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Abstract: Conventional cropping systems rely on targeted short-term fertility management, whereas organic systems depend, in part, on long-term increase in soil fertility as determined by crop rotation and management. Such differences influence soil nitrogen (N) cycling and availability through the year. The main objective of this study was to compare nitrous oxide (N2O) emissions from soil under winter wheat (Triticum aestivum L.) within three organic and one conventional cropping system that differed in type of fertilizer, presence of catch crops and proportion of N2-fixing crops. The study was replicated in two identical long-term crop rotation experiments on sandy loam soils under different climatic conditions in Denmark (Flakkebjerg—eastern Denmark and Foulum—western Denmark). The conventional rotation received 165–170kgNha−1 in the form of NH4NO3, while the organic rotations received 100–110kgNha−1 as pig slurry. For at least 11 months, as from September 2007, static chambers were used to measure N2O emissions at least twice every calendar month. Mean daily N2O emissions across the year ranged from 172 to 438μg N m−2 d−1 at Flakkebjerg, and from 173 to 250μg N m−2 d−1 at Foulum. A multiple linear regression analysis showed inter-seasonal variations in emissions (P <0.001), but annual N2O emissions from organic and conventional systems were not significantly different despite the lower N input in organic rotations. The annual emissions ranged from 54 to 137mgNm−2, which corresponded to 0.5–0.8% of the N applied in manure or mineral fertilizer. Selected soil attributes were monitored to support the interpretation of N2O emission patterns. A second multiple linear regression analysis with potential drivers of N2O emissions showed a negative response to soil temperature (P =0.008) and percent water-filled pore space (WFPS) (P =0.052) at Foulum. However, there were positive interactions of both factors with NO3-N, i.e., high N2O emissions occurred during periods when high soil nitrate levels coincided with high soil temperature (P =0.016) or high soil water content (P =0.056). A positive effect (P =0.03) of soil temperature was identified at Flakkebjerg, but the number of soil samplings was limited. Effects of cropping system on N2O emissions were not observed. [Copyright &y& Elsevier]

Soil organic matter is a thoroughly studied yet not fully understood soil constituent. Thevarious turnover rates and qualities of such pools only complicate investigation. However,stable carbon isotopes are a powerful tool to study soil system processes, and hence give thechance of a greater insight into the nature of soil organic matter. Here, it is attempted to constrain the transport and turnover of plant-derivedorganic matter by the use of stable C isotopes taking advantage of C3-C4 plantshifts.Two field sites subjected to a spring barley- to silage maize crop shift were selected for thispurpose. The sites are located just outside Humlum and Vildbjerg in Western Jutland,Denmark. The field sites have had maize crops continuously during the last fourteen andtwelve years respectively. Triplicate soil cores from the two field sites were excavated downto 1.6 m depth. Additionally, soil cores from neighbouring control fields, kept with C3 plantsonly, were collected. The Humlum soil consist of clay-rich meltwater deposits, whereas theVildbjerg soil consist of sandy meltwater-deposits. Grain size distribution is the mostprominent difference between the two sites. Samples were partitioned into depth intervals based on soil characteristics. Bulk soilmaterial was analysed for total C and δ13C by IRMS. Respiration experiments were performed on field moist soil. Subsamples were inserted insealed jars at 20 ˚ C in ambient atmosphere. Headspace gas was sampled at 0, 24, 48, 72 and144 hours. Gas sample concentration and δ13C of the evolved CO2 were analysed byIRMS. For the Vildbjerg site, no significant difference was seen between the maize- and controlfield in bulk soil C-concentration nor in bulk soil δ13C. The Humlum site did not show any significant difference in bulk soil C-concentrationbetween the maize- and the control field. However, the Ap horizon of the maize field had aless negative δ13C value compared to the Ap horizon of the control field. This is inaccordance with the C3-C4 plant shift. The incubations resulted in respired CO2with less negative δ13C from both maize fieldssites compared to the control fields. For the Vildbjerg soil, this discrimination was evident toa depth of 40 cm, whereas for the Humlum soil the difference was seen to at least 80 cm. TheC4 plant-derived organic carbon only constitutes a minor portion of the total Cpool, due to little contribution to the δ13C of the bulk soil. However, the recentlyintroduced maize-derived organic matter proved to respire easily, and hence wasmore labile than the older persistent carbon pool, which held more negative δ13Csignatures. The comparable crop histories of the two maize fields make it plausible that the input ofmaize-derived organic matter has been similar at both sites. Consequently, differences inδ13C signatures between the two sites are likely to be the result of different soil properties. [ABSTRACT FROM AUTHOR]

Rationale: Pulse-labeling with (13)CO2 and the subsequent analysis of (13)C-carbon via isotope ratio mass spectrometry (IRMS) have been shown to be an excellent method to investigate the terrestrial carbon cycle. Improving (13)CO2 manipulation experiments will facilitate our understanding of carbon cycling processes., Methods: A mobile field setup for in situ (13)CO2 pulse-labeling was developed for low vegetation field experiments. Two pulse-labeling experiments were conducted in a Danish heathland in September 2010 (Exp1) and May 2011 (Exp2). A flow-through system was developed where labeling chambers were supplied with (13)CO2-enriched air from a gas reservoir. Reservoir and chamber air was sampled over the course of the experiments and analyzed for CO2 concentration and isotopic composition on a GasBench II interfaced with an isotope ratio mass spectrometer. The soil CO2 efflux and the atom% excess in soil respiration were assessed after the (13)CO2-pulse to verify the setup performance., Results: The carbon dioxide concentrations and (13)CO2 enrichments were stable during the experiments. The CO2 concentrations conformed to the aimed values, whereas the (13)CO2 enrichments were lower than expected. The sources of error for the deviation in observed atom% (13)CO2 values are discussed, and a measurement procedure is suggested for samples highly enriched in (13)C by using adjusted resistor settings of the mass spectrometer. However, more work has to be done. Enrichment patterns in soil respiration agree with published observations indicating satisfactory performance of the developed system., Conclusions: A mobile flow-through system suitable for continuous in situ (13)CO2 pulse-labeling was successfully developed that is easily applicable in remote natural ecosystems., (Copyright © 2013 John Wiley & Sons, Ltd.)

Our understanding of forest biosphere-atmosphere interactions is fundamental for predicting forest ecosystem responses to climatic changes. Currently, however, our knowledge is incomplete partly due to inability to separate the major components of soil CO(2) effluxes, viz. root respiration, microbial decomposition of soil organic matter and microbial decomposition of litter material. In this study we examined whether the delta(13)C characteristics of solid organic matter and respired CO(2) from different soil-C components and root respiration in a Danish beech forest were useful to provide information on the root respiration contribution to total CO(2) effluxes. The delta(13)C isotopic analyses of CO(2) were performed using a FinniganMAT Delta(PLUS) isotope-ratio mass spectrometer coupled in continuous flow mode to a trace gas preparation-concentration unit (PreCon). Gas samples in 2-mL crimp seal vials were analysed in a fully automatic mode with an experimental standard error +/-0.11 per thousand. We observed that the CO(2) derived from root-free mineral soil horizons (A, B(W)) was more enriched in (13)C (delta(13)C range -21.6 to -21.2 per thousand ) compared with CO(2) derived from root-free humus layers (delta(13)C range -23.6 to -23.4 per thousand ). The CO(2) evolved from root respiration in isolated young beech plants revealed a value intermediate between those for the soil humus and mineral horizons, delta(13)C(root) = -22.2 per thousand, but was associated with great variability (SE +/- 1.0 per thousand ) due to plant-specific differences. delta(13)C of CO(2) from in situ below-ground respiration averaged -22.8 per thousand, intermediate between the values for the humus layer and root respiration, but variability was great (SE +/- 0.4 per thousand ) due to pronounced spatial patterns. Overall, we were unable to statistically separate the CO(2) of root respiration vs. soil organic matter decomposition based solely on delta(13)C signatures, yet the trend in the data suggests that root respiration contributed approximately 43% to total respiration. The vertical gradient in delta(13)C, however, might be a useful tool in partitioning respiration in different soil layers. The experiment also showed an unexpected (13)C-enrichment of CO(2) (>3.5 per thousand ) compared with the total-C signatures in the individual soil-C components. This may suggest that analyses of bulk samples are not representative for the C-pools actively undergoing decomposition., (Copyright 2004 John Wiley & Sons, Ltd.)