Chunyan Liu

N2O, CH4 and CO2 emissions from seasonal tropical rainforests and a rubber plantation in Southwest China

The main focus of this study was to evaluate the effects of soil moisture and temperature on temporal variation of N2O, CO2 and CH4 soil-atmosphere exchange at a primary seasonal tropical rainforest (PF) site in Southwest China and to compare these fluxes with fluxes from a secondary forest (SF) and a rubber plantation (RP) site. Agroforestry systems, such as rubber plantations, are increasingly replacing primary and secondary forest systems in tropical Southwest China and thus effect the N2O emission in these regions on a landscape level. The mean N2O emission at site PF was 6.0xa0±xa00.1xa0SExa0μgxa0Nxa0m−2xa0h−1. Fluxes of N2O increased from <5xa0μgxa0Nxa0m−2xa0h−1 during dry season conditions to up to 24.5xa0μgxa0Nxa0m−2xa0h−1 with re-wetting of the soil by the onset of first rainfall events. Comparable fluxes of N2O were measured in the SF and RP sites, where mean N2O emissions were 7.3xa0±xa00.7xa0SExa0μgxa0Nxa0m−2xa0h−1 and 4.1xa0±xa00.5xa0SExa0μgxa0Nxa0m−2xa0h−1, respectively. The dependency of N2O fluxes on soil moisture levels was demonstrated in a watering experiment, however, artificial rainfall only influenced the timing of N2O emission peaks, not the total amount of N2O emitted. For all sites, significant positive correlations existed between N2O emissions and both soil moisture and soil temperature. Mean CH4 uptake rates were highest at the PF site (−29.5xa0±xa00.3xa0SExa0μgxa0Cxa0m−2xa0h−1), slightly lower at the SF site (−25.6xa0±xa01.3xa0SExa0μgxa0Cxa0m−2xa0h−1) and lowest for the RP site (−5.7xa0±xa00.5xa0SExa0μgxa0Cxa0m−2xa0h−1). At all sites, CH4 uptake rates were negatively correlated with soil moisture, which was also reflected in the lower uptake rates measured in the watering experiment. In contrast to N2O emissions, CH4 uptake did not significantly correlate with soil temperature at the SF and RP sites, and only weakly correlated at the PF site. Over the 2xa0month measurement period, CO2 emissions at the PF site increased significantly from 50xa0mgxa0Cxa0m−2xa0h−1 up to 100xa0mgxa0Cxa0m−2xa0h−1 (mean value 68.8xa0±xa00.8xa0SExa0mgxa0Cxa0m−2xa0h−1), whereas CO2 emissions at the SF and RP site where quite stable and varied only slightly around mean values of 38.0xa0±xa01.8xa0SExa0mgxa0Cxa0m−2xa0h−1 (SF) and 34.9xa0±xa01.1xa0SExa0mgxa0Cxa0m−2xa0h−1 (RP). A dependency of soil CO2 emissions on changes in soil water content could be demonstrated for all sites, thus, the watering experiment revealed significantly higher CO2 emissions as compared to control chambers. Correlation of CO2 emissions with soil temperature was significant at the PF site, but weak at the SF and not evident at the RP site. Even though we demonstrated that N and C trace gas fluxes significantly varied on subdaily and daily scales, weekly measurements would be sufficient if only the sink/ source strength of non-managed tropical forest sites needs to be identified.

Fluxes of nitrous oxide, methane and carbon dioxide during freezing-thawing cycles in an Inner Mongolian steppe

Fluxes of nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) were followed at winter-grazed (WG) and ungrazed steppe (UG99) in Inner Mongolia during the winter–spring transition of 2006. Mean fluxes during the period March 12–May 11 were 8.2u2009±u20090.5 (UG99) and 1.5u2009±u20090.2xa0μg N2O–N m−2xa0h−1 (WG) for N2O, 7.2u2009±u20090.2 (UG99) and 3.0u2009±u20090.1xa0mg CO2–C m−2xa0h−1 (WG) for CO2 and −42.5u2009±u20090.9 (UG99) and −14.1u2009±u20090.3xa0μg CH4–C m−2 h−1 (WG) for CH4. Our data show that N2O emissions from semi-arid steppe are strongly affected by freeze–thawing. N2O emissions reached values of up to 75xa0μg N2O–N m−2xa0h−1 at the UG99 site, but were considerably lower at the WG site. The observed differences in N2O, CH4 and CO2 fluxes between the ungrazed and grazed sites were ascribed to the reduced plant biomass at the grazed site, and—most important—to a reduction in soil moisture, due to reduced snow capturing during winter. Thus, winter-grazing significantly reduced N2O emission but on the other hand also reduced the uptake of atmospheric CH4. To finally evaluate which of the both effects is most important for the non-CO2 greenhouse gas balance measurements covering an entire year are needed.

Nitrous oxide and nitric oxide emissions from an irrigated cotton field in Northern China

Cotton is one of the major crops worldwide and delivers fibers to textile industries across the globe. Its cultivation requires high nitrogen (N) input and additionally irrigation, and the combination of both has the potential to trigger high emissions of nitrous oxide (N2O) and nitric oxide (NO), thereby contributing to rising levels of greenhouse gases in the atmosphere. Using an automated static chamber measuring system, we monitored in high temporal resolution N2O and NO fluxes in an irrigated cotton field in Northern China, between January 1st and December 31st 2008. Mean daily fluxes varied between 5.8 to 373.0xa0µg N2O-Nxa0m−2u2009h−1 and −3.7 to 135.7xa0µg NO-Nxa0m−2u2009h−1, corresponding to an annual emission of 2.6 and 0.8xa0kgxa0N ha−1u2009yr−1 for N2O and NO, respectively. The highest emissions of both gases were observed directly after the N fertilization and lasted approximately 1xa0month. During this time period, the emission was 0.85 and 0.22xa0kgxa0N ha−1 for N2O and NO, respectively, and was responsible for 32.3% and 29.0% of the annual total N2O and NO loss. Soil temperature, moisture and mineral N content significantly affected the emissions of both gases (pu2009<u20090.01). Direct emission factors were estimated to be 0.95% (N2O) and 0.24% (NO). We also analyzed the effects of sampling time and frequency on the estimations of annual cumulative N2O and NO emissions and found that low frequency measurements produced annual estimates which differed widely from those that were based on continuous measurements.

Effects of nitrogen fertilizer on CH4 emission from rice fields: multi-site field observations

There is an ongoing discussion of the possible effects of nitrogen (N) application on methane (CH4) emission from rice fields. However, the Intergovernmental Panel on Climate Change (IPCC) methodologies for estimating the national inventory of CH4 emission from paddy rice production do not consider the effects of N addition. To assess the lack of knowledge about N addition effects on inventory estimates, we recently launched a multi-site observation campaign in major rice cultivation regions of China. The observations showed that, across various climate zones, the application of ammonium-based fertilizers at the commonly-adopted levels for fields in China (150 or 250xa0kg N ha−1) generally inhibited accumulative CH4 emission during rice season (by 28–30% on average) as compared to no N addition. An increase in application from the moderate level of 150xa0kg N ha−1 to the high rate of 250xa0kg N ha−1 did not significantly modify CH4 emission. Our results suggest that disregarding the effect of N fertilization by the IPCC methodologies may not significantly bias CH4 inventory estimates of China. In regions with much lower N addition levels, however, disregarding the effect of N fertilization may result in the underestimation of regional CH4 emission, since these emissions were mainly derived from studies in regions with relatively high N addition rates.

Microbial N Turnover and N-Oxide (N2O/NO/NO2) Fluxes in Semi-arid Grassland of Inner Mongolia

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N2O, NO and NO2 were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1xa0mgxa0Nxa0kg−1 soil dry weight day−1. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N2O and NOx flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N2O fluxes varied between 0.39 and 1.60xa0μgxa0N2O-Nxa0m−2xa0h−1, equivalent to 0.03–0.14xa0kgxa0N2O-Nxa0ha−1xa0y−1, and were considerably lower than previously reported for the same region. NOx flux rates ranged between 0.16 and 0.48xa0μgxa0NOx-Nxa0m−2xa0h−1, equivalent to 0.01–0.04xa0kgxa0NOx-Nxa0ha−1xa0y−1, respectively. N2O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance.

Characteristics of multiple‐year nitrous oxide emissions from conventional vegetable fields in southeastern China

[1]xa0The annual and interannual characteristics of nitrous oxide (N2O) emissions from conventional vegetable fields are poorly understood. We carried out 4 year measurements of N2O fluxes from a conventional vegetable cultivation area in the Yangtze River delta. Under fertilized conditions subject to farming practices, approximately 86% of the annual total N2O release occurred following fertilization events. The direct emission factors (EFd) of the 12 individual vegetable seasons investigated ranged from 0.06 to 14.20%, with a mean of 3.09% and a coefficient of variation (CV) of 142%. The annual EFd varied from 0.59 to 4.98%, with a mean of 2.88% and an interannual CV of 74%. The mean value is much larger than the latest default value (1.00%) of the Intergovernmental Panel on Climate Change. Occasional application of lagoon-stored manure slurry coupled with other nitrogen fertilizers, or basal nitrogen addition immediately followed by heavy rainfall, accounted for a substantial portion of the large EFds observed in warm seasons. The large CVs suggest that the emission factors obtained from short-term observations that poorly represent seasonality and/or interannual variability will inevitably yield large uncertainties in inventory estimation. The results of this study indicate that conventional vegetable fields associated with intensive nitrogen addition, as well as occasional applications of manure slurry, may substantially account for regional N2O emissions. However, this conclusion needs to be further confirmed through studies at multiple field sites. Moreover, further experimental studies are needed to test the mitigation options suggested by this study for N2O emissions from open vegetable fields.

Importance of point sources on regional nitrous oxide fluxes in semi-arid steppe of Inner Mongolia, China

The aim of the present work was to estimate the contribution of different point and diffuse sources to the regional N2O emission strength of steppe in the Xilin river catchment, Inner Mongolia, People’s Republic of China. Transect studies showed that the topographic effect on N2O emissions from upland soils was negligible and that upland steppe is only a very weak net source of N2O during the growing season (0.8xa0±xa00.4xa0μg N2O–Nxa0m−2xa0h−1). Slightly higher emissions were found for riparian areas (1.8xa0±xa00.3xa0μg N2O–Nxa0m−2xa0h−1), which cover ∼4% of the landscape. Even faeces or urine additions stimulated N2O emissions from steppe soils only weakly (<2.5xa0μg N2O–Nxa0m−2xa0h−1 for a 5xa0days period). Due to low moisture contents, N2O emissions from dung heaps were also rather low (6.2xa0±xa00.8xa0μg N2O–Nxa0kg−1xa0dry matterxa0h−1). In contrast, three orders of magnitude higher N2O emissions were found at sheepfolds (2.45xa0mg N2O–Nxa0m−2xa0h−1 on average). By calculating N2O emissions on a landscape scale, we show that point sources, and especially sheepfolds, become the dominating regional N2O source during the growing season if stocking rates are >1xa0sheepxa0ha−1. Our results indicate that the common grazing management in the Xilin river region leads to a translocation of nitrogen from large source areas towards defined spots. This finding is further supported by measurements of NH3 concentrations at different sites. Since most of the nitrogen accumulated in these hot spots is finally lost through burning of the dried excrements by the farmers for heating and cooking purposes, the ecosystem faces a significant human perturbation of regional N cycling, which may contribute to an accelerated degradation of steppe in the Xilin river region.

Quantifying net ecosystem carbon dioxide exchange of a short-plant cropland with intermittent chamber measurements

[1]xa0An approach for quantifying the net ecosystem exchange (NEE) of carbon dioxide, which is subject to a rectangular hyperbolic relationship between NEE and photosynthetic active radiation, was adapted to a typical wheat-rice rotation ecosystem under a subtropical monsoon climate in the Yangtze River delta. Adaptation schemes were established; these relied upon intermittent measurements and thereby parameterization of photosynthesis, canopy respiration, root respiration, root-to-shoot ratio, and plant growth, using manual chambers and conventional methods. To apply the adapted approach for NEE estimation at daily, seasonal, and annual scales, data from hourly air temperature, hourly photosynthetic active radiation, shoot biomass at maturity, measured soil heterotrophic respiration during the nonflooded growing season, and intermittently observed ecosystem respiration are all required. Indirect verification showed that this approach was capable of yielding seasonal NEE estimates comparable with those of field measurements using meteorological techniques such as eddy covariance (EC). The daily NEE fluxes were calculated for two wheat-rice rotations. Then the total NEE during the rice-growing seasons, the wheat-growing seasons, the nonrice periods of the year, and the entire rotation cycles were respectively estimated as −7.08 to −7.54, −1.49 to −1.58, −0.83 to −0.92, and −7.91 to −8.46 t C ha−1 in the 2001–2002 rotation, and −7.35 to −7.82, −2.63 to −2.80, −2.17 to −2.33, and −9.51 to −10.15 t C ha−1 in the 2002–2003 rotation. Slight carbon gains occurred during the rice seasons (−0.14 to −0.62 t C ha−1), but obvious carbon losses occurred during the nonrice periods (2.27 to 3.13 t C ha−1) and over the entire rotation cycle (1.66 to 2.96 t C ha−1). These carbon losses were due to low rates of crop residue incorporation and lack of organic manure application. This study implies that the adapted approach applies for field trials requiring multiple field plots of a short-plant ecosystem. This approach may provide a methodological alternative to fill the measurement gap for quantifying NEE on fragmented terrains at high temporal and spatial resolutions.

Annual emissions of nitrous oxide and nitric oxide from rice-wheat rotation and vegetable fields: a case study in the Tai-Lake region, China

Background and aimsKnowledge on nitrous oxide (N2O) and nitric oxide (NO) emissions from typical cropping systems in the Tai-Lake region is important for estimating regional inventory and proposing effective N2O and NO mitigation options. This study aimed at a) characterizing the seasonal and annual emissions of both gases from the major cropping systems, and b) determining their direct emission factors (EFds) as the key parameters for inventory compilation.MethodsMeasurements of N2O and NO emissions were conducted year-round in the Tai-Lake region using a static opaque chamber method. The measurements involved a typical rice-wheat rotation ecosystem and a vegetable field. The two types of croplands were subjected to both a fertilized treatment and a control treatment without nitrogen addition. In the rice-wheat ecosystem, N2O emissions were measured throughout an entire year-round rotation spanning from June 2003 to June 2004, whereas NO emissions were measured only during the non-rice period. In the vegetable field, both N2O and NO emissions were measured from November 2003 to November 2004.ResultsDuring the investigation period, the average cumulative N2O and NO emissions under the fertilized conditions amounted to 3.80 and 0.80 (during the non-rice period for NO) kgxa0Nu2009ha−1, respectively, in the rice-wheat field, and 20.81 and 47.13xa0kgxa0N ha−1, respectively, in the vegetable field. The average total N2O and NO emissions under the control conditions were 1.39 and 0.29 (during the non-rice period for NO) kgxa0Nu2009ha−1, respectively, in the rice−wheat rotation, and 2.98 and 0.80xa0kgxa0N ha−1, respectively, in the vegetable field. The direct emission factor (EFd, which is defined as the loss rate of applied nitrogen via N2O or NO emissions in the current season or year) of N2O was annually determined to be 0.56xa0% in the rice-wheat field, while the seasonal EFd of NO was 0.34xa0% during the non-rice period of the rotation cycle. In the vegetable field, the seasonal EFds of N2O and NO varied from 0.15xa0% to 14.50xa0% and 0.80xa0% to 28.21xa0%, respectively, among different crop seasons; and the annual EFds were 1.38xa0% and 3.59xa0%, respectively.ConclusionsThis study suggests that conventional vegetable fields associated with intensive synthetic nitrogen application, as well as addition of manure slurry, may substantially contribute to the regional N2O and NO emissions though they account for a relatively small portion of the farmlands in the Tai-Lake region. However, further studies to be conducted at multiple field sites with conventional vegetable and rice-based fields are needed to test this conclusion.

Dinitrogen fixation by biological soil crusts in an Inner Mongolian steppe

Eurasian steppe ecosystems are nitrogen-limited and suffer additionally from high grazing intensities in many areas. Soil surface-bound cyanobacteria are able to fix nitrogen and can be the major source of plant available nitrogen in such ecosystems. In this study, the abundance and dinitrogen fixation capacity of the most common soil surface-bound microbial and lichen species were determined at an ungrazed, a winter-grazed, and a heavily grazed steppe site in the Xilin River catchment, Inner Mongolia, People’s Republic of China. The microorganisms were identified as Nostoc spec. and the lichen species as Xanthoparmelia camtschadalis (Ach.) Hale by a combination of classical light microscopy, confocal laser scanning microscopy and molecular analysis of the internal transcribed spacer (ITS1) region of ribosomal RNA. Both species were found exclusively at grazed steppe sites, with a clear difference in abundance depending on the grazing intensity. At the winter-grazed site, Nostoc was more abundant than Xanthoparmelia; for the heavily grazed site, the opposite was found. N2 fixation was quantified with both the acetylene reduction method and 15N2 incubation. Cyanobacterial colonies of Nostoc fixed N2 vigorously, whereas X. camtschadalis did not at all. The fraction of nitrogen derived from the fixation of molecular nitrogen in Nostoc was 73%, calculated from 15N natural abundance measurements of Nostoc with X. camtschadalis as reference. The conservatively calculated N2 uptake by Nostoc was 0.030–0.033xa0kg N ha−1 for the heavily grazed site and 0.080–0.087xa0kg N ha−1 for the winter-grazed site for the growing seasons of 2004 and 2005, respectively. Together with previous findings, this study demonstrates that N2 fixation by Nostoc can potentially replace significant amounts, if not all, of the nitrogen lost in the form of N2O and NO soil emissions in this steppe ecosystem.