discrepancies were present in fields greater than one month after initial fertilization, reflecting a microbial community poised to denitrify the initial N pulse. Potential surficial nitrification rates varied between 1.3% of the slurry-measured denitrification potential in a heavily-fertilized site to 100% in an unfertilized site. Compared to the use of urea, fish bone meal fertilizer use resulted in decreased N loss through denitrification in the surface sediment, according to both porewater modeling and IPT measurements. In addition, sub-surface porewater profiles point to root-mediated coupled nitrification/denitrification as a potential N loss pathway that is not captured in surface-based incubations. Profile-based surface plus subsurface coupled nitrification/denitrification estimates were between 1.1 and 12.7 times denitrification estimates from the surface only. These results suggest that the use of a classic isotope pairing technique that employs 15NO3- in fertilized agricultural systems can lead to a drastic overestimation of in-situ denitrification rates and that root-associated subsurface coupled nitrification/denitrification may be a major N loss pathway in these flooded agricultural systems. Preliminary results from the whole core experiments confirm our hypothesis that the major pathway of N loss was root-mediated nitrification/denitrification followed by a flow of 29+30N2 through the aerenchyma. The vegetated wind treatment exhibited the highest concentrations of labeled N2 in the subsurface during all time periods. In contrast, the vegetated no wind treatment had much higher aerenchyma 29+30N2 concentration, accounting for approximately 100% of the subsurface N2 accumulation by day three of the incubation. Surface water N2 concentrations were also highest in the no wind treatment. After nine days the 29+30N2 concentrations dropped by approximately 70%, with little difference remaining among the treatments, indicating limitation by 15NH4+ diffusion. These results indicate that N2 is preferentially transported through the aerenchyma in taro and probably other plants grown in flooded agricultural fields. However, increased wind stress reduced transport through the aerenchyma and resulted in greater N2 accumulation in the subsurface, which indicates the importance of mass flow transport of air and its effect on oxygenation at the root tips. The results indicate that the complexity of N cycling in flooded agricultural systems may confound attempts to estimate in-situ N losses through porewater modeling, classic isotope pairing techniques, or N flux chambers. The whole-core technique presented here allows for the measurement of multiple N pools and fates while minimizing system disturbance and more accurately representing field conditions.
Publications
- No publications reported this period
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Progress 08/01/08 to 07/31/09
Outputs
OUTPUTS: During the first year project activities were devoted to fulfilling objective 1 (to determine the distribution of anammox in flooded taro soils of Hawaii and evaluate its contribution to N2 production). The first step involved adapting the isotope pairing experimental procedure developed to measure anammox activity in ocean sediments to the freshwater taro sediments. The experimental procedures were finalized in February 2009. Three field campaigns were conducted to collect soil cores from taro farms in Hanalei (Kauai) and Waiahole and Haleiwa (Oahu). Soil cores were segmented into 10-15 depth increments and analyzed for pore water N (NH4+, NO2-, NO3-), porosity, and dissolved oxygen. Anammox activity, denitrification potential, and nitrification potential was determined on the 0-0.5 cm depth increment in all cores. Soil samples were form each core were packaged and shipped to Dr. Tiedje's lab at MSU for microbial analyses, which are currently underway. We are currently analyzing the results from the laboratory work and plan to develop a manuscript. In August, the PI traveled to Kauai (the primary taro production area in Hawaii) to make a presentation on nitrogen transformations in taro soils to the Kauai Taro Growers Association. PARTICIPANTS: Jonathan Deenik (PI): responsible for overall coordination of the project, worked closely with PIs to develop and adapt isotope pairing experiments to taro sediments, supervised Post Doc and two laboratory technicians. James Tiedje (Co-PI): responsible for microbial characterization of taro sediments, direct supervision of Post Doc at MSU Brian Popp (Co-PI): responsible for overseeing isotope experiments and ensuring data quality from mass spectroscopy work, involved in method development for nitrification potential. Greg Bruland (Co-PI): participated in soil sampling campaigns and involved in methodology for anammox/denitrification and nitrification potential. Jaclyn Mueller (Lab Technician): assisted in field work, sample preparation, nutrient analysis, and mass spectroscopy work. Garvin Brown (Lab Technician: assisted in field work, sample preparation, nutrient analysis, and mass spectroscopy work. Pia Engstrom (Collaborator from Gothenburg University: advised the project on the isotope pairing experiments and assisted in adapting the method to taro sediments. Ryan Penton (Post Doc) responsible for isotope pairing experiments and microbial characterization of taro sediments. TARGET AUDIENCES: Taro farmers throughout the state of Hawaii are the primary target audience. The PI has travelled to all of the primary taro growing regions in the state and communicated the objectives and significance of the project as they relate to nitrogen management in taro production. Findings to date have been communicated through a formal presentation to the Kauai Taro Growers Association. PROJECT MODIFICATIONS: Nothing significant to report
Impacts
Anammox activity was very low to negligible in all the cores. Mean 29N production rates ranged from 0 to 1.18 nM/mL/hr. Denitrification potential varied from very low (0.17 nM/mL/hr) to 118 nM/mL/hr with a mean rate of 29.6 nM/mL/hr. Nitrification potential was consistently two orders of magnitude greater than denitrification across the sites with a mean of 6,090 nM/mL/hr and a range of 768 to 23,430 nM/mL/hr. Results so far indicate that while anammox bacteria are present in taro sediments they are not a significant contributor to N loss. The high nitrification potential measured at all sites coupled with a significant potential for denitrification has important ramifications for N management in a taro system. Our data suggest that urea applied to the taro floodwaters and hydrolyzes to NH4+ in the aerobic surface layer is rapidly oxidized to NO3-, which is subsequently denitrified. Current fertilization practices in taro fields in Hawaii place urea or ammonium sulfate in the aerobic layer where it is subject to loss through nitrification coupled with denitrification. The PI has communicated these findings to the Kauai Taro Growers Association and discussions are underway to establish field trials testing alternative N management practices to mitigate the loss of applied N to denitrification.
Publications
- No publications reported this period
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