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The alpine meadow ecosystem is one of the major vegetation biomes on the Qinghai-Tibetan Plateau, which hold substantial quantities of soil organic carbon. Pronounced grassland degradations (induced by overgrazing/climate change and further exacerbated by the subterranean rodent activities) that have widely occurred in this ecosystem may significantly alter the non-growing season carbon turnover processes such as carbon dioxide (CO 2 ) efflux, but little is known about how the non-growing season CO 2 emissions respond to the degradation (particularly the exacerbated degradations by plateau zokor), as most previous studies have focused primarily on the growing season. In this study, the effects of four degradation levels (i.e., the healthy meadow (HM), degraded patches (DP), 2-year-old zokor mounds (ZM2), and current-year zokor mounds (ZM1)) on CO 2 emissions and corresponding environmental and agronomic variables were investigated over the two non-growing seasons under contrasting climatic conditions (a normal season in 2013-2014 and a "warm and humid" season in 2014-2015). The temporal variation in the non-growing season CO 2 emissions was mainly regulated by soil temperature, while increasing degradation levels reduced the temperature sensitivity of CO 2 emissions due to a reduction in soil water content. The cumulative CO 2 emissions across the non-growing season were 587-1283 kg C ha -1 for all degradation levels, which varied significantly (p < 0.05) interannually. The degradation of alpine meadows significantly (p < 0.05) reduced the vegetation cover and aboveground net primary productivity as well as the belowground biomass, which are typically thought to decrease soil CO 2 emissions. However, the non-growing season CO 2 emissions for the degraded meadow, weighted by the areal extent of the DP, ZM2, and ZM1, were estimated to be 641-1280 kg C ha -1 , which was significantly higher (p < 0.05) as compared with the HM in the warm and humid season of 2014-2015 but not in the normal season of 2013-2014. Additionally, grassland degradation substantially increased the productivity-scaled non-growing season CO 2 emissions, which showed an exponential trend with increasing degradation levels. These results suggest that there is a strong connection between grassland degradation and soil carbon loss, e.g., in the form of CO 2 release, pointing to the urgent need to manage degraded grassland restoration that contributes to climate change mitigation.

The gas-flow-soil-core (GFSC) technique allows to directly measure emission rates of denitrification gases of incubated soil cores. However, the technique was still suffering some drawbacks such as inadequate accuracy due to asynchronous detection of dinitrogen (N2) and other gases and low measurement frequency. Furthermore, its application was limited due to intensive manual operation. To overcome these drawbacks, we updated the GFSC system as described by Wang et al. (2011) by (a) using both a chemiluminescent detector and a gas chromatograph detector to measure nitric oxide (NO), (b) synchronizing the measurements of N2, NO, nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4), and (c) fully automating the sampling/analysis of all the gases. These technical modifications significantly reduced labor demands by at least a factor of two, increased the measurement frequency from 3 to 6 times per day and resulted in remarkable improvements in measurement accuracy (with detection limits of 0.5, 0.01, 0.05, 2.3 and 0.2μgN or Ch(-1)kg(-1)ds, or 17, 0.3, 1.8, 82, and 6μgN or Cm(-2)h(-1), for N2, N2O, NO, CO2, and CH4, respectively). In some circumstances, the modified system measured significantly more N2 and CO2 and less N2O and NO because of the enhanced measurement frequency. The modified system distinguished the differences in emissions of the denitrification gases and CO2 due to a 20% change in initial carbon supplies. It also remarkably recovered approximately 90% of consumed nitrate during incubation. These performances validate the technical improvement, and indicate that the improved GFSC system may provide a powerful research tool for obtaining deeper insights into the processes of soil carbon and nitrogen transformation during denitrification., (Copyright © 2013. Published by Elsevier Ltd.)

Here we describe a newly designed system with three stand-alone working incubation vessels for simultaneous measurements of N(2), N(2)O, NO, and CO(2) emissions from soil. Due to the use of a new micro thermal conductivity detector and the redesign of vessels and gas sampling a so-far unmatched sensitivity (0.23 μg N(2)-N h(-1) kg(-1) ds or 8.1 μg N(2)-N m(-2) h(-1)) for detecting N(2) gas emissions and repeatability of experiments could be achieved. We further tested different incubation methods to improve the quantification of N(2) emission via denitrification following the initialization of soil anaerobiosis. The best results with regard to the establishment of a full N balance (i.e., the changes in mineral N content being offset by simultaneous emission of N gases) were obtained when the anaerobic soil incubation at 25 °C was preceded by soil gas exchange under aerobic conditions at a lower incubation temperature. The ratios of N and C gas emission changed very dynamically following the initialization of anaerobiosis. For soil NO(3)(-) contents of 50 mg N kg(-1) dry soil (ds) and dissolved organic carbon (DOC) concentrations of approximately 300 mg C kg(-1) ds, the cumulative emissions of N(2), N(2)O, and NO were 24.3 ± 0.1, 12.6 ± 0.4, and 10.1 ± 0.3 mg N kg(-1) ds, respectively. Thus, N gas emissions accounted (on average) for 46.2% (N(2)), 24.0% (N(2)O), and 19.2% (NO) of the observed changes in soil NO(3)(-). The maximum N(2) emission reached 1200 μg N h(-1) kg(-1) ds, whereas the peak emissions of N(2)O and NO were lower by a factor of 2-3. The overall N(2):N(2)O and NO:N(2)O molar ratios were 1.6-10.0 and 1.6-2.3, respectively. The measurement system provides a reliable tool for studying denitrification in soil because it offers insights into the dynamics and magnitude of gaseous N emissions due to denitrification under various incubation conditions.

A method for measuring NO and NO2 exchanges between cultivated lands and the atmosphere in a FACE (free-air CO2 enrichment) study is described. With this method, gas is sampled with a technique of static-opaque-chamber and the fluxes of NO and NO2 exchanges are determined by analyzing the NO and NO2 concentrations with a chemiluminescent NOx analyzer. Application of this method in the FACE study of a rice-wheat ecosystem has indicated that reliable data on the exchange fluxes could be obtained. Over the non-waterlogged period of a rice-wheat rotation, net emission of NO from the fields was observed, while net uptake of NO2 occurred. The daily net emission of NO did not correlated with the soil temperature, but negatively depended upon soil moisture (R2 = 0.82, P < 0.001). A significant seasonal variation in the net uptake of NO2 was observed, which was regulated by wheat growth status. The daily uptake of NO2 depended upon both soil temperature and soil moisture. The dependence for each could be described with a parabola function (for soil temperature: R2 = 0.74, P < 0.001; for soil moisture: R2 = 0.69, P < 0.001). An elevation of atmospheric CO2 by 200 +/- 40 mumol.mol-1 mitigated the net emission of NO by 19% (t-test P = 0.096) and might be possible to reduce the net uptake of NO2 by 10% (t-test P = 0.26), which was likely due to the stimulated wheat growth.

Effects of elevated atmospheric CO2 on CH4 and N2O emissions during the paddy rice-growing season were examined in a FACE (free-air carbon dioxide enrichment) study. The emission fluxes of CH4 and N2O from paddy rice fields were measured using methods based on static opaque-chamber and gas chromatography techniques. Synthetic fertilizer N was amended for the rice-growing season at two rates, 150 and 250 kgN.hm-2 and the atmospheric CO2 was enriched by 200 mumol.mol-1. At both N levels, the preliminary results indicate that no significant effect of CO2 enrichment on CH4 and N2O emissions from the rice paddy fields was detected. The result on CH4 emissions is inconsistent with the most literatures, and the result on N2O emissions is consistent with the most literatures.

The latest researches on the effects of elevated atmospheric CO2 on the exchanges of trace gases (e.g., CO2, CH4 and N2O) between the atmosphere and ecosystems were reviewed. The techniques and methods involved in the researches were introduced firstly. Then the review mainly focused upon the results from those studies using the open-top-chamber (OTC) methods and the free-air carbon dioxide enrichment (FACE) system. Generally, elevated atmospheric CO2 may stimulate biomass accumulation, and enlarge C/N ratio in plant tissue so as to reduce the decomposition of organic matter. This action could increase CO2 sequestration in terrestrial ecosystems. Elevated atmospheric CO2 could impact on methanogenic bacteria and CH4 emissions. An increase in CH4 emissions from wetland may appear. The argument among the responses of N2O emissions to elevated CO2, however, was inconsistent. So far, no study on other trace gases was reported. More efforts should be taken in the research on the effects of elevated atmospheric CO2 on the exchange of trace gases.

We firstly introduced the method for determining the net ecosystem exchange fluxes of CO2 (NEE) between croplands and atmosphere, based on field measurements using static opaquechamber/gas chromatography methods was introduced, and the application of this method in the FACE (free-air CO2 enrichment) study to examine the effects of elevated CO2 on the NEE over a typical paddy ecosystem was carried out, because of lacking in observation data for some necessary parameters, e.g., dark maintenance respiration coefficient, only the minimum value of NEE (NEEmin) was calculated based on opaque-chamber measurements. The NEEmin data indicate that CO2 elevated by 200 +/- 40 mumol.mol-1 significantly increased the ecosystem uptake of atmospheric CO2 by a factor ca. 3. To accurately determine the NEE based on opaquechamber measurements, dark maintenance respiration coefficient, above-ground biomass and root: shoot, i.e. R:S, ratio of root to shoot should be observed over the whole growing season.

A method was specially designed and applied for measuring CO2 concentration of soil air over the non-waterlogged period of a rice-wheat rotation on an available area of about 1.6 m2 in a FACE (free-air CO2 enrichment) study. Based on measuring the CO2 concentration over the soil profile of 0-30 cm in depth using this method, the CO2 profile in the soils of wheat fields under elevated and ambient CO2 and the bare land under ambient CO2 was investigated and some preliminary results were obtained. Within 0-30 cm in soil depth, CO2 in the pores of the upper soil layers vertically diffused upwards much more quickly than that in the lower soil layers. During the period with active wheat growth, elevated atmospheric CO2 by 200 +/- 40 mumol.mol-1 significantly increased the CO2 concentration in soil air within 0-30 cm in depth by 14% +/- 5% (t-test, P < 0.001).

Livestock sheds are local and regional hotspots of greenhouse gases (GHG) emissions, but only very few studies analyse the intensities of GHG emissions from this source. The objective of this study was to quantify annual CH4, CO2 and N2O emissions from inside and outside of sheepfolds and summer cattle sheds in a typical agro‐pastoral ecotone using static chamber technique. Both sheepfolds and cattle shed functioned as huge net sources of CH4 and N2O at annual scale. Animal presence increased CH4, CO2 and N2O effluxes for up to 1100 times compared to the animal sheds without animals. N2O emissions boosted for 160%–280% during and after rainfall and spring‐thaw events. The CH4 and CO2 fluxes increased exponentially with faeces temperature for the outside sheepfold and summer cattle shed. The annual GHG emissions from both sheepfolds and summer cattle shed were 56 t CO2 equivalents ha−1, of which N2O contributed to 94%. Sheepfold dominated the total GHG emissions from animal sheds and accounted for 83% of the annual GHG flux. Annual emission on a per animal basis was 15, 0.2 and 28 kg CO2 eq year−1 sheep−1 and 26, 10 and 140 kg CO2 eq year−1 cattle−1 for N2O, CH4 and CO2, respectively. The annual N2O emissions from animal sheds were 70–250 times larger than nearby grassland soils, which were also net sink for atmospheric CH4. Concluding, animal sheds are very intensive local hotspots of GHG emissions, which should be considered at the local and regional scales. [ABSTRACT FROM AUTHOR]

The alpine meadow ecosystem is one of the major vegetation biomes on the Qinghai-Tibetan Plateau, which hold substantial quantities of soil organic carbon. Pronounced grassland degradations (induced by overgrazing/climate change and further exacerbated by the subterranean rodent activities) that have widely occurred in this ecosystem may significantly alter the non-growing season carbon turnover processes such as carbon dioxide (CO2) efflux, but little is known about how the non-growing season CO2 emissions respond to the degradation (particularly the exacerbated degradations by plateau zokor), as most previous studies have focused primarily on the growing season. In this study, the effects of four degradation levels (i.e., the healthy meadow (HM), degraded patches (DP), 2-year-old zokor mounds (ZM2), and current-year zokor mounds (ZM1)) on CO2 emissions and corresponding environmental and agronomic variables were investigated over the two non-growing seasons under contrasting climatic conditions (a normal season in 2013-2014 and a “warm and humid” season in 2014-2015). The temporal variation in the non-growing season CO2 emissions was mainly regulated by soil temperature, while increasing degradation levels reduced the temperature sensitivity of CO2 emissions due to a reduction in soil water content. The cumulative CO2 emissions across the non-growing season were 587-1283 kg C ha−1 for all degradation levels, which varied significantly (p < 0.05) interannually. The degradation of alpine meadows significantly (p < 0.05) reduced the vegetation cover and aboveground net primary productivity as well as the belowground biomass, which are typically thought to decrease soil CO2 emissions. However, the non-growing season CO2 emissions for the degraded meadow, weighted by the areal extent of the DP, ZM2, and ZM1, were estimated to be 641-1280 kg C ha−1, which was significantly higher (p < 0.05) as compared with the HM in the warm and humid season of 2014-2015 but not in the normal season of 2013-2014. Additionally, grassland degradation substantially increased the productivity-scaled non-growing season CO2 emissions, which showed an exponential trend with increasing degradation levels. These results suggest that there is a strong connection between grassland degradation and soil carbon loss, e.g., in the form of CO2 release, pointing to the urgent need to manage degraded grassland restoration that contributes to climate change mitigation. [ABSTRACT FROM AUTHOR]

Background and aims: The direct measurement of denitrification dynamics and its product fractions is important for parameterizing process-oriented model(s) for nitrogen cycling in various soils. The aims of this study are to a) directly measure the denitrification potential and the fractions of nitrogenous gases as products of the process in laboratory, b) investigate the effects of the nitrate (NO) concentration on emissions of denitrification gases, and c) test the hypothesis that denitrification can be a major pathway of nitrous oxide (NO) and nitric oxide (NO) production in calcic cambisols under conditions of simultaneously sufficient supplies of carbon and nitrogen substrates and anaerobiosis as to be found to occur commonly in agricultural lands. Methods: Using the helium atmosphere (with or without oxygen) gas-flow-soil-core technique in laboratory, we directly measured the denitrification potential of a silt clay calcic cambisol and the production of nitrogen gas (N), NO and NO during denitrification under the conditions of seven levels of NO concentrations (ranging from 10 to 250 mg N kg dry soil) and an almost constant initial dissolved organic carbon concentration (300 mg C kg dry soil). Results: Almost all the soil NO was consumed during anaerobic incubation, with 80-88 % of the consumed NO recovered by measuring nitrogenous gases. The results showed that the increases in initial NO concentrations significantly enhanced the denitrification potential and the emissions of N and NO as products of this process. Despite the wide range of initial NO concentrations, the ratios of N, NO and NO products to denitrification potential showed much narrower ranges of 51-78 % for N, 14-36 % for NO and 5-22 % for NO. Conclusions: These results well support the above hypothesis and provide some parameters for simulating effects of variable soil NO concentrations on denitrification process as needed for biogeochemical models. [ABSTRACT FROM AUTHOR]

Abstract: Greenhouse gas fluxes from rice paddies under nitrogen fertilization merit serious attention because nitrogen fertilizer is increasingly used for the intensification of rice cultivation. A 3-year field study was conducted to measure methane (CH4) and nitrous oxide (N2O) fluxes simultaneously in a sandy loam paddy field under three nitrogen application rates (0, 150 and 250kgNha−1) in the Yangtze River Delta from 2005 through 2007. The rice paddies were under a typical Chinese water regime, characterized by intermittent irrigation with midseason drainage. The results revealed a trade-off between CH4 and N2O emissions as influenced by the application of urea-based fertilizers, i.e., the nitrogen fertilization reduced CH4 emissions from rice paddies but increased N2O emissions. The seasonal CH4 emissions averaged 155.9kgCha−1 in the absence of nitrogen amendment. Compared to no nitrogen addition, the seasonal CH4 emissions were decreased by 27% and 53% in the fertilized plots at rates of 150 and 250kgNha−1, respectively. It was most likely that the sandy loam texture combined with the addition of urea-based fertilizers stimulated growth and activity of methane oxidizers. In contrast, nitrogen addition increased N2O emissions 2.5 times for an application rate of 150kgNha−1 and 6.0 times at 250kgNha−1, compared to no nitrogen addition (0.38kgNha−1). The direct emission factor of fertilizer N for N2O was estimated to be 0.39–1.22% for rice fields, with a mean value of 0.77%. The overall emissions of CH4 and N2O, expressed as carbon dioxide equivalents were affected by the nitrogen addition rate, with the minimum emissions occurring at 250kgNha−1. This result indicated that the commonly applied nitrogen rate in this region might be an effective option for mitigating the combined impacts of rice production. Our results also demonstrate the presence of large interannual variations in the CH4 and N2O fluxes. It is probable that these variations resulted from differences in the amount and distribution of precipitation. [Copyright &y& Elsevier]

Chamber techniques can easily be applied to field trials with multiple small plots measuring carbon- and nitrogen-trace gas fluxes. Nevertheless, such chamber measurements are usually made weekly and rarely more frequently than once daily. However, automatic chambers do allow flux measurements on sub-daily time scales. It has been hypothesized that sub-daily measurements provide more reliable results, as diurnal variations are captured better compared to manual measurements. To test this hypothesis we compared automatic and manual measurements of N2O, CO2 and CH4 fluxes from tilled and non-tilled plots of a rice–wheat rotation ecosystem over a non-waterlogged period. Our results suggest that both techniques, i.e., either manual or automatic chambers of N2O and CO2 emissions resulted in biased fluxes. The manual measurements were adequate to capture either day-to-day or seasonal dynamics of N2O, CO2 and CH4 exchanges, but overestimated the cumulative N2O and CO2 emissions by 18% and 31%, respectively. This was due to neglecting temperature-dependent diurnal variations of C and N trace gas fluxes. However, the automatic measurements underestimated the cumulative emissions of N2O and CO2 by 22% and 17%, respectively. This underestimation resulted from chamber effects upon soil moisture during rainfall processes. No significant difference was detected between the two methods in CH4 exchanges over the non-waterlogged soils. The bias of manual chambers may be significant when pronounced diurnal variations occur. The bias of automatic measurements can only be avoided/minimized if chamber positions are frequently changed and/or if chambers are automatically opened during rainfall events. We therefore recommend using automatic chambers together with continuous measurements of soil chamber moisture to allow for soil moisture correction of fluxes or to correct flux estimates as derived by manual chambers for possible diurnal variations. [Copyright &y& Elsevier]

Using the free-air CO2 enrichment (FACE) techniques, we carried out a 3-year mono-factorial experiment in temperate paddy rice fields of Japan (1998–2000) and a 3-year multifactorial experiment in subtropical paddy rice fields in the Yangtze River delta in China (2001–2003), to investigate the methane (CH4) emissions in response to an elevated atmospheric CO2 concentration (200±40 mmol mol−1 higher than that in the ambient atmosphere). No significant effect of the elevated CO2 upon seasonal accumulative CH4 emissions was observed in the first rice season, but significant stimulatory effects (CH4 increase ranging from 38% to 188%, with a mean of 88%) were observed in the second and third rice seasons in the fields with or without organic matter addition. The stimulatory effects of the elevated CO2 upon seasonal accumulative CH4 emissions were negatively correlated with the addition rates of decomposable organic carbon ( P<0.05), but positively with the rates of nitrogen fertilizers applied in either the current rice season ( P<0.05) or the whole year ( P<0.01). Six mechanisms were proposed to explain collectively the observations. Soil nitrogen availability was identified as an important regulator. The effect of soil nitrogen availability on the observed relation between elevated CO2 and CH4 emission can be explained by (a) modifying the C/N ratio of the plant residues formed in the previous growing season(s); (b) changing the inhibitory effect of high C/N ratio on plant residue decomposition in the current growing season; and (c) altering the stimulatory effects of CO2 enrichment upon plant growth, as well as nitrogen uptake in the current growing season. This study implies that the concurrent enrichment of reactive nitrogen in the global ecosystems may accelerate the increase of atmospheric methane by initiating a stimulatory effect of the ongoing dramatic atmospheric CO2 enrichment upon methane emissions from nitrogen-poor paddy rice ecosystems and further amplifying the existing stimulatory effect in nitrogen-rich paddy rice ecosystems. [ABSTRACT FROM AUTHOR]

Highlights: [•] A technical system is improved to measure soil N2, N2O, NO, CO2 and CH4 emissions. [•] Two methods are applied for measuring NO concentrations of different levels. [•] Synchronization and automation enable more accurate quantification of the gases. [Copyright &y& Elsevier]

We evaluated the applicability of a closed-path gas analyzer with two mid-infrared quantum cascade lasers (QCLs) for simultaneous measurement of nitrous oxide (N 2 O), methane (CH 4) and carbon dioxide (CO 2) fluxes from a cropland using the eddy covariance (EC) technique. The measurements were carried out in a typical vegetable field in the subtropical China during the wintertime, when the gas fluxes are at their lowest level in the year. A new approach was proposed to optimize the determination of lag times between the wind and gas concentration data, which was proven efficient to increase the reliability of the measured fluxes when the gas exchanges are weak. The dual-QCL analyzer showed a median precision (1 σ) of 0.14 nmol mol−1 for N 2 O, 3.3 nmol mol−1 for CH 4 and 0.36 μmol mol−1 for CO 2 at sampling frequency of 10 Hz under the field conditions. Such precisions are better than, or comparable with, those of other commonly used closed-path or open-path gas analyzers, which are capable of measuring ony one or two ot the three gases. The detection limit of the EC system for measuring half-hourly fluxes were 0.05 nmol m−2 s−1 for N 2 O, 1.12 nmol m−2 s−1 for CH 4 and 0.14 μmol m−2 s−1 for CO 2. The results showed that 100% of the N 2 O, 85% of the CH 4 and 96% of the CO 2 fluxes were larger than the above detection limits. This study suggests that the EC technique using a closed-path gas analyzer with two quantum cascade lasers is qualified for reliable and simultaneous measurements of N 2 O, CH 4 and CO 2 fluxes from a subtropical cropland throughout the year. Moreover, EC method based on this type of gas analyzer provides an additional option for long-term and simultaneous flux measurements of the three greenhouse gases in a wide range of agricultural and natural ecosystems. Unlabelled Image • The performance of an EC system with a closed-path dual-laser analyzer was evaluated. • A new method is proposed to optimize the lag time between wind and gas concentration. • The dual-laser analyzer showed good performance under the field conditions. • The EC system is qualified for simultaneous measurements of N 2 O, CH 4 and CO 2 fluxes. [ABSTRACT FROM AUTHOR]

• Comparing tropical rainforest (TRF) to rubber plantation (RP) emissions, CH 4 sink decreased and CO 2 emission was similar. • Leaf area index and soil water content mainly influences soil CO 2 and CH 4 emissions in RP respectively. • Soil water content, soil temperature, and dead fine roots were the most important factors of soil CO 2 and CH 4 emissions in TRF. • Soil temperature and fine root growth explained the difference in soil CO 2 and CH 4 between TRF and RP. Soil CO 2 (a) and CH 4 (b) fluxes in tropical rainforest (black circles) and rubber plantation (open circles) at Xishuangbanna, Southwest China. [Display omitted] [ABSTRACT FROM AUTHOR]