Jianwen Zou

Effects of water regime during rice-growing season on annual direct N2O emission in a paddy rice–winter wheat rotation system in southeast China

Annual paddy rice-winter wheat rotation constitutes one of the typical cropping systems in southeast China, in which various water regimes are currently practiced during the rice-growing season, including continuous flooding (F), flooding-midseason drainage-reflooding (F-D-F), and flooding-midseason drainage-reflooding and moisture but without waterlogging (F-D-F-M). We conducted a field experiment in a rice-winter wheat rotation system to gain an insight into the water regime-specific emission factors and background emissions of nitrous oxide (N(2)O) over the whole annual cycle. While flooding led to an unpronounced N(2)O emission during the rice-growing season, it incurred substantial N(2)O emission during the following non-rice season. During the non-rice season, N(2)O fluxes were, on average, 2.61 and 2.48 mg N(2)O-Nm(-)(2) day(-1) for the 250 kg N ha(-1) applied plots preceded by the F and F-D-F water regimes, which are 56% and 49% higher than those by the F-D-F-M water regime, respectively. For the annual rotation system experienced by continuous flooding during the rice-growing season, the relationship between N(2)O emission and nitrogen input predicted the emission factor and background emission of N(2)O to be 0.87% and 1.77 kg N(2)O-Nha(-1), respectively. For the plots experienced by the water regimes of F-D-F and F-D-F-M, the emission factors of N(2)O averaged 0.97% and 0.85%, with background N(2)O emissions of 2.00 kg N(2)O-Nha(-1) and 1.61 kg N(2)O-Nha(-1) for the annual rotation system, respectively. Annual direct N(2)O-N emission was estimated to be 98.1 Gg yr(-1) in Chinese rice-based cropping systems in the 1990s, consisting of 32.3 Gg during the rice-growing season and 65.8 Gg during the non-rice season, which accounts for 25-35% of the annual total emission from croplands in China.

Response of soil carbon dioxide fluxes, soil organic carbon and microbial biomass carbon to biochar amendment: a meta-analysis

Biochar as a carbon‐rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta‐analysis procedures to examine responses of soil carbon dioxide (CO2) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO2 fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO2 fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N‐fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land‐use types. Responses of soil CO2 fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO2 fluxes and MBC content, respectively. Both soil CO2 fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO2 fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO2 flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1–9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land‐use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change.

Lower resistance and higher tolerance of invasive host plants: biocontrol agents reach high densities but exert weak control

Invasive plants often have novel biotic interactions in their introduced ranges. Their defense to herbivory may differ from their native counterparts, potentially influencing the effectiveness of biological control. If invasive plants have decreased resistance but increased tolerance to enemies, insect herbivores may rapidly build up their populations but exert weak control. Moreover, resource availability to plants may affect the efficacy of biological control agents. We tested these predictions using Chinese tallow tree (Triadica sebifera) and two specialist herbivores (Heterapoderopsis bicallosicollis and Gadirtha inexacta) that are candidates for biological control. We performed a pair of field common garden experiments in China in which Triadica seedlings from the native or introduced range were grown in low or high light conditions and subjected to different levels of herbivory by each herbivore in a factorial design. We found that Heterapoderopsis achieved greater densities on tallow trees from the introduced range or when trees were grown in high light conditions. When Gadirtha was raised in the lab on tallow tree foliage we found that it performed better (larger pupal size) when fed foliage from introduced populations. However, introduced populations generally had greater herbivore tolerance such that the impact of each agent on plant performance was lower than on native populations despite higher herbivore loads. Tallow trees grew more slowly and achieved smaller sizes in lower light levels, but the impact of biological control agents was comparable to that found for higher light levels. Plants from introduced populations grew larger than those from native populations in all conditions. Our results suggest that reduced resistance and increased tolerance to herbivory in introduced populations may impede success of biological control programs. Biological control practitioners should include plants from the introduced range in the prerelease evaluation, which will help predict insect impact on target weeds.

Estimates of synthetic fertilizer N-induced direct nitrous oxide emission from Chinese croplands during 1980-2000.

There is increasing concern that agricultural intensification in China has greatly increased N(2)O emissions due to rapidly increased fertilizer use. By linking a spatial database of precipitation, synthetic fertilizer N input, cropping rotation and area via GIS, a precipitation-rectified emission factor of N(2)O for upland croplands and water regime-specific emission factors for irrigated rice paddies were adopted to estimate annual synthetic fertilizer N-induced direct N(2)O emissions (FIE-N(2)O) from Chinese croplands during 1980-2000. Annual FIE-N(2)O was estimated to be 115.7 Gg N(2)O-N year(-1) in the 1980s and 210.5 Gg N(2)O-N year(-1) in the 1990s, with an annual increasing rate of 9.14 Gg N(2)O-N year(-1) over the period 1980-2000. Upland croplands contributed most to the national total of FIE-N(2)O, accounting for 79% in 1980 and 92% in 2000. Approximately 65% of the FIE-N(2)O emitted in eastern and southern central China.

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.

Static opaque chamber-based technique for determination of net exchange of CO2 between terrestrial ecosystem and atmosphere

Terrestrial carbon cycling is one of the hotspots in global change issues. In this paper, we presented the rationale for determination of net exchange of CO2 between terrestrial and the atmosphere (NEE) and the methods for measuring several relevant components. Three key processes for determination of NEE were addressed, including the separation of shoot autotrophic respiration from total CO2 emissions of the ecosystem, the partition of root respiration from soil CO2 efflux, and the quantification of rhizodeposition C from NPP. With an understanding of the processes involved in the CO2 exchange between terrestrial and the atmosphere, we estimated NEE of rice ecosystem in Nanjing based on field measurements of CO2 emissions and several relevant biotic components as well as abiotic factors. The field measurements of CO2 emissions were made over the rice-growing seasons in 2001 and 2002 with the static opaque chamber method. Calculations indicated that the seasonal pattern of NEE is comparable for two seasons. Either net carbon emission or fractional carbon fixation occurred during 3 weeks after rice transplanting and thereafter net carbon fixation appeared with an increasing trend as rice growing. Higher net carbon fixation occurred in the rice developmental period from elongating to heading. A decline trend in the fixation was documented after rice heading. The mean daily NEE was -6.06 gC·m−2 in 2001 season and -7.95 gC·m−2 in 2002 season, respectively. These values were comparable to the results obtained by Campbell et al. who made field measurements with the Bowen ratio-energy balance technique in irrigated rice, Texas USA. Moreover, the mean daily NEE in this study was also comparable to the values obtained from a Japanese rice paddy with the eddy covariance method under the similar water regime, either drainage course or waterlogged. It is concluded that NEE determined by the static opaque chamber method is comparable and in agreement with those measured by Bowen ratioenergy balance and eddy covariance methods.

Relationship between nitrous oxide emission and winter wheat production

A 3-year field study in southeast China was performed to examine the relationship between N2O emission and winter wheat production. Over the 2002–03, 2003–04 and 2004–05 wheat-cropping seasons, N2O emissions depended on nitrogen addition, plowing practice, and preceding crop type treatments, and showed a pronounced inter-annual variation. N2O–N emission factor, the proportion of fertilizer N released as N2O–N from the wheat field, varied from 1.33% to 2.97%. The relationship between N2O emission (y) and crop yield (x) was well explained by the function y = 3.773Ln(x) + 1.46. Similarly, the function y = 4.445Ln(x) − 3.52 can be employed to address the relationship between N2O emission (y) and above ground biomass (x). About 84% and 87% of variation in seasonal N2O emission were explained by the two functions, while only 66% of the variation was represented by the N input with a linear relationship. The results of this study suggest that seasonal N2O emission of soil under winter wheat could be better predicted by crop yield and biomass than by N input.

Fe(III) fertilization mitigating net global warming potential and greenhouse gas intensity in paddy rice-wheat rotation systems in China.

A complete accounting of net greenhouse gas balance (NGHGB) and greenhouse gas intensity (GHGI) affected by Fe(III) fertilizer application was examined in typical annual paddy rice-winter wheat rotation cropping systems in southeast China. Annual fluxes of soil carbon dioxide (CO(2)), methane (CH(4)) and nitrous oxide (N(2)O) were measured using static chamber method, and the net ecosystem exchange of CO(2) (NEE) was determined by the difference between soil CO(2) emissions (R(H)) and net primary production (NPP). Fe(III) fertilizer application significantly decreased R(H) without adverse effects on NPP of rice and winter wheat. Fe(III) fertilizer application decreased seasonal CH(4) by 27-44%, but increased annual N(2)O by 65-100%. Overall, Fe(III) fertilizer application decreased the annual NGHGB and GHGI by 35-47% and 30-36%, respectively. High grain yield and low greenhouse gas intensity can be reconciled by Fe(III) fertilizer applied at the local recommendation rate in rice-based cropping systems.

Response of nitric and nitrous oxide fluxes to N fertilizer application in greenhouse vegetable cropping systems in southeast China

It is of great concern worldwide that active nitrogenous gases in the global nitrogen cycle contribute to regional and global-scale environmental issues. Nitrous oxide (N2O) and nitric oxide (NO) are generally interrelated in soil nitrogen biogeochemical cycles, while few studies have simultaneously examined these two gases emission from typical croplands. Field experiments were conducted to measure N2O and NO fluxes in response to chemical N fertilizer application in annual greenhouse vegetable cropping systems in southeast China. Annual N2O and NO fluxes averaged 52.05 and 14.87 μg N m−2 h−1 for the controls without N fertilizer inputs, respectively. Both N2O and NO emissions linearly increased with N fertilizer application. The emission factors of N fertilizer for N2O and NO were estimated to be 1.43% and 1.15%, with an annual background emission of 5.07 kg N2O-N ha−1 and 1.58 kg NO-N ha−1, respectively. The NO-N/N2O-N ratio was significantly affected by cropping type and fertilizer application, and NO would exceed N2O emissions when soil moisture is below 54% WFPS. Overall, local conventional input rate of chemical N fertilizer could be partially reduced to attain high yield of vegetable and low N2O and NO emissions in greenhouse vegetable cropping systems in China.

Decomposition of Phragmites australis litter retarded by invasive Solidago canadensis in mixtures: an antagonistic non-additive effect

Solidago canadensis is an aggressive invader in China. Solidago invasion success is partially attributed to allelopathic compounds release and more benefits from AM fungi, which potentially makes the properties of Solidago litter different from co-occurring natives. These properties may comprehensively affect litter decomposition of co-occurring natives. We conducted a field experiment to examine litter mixing effects in a Phragmites australis dominated community invaded by Solidago in southeast China. Solidago had more rapid mass and N loss rate than Phragmites when they decomposed separately. Litter mixing decreased N loss rate in Phragmites litter and increased that of Solidago. Large decreases in Phragmites mass loss and smaller increases in Solidago mass loss caused negative non-additive effect. Solidago litter extracts reduced soil C decomposition and N processes, suggested an inhibitory effect of Solidago secondary compounds. These results are consistent with the idea that nutrient transfer and secondary compounds both affected litter mixtures decomposition.