Stable Isotopes Reveal Rapid Cycling of Soil Nitrogen after Manure Application.

Understanding the fate of applied nitrogen (N) in agricultural soils is important for agronomic, environmental, and human health reasons, but it is methodologically difficult to study at the field scale. Natural abundance stable isotope measurements (δN) were used in this field study with micrometeorological measurements of nitrous oxide (NO) emissions to identify the biogeochemical processes responsible for rapid N transformations immediately after application of liquid dairy manure. Fifteen samplings occurred between 16 Mar. 2012 and 5 Apr. 2013, with a focus on spring manure application (before and after) and a winter snowmelt period. Concentrations and δN values of ammonium (NH), nitrate (NO), NO, and total N were measured throughout the year. Approximately 56 (±7)% of the NH-N applied in the spring could not be accounted for 3 d after manure application and was presumably lost by ammonia volatilization before it was tilled into the soil and/or removed from the inorganic N pool by microbial assimilation. Almost all of the remaining manure-NH (95 ± 1.1%) was converted within 3 wk to NO and NO by nitrification and nitrifier-denitrification, respectively. The in situ N isotope effect for nitrification (ε) was calculated to be -32.0 (±5.3)‰. Overall, field-scale measurements of δN at natural abundance provided valuable information that was used to distinguish sources of NH (manure vs. soil organic N) and to follow the production and consumption of NO and the pathways of NO production in soil.

From the ground up: global nitrous oxide sources are constrained by stable isotope values.

Rising concentrations of nitrous oxide (N2O) in the atmosphere are causing widespread concern because this trace gas plays a key role in the destruction of stratospheric ozone and it is a strong greenhouse gas. The successful mitigation of N2O emissions requires a solid understanding of the relative importance of all N2O sources and sinks. Stable isotope ratio measurements (δ15N-N2O and δ18O-N2O), including the intramolecular distribution of 15N (site preference), are one way to track different sources if they are isotopically distinct. 'Top-down' isotope mass-balance studies have had limited success balancing the global N2O budget thus far because the isotopic signatures of soil, freshwater, and marine sources are poorly constrained and a comprehensive analysis of global N2O stable isotope measurements has not been done. Here we used a robust analysis of all available in situ measurements to define key global N2O sources. We showed that the marine source is isotopically distinct from soil and freshwater N2O (the continental source). Further, the global average source (sum of all natural and anthropogenic sources) is largely controlled by soils and freshwaters. These findings substantiate past modelling studies that relied on several assumptions about the global N2O cycle. Finally, a two-box-model and a Bayesian isotope mixing model revealed marine and continental N2O sources have relative contributions of 24-26% and 74-76% to the total, respectively. Further, the Bayesian modeling exercise indicated the N2O flux from freshwaters may be much larger than currently thought.

Stable oxygen isotope ratios of nitrate produced from nitrification: (18)O-labeled water incubations of agricultural and temperate forest soils.

In many nitrate (NO(3)(-)) source partitioning studies, the delta(18)O value for NO(3)(-) produced from nitrification is often assumed to reflect the isotopic compositions of environmental water (H(2)O) and molecular oxygen (O(2)) in a 2:1 ratio. Most studies that have measured or observed this microbial endmember have found that the delta(18)O-NO(3)(-) was more positive (up to +15 per thousand higher) than the assumed value. Current understanding of the mechanism(s) responsible for this discrepancy is limited. Incubations of one temperate forest soil (organic) and two agricultural soils (mineral) were conducted with (18)O-labeled H(2)O to apportion the sources of oxygen in NO(3)(-) generated from nitrification. The NO(3)(-) produced in all soils had delta(18)O values that could not be explained by a simple endmember mixing ratio of 2:1. A more comprehensive model describing the formation of microbial NO(3)(-) was developed, which accounts for oxygen exchange between H(2)O and NO(2)(-), and includes terms for kinetic and equilibrium isotope effects. Oxygen isotope exchange (i.e., the fraction of NO(3)(-)-oxygen that originates from the abiotic exchange of H(2)O and NO(2)(-)) varied widely between the temperate forest soil (0.37) and the two agricultural soils (0.52 and 0.88). At present, the microbial endmember for nitrification cannot be successfully predicted.