Urea concentration affects short-term N turnover and N 2 O production in grassland soil Søren O. Petersen 1*, S. Stamatiadis 2, C. Christofides 2, S. Yamulki 3 and R. Bol 3 1 Danish Institute of Agricultural Sciences, Dept. of Agroecology, Tjele, DK; 2 GAIA Centre, Ecology & Biotechnology Laboratory, Kifissia, GR; 3 Institute of Grassland and Environmental Research, Soils & Agroecology Dept., North Wyke, UK Background For Western Europe it is estimated that, on average, 8% of total N excreted by dairy cattle is deposited during grazing (IPCC, 1997). The intake and excretion of N is influenced by factors such as feed composition, lactation stage and pasture quality, and the excretion of excess N as urea in the urine can therefore vary considerably. It is well-known that plant roots may be scorched by urine deposition due to high levels of ammonia in the soil following urea hydrolysis. We hypothesized that ammonia could also be a stress factor for soil organisms, including nytrifying and denitrifying bacteria, and hence influence N 2 O emissions. This laboratory study was conducted to investigate short-term effects of urea concentration on N 2 O emissions and mechanisms behind. Experimental set-up Solutions containing 0 (CTL), 5 (LU) and 10 g L -1 urea-N (HU) were added to sieved and repacked soil cores with pasture soil (sandy loam with 2.7% C, 0.18% N, pH CaCl2 of 5.5, and CEC of 87 meq kg -1 ) at a rate of 4 L m -2. Also, 5 g L -1 urea-N was added to soil amended with 50 μg cm -3 nitrate-N in order to simulate N turnover in overlapping urine spots (LUN). A control with nitrate alone (N) was also included. The urea was labelled with 25 atom% 15 N. Final soil moisture was 60% WFPS. All treatments were incubated at 14  C. Carbon dioxide and N 2 O evolution rates were determined after c. 0.2, 0.5, 1, 3, 6 and 9 d. At the four last samplings, the replicates used for gas flux measurements were then destructively sampled for determination of the variables listed below. Regulation of N 2 O emissions Emissions of N 2 O during 0-9 d decreased in the order LU>HU>LUN>>CTL=N (Fig. 5). In HU, the emission of N 2 O increased dramatically between day 6 and 9, parallel to a dramatic accumulation of nitrite in this treatment, which indicated an imbalance between NH 4 + and NO 2 - oxidation (Fig. 6). The EC levels in LU, HU and LUN corresponded to osmotic potentials of to MPa after 1 d, decreasing to between and MPa after 9 d. A negative interaction between osmotic stress and high NH 4 + concentrations has been observed, particularly for nitrite oxidation (Harada and Kai, 1968). The level of NH 3 (aq) calculated for the HU treatment suggested that nitrification rates could be significantly reduced (Monaghan and Barraclough, 1992), as was also observed in this study (cf Fig. 1). The potential for ammonium oxidation (PAO) was not, however, reduced in HU compared to the other urea treatments (Fig. 7), indicating that the inhibition of NH 4 + oxidation in the soil was reversible. Denitrifying enzyme activity (DEA) was clearly affected by the urea amendment, probably as a result of the change in pH (Simek et al., 2002). The time course of N 2 O emissions, and the correspondence with nitrite accumulation in HU indicates that ammonium oxidation was the main source of N 2 O in the system investigated. The N dynamics observed were consistent with nitrifier-denitrification (Wrage et al., 2001). Urine composition and N 2 O emission potential Accumulated N 2 O emisssions in this short-term study corresponded to only % of urea-N added, but emissions could be higher from pastures on more fine-textured soil, or pastures with fertilizer inputs. There were indications of microbial stress at high urinary urea concentration, and evidence for at interaction with N 2 O emissions. Management practices which reduce the level of surplus N excreted during grazing may reduce the potential for N 2 O emissions induced by microbial stress. This study was conducted as part of the FP5 project ’Greenhouse Gas Mitigation for Organic and Conventional Dairy Production’ (MIDAIR). It also contributes to the Danish project ’Dinitrogen Fixation and Nitrous Oxide Losses in Organically Farmed Grass-Clover Pastures: An Integrated Experimental and Modelling Approach’. Urea-N recovery Total recovery of urea-N during the experiment was 84  1% (Fig. 1). Soil nitrate accumulated exponentially to concentrations of 90, 60 and 100 mg N kg -1 in LU, HU and LUN after 9 d. Of this, 47, 40 and 58 mg N kg -1 was derived from urea. Nitrification was thus delayed in the HU treatment. Between 33 and 52% of the nitrate produced was derived from soil N, although initial soil NH 4 + was <5 mg N kg -1. This suggests a significant initial turnover of the NH 4 + pool. Total concentrations of NH 4 + after 1 d corresponded to 51-61% of urea-N added, and after 3 d to 80-85%. The transient disappearance could be due to microbial assimilation in response to the sudden decrease in osmotic potential. References: Harada, T. and Kai, H. (1968) Studies on the environmental conditions controlling nitrification in soil. Soil Sci. Plant Nutr. 14: Monaghan, R.M. and Barraclough, D. (1992) Some chemical and physical factors affecting the rate and dynamics of nitrification in urine-affected soil. Plant Soil 143: Simek, M., Jisova, L. and Hopkins, D.W. (2002) What is the so-called optimum pH for denitrification in soil? Soil Biol. Biochem. 34: Wrage, N., Velthof, G.L., van Beusichem, M.L. and Oenema, O. (2001) Role of nitrifier denitrification in the production of nitrous oxide. Soil Biol. Biochem. 33: Fig. 5. Nitrous oxide evolution rates. Fig. 6. Nitrite concentrations in the soil Fig. 7. Rates of potential NH 4 + oxidation and denitrifying enzyme activity on Day 3. Fig. 1. Recovery of 15N after 3, 6 and 9 d in urea-amended soil. CO 2 evolution and microbial growth Accumulated CO 2 evolution, disregarding the calculated contribution from CO 2 added in urea, was twice as high from HU as from LU (Fig. 2). The reason for the lower CO 2 emission from LUN is not clear. The level of microbial biomass, as reflected in concentrations of PLFA (Fig. 3), was higher in LUN than in the other treatments, but the absence of higher respiration rates indicates that this may have been due to a difference in extractability. In the HU treatment, an initial decrease in biomass was followed by a phase (Day 3 to 9) with extensive growth. The ratios of cy17:0/16:1  7c (Fig. 4) also indicated that the high urea concentration resulted in stress followed by rapid microbial turnover. Fig. 2. Accumlated CO2 evolution per treatment. Fig. 3. Concentrations of PLFA, Fig. 4. Ratios of cy17:0-to- used here as an index of biomass. 16:1  7c (a stress biomarker). Archived at