Xunhua Zheng

Impacts of soil moisture on nitrous oxide emission from croplands: a case study on the rice-based agro-ecosystem in Southeast China

Abstract Based on the in situ measurement of soil moisture and nitrous oxide (N2O) emission from a rice–wheat rotation ecosystem of southeast China and on the simulated experiments in laboratory, the impact of soil moisture on N2O emission is investigated. By analyzing the experimental data in detail, some results could be outlined as follows: (a) It is soil moisture and temperature instead of N fertilization that determines the seasonal variation pattern of N2O emission from the rice-based crop rotation ecosystem of southeast China. (b) Soil moisture is the most sensitive factor to regulate N2O emission from croplands. (c) Explosive emission of N2O from the rice-based agro-ecosystem was found to happen at the soil moisture within (110±5)% soil water holding capacity or field capacity (SWHC) or (99±9)% water-filled pore space (WFPS). When soil moisture of the rice–wheat fields is less than 105% SWHC, the N2O emission was observed to increase exponentially vs. soil moisture. In contrast, N2O emission was found to decrease reciprocally vs. soil moisture more than 115% SWHC. (d) The response of the N2O emission rate from soils in fields to variations of soil moisture may be well described with a general empirical equation. For x⩽C0% SWHC, F=A e −B(x−C 0 ) 2 +D e Ex . For x⩾C0% SWHC, F=A e −B(x−C 0 ) 2 + e G x −H . The equation to describe the relationship between soil moisture and N2O emission rates from incubated soil is different from that for fitting data observed in fields. Reasons for the difference still remains uncertain.

Grazing-induced reduction of natural nitrous oxide release from continental steppe

Atmospheric concentrations of the greenhouse gas nitrous oxide (N2O) have increased significantly since pre-industrial times owing to anthropogenic perturbation of the global nitrogen cycle, with animal production being one of the main contributors. Grasslands cover about 20 per cent of the temperate land surface of the Earth and are widely used as pasture. It has been suggested that high animal stocking rates and the resulting elevated nitrogen input increase N2O emissions. Internationally agreed methods to upscale the effect of increased livestock numbers on N2O emissions are based directly on per capita nitrogen inputs. However, measurements of grassland N2O fluxes are often performed over short time periods, with low time resolution and mostly during the growing season. In consequence, our understanding of the daily and seasonal dynamics of grassland N2O fluxes remains limited. Here we report year-round N2O flux measurements with high and low temporal resolution at ten steppe grassland sites in Inner Mongolia, China. We show that short-lived pulses of N2O emission during spring thaw dominate the annual N2O budget at our study sites. The N2O emission pulses are highest in ungrazed steppe and decrease with increasing stocking rate, suggesting that grazing decreases rather than increases N2O emissions. Our results show that the stimulatory effect of higher stocking rates on nitrogen cycling and, hence, on N2O emission is more than offset by the effects of a parallel reduction in microbial biomass, inorganic nitrogen production and wintertime water retention. By neglecting these freeze–thaw interactions, existing approaches may have systematically overestimated N2O emissions over the last century for semi-arid, cool temperate grasslands by up to 72 per cent.

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.

NET PRIMARY PRODUCTION OF CHINESE CROPLANDS FROM 1950 TO 1999

Considerable efforts have been made to assess the contribution of forest and grassland ecosystems to the global carbon budget, while less attention has been paid to agriculture. Net primary production (NPP) of Chinese croplands and driving factors are seldom taken into account in the regional carbon budget. We studied crop NPP by analyzing the documented crop yields from 1950 to 1999 on a provincial scale. Total NPP, including estimates of the aboveground and belowground components, was calculated from harvested yield data by (1) conversion from economic yield of the crop to aboveground mass using the ratio of aboveground residue production to the economic yield, (2) estimation of belowground mass as a function of aboveground mass, and (3) conversion from total dry mass to carbon mass. This approach was applied to 13 crops, representing 86.8% of the total harvested acreage of crops in China. Our results indicated that NPP in Chinese croplands increased markedly during this period. Averaging for each decade, the amount of NPP was 146 +/- 32, 159 +/- 34, 260 +/- 55, 394 +/- 85, and 513 +/- 111 Tg C/yr (mean +/- SD) in the 1950s, 1960s, 1970s, 1980s, and 1990s, respectively. This increase may be attributed to synthetic fertilizer application. A further investigation indicated that the climate parameters of temperature and precipitation determined the spatial variability in NPP. Spatiotemporal variability in NPP can be well described by the consumption of synthetic fertilizer and by climate parameters. In addition, the total amount of residue C and root C retained by the soils was estimated to be 618 Tg, with a range from 300 to 1040 Tg over the 50 years.

Atmospheric CO2 enrichment facilitates cation release from soil

Atmospheric CO(2) enrichment generally stimulates plant photosynthesis and nutrient uptake, modifying the local and global cycling of bioactive elements. Although nutrient cations affect the long-term productivity and carbon balance of terrestrial ecosystems, little is known about the effect of CO(2) enrichment on cation availability in soil. In this study, we present evidence for a novel mechanism of CO(2)-enhancement of cation release from soil in rice agricultural systems. Elevated CO(2) increased organic C allocation belowground and net H(+) excretion from roots, and stimulated root and microbial respiration, reducing soil redox potential and increasing Fe(2+) and Mn(2+) in soil solutions. Increased H(+), Fe(2+), and Mn(2+) promoted Ca(2+) and Mg(2+) release from soil cation exchange sites. These results indicate that over the short term, elevated CO(2) may stimulate cation release from soil and enhance plant growth. Over the long-term, however, CO(2)-induced cation release may facilitate cation losses and soil acidification, negatively feeding back to the productivity of terrestrial ecosystems.

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.

A comparison between measured and modeled N2O emissions from Inner Mongolian semi-arid grassland

The objectives of this study were (1) to determine the effect of land use on N2O emissions from Inner Mongolian semi-arid grasslands of China and (2) to evaluate the process-based DNDC model to extrapolate our field measurements from a limited number of sites to a larger temporal and spatial scale. The results suggest the following. Rainfall event was the dominant controlling factor for the seasonal variations of the N2O fluxes. The seven selected sites exhibited a similar seasonal trend in N2O emission, despite their different vegetation, land use and textures. In the typical steppe, N2O fluxes generally decrease with decreasing soil organic C (SOC) and total N content, indicating that soil C and N pools are very important in determining the spatial magnitude of the N2O flux. N2O emissions were very small during the entire growing season, averaging only 0.76 g N2O-N ha−1 day−1 for the five typical steppe sites, 0.35 g N2O-N ha−1 day−1 for the mown meadow steppe site, and 0.83 g N2O-N ha−1 day−1 from the cropped meadow steppe site. No enhanced effect due to overgrazing was observed for the N2O emission from the semi-arid grasslands. This was mainly results from the decreased SOC content due to overgrazing, which may have reduced the promoting effect of increased soil bulk density by trampling and animal excreta. Except for the mown steppe site, the model predictions of the N2O flux for the six different sites agree well with the observed values (r2 ranging from 0.35 to 0.68). It would be concluded that the DNDC model captured the key driving process for N2O emission. Nitrification was the predominant process, contributing 64–88% to the N2O emission. However, in terms of the magnitude of the N2O emission, further modifications should focus on the underestimated N2O flux during the spring and autumn periods (nitrification, freeze/thaw cycles) and the effect of topography and the mowing on N2O emission.

Soil‐atmosphere exchange potential of NO and N2O in different land use types of Inner Mongolia as affected by soil temperature, soil moisture, freeze‐thaw, and drying‐wetting events

[1] Changes in precipitation and temperature in Asian continental steppelands may affect soil physical, chemical and biological processes that control the biosphere-atmosphere exchange of N-trace gases. The changes include regional desertification, global warming and strong El Nino events that impact the large steppe land area in China and Mongolia. The area is so large that feedbacks to the global greenhouse gas balance may occur. In this study we investigated how changes in soil moisture and temperature, and especially drying-rewetting and freeze-thaw events, affect nitric oxide (NO) and nitrous oxide (N 2 O) fluxes from large intact soil cores taken from representative land use/cover types in the region of the Xilin River catchment, Inner Mongolia. These soil cores were incubated under varying conditions with respect to temperature (ranging from -10 to 15°C) and simulated rainfall (25, 45 and 65 mm). Following drying-rewetting and freeze-thaw transitions, we observed pulses of NO and N 2 O emissions from the soils of typical steppe, mountain meadow, sand dune and marshland. A comparable trend in soil CO 2 emissions and soil air N 2 O concentrations indicated that the high substrate availability and rapid recovery of microbial activity after soil wetting and thawing resulted in high gas fluxes. Across the whole temperature range, NO and N 2 O fluxes from all soils, except for N 2 O emissions from marshland soils, showed a positive exponential relationship with soil temperature. A combination of soil temperature and soil moisture explained most of the observed variations in NO (up to 74-90%) and N 2 O (up to 67-89%) fluxes for individual soils. Spatial differences in NO emissions between land use/cover types could be explained by differences in soil organic carbon and pH, whereas spatial variations of N 2 O fluxes were primarily correlated with differences in soil microbial biomass. On the basis of the incubation under controlled conditions, the average annual flux, weighted by the areal extent of the different investigated land use/cover types in the region, was estimated at ~3.9 ± 1.1 kg N ha -1 yr -1 for NO and 0.53 ± 0.20 kg N ha -1 yr -1 for N 2 O, respectively. It is noteworthy that our measurements were conducted using soil cores without a vegetation cover, which probably resulted in an overestimation of N-trace gas fluxes. However, our results indicate that the rarely determined NO formation appears to be a significant pathway in the N cycle of semiarid steppe, which is highly sensitive to the climatic change taking place in these regions, especially an increase in intensity and frequency of drying-wetting and freeze-thaw cycles.

Contribution of plants to N2O emissions in soil-winter wheat ecosystem: pot and field experiments

Outdoor pot and field experiments were conducted to assess the role of growing plants in agricultural ecosystem N2O emissions. N2O emissions from plants were quantified as the difference in soil-crop system N2O emissions before and immediately after cutting plants during the main growth stages in 2001–02 and 2002–03 winter wheat seasons. Emissions of N2O from plants depended on biomass within the same plant developmental status. Field results indicated that the seasonal contribution of N2O emissions from plants to ecosystem fluxes averaged 25%, ranging from 10% at wheat tillering to 62% at the heading stage. The fluxes of N2O emissions from plants varied between 0.3 and 3.9 mg N2O-N m−2 day−1 and its seasonal amount was equivalent to 0.23% of plant N released as N2O. A N2O emission coefficient (N2OE, mg N2O-N g−1 C day−1), defined as N2O-N emission in milligrams from per gram carbon of plant dry matter within a day, was represented by a 5-fold variation ranging from 0.021 to 0.004 mg N2O-N g C−1 day−1. A linear relationship (y=0.4611x+0.0015, r2=0.9352, p < 0.001) between N2OE (y) and plant dark respiration rate (x, mg CO2-C g C−1 day−1) suggested that in the absence of photosynthesis, some N2O production in plant N assimilation was associated with plant respiration. Although this study could not show whether N2O was produced or transferred by winter wheat plants, these results indicated an important role for higher plant in N2O exchange. Identifying its potential contribution is critical for understanding agricultural ecosystem N2O sources.