Urea concentration affects short-term N turnover and N2O production in grassland soil

Summary

Introduction
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. Urea can lead to high ammonium levels in the soil which may influence N dynamics and gaseous emissions. This laboratory study was conducted to investigate short-term effects of urea concentration on N2O emissions.
Methods
Solutions containing 0 (CTL), 5 (LU) and 10 g l-1 urea-N (HU) were added to sieved and repacked soil cores of pasture soil 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). The urea was labelled with 25 atom% 15N. Final soil moisture was 60% WFPS. All treatments were incubated at 14C. Carbon dioxide and N2O 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 destructively sampled for determination of pH, electrical conductivity (EC), inorganic and total N, as well as dissolved organic C and phospholipid fatty acid composition. On Day 3, soil was also subsampled for determination of potential ammonium oxidation (PAO) and denitrifying enzyme activity (DEA). The amount and isotopic composition of soil N, nitrate, N2O and N2 was determined by IR-MS; labelling of N2 was insignificant.
Results and discussion
Accumulated CO2 evolution, corrected for CO2 added in urea, was twice as high from HU as from LU, whereas CO2 from LUN was at the level of the CTL treatment after correction. The lower CO2 emission from LUN was associated with a consistent increase in microbial biomass, as reflected in concentrations of PLFA, which suggested that the lower CO2 evolution rate was due to C assimilation rather than growth inhibition. The EC levels in LU, HU and LUN corresponded to osmotic potentials of -0.05 to -0.12 MPa after 1 d, decreasing to between -0.14 and -0.19 MPa after 9 d. These potentials would not normally be stressful, but a negative interaction with high ammonium concentrations has been observed for ammonium oxidation and, particularly, for nitrite oxidation (Harada and Kai, 1968; Stark and Firestone, 1995). PAO measurements after 3d did not indicate any detrimental effects on ammonium oxidisers. Total recovery of urea-N during the experiment was 801.5% (meanS.E.). 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 were derived from urea. Nitrification was thus delayed in the HU treatment. Here, a dramatic increase in nitrite concentration to 8 mg N kg-1 was observed between 6 and 9 d, suggesting a selective inhibition of nitrite oxidation. The fact that 33-52% of the nitrate produced was derived from soil N, points to a significant initial turnover of the ammonium pool. Total concentrations of ammonium 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. After 6 and 9 d, soil inorganic N corresponded to approximately 100% in LU and LUN, and to 90% in HU. Emissions of N2O during 0-9 d decreased in the order LU>HU>LUN>>CTL and corresponded to 0.1-0.2% of urea-N added. Emission rates for N2O derived from soil were relatively constant in LU, HU and LUN. In HU, the emission of N2O derived from urea increased dramatically between day 6 and 9, parallel to the accumulation of nitrite. Apparently the higher ammonium concentration resulted in accumulation of nitrite which, in turn, led to an accelerated loss of N2O via ammonium oxidation. This implies that management practices which reduce excess N in cattle urine may substantially reduce N2O emissions from grazed pastures.