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1. Background & Objectives The nitrification inhibitor dicyandiamide (DCD) is used for the purpose of increasing nitrogen efficiency in agriculture. It slows down the conversion of ammonium (NH4+) to nitrate (NO3-), which can reduce NO3- leaching and nitrous oxide (N2O) emissions. However, a detailed understanding of how DCD affects the complex nitrogen (N) transformations taking place in the soil after addition of slurry is lacking. The impact on the N-cycling microbial communities remains unclear and the extent to which N2O and N2 emissions are affected by DCD is not well known. The main objectives of this study were to determine the effects of DCD, after cattle slurry application, on (1) net and gross N transformations, (2) emissions of N2O and N2, and (3) the abundance of functional genes involved in nitrification and denitrification.
2. Materials & Methods A microcosm study was carried out in the laboratory, using three contrasting Irish grassland soils.
Cattle slurry (6.7 tonnes ha-1), with or without amendment with DCD (4.4 kg ha-1) was applied on top of sieved soil samples that were repacked to a bulk density of 0.88 g cm-3. Either the NH4+ or the NO3- pool was isotopically labelled, in parallel N treatments, for the quantification of gross N transformation rates. The amounts of water in the treatments were adjusted to achieve a soil waterfilled pore space of 65 % after application and the samples were incubated at 15 ºC. Gas sampling and/or soil extraction was done at 0 h, 2 h, 3.5 h, 5.5 h, 7.5 h, 2 d, 6 d, 10 d, 15 d and 20 d after amendment. Gross N transformations were obtained using a 15N-tracing model (Müller et al., 2007).
PLFA analysis was used to provide a fingerprint of the viable microbial community structure, and abundance of ammonia oxidizers and denitrifiers were monitored by targeting functional genes, using qPCR.
3. Results & Discussion Net nitrification, over 20 days, decreased by 89 % (P 0.05) when cattle slurry was amended with DCD before application to soil (Figure 1). The inhibition of nitrification by DCD also significantly (P 0.05) reduced cumulative emissions of N2O and N2, by 27 and 52 %, respectively (Figure 2).
The N2O/(N2O+N2) ratio was not affected. Further results on gross N transformation rates and effects of DCD on the N-cycling microbial communities will be presented.
Nitrogen Workshop 2012 Figure 1. Mean NH4+ and NO3- concentrations in grassland soil, over 20 days after application of cattle slurry, with and without a DCD amendment. The graph shows averages from three sites in Ireland.
Figure 2. Cumulative N2O and N2 emissions, over 20 days after application of cattle slurry, with and without a DCD amendment.
The graph shows averages from three grassland sites in Ireland.
4. Conclusion In this laboratory study, DCD was shown to be an efficient inhibitor of nitrification when used as an amendment to cattle slurry applied to grassland soil. The decrease in nitrification also led to significant decreases in N2O and N2 emissions. Further results on gross N transformation rates and effects of DCD on the N-cycling microbial communities will be presented.
References Müller C., Rütting T., Kattge J., Laughlin R.J. and Stevens R.J. 2007. Estimation of parameters in complex 15N tracing models by Monte Carlo sampling. Soil Biology & Biochemistry 39, 715-726.
The fate of urine nitrogen with use of a nitrification inhibitor Selbie, D.a,b, Cameron, K.C.b, Di, H.J.b, Moir, J.L.b, Lanigan, G.a, Laughlin R.J. c, Richards, K.G.a a Teagasc, Johnstown Castle, Environmental Research Centre, Co. Wexford, Ireland b Soil & Physical Sciences Dept., Lincoln University, Canterbury, New Zealand c Agri-Food and Biosciences Institute, Newforge Lane, Belfast, BT9 5PX, Northern Ireland
1. Background & Objectives Pasture-grazed ruminants in Ireland contribute a significant proportion of nitrogen (N) loss to the environment through excreta deposition. Feed N utilisation by the ruminant animal is low with 60-90% of ingested N returned to the soil/pasture system in the excreta, particularly in the urine (Haynes and Williams, 1993). The urine N loading rate in a single cattle urine patch is approximately 1000 kg N ha-1 (Haynes and Williams, 1993). N balance studies have estimated the fate of urine N on a range of soils (Clough et al., 1998). Application of dicyandiamide (DCD) nitrification inhibitor has consistently reduced N leaching and N2O emissions from urine patches (Di and Cameron, 2007; de Klein et al., 2011), but produced variable pasture N responses (Di and Cameron, 2007; Zaman and Blennerhassett, 2010). A N balance study was conducted on grassland lysimeters in Ireland to investigate the fate of urine N with and without the application of DCD nitrification inhibitor.
2. Materials & Methods Urine labelled with the 15N isotope was applied to soil monolith lysimeters at a rate of 1000 kg N ha-1 with and without DCD in late autumn. DCD was applied in solution form at 30 kg DCD ha-1 in two split applications, the first following urine application in late autumn and the second in late winter. Drainage water was analysed using standard methods for nitrate (NO3-), ammonium (NH4+) and total N. Nitrous oxide (N2O) and di-nitrogen (N2) were sampled from static chambers (Hutchinson and Mosier, 1981) and quantified by gas chromatography for N2O and by isotope ratio mass spectrometry (IRMS) for N2. Dissolved N2O in drainage water was extracted using helium gas and analysed for N2O above. Pasture was harvested monthly and analysed for total N content using standard methods. Soil cores were extracted from lysimeters at the end of the experiment from varying depths and analysed for NO3- and NH4+ and total N. 15N enrichment of water and soil extracts was estimated using a diffusion method (Chen and Dittert, 2008) and together with pasture and gas, analysed using IRMS.
3. Results & Discussion Mass N recoveries from water, gas and pasture fractions are shown in Table 1. In terms of mass recovery, pasture N uptake was the largest sink (44.5%) under a 1000 kg N ha-1 urine patch without DCD, followed by N leaching (24.3%) and then by gaseous emissions (1.9%).
The incomplete mass balance in the U1000 treatment shows that 29.3% of urine N was unaccounted for, and may have been immobilized in the soil. Application of DCD appeared to reduce total N in water, gas and pasture fractions, giving rise to an unaccounted for N fraction of 35.2% of N applied, which was larger than 29.3% where no DCD was applied.
Nitrogen Workshop 2012 Table 1. Mass balance of urine applied at 1000 kg N ha-1 with and without DCD nitrification inhibitor
4. Conclusions The urine N mass balance showed that pasture N uptake accounted for the largest proportion of the urine N applied, followed by N leaching and then gaseous emissions. Application of DCD reduced N found in water, gas and pasture, leading to a greater fraction of N unaccounted for. A complete 15N balance will be presented at the conference which includes the soil fraction as well as water, gas and pasture fractions.
References Chen, R.R. and Dittert, K. 2008. Diffusion technique for N-15 and inorganic N analysis of low-N aqueous solutions and Kjeldahl digests. Rapid Communications in Mass Spectrometry 22, 1727-1734.
Clough, T.J., Ledgard, S.F., Sprosen, M.S. and Kear, M.J. 1998. Fate of N-15 labelled urine on four soil types.
Plant and Soil 199, 195-203.
de Klein, C.A.M., Cameron, K.C., Di, H.J., Rys, G., Monaghan, R.M. and Sherlock, R.R. 2011. Repeated annual use of the nitrification inhibitor dicyandiamide (DCD) does not alter its effectiveness in reducing N2O emissions from cow urine. Animal Feed Science and Technology 166-167, 480-491.
Di, H.J. and Cameron, K.C. 2007. Nitrate leaching losses and pasture yields as affected by different rates of animal urine nitrogen returns and application of a nitrification inhibitor - a lysimeter study. Nutrient Cycling in Agroecosystems 79, 281-290.
Haynes, R.J. and Williams, P.H. 1993. Nutrient cycling and soil fertility in the grazed pasture ecosystem.
Advances in Agronomy 46, 119-199.
Hutchinson, G.L. and Mosier, A.R. 1981. Improved soil cover method for field measurement of nitrous-oxide fluxes. Soil Science Society of America Journal 45, 311-316.
Zaman, M. and Blennerhassett, J.D. 2010. Effects of the different rates of urease and nitrification inhibitors on gaseous emissions of ammonia and nitrous oxide, nitrate leaching and pasture production from urine patches in an intensive grazed pasture system. Agriculture Ecosystems & Environment 136, 236-246.
Search for the missing N: Excess N2 in groundwater and streams Fox, R.J.a, Fisher, T.R.a, Gustafson, A.B. a, Jordan, T.E.b, Knee, K.b, Brenner, D. b a Horn Point Laboratory, Center for Environmental Science, University of Maryland, Cambridge MD 21613, USA b Smithsonian Environmental Research Center, PO Box 28, Edgewater MD 21037, USA
1. Background & Objectives The majority of known nitrogen inputs to watersheds cannot be accounted for by stream discharge. Howarth et al. (1996), Jordan and Weller (1996), Boyer et al. (2002), and Schaefer and Alber (2007) have shown that export of N from North American rivers draining into the Atlantic accounts for only 9-25% of the net anthropogenic N inputs (NANI) to their watersheds; the missing N (75-91% of NANI) must be either (1) stored in soils or biomass or (2) lost to the atmosphere via denitrification or other biological N2 and N2O production (Van Breeman et al., 2002). We hypothesize that the majority of the missing N is converted to biological N2 within the watershed. Here we present evidence for the biological production of N2 and N2O in soils of a Mid-Atlantic coastal plain watershed in North America.
3. Results & Discussion Excess N2 and N2O were common in all three environments. Groundwater under well-drained agricultural fields has high O2 (50-80% Figure 2. % O2 saturation was inversely related to saturation), high NO3- (500-1000 M), little excess N2- N in flowing stream water in fall 2009.
Nitrogen Workshop 2012 excess N2-N (20-50 M, e.g, CFF3, CFC2, CFC1 in Fig. 1), and low to moderate N2O-N (0.02-4.8 M). In contrast, under wetlands with hydric soils near agricultural fields, there are low concentrations of O2 (0-50% saturation), low NO3- (0-100 M), high excess N2-N (50-500 M, e.g, EFWET1A, JLAG4, JLAG3 & 3A, Fig. 1), and variable N2O-N (0.02 to 75 M). In flowing stream waters excess N2-N had moderate concentrations (0-100 M) which were inversely related to O2 (Figure 2). In the vadose zone direct estimates of excess N2 were made but were variable due to the assumption of Figure 3. Excess N2 profiles measured in a constant atmospheric Ar for calculations (Figure 3). riparian area next to an agricultural field using the Higher partial pressures of N2 were always present CIMS (vadose zone) and MIMS (groundwater) methods. Concentrations are expressed in partial in groundwater, and decreased towards the soil pressure units to compare the dissolved and gas surface (Figure 3). The CIMS method requires phase concentrations. The lower three points are further development in order to make more accurate within groundwater, and the upper points are estimates of excess N2 concentrations within vadose within the vadose zone. The depth to the water zone gas, but currently we are able to measure small table varies significantly and the water table depth for each sampling time is denoted as a labelled significant increases and decreases in N2/Ar.
4. Conclusion The measured concentrations of N2 in groundwater, stream water, and the vadose zone show strong evidence that denitrification or N2 production through other processes is the source for the majority of the missing N within this watershed. In future research we hope to improve our estimates of N2 and N2O to the atmosphere in order to test the hypothesis that the missing N is converted biologically into N2 and N2O, which then evades into the atmosphere through stream and soil surfaces. We also hope to integrate these measurements spatially to estimate fluxes of biological N2 and N2O to the atmosphere.
References Boyer, E.W., C.L. Goodale, N.A. Jaworki, R.W. Howarth. 2002. Anthropogenic nitrogen sources and relationships to riverine nitrogen export in the northeastern U.S.A. Biogeochemistry. 57/58, 137-169.
Fox, R. J. 2011. Dynamics of metabolic gases in groundwater and the vadose zone of soils on Delmarva. PhD thesis, University of Maryland, 318 pages.
Howarth, R.W., G. Billen, D. Swaney, A. Townsend, N. Jaworski, K. Lajtha, J.A. Downing, R. Elmgren, N. Caraco, T.
Jordan, F. Berendse, J. Freney, V. Kudeyardov, P. Murdoch, and Z. Zhao-Liang. 1996. Regional nitrogen budgets and riverine N & P fluxes for the drainages to the North Atlantic Ocean: Natural and human influences. Biogeochemistry.
Jordan, T.E., and D. W. Weller. 1996. Human contributions to terrestrial nitrogen flux. BioScience. 46(9):655-664.
Kana, T.M., C. Darkangelo, M.D. Hunt, J.B. Oldham, G.E. Bennett, and J.C. Cornwell. 1994. Membrane inlet mass spectrometer for rapid high-precision determination of N2, O2 and Ar in environmental water samples. Analytical Chemistry. 66(23), 4166-4170.
Schaefer, S. C. and M. Alber. 2007. Temperature controls a latitudinal gradient in the proportion of watershed nitrogen exported to coastal ecosystems. Biogeochemistry. 85, 333-346.
Van Breemen, N., E. W. Boyer, C. L. Goodale, N. A. Jaworski, K. Paustian, S. P. Seitzinger, K. Lajtha, B. Mayer, D.
Van Dam, R. W. Howarth, K. J. Nadelhoffer, M. Eve, and G. Billen. 2002. Where did all the nitrogen go? Fate of nitrogen inputs to large watersheds in the northeastern U.S.A. Biogeochemistry. 57/58, 267-293.
Characterising dissolved organic matter flux in UK freshwaters: Sources, Transport and Delivery Yates, C.A. Johnes, P. J. Spencer, R. G. M.
Department of Geography and Environmental Science, University of Reading, Reading, UK