Shenghui Han

The Asian Nitrogen Cycle Case Study

Abstract We analyzed nitrogen budgets at national and regional levels on a timeline from 1961–2030 using a model, IAP-N 1.0. The model was designed based upon the Inter-governmental Panel on Climate Change (IPCC) methods using Asia-specific parameters and a Food and Agriculture Organization of the United Nations (FAO) database. In this paper we discuss new reactive-nitrogen and its various fates, and environmental nitrogen enrichment and its driving forces. The anthropogenic reactive nitrogen of Asia dramatically increased from ∼ 14.4 Tg N yr−1 in 1961 to ∼ 67.7 Tg N yr−1 in 2000 and is likely to be 105.3 Tg N yr−1 by 2030. Most of the anthropogenic reactive-nitrogen has accumulated in the environment. We found that an increasing demand for food and energy supplies and the lack of effective measures to improve the efficiency of fertilizer nitrogen use, as well as effective measures for the prevention of NOx emissions from fossil-fuel combustion, are the principal drivers behind the environmental nitrogen-enrichment problem. This problem may be finally solved by substituting synthetic nitrogen fertilizers with new high-efficiency nitrogen sources, but solutions are dependent on advances in biological technology.

Effects of elevated CO2 and N fertilization on CH4 emissions from paddy rice fields

[1] The authors employed free-air carbon dioxide enrichment facilities for investigating the effects of elevating the present atmospheric CO 2 by 200 μmol mol -1 and increasing the application rate of urea-based fertilizers from 150 to 250 kg N ha -1 on CH 4 emissions from paddy rice fields in southeastern China. The elevated CO 2 significantly stimulated methane emission, which was mainly due to the stimulation in rice growth. Intensifying N fertilization mitigated the CH 4 emission under the ambient CO 2 but stimulated the CH 4 emission under the elevated CO 2 . This suggests that N fertilization has a potential to stimulate both CH 4 production and CH 4 oxidation. Thus the net effect of N fertilization on CH 4 emission from paddy rice fields most likely depends upon the counterbalance between the nitrogen-induced increases in CH 4 production and CH 4 oxidation, as a N excess may result in the inhibition of methane emission, whereas a N limitation may result in the stimulation of methane emission.

Estimates of methane emissions from Chinese rice paddies by linking a model to GIS database

Abstract Methane is one of the principal greenhouse gases. Irrigated rice paddies are recognized as contributing to atmospheric methane concentration. Methane emissions from rice paddies are among the most uncertain estimates in rice-growing countries. Efforts have been made over the last decade to estimate CH 4 emissions from Chinese rice paddies via the model method. However, these estimates are very vague due to different models and upscaling methods. A reduction in these uncertainties may be achieved by coupling field-scale models with regional databases. The objective of this article is to develop a methodology of coupling a CH 4 emission model with regional databases by which CH 4 emissions from Chinese rice paddies can then be estimated. CH4MOD, a model for simulating CH 4 emissions from rice paddies with minimal input by using commonly available parameters, is of great potential in terms of upscaling as it has provided a realistic estimate of the observed results from various soils, climates and agricultural practices. By linking spatial databases to CH4MOD, CH 4 emissions from Chinese rice paddies in the 2000 rice-growing season were simulated on a day-by-day basis. The spatial databases were created by GIS with a spatial resolution of 10km×10km, including soil sand percentage, amounts of crop straw and roots from the previous season and farm manure, the water management pattern, dates of rice transplanting and harvesting, acreage of rice planted, rice grain yield and daily air temperature. ARCGIS software was used to meet all GIS needs, including data access, projection definition, overlaying of different vector layers, creation of grids (a raster format of ARCGIS software) by converting vector data, and the data conversion between grids and ASCII formats. Methane emissions from rice paddies in mainland China in the 2000 rice-growing season were estimated to be 6.02 Tg (1 Tg = 10 9 kg). Of the total, approximately 49% (2.93Tg) is emitted during the single rice-growing season, and 27% (1.63Tg) and 24% (1.46Tg) are from the early and late rice-growing seasons respectively. It was concluded that regional CH 4 emissions from rice paddies could be estimated by coupling CH4MOD with regional databases with a high spatial resolution. A further effort should be made to improve the quality of the spatial databases, especially in terms of the amount of added organic matter and the water regime. It is also necessary to evaluate the uncertainties of the present estimates in order to improve the overall accuracy.

Annual methane uptake by typical semiarid steppe in Inner Mongolia

[1] Steppe ecosystems cover approximately 10% of the global land surface. Recent measurements have shown that steppe soils function as a significant sink for atmospheric methane (CH 4 ). However, precise quantification of the annual CH 4 uptake by steppe is challenged by infrequent measurements of exchange rates, which often only cover the growing season. In order to understand the annual dynamics and magnitude of CH 4 exchange, especially contribution of nongrowing season to the cumulative annual CH 4 exchange, we conducted year-round CH 4 flux measurements at high temporal resolution at two adjacent steppe sites. One was ungrazed and fenced since 1999 (UG99) and the other was grazed during the winter (WG01). The measurements were supplemented with observations of CH 4 concentrations in the soil profile. Sites were located in typical Leymus chinensis steppe in Inner Mongolia, China. The results show that the typical semiarid steppe functioned exclusively as a sink for atmospheric CH 4 throughout the entire year. Even during the spring soil thawing, a period with high water content in the top soil, CH 4 uptake was dominant. The seasonality of CH 4 uptake displayed a strong dependency on the seasonal variation in soil temperature. Soil moisture increased in importance when temperature was not the limiting factor. For example, CH 4 rates decreased sharply following summer rainfall events. The annual CH 4 uptake by the ungrazed UG99 and the winter-grazed WG01 sites was 3.7 and 2.1 kg C ha -1 , respectively. The contribution of the nongrowing season (October-April) to the cumulative annual CH 4 uptake was approximately 30% (25%-36%). Additionally, our data suggest that winter grazing significantly alters the capacity of steppe soils for CH 4 uptake. However, more measurements at paired ungrazed/grazed sites are needed to assess how grazing might affect the CH 4 uptake capacity of steppe soils at a larger regional or global scale.

Annual N 2 O emissions from conventionally grazed typical alpine grass meadows in the eastern Qinghai–Tibetan Plateau

Annual nitrous oxide (N2O) emissions from high-altitude alpine meadow grasslands have not been effectively characterized because of the scarcity of whole-year measurements. The authors performed a year-round measurement of N2O fluxes from three conventionally grazed alpine meadows that represent the typical meadow landscape in the eastern Qinghai-Tibetan Plateau (QTP). The results showed that annual N2O emissions averaged 0.123±0.053 (2SD, i.e., the double standard deviation indicating the 95% confidence interval) kgNha-1yr-1 across the three meadow sites. N2O flux pulses during the spring freezing-thawing period (FTP) were observed at only one site, indicating a large spatial variability in association with soil moisture differences. Approximately 34-57% (mean: 46%) of the annual N2O emissions occurred in the non-growing season, highlighting the substantial importance of accurate flux observations during this period. The simultaneous observations showed conservative, marginal nitric oxide (NO) fluxes of 0.058±0.032 (2SD) kgNha-1yr-1. The N2O fluxes across the three field sites correlated negatively with the soil nitrate concentrations during the entire year-round period (P<0.05). Furthermore, a significant joint regulatory effect of topsoil temperature and moisture on the N2O and NO fluxes was observed during the relatively warm periods. Based on the results of the present and previous studies, a simple extrapolation roughly estimated the annual total N2O emission from Chinese grasslands to be 73±15 (2SD) GgNyr-1 (1Gg=109g). A linear dependence of the annual N2O fluxes on the aboveground net primary productivity (ANPP) was also found. This result may provide a simple approach for estimating the N2O emission inventories of frigid alpine or temperate grasslands that are ungrazed either in the summer or year round. However, further confirmation of this relationship with a wider ANPP range is still needed in the future studies.

Designing a regional nitrogen cycle module of grassland for the IAP-N model

Assessment of the nitrogen (N) balance and its long-term trend is necessary for management practices because of the negative environmental effects caused by an imbalance of reactive N in grassland ecosystems. In this study, we designed a module for the IAP-N (Improving Anthropogenic Practices of managing reactive Nitrogen) model to enable it to assess the N budget of regional grasslands. The module was developed to quantify the individual components of the N inputs and outputs for grassland ecosystems using livestock and human populations, grassland area, and fossil-energy consumption data as the model inputs. In this paper, the estimation approaches for individual components of N budget, data acquisition, and parameter selection are described in detail. The model was applied to assess the N budget of Inner Mongolia in 2006 at the county scale. The simulation results show that the most important pathway of N outputs from the grassland was livestock intake. The N output from livestock intake was especially large in the middle of Inner Mongolia. Biological fixation, atmospheric deposition, and livestock excreta deposition were comparably important for the N inputs into the grassland. The N budget for Inner Mongolia grassland in 2006 was −1.7×108±0.6×108 kg. The case study for Inner Mongolia shows that the new grassland module for the IAP-N model can capture the characteristics of the N budget in a semiarid grassland.

Influences of observation method, season, soil depth, land use and management practice on soil dissolvable organic carbon concentrations: A meta-analysis

Quantifications of soil dissolvable organic carbon concentrations, together with other relevant variables, are needed to understand the carbon biogeochemistry of terrestrial ecosystems. Soil dissolvable organic carbon can generally be grouped into two incomparable categories. One is soil extractable organic carbon (EOC), which is measured by extracting with an aqueous extractant (distilled water or a salt solution). The other is soil dissolved organic carbon (DOC), which is measured by sampling soil water using tension-free lysimeters or tension samplers. The influences of observation methods, natural factors and management practices on the measured concentrations, which ranged from 2.5-3970 (mean: 69) mg kg-1 of EOC and 0.4-200 (mean: 12) mg L-1 of DOC, were investigated through a meta-analysis. The observation methods (e.g., extractant, extractant-to-soil ratio and pre-treatment) had significant effects on EOC concentrations. The most significant divergence (approximately 109%) occurred especially at the extractant of potassium sulfate (K2SO4) solutions compared to distilled water. As EOC concentrations were significantly different (approximately 47%) between non-cultivated and cultivated soils, they were more suitable than DOC concentrations for assessing the influence of land use on soil dissolvable organic carbon levels. While season did not significantly affect EOC concentrations, DOC concentrations showed significant differences (approximately 50%) in summer and autumn compared to spring. For management practices, applications of crop residues and nitrogen fertilizers showed positive effects (approximately 23% to 91%) on soil EOC concentrations, while tillage displayed negative effects (approximately -17%), compared to no straw, no nitrogen fertilizer and no tillage. Compared to no nitrogen, applications of synthetic nitrogen also appeared to significantly enhance DOC concentrations (approximately 32%). However, further studies are needed in the future to confirm/investigate the effects of ecosystem management practices using standardized EOC measurement protocols or more DOC cases of field experiments.