The large N2O emissions from agricultural lands are of particular concern given both its high global warming potential relative to CH4 and CO2 and its contribution to stratospheric ozone depletion.Conservation tillage, including no-tillage and reduced tillage management, is increasingly being adopted on agricultural lands worldwide.About 10% of global arable lands, i.e.∼125 million hectares, are currently managed using conservation tillage.The adoption of conservation tillage has demonstrated important benefits for soil carbon sequestration in topsoil, soil erosion, soil quality and crop yields.However, there is considerable debate concerning the effects of conservation tillage on climate change mitigation due to the highly variable effects of conservation tillage on N2O emissions.Various studies found increase , decrease , and no differences in N2O emissions resulting from adoption of conservation tillage practices.These inconsistent effects may be associated with the duration of conservation tillage practices with short-term application reported to stimulate N2O emission while long-term application decreases N2O emission.Additionally, climate regimes and various soil properties are reported to have a strong effect on soil N2O emission.Soil N2O emission primarily results from nitrification and denitrification processes in soil.Under relatively aerobic conditions, NH4+ is converted to NO3− along with N2O emission by autotrophic nitrifiers, In contrast, under anaerobic conditions, heterotrophic denitrifiers convert NO3− to N2O and N2.Soil aeration status is a dominant factor controlling nitrification and denitrification processes and their potential N2O production.In addition,hydroponic nft soil physical and chemical properties, such as soil texture, pH, organic content, clay content, etc., play significant roles in N2O emission dynamics.
For instance, fine-textured soils often have higher N2O emissions than coarse-textured soils due to slower O2 diffusion rates leading to lower soil O2 concentrations that favor denitrification.However, other studies have shown lower N2O emission from fine-textured soils as low gas diffusivity allowed greater time for more complete reduction of N2O to N2.Higher microbially-labile organic matter contents also favor enhanced denitrification by providing substrate for heterotrophic denitrifier growth, which leads to more rapid O2 consumption.High N2O emission may also be favored in alkaline soils due to more suitable growth conditions for both nitrifiers and denitrifiers.Furthermore, agricultural practices, such as N fertilization, crop species, crop rotation and water management may have strong influences on N2O emission.Numerous studies have investigated the impacts of soil properties and agricultural practices on soil N2O emissions in conservation tillage systems and found diverse and contradictory results that hinder the overall assessment of conservation tillage impacts on climate change mitigation.Van Kessel et al.investigated changes in N2O emission in response to different categorical conservation tillage practices and found strong influences from the duration of conservation practices and climate regimes.Their meta analysis focused on the magnitude of N2O emission under contrasting conservation tillage regimes, but did not consider specific soil properties and widely-used agricultural practices.Zhao et al.analyzed the relationship between specific-conditions and greenhouse gas emissions in no-till farming systems using meta-regression based on a regional database in China, but this analysis was limited in scale.
A detailed assessment of the influence of conservation tillage practices on soil N2O emission is critical to determine the potential for conservation tillage practices to mitigate climate change.This study aimed to assess the effects of conservation tillage on soil N2O emission relative to conventional tillage by conducting a meta-analysis of peer reviewed field studies.Specifically, we attempt to address the following questions: i) How do climate regime and experimental duration affect soil N2O emissions following application of conservation tillage practices? ii) Do initial soil properties affect the response of N2O emission to conservation tillage practices? and iii) Can agricultural practices mitigate N2O emission associated with conservation tillage? This comprehensive meta-analysis is significant for developing strategies for the future expansion of conservation tillage and for enhancing agricultural practices to mitigate greenhouse gas emission from agricultural lands.Conservation tillage is promoted as an effective method for carbon sequestration and thus a possible mitigation strategy for climate change.However, considerable controversy exists concerning how conservation tillage affects soil N2O emissions, which may offset potential carbon-related climate change mitigation benefits.N2O emissions are primarily controlled by the microbiological processes of nitrification and denitrification.Whereas the heterotrophic denitrification process occurs under anaerobic conditions, nitrification is an aerobic process.Therefore, the integrated effects of soil physical, chemical and biological factors, such as soil aeration, pH, temperature, moisture, texture and substrate availability, function together to affect soil N2O emission dynamics.Furthermore, agricultural practices, such as irrigation, fertilization and cropping systems, play important direct/indirect roles in soil N2O emissions.Given the wide range of integrative factors affecting N2O emissions, a comprehensive meta-analysis can provide a powerful approach for gaining important insights into the importance of specific factors regulating N2O emission across a wide range of spatial and temporal scales.Overall, the implementation of conservation tillage significantly affected soil N2O emission in this meta-analysis ; however, non-significant differences were observed for different conservation tillage practices.
These results are consistent with an analysis by Van Kessel et al..Meta-regression results indicated some detailed information concerning differences in no-tillage versus reduced tillage practices on soil N2O emission.At the initiation of conservation tillage, soil compaction can moderate soil aeration and stimulate N2O emission through denitrification.However, substrate limitation suppressed this initial stimulation leading to decreased N2O emission over time.With retention of residues, sufficient substrate is available to support the N2O-producing heterotrophic microbial community.With sufficient substrate availability, soil aeration becomes a dominant factor regulating N2O emission in conservation tillage systems.As shown in Fig.6b, N2O emission rate was regulated by the interactions of conservation tillage and soil texture.N2O emission in both conservation tillage practices displayed a significant positive correlation in the fine particle size classes , which was consistent with the higher N2O emissions in fine-textured soils observed by Choudhary et al..Conservation tillage may improve bulk density and water holding capacity, especially in the fine-textured soils which are prone to generate anaerobic microsite hot spots for N2O production in otherwise aerobic soils.Significant differences were recorded in conservation tillage induced soil N2O emissions among climate regimes.Temperature and precipitation are the primary factors regulating N2O emission across climate regimes.A significant negative correlation was recorded between effect size and precipitation , consistent with the findings of Van Kessel et al.who reported a larger mean effect size in dry climates than humid climates upon implementation of reduced tillage.Increasing amounts of precipitation lead to higher soil moisture and lower soil oxygen concentrations, which strongly regulate nitrification and denitrification dynamics.Higher water-fill pore space was observed in no-tillage systems compared to conventional tillage during the dry season, but no difference was observed during the normal wet portion of the year.WFPS differences were more pronounced between tillage practices under lower precipitation scenarios, which resulted from increased denitrification induced N2O emission in conservation tillage relative to conventional tillage.In addition, the N2O emission effect size showed a weak positive correlation with temperature , which was further supported by the higher increase of N2O emissions in tropical and warm temperate climate regimes.These findings are similar to those found by Zhao et al..Nitrification is favored at optimal soil temperature and moisture conditions of 25–40 °C and WFPS of 30–70%, respectively.Within the optimal conditions, nitrifier activities are enhanced with increasing soil temperature leading to the potential for increased soil N2O emissions.However, contradictory results have shown higher N2O emissions from soils in conventional tillage versus no-tillage with increasing temperature.
Conservation tillage-induced N2O emissions were affected by experimental duration.Short- to medium-term implementation of conservation tillage significantly increased soil N2O emission, especially in the first 3 years following the initiation of conservation tillage.However, a negative mean effect size was measured for studies with long-term experimental duration.Similar changes in N2O emissions with duration of conservation tillage were reported by Six et al., with an increase of N2O emissions in the first 10 years and a decrease thereafter.These changes associated with duration of conservation tillage may be attributed to attainment of new steady-state soil conditions,hydroponic channel such as soil structure, compaction, WFPS and aeration, which are not optimal for soil microbes to produce N2O by denitrification and/or nitrification processes.Soil N2O emissions are strongly correlated with soil denitrification nitrification processes that are driven by soil microbes, which in turn are largely affected by several soil properties.SOC and generalized soil texture had no significant differences on N2O emissions following implementation of conservation tillage.A trend of increasing N2O emission with increasing SOC content was reported by Li et al..High SOC provides more substrate for heterotrophic denitrifiers, which should favor enhanced denitrification and N2O emissions.Soil texture strongly affects soil aeration and thus is often implicated as an important factor regulating N2O emissions.As nitrification is considered to be the dominant process generating N2O in generally well aerated, coarse-textured soils , application of conservation tillage in these soils may result in soil compaction and poor aeration, suppressing nitrification and its associated N2O emissions.In contrast, denitrification is often the primary N2Ogenerating process in fine-textured soils due to a generally higher prevalence of anoxic microsites.Therefore, application of conservation tillage to fine-textured soils may result in the development of additional anaerobic conditions through compaction and greater water retention owing to the higher micropore content of compacted soils, which favor the development of additional anaerobic microsites for denitrification.Soil pH and clay content were identified to significantly affect the N2O emission effect size from the implementation of conservation tillage.Our analysis indicated a significant increase of N2O emissions in acidic and alkaline soils but not in neutral soils.Greater N2O emissions in acidic soils have been previously reported.In acidic soils, stepwise denitrification was purported to be suppressed by an attenuation of reductase activities that hinder N2O conversion to N2, resulting in the accumulation of N2O in acidic soils.In contrast, nitrifiers generally perform better in neutral to slightly alkaline soils , which may contribute to increased N2O emissions in alkaline soils.The effect of clay content on N2O emission in our analysis contradicts the expectations of increasing N2O emission with increasing clay content.The significant increase of N2O emission in soils with low clay content was mostly associated with medium-textured soils, which was consistent with the results of our soil texture evaluation.However, the sample size for low clay content soils was small, which could bias the results.More comparisons are necessary for a rigorous exploration of the effect of clay content on soil N2O emission in conservation tillage systems.
As expected, increasing N application rates led to increased N2O emissions.Similar results were reported from short term trials evaluating the influence of N application on N2O emissions in Mediterranean soils.Enhanced inorganic N from fertilization would be expected to intensify nitrification-denitrification processes resulting in increased N2O production.A linear response of N2O emission to N application rate was identified when the N fertilizer rate was less or equal to that required to achieve maximum crop yield, while an exponential increase in N2O emission was observed in soils with higher N inputs.Contrasting water management practices showed a significant in- fluence on soil N2O emission in conservation tillage systems.Irrigation significantly increased soil N2O emissions, consistent with the findings of Cayuela et al..The drying and wetting cycles created by irrigation provide an ideal environment for coupled nitrification denitrification.Nitrate production during the dry period is available for denitrification when irrigation increases the WFPS leading to potential anaerobic conditions.A significant difference was found between residue retained and residue removed treatments following implementation of conservation tillage, consistent with the report by Baggs et al..Retention of residues provides substrate for microbial growth through mineralization, which should increase denitrifier and nitrifier abundance depending on oxygen content.Inorganic N released from residue by mineralization would further stimulate the N2O production processes.Finally, consumption of soil O2 from enhanced organic matter decomposition may contribute to a greater prevalence of anaerobic conditions that favor denitrification.Our analysis indicated that crop rotation reduced N2O emission from conservation tillage as compared to non-rotation systems.Previously, no significant effect of crop rotation on N2O emission was found by Omonode et al..As our analysis indicated a relatively weak significance level for crop rotation effects on soil N2O emission dynamics, further investigations are warranted to better understand the complex interactions between crop rotation and N2O emission.Our meta-analysis showed a crop-specific effect on conservation tillage induced N2O emissions.The higher N2O emissions from maize, wheat and rice may be related to the higher N fertilizer application rates for these crops as compared to the lower and insignificant effects from beans and other crop types that generally receive lower N fertilization rates.The relatively small increase of N2O emissions determined in rice paddies following conservation tillage was similar to that reported by Zhang et al.and is possibly due to the dominance of anaerobic conditions that favor complete denitrification and thus a lower yield of N2O relative to N2.