Quantifying N2O reduction to N2 during denitrification in soils via isotopic mapping approach: Model evaluation and uncertainty analysis.

The last step of denitrification, i.e. the reduction of N2O to N2, has been intensively studied in the laboratory to understand the denitrification process, predict nitrogen fertiliser losses, and to establish mitigation strategies for N2O. However, assessing N2 production via denitrification at large spatial scales is still not possible due to lack of reliable quantitative approaches. Here, we present a novel numerical "mapping approach" model using the δ15Nsp/δ18O slope that has been proposed to potentially be used to indirectly quantify N2O reduction to N2 at field or larger spatial scales. We evaluate the model using data obtained from seven independent soil incubation studies conducted under a He-O2 atmosphere. Furthermore, we analyse the contribution of different parameters to the uncertainty of the model. The model performance strongly differed between studies and incubation conditions. Re-evaluation of the previous data set demonstrated that using soils-specific instead of default endmember values could largely improve model performance. Since the uncertainty of modelled N2O reduction was relatively high, further improvements to estimate model parameters to obtain more precise estimations remain an on-going matter, e.g. by determination of soil-specific isotope fractionation factors and isotopocule endmember values of N2O production processes using controlled laboratory incubations. The applicability of the mapping approach model is promising with an increasing availability of real-time and field based analysis of N2O isotope signatures.

Improvement of the 15 N gas flux method for in situ measurement of soil denitrification and its product stoichiometry.

RATIONALE: Field measurement of denitrification in agricultural ecosystems using the 15 N gas flux method has been limited by poor sensitivity because current isotope ratio mass spectrometry is not precise enough to detect low 15 N2 fluxes in the presence of a high atmospheric N2 background. For laboratory studies, detection limits are improved by incubating soils in closed systems and under N2 -depleted atmospheres. METHODS: We developed a new procedure to conduct the 15 N gas flux method suitable for field application using an artificially N2 -depleted atmosphere to improve the detection limit at the given precision of mass spectrometry. Laboratory experiments with and without 15 N-labelling and using different flushing strategies were conducted to develop a suitable field method. Subsequently, this method was tested in the field and results were compared with those obtained from the conventional 15 N gas flux method. RESULTS: Results of the two methods were in close agreement showing that the denitrification rates determined were not biased by the flushing procedure. Best sensitivity for N2 + N2 O fluxes was 10 ppb, which was 80-fold better than that of the reference method. Further improvement can be achieved by lowering the N2 background concentration below the values established in the present study. CONCLUSIONS: In view of this progress in sensitivity, the new method will be suitable to measure denitrification dynamics in the field beyond peak events.

N2O source partitioning in soils using (15)N site preference values corrected for the N2O reduction effect.

RATIONALE: The aim of this study was to determine the impact of isotope fractionation associated with N2O reduction during soil denitrification on N2O site preference (SP) values and hence quantify the potential bias on SP-based N2O source partitioning. METHODS: The N2O SP values (n = 431) were derived from six soil incubation studies in N2-free atmosphere, and determined by isotope ratio mass spectrometry (IRMS). The N2 and N2O concentrations were measured directly by gas chromatography. Net isotope effects (NIE) during N2O reduction to N2 were compensated for using three different approaches: a closed-system model, an open-system model and a dynamic apparent NIE function. The resulting SP values were used for N2O source partitioning based on a two end-member isotopic mass balance. RESULTS: The average SP0 value, i.e. the average SP values of N2O prior to N2O reduction, was recalculated with the closed-system model, resulting in -2.6 ‰ (±9.5), while the open-system model and the dynamic apparent NIE model gave average SP0 values of 2.9 ‰ (±6.3) and 1.7 ‰ (±6.3), respectively. The average source contribution of N2O from nitrification/fungal denitrification was 18.7% (±21.0) according to the closed-system model, while the open-system model and the dynamic apparent NIE function resulted in values of 31.0% (±14.0) and 28.3% (±14.0), respectively. CONCLUSIONS: Using a closed-system model with a fixed SP isotope effect may significantly overestimate the N2O reduction effect on SP values, especially when N2O reduction rates are high. This is probably due to soil inhomogeneity and can be compensated for by the application of a dynamic apparent NIE function, which takes the variable reduction rates in soil micropores into account.

Soil denitrification potential and its influence on N2O reduction and N2O isotopomer ratios.

RATIONALE: N2O isotopomer ratios may provide a useful tool for studying N2O source processes in soils and may also help estimating N2O reduction to N2. However, remaining uncertainties about different processes and their characteristic isotope effects still hamper its application. We conducted two laboratory incubation experiments (i) to compare the denitrification potential and N2O/(N2O+N2) product ratio of denitrification of various soil types from Northern Germany, and (ii) to investigate the effect of N2O reduction on the intramolecular (15)N distribution of emitted N2O. METHODS: Three contrasting soils (clay, loamy, and sandy soil) were amended with nitrate solution and incubated under N2 -free He atmosphere in a fully automated incubation system over 9 or 28 days in two experiments. N2O, N2, and CO2 release was quantified by online gas chromatography. In addition, the N2O isotopomer ratios were determined by isotope-ratio mass spectrometry (IRMS) and the net enrichment factors of the (15)N site preference (SP) of the N2O-to-N2 reduction step (η(SP)) were estimated using a Rayleigh model. RESULTS: The total denitrification rate was highest in clay soil and lowest in sandy soil. Surprisingly, the N2O/(N2O+N2) product ratio in clay and loam soil was identical; however, it was significantly lower in sandy soil. The IRMS measurements revealed highest N2O SP values in clay soil and lowest SP values in sandy soil. The η(SP) values of N2O reduction were between -8.2 and -6.1‰, and a significant relationship between δ(18)O and SP values was found. CONCLUSIONS: Both experiments showed that the N2O/(N2O+N2) product ratio of denitrification is not solely controlled by the available carbon content of the soil or by the denitrification rate. Differences in N2O SP values could not be explained by variations in N2O reduction between soils, but rather originate from other processes involved in denitrification. The linear δ(18)O vs SP relationship may be indicative for N2O reduction; however, it deviates significantly from the findings of previous studies.

Novel laser spectroscopic technique for continuous analysis of N2O isotopomers--application and intercomparison with isotope ratio mass spectrometry.

RATIONALE: Nitrous oxide (N(2)O), a highly climate-relevant trace gas, is mainly derived from microbial denitrification and nitrification processes in soils. Apportioning N(2)O to these source processes is a challenging task, but better understanding of the processes is required to improve mitigation strategies. The N(2)O site-specific (15)N signatures from denitrification and nitrification have been shown to be clearly different, making this signature a potential tool for N(2)O source identification. We have applied for the first time quantum cascade laser absorption spectroscopy (QCLAS) for the continuous analysis of the intramolecular (15)N distribution of soil-derived N(2)O and compared this with state-of-the-art isotope ratio mass spectrometry (IRMS). METHODS: Soil was amended with nitrate and sucrose and incubated in a laboratory setup. The N(2)O release was quantified by FTIR spectroscopy, while the N(2)O intramolecular (15)N distribution was continuously analyzed by online QCLAS at 1 Hz resolution. The QCLAS results on time-integrating flask samples were compared with those from the IRMS analysis. RESULTS: The analytical precision (2σ) of QCLAS was around 0.3‰ for the δ(15)N(bulk) and the (15)N site preference (SP) for 1-min average values. Comparing the two techniques on flask samples, excellent agreement (R(2)= 0.99; offset of 1.2‰) was observed for the δ(15)N(bulk) values while for the SP values the correlation was less good (R(2 )= 0.76; offset of 0.9‰), presumably due to the lower precision of the IRMS SP measurements. CONCLUSIONS: These findings validate QCLAS as a viable alternative technique with even higher precision than state-of-the-art IRMS. Thus, laser spectroscopy has the potential to contribute significantly to a better understanding of N turnover in soils, which is crucial for advancing strategies to mitigate emissions of this efficient greenhouse gas.

Effects of active silicon uptake by rice on 29Si fractionation in various plant parts.

Rice (Oryza sativa L.) accumulates large amounts of silicon which improves its growth and health due to enhanced resistance to biotic and abiotic stresses. Silicon uptake and loading to xylem in rice are predominantly active processes performed by transporters encoded by the recently identified genes Lsi1 (Si influx transporter gene) and Lsi2 (Si efflux transporter gene). Silicon deposition in rice during translocation to upper plant tissues is known to discriminate against the heavier isotopes (29)Si and (30)Si, resulting in isotope fractionation within the plant. We analyzed straw and husk samples of rice mutants defective in Lsi1, Lsi2 or both for silicon content and delta(29)Si using isotope ratio mass spectrometry (IRMS) and compared these results with those for the corresponding wild-type varieties (WT). The silicon content was higher in husk than in straw. All the mutant rice lines showed clearly lower silicon content than the WT lines (4-23% Si of WT). The delta(29)Si was lower in straw and husk for the uptake defective mutant (lsi1) than for WT, albeit delta(29)Si was 0.3 per thousand higher in husk than in straw in both lines. The effect of defective efflux (lsi2) differed for straw and husk with higher delta(29)Si in straw, but lower delta(29)Si in husk while WT showed similar delta(29)Si in both fractions. These initial results show the potential of Si isotopes to enlighten the influence of active uptake on translocation and deposition processes in the plant.