Effects of biochar,earthworms, and litter addition on soil microbial activity and abundance in a temperate agricultural soil

Biochar application to arable soils could be effective for soil C sequestration and mitigation of greenhouse gas (GHG) emissions. Soil microorganisms and fauna are the major contributors to GHG emissions from soil, but their interactions with biochar are poorly understood. We investigated the effects of biochar and its interaction with earthworms on soil microbial activity, abundance, and community composition in an incubation experiment with an arable soil with and without N-rich litter addition. After 37 days of incubation, biochar significantly reduced CO₂ (up to 43 %) and N₂O (up to 42 %), as well as NH₄ ⁺-N and NO₃ ^?-N concentrations, compared to the control soils. Concurrently, in the treatments with litter, biochar increased microbial biomass and the soil microbial community composition shifted to higher fungal-to-bacterial ratios. Without litter, all microbial groups were positively affected by biochar × earthworm interactions suggesting better living conditions for soil microorganisms in biochar-containing cast aggregates after the earthworm gut passage. However, assimilation of biochar-C by earthworms was negligible, indicating no direct benefit for the earthworms from biochar uptake. Biochar strongly reduced the metabolic quotient qCO₂ and suppressed the degradation of native SOC, resulting in large negative priming effects (up to 68 %). We conclude that the biochar amendment altered microbial activity, abundance, and community composition, inducing a more efficient microbial community with reduced emissions of CO₂ and N₂O. Earthworms affected soil microorganisms only in the presence of biochar, highlighting the need for further research on the interactions of biochar with soil fauna.     

Contribution of fallow periods between rice crops to seasonal GHG emissions: effect of water and tillage management

ABSTRACT

Irrigated rice cultivation is a major source of greenhouse gas (GHG) emissions from agriculture. Methane (CH₄) and nitrous oxide (N₂O) are emitted not only throughout the growing season but also in the fallow period between crops. A study was conducted for two transition periods between rice crops (dry to wet season transition and wet to dry season transition) in the Philippines to investigate the effect of water and tillage management on CH₄ and N₂O emissions as well as on soil nitrate and ammonium. Management treatments between rice crops included (1) continuous flooding (F), (2) soil drying (D), (3) soil drying with aerobic tillage (D + T), and (4) soil drying and wetting (D + W). The static closed chamber method was used to measure CH₄ and N₂O fluxes.

Soil nitrate accumulated and N₂O was emitted in treatments with soil drying. Nitrate disappeared while ammonium gradually increased after the soil was flooded during land preparation, indicating net nitrogen mineralization. N₂O emissions were highest in both transition periods in D + W (437 and 645 µg N₂O m^?2 h^?1). Methane emissions were significant in only the F treatment. The highest global warming potential (GWP) in the transition between rice crops occurred in F, with CH₄ contributing almost 100% to the GWP. The GWP from other treatments was lower than F, with about 60–99% of the GWP attributed to N₂O emissions in treatments with soil drying. The GWP in the transition between rice crops represented up to 26% of the total GWP from harvest to harvest. This study demonstrates that the transition period can be an important source of GHG emissions with relative importance of CH₄ and N₂O depending on the soil water regime. Therefore, the transition period should not be disregarded when estimating GHG emissions for rice cropping systems.     

Dissolved organic matter and inorganic N jointly regulate greenhouse gases fluxes from forest soils with different moistures during a freeze-thaw period

ABSTRACT

Antecedent soil moisture before freezing can affect greenhouse gases (GHG) fluxes from soils during thaw, but their critical threshold values for GHG fluxes and the underlying mechanisms are still not clear. By using packed soil-core incubation experiments, we have studied nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) fluxes from a mature broadleaf and Korean pine-mixed forest soil and an adjacent white birch forest soil with nine levels of soil moisture ranging from 10 to 90% water-filled pore space (WFPS) during a 2-month freezing at ?8°C and the following 10-day thaw at 10°C. The threshold values of soil moisture ranged from 50 to 70% WFPS for CH₄ uptake and from 70 to 90% WFPS for N₂O and CO₂ emissions from the two soils during the freeze-thaw period. Under the optimum soil moisture condition, fulvic-like compounds with high bioavailability contributed more than 60% of dissolved organic matter (DOM) in the soil. Cumulative N₂O emissions from forest soils during the freeze-thaw period were greatest when the concentration ratio of nitrate-N to dissolved organic carbon (DOC) was 0.04 g N g^?1 C. Cumulative soil CO₂ emissions and CH₄ uptake during the freeze-thaw period were both regulated by the interaction between soil DOC and net N mineralization. The activities of β-1,4-glucosidase and β-1,4-N-acetyl-glucosaminidase, microbial biomass C and N, and the microbial biomass C-to-N ratios, were all significantly correlated to the soil N₂O, CO₂, and CH₄ fluxes. Overall, upon a freeze-thaw period with different soil moistures, GHG fluxes from forest soils were jointly regulated by inorganic N and DOC concentrations, and related to the labile components of DOM released into the soil, which could be strictly controlled by the related microbial properties.     

Greenhouse Gas Emissions following Conversion of a Reclaimed Minesoil to Bioenergy Crop Production

This study provides a comparative assessment of greenhouse gas (GHG) emissions when converting a reclaimed minesoil that was previously under meadow to miscanthus (Miscanthus  × giganteus ) and maize (Zea mays L.) land uses in Ohio, USA. Additionally, effluent from an anaerobic digester at rates of 0, 75, 150, and 225 kg N ha⁻¹ rates was also assessed for C and nutrient fertilization. Results from the study show that land use conversion to maize had the highest net release of GHG equivalent of 6·6 Mg CO_2equ ha⁻¹ y⁻¹, on average, across effluent application rates. Under miscanthus land use with no and high effluent application rates, net GHG equivalent on average was 4·3 Mg CO_2equ ha⁻¹ y⁻¹, which was larger when compared with that under the meadow land use (1·6 Mg CO_2equ ha⁻¹ y⁻¹). Miscanthus land use under medium rates of effluent application had similar net GHG equivalent (7·1 Mg CO_2equ ha⁻¹ y⁻¹) to the maize land use. The application of effluent did increase CO₂–C and N₂O–N emissions; but increases in above‐ground–below‐ground biomass production (1·6 Mg C ha⁻¹) in the meadow land use and C input from effluent retained in the soil in the miscanthus and maize land uses offset most of the effluent‐induced GHG equivalent emissions. Contribution of cumulative N₂O–N to GHG equivalent emissions in general was 11% when no effluent was applied and 22% when effluent was applied across land uses. Findings from this study show that land use changes from antecedent meadow to maize and miscanthus during the first year of establishment would result in net increase of GHG emissions. Published 2017. This article is a U.S. Government work and is in the public domain in the USA     

Greenhouse gas emissions from Silphium perfoliatum and silage maize cropping on Stagnosols

Background

The sustainability of bioenergy is strongly affected by direct field-derived greenhouse gas (GHG) emissions and indirect emissions form land-use change. Marginal land in low mountain ranges is suitable for feedstock production due to small impact on indirect land-use change. However, these sites are vulnerable to high N₂O emissions because of their fine soil texture and hydrology.

Aims

The perennial cup plant (Silphium perfoliatum L.) might outperform silage maize (Zea mays L.) on cold, wet low mountain ranges sites regarding yield and ecosystem services. The aim of this study was to assess whether the cultivation of cup plant also provides GHG mitigation potential compared to the cultivation of maize.

Methods

A t-year field experiment was conducted in a low mountain range region in western Germany to compare area and yield-scaled GHG emissions from cup plant and maize fields. GHG emissions were quantified using the closed chamber method.

Results

Cup plant fields emitted an average of 3.6 ± 4.3 kg N₂O-N ha^–1 year^–1 (–85%) less than maize fields. This corresponded to 74.0 ± 94.1 g CO_2-eq kWh^–1 (–78%) less emissions per produced electrical power. However, cup plant had a significantly lower productivity per hectare (–34%) and per unit of applied nitrogen (–32%) than maize.

Conclusion

Cup plant as a feedstock reduces direct field-derived GHG emissions compared to maize but, due to lower yields cup plant, likely increases emissions associated with land-use changes. Therefore, the increased sustainability of bioenergy from biogas by replacing maize with cup plant is heavily dependent on the performance of maize at these sites and on the ecosystem services of cup plant in addition to GHG savings.     

Reduction in greenhouse gas emissions by no-tilling rice cultivation in Hachirogata polder,northern Japan: Life-cycle inventory analysis

Abstract

The scenarios for conventional puddling and no-tilling rice (Oryza sativa L.) cultivation were compared in terms of greenhouse gas (GHG) emissions from paddy fields, fuel consumption and manufacturing of invested materials using a life cycle inventory (LCI) based analysis. Only the differences between the scenarios were examined. The no-tilling scenario omitted both tilling and puddling, but included spraying of a non-selective herbicide and used a transplanter equipped with a rotor. Fertilization was a basal single application of controlled release fertilizer in nursery boxes for all scenarios. After transplanting, there were no differences in machine work, invested materials or rice yields between the scenarios. The no-tilling scenario saved on fuel consumption, totaling carbon dioxide (CO₂) output of 42 kg ha^?1, which was equal to the 6% reported GHG emissions from fuel consumption by operating machines during rice production in Japan. Methane (CH₄) and nitrous oxide (N₂O) emissions from the paddy fields were also monitored and compared for the scenarios. Methane has a major effect on global warming as part of the GHG emitted from paddy fields. The cumulative CH₄ emissions from the no-tilling cultivation were 43% lower than those from conventional puddling cultivation because the plow layer was more oxidative in no-tilling cultivation. The N₂O emissions were not significantly different between the cultivation scenarios. There were no significant differences in soil respiration, soil carbon contents or straw yields between the cultivation scenarios. The effect of tillage on CO₂ flux in the paddy fields did not seem to be significant in this study. Consequently, the GHG emissions from the no-tilling field counted as CO₂ using global warming potentials were 1,741 kg CO₂ ha^?1 lower than those from the conventional puddling field. In conclusion, no-tilling rice cultivation has the potential to save 1,783 kg CO₂ ha^?1 calculated using the sum of fuel consumption and GHG emissions from paddy fields. No-tilling rice cultivation is considered to be environmentally friendly agriculture with respect to reducing GHG emissions.     

Effects of greenhouse intensive cultivation and organic amendments on greenhouse gas emission according to a soil incubation study

Soil cultivation changes and usage of agricultural wastes can have profound impacts on greenhouse gas (GHG) emission from soil. In this study, the effects of soil cultivation and organic amendment on GHG emission were investigated using aerobic incubation. Surface soil (0–20 cm) from (1) rice–legume consecutive rotation (Rice) and (2) recently (<3 years) converted from rice field to plastic-covered intensive vegetable and flower production (VegC) were collected in Kunming, P.R. China. Rose (Rosa rugosa Thunb.) residues and cattle manure were applied at 5% by weight. Results indicated that N₂O and CO₂ fluxes were significantly influenced by soil cultivation, organic amendment, incubation time and their interaction (p <0.05). Applying cattle manure increased, while rose residue decreased, cumulative N₂O emissions from soil (84 days). Rose residue application significantly increased cumulative CO₂ emissions with peak values of 6371 (Rice) and 7481 mg kg^?1 (VegC), followed by cattle manure addition figure of 2265 (VegC) and 3581 mg kg^?1 (Rice). Both were significantly higher (p <0.05) than the un-amended Control at 709 (VegC) and 904 mg kg^?1 (Rice). Our study demonstrates that a low C/N ratio in cattle manure is better than a high C/N ratio in rose residue in regard to reducing the global warming potential of agricultural soil.     

Nitrogen addition impacts on the emissions of greenhouse gases depending on the forest type: a case study in Changbai Mountain,Northeast China

Purpose

Anthropogenic-induced greenhouse gas (GHG) emission rates derived from the soil are influenced by long-term nitrogen (N) deposition and N fertilization. However, our understanding of the interplay between increased N load and GHG emissions among soil aggregates is incomplete.

Materials and methods

Here, we conducted an incubation experiment to explore the effects of soil aggregate size and N addition on GHG emissions. The soil aggregate samples (0–10 cm) were collected from two 6-year N addition experiment sites with different vegetation types (mixed Korean pine forest vs. broad-leaved forest) in Northeast China. Carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) production were quantified from the soil samples in the laboratory using gas chromatography with 24-h intervals during the incubation (at 20 °C for 168 h with 80 % field water capacity).

Results and discussion

The results showed that the GHG emission/uptake rates were significantly higher in the micro-aggregates than in the macro-aggregates due to the higher concentration of soil bio-chemical properties (DOC, MBC, NO₃ ^?, NH₄ ⁺, SOC and TN) in smaller aggregates. For the N addition treatments, the emission/uptake rates of GHG decreased after N addition across aggregate sizes especially in mixed Korean pine forest where CO₂ emission was decreased about 30 %. Similar patterns in GHG emission/uptake rates expressed by per soil organic matter basis were observed in response to N addition treatments, indicating that N addition might decrease the decomposability of SOM in mixed Korean pine forest. The global warming potential (GWP) which was mainly contributed by CO₂ emission (>98 %) decreased in mixed Korean pine forest after N addition but no changes in broad-leaved forest.

Conclusions

These findings suggest that soil aggregate size is an important factor controlling GHG emissions through mediating the content of substrate resources in temperate forest ecosystems. The inhibitory effect of N addition on the GHG emission/uptake rates depends on the forest type.

     

Continuous manuring combined with chemical fertilizer affects soil microbial residues in a Mollisol

In this study, the influence of 10 years’ continuous application of organic manure at various rates combined with chemical fertilizer on microbial residues was evaluated in a highly fertile temperate soil. The presence and origin of microbial residues were indicated by amino sugar analysis. The treatments were: (1) CK, unfertilized control; (2) OM₀, only chemical fertilizer, no manure added; (3) OM₁, organic manure added at 7.5 Mg?ha^?1?year^?1 plus chemical fertilizer; (4) OM₂, organic manure added at 15 Mg?ha^?1?year^?1 plus chemical fertilizer; and (5) OM₃, organic manure added at 22.5 Mg?ha^?1?year^?1 plus chemical fertilizer. Fertilization significantly increased the total amino sugar concentrations, especially in the plots with higher manure addition rates (OM₂ and OM₃ plots, P?<?0.05). This suggests a positive effect of organic manure combined with chemical fertilizer on the accumulation of microbial residues in soil. However, the highest manure rate (OM₃) did not lead to further increase in the total amino sugar pool as compared with the moderate manure rate (OM₂). This suggests manure addition “saturates” in its effect on microbial residue build-up. The different patterns of individual amino sugars suggest a change in the quality of microbial-derived soil organic matter after 10 years.     

Carbon Dioxide Emission from Black Soil as Influenced by Land‐Use Change and Long‐Term Fertilization

Land‐use change and soil management play a vital role in influencing losses of soil carbon (C) by respiration. The aim of this experiment was to examine the impact of natural vegetation restoration and long‐term fertilization on the seasonal pattern of soil respiration and cumulative carbon dioxide (CO₂) emission from a black soil of northeast China. Soil respiration rate fluctuated greatly during the growing season in grassland (GL), ranging from 278 to 1030 mg CO₂ m^?2 h^?1 with an average of 606 mg CO₂ m^?2 h^?1. By contrast, soil CO₂ emission did not change in bareland (BL) as much as in GL. For cropland (CL), including three treatments [CK (no fertilizer application), nitrogen, phosphorus and potassium application (NPK), and NPK together with organic manure (OM)], soil CO₂ emission gradually increased with the growth of maize after seedling with an increasing order of CK < NPM < OM, reaching a maximum on 17 August and declining thereafter. A highly significant exponential correlation was observed between soil temperature and soil CO₂ emission for GL during the late growing season (from 3 August to 28 September) with Q₁₀ = 2.46, which accounted for approximately 75% of emission variability. However, no correlation was found between the two parameters for BL and CL. Seasonal CO₂ emission from rhizosphere soil changed in line with the overall soil respiration, which averaged 184, 407, and 584 mg CO₂ m^?2 h^?1, with peaks at 614, 1260, and 1770 mg CO₂ m^?2 h^?1 for CK, NPK, and OM, respectively. SOM‐derived CO₂ emission of root free‐soil, including basal soil respiration and plant residue–derived microbial decomposition, averaged 132, 132, and 136 mg CO₂ m^?2 h^?1, respectively, showing no difference for the three CL treatments. Cumulative soil CO₂ emissions decreased in the order OM > GL > NPK > CK > BL. The cumulative rhizosphere‐derived CO₂ emissions during the growing season of maize in cropland accounted for about 67, 74, and 80% of the overall CO₂ emissions for CK, NPK, and OM, respectively. Cumulative CO₂ emissions were found to significantly correlate with SOC stocks (r = 0.92, n = 5, P < 0.05) as well as with SOC concentration (r = 0.97, n = 5, P < 0.01). We concluded that natural vegetation restoration and long‐term application of organic manure substantially increased C sequestration into soil rather than C losses for the black soil. These results are of great significance to properly manage black soil as a large C pool in northeast China.     

Daytime,Temporal, and Seasonal Variations of N2O Emissions in an Upland Cropping System of the Humid Tropics

Abstract

Nitrous oxide (N₂O) contributes to global climate change, and its emission from soil–crop systems depend on soil, environmental, and anthropogenic factors. Thus, we evaluated the variability of N₂O emissions measured by microchambers (cross section: 184 cm²) from a groundnut–fallow–maize–fallow cropping system of the humid tropics. The crops received inorganic nitrogen (N) plus crop residues (NC), inorganic N alone as ammonium sulfate (RN), and half of the inorganic N along with crop residues and chicken manure (N_1/2CM), amounting for the crop rotation to 322, 180, and 400 kg N ha^?1 yr^?1, respectively. The N₂O fluxes during the groundnut–maize crop rotation were log‐normally distributed, and the frequency distributions were positively skewed. Daytime changes in N₂O fluxes were inconsistent, and the 50% of total N₂O emission during the 12 h measurement periods was attained earlier under maize (～11∶00 h) than groundnut covers (～13∶00 h). Spatial variability in each treatment with eight gas chambers was large but smaller during the cropping periods than the fallow, indicating masking efficiency of crop covers for the soil heterogeneity that was accelerated presumably by antecedent climatic variables. The temporal variability of N₂O emissions was also large (coefficients of variation, CV, ranged from 60 to 81%), involving both input differences between treatments and measurement periods. As such, the relative deviation from the annual mean of total N₂O emission was high during the period after a large N application with a maximum of +480%, due to addition of chicken manure. The seasonal contribution of summer and monsoon to N₂O emissions was insignificant. However, intensive rainfall negatively (?0.65**) and the amount of added N from either source positively (0.83***) correlated with the integrated N₂O emissions, and those were exponential. Results suggest that around noon (12∶00 h) gas collection could represent well the daily N₂O fluxes, increasing the number or size of the gas chambers could minimize the large variability, and mainly the rainfall and N inputs regulated its emissions in the humid tropics of Malaysia.     

Impact of Shrub Willow (<Emphasis Type="Italic">Salix</Emphasis> spp.) as a Potential Bioenergy Feedstock on Water Quality and Greenhouse Gas Emissions

The development of shrub willow as a bioenergy feedstock contributes to renewable energy portfolios in many countries with temperate climates and marginal croplands. As willow is developed commercially in the US Northeast, there is a need to better understand its impact on water quality and greenhouse gas (GHG) emissions compared to alternative land uses (e.g., corn, hay). We measured the impact of cultivated willow of various ages (2 and 5 years) and management strategies (fertilized vs. unfertilized) compared to corn and hay on water table depth, soil water NO₃ ^? and PO₄ ^3? concentrations, and N₂O, CH₄, and CO₂ fluxes at the soil-atmosphere interface during a drier than normal year in heavy clay soils with marginal agricultural value in upstate New York, USA. Soil water concentrations resulted in higher PO₄ ^3? in willow and higher NO₃ ^? in corn and hay, although willow is unlikely to negatively impact water quality with respect to phosphorus due to shorter periods of hydrologic connectivity in willow and hay than in corn. Gas fluxes varied spatially and temporally with hot moments of CH₄ and N₂O in corn and hay and seasonally variable CO₂ in willow. While CH₄ did not vary between fields, N₂O was higher in corn and hay, and CO₂ in willow, resulting in no net difference between CO₂ equivalent (CH₄, CO₂, and N₂O) emissions between fields. Converting marginal cropland on clay soils from corn or hay to willow left overall GHG emissions unaffected, slightly increased PO₄ ^3?, and decreased NO₃ ^? concentrations in soil water.     

Dynamics of maize (Zea mays L.) leaf straw mineralization as affected by the presence of soil and the availability of nitrogen

An incubation experiment was carried out with maize (Zea mays L.) leaf straw to analyze the effects of mixing the residues with soil and N amendment on the decomposition process. In order to distinguish between soil effects and nitrogen effects for both the phyllospheric microorganisms already present on the surface of maize straw and soil microorganisms the N amendment was applied in two different placements: directly to the straw or to the soil. The experiment was performed in dynamic, automated microcosms for 22 days at 15 °C with 7 treatments: (1) untreated soil, (2) non-amended maize leaf straw without soil, (3) N amended maize leaf straw without soil, (4) soil mixed with maize leaf straw, (5) N amended soil, (6) N amended soil mixed with maize leaf straw, and (7) soil mixed with N amended maize leaf straw. ¹⁵NH₄¹⁵NO₃ (5 at%) was added. Gas emissions (CO₂, ¹³CO₂ and N₂O) were continuously recorded throughout the experiment. Microbial biomass C, biomass N, ergosterol, δ¹³C of soil organic C and of microbial biomass C as well as ¹⁵N in soil total N, mineral N and microbial biomass N were determined in soil samples at the end of the incubation. The CO₂ evolution rate showed a lag-phase of two days in the non-amended maize leaf straw treatment without soil, which was completely eliminated when mineral N was added. The addition of N generally increased the CO₂ evolution rate during the initial stages of maize leaf straw decomposition, but not the cumulative CO₂ production. The presence of soil caused roughly a 50% increase in cumulative CO₂ production within 22 days in the maize straw treatments due to a slower decrease of CO₂ evolution after the initial activity peak. Since there are no limitations of water or N, we suggest that soil provides a microbial community ensuring an effective succession of straw decomposing microorganisms. In the treatments where maize and soil was mixed, 75% of microbial biomass C was derived from maize. We concluded that this high contribution of maize using microbiota indicates a strong influence of organisms of phyllospheric origin to the microbial community in the soil after plant residues enter the soil.     

土壤有机碳对沼渣或牛粪施用后温室气体排放潜力的影响—盆栽试验结果

A change in the European Union energy policy has markedly promoted the expansion of biogas production.Consequently,large amounts of nutrient-rich residues are being used as organic fertilizers.In this study,a pot experiment was conducted to simulate the high-risk situation of enhanced greenhouse gas (GHG) emissions following organic fertilizer application in energy maize cultivation.We hypothesized that cattle slurry application enhanced CO2 and N2O fluxes compared to biogas digestate because of the overall higher carbon (C) and nitrogen (N) input,and that higher levels of CO2 and N2O emissions could be expected by increasing soil organic C (SOC) and N contents.Biogas digestate and cattle slurry,at a rate of 150 kg NH4+-N ha-1,were incorporated into 3 soil types with low,medium,and high SOC contents (Cambisol,Mollic Gleysol,and Sapric Histosol,termed Clow,Cmedium,and Chigh,respectively).The GHG exchange (CO2,CH4,and N2O) was measured on 5 replicates over a period of 22 d using the closed chamber technique.The application of cattle slurry resulted in significantly higher CO2 and N2O fluxes compared to the application of biogas digestate.No differences were observed in CH4 exchange,which was close to zero for all treatments.Significantly higher CO2 emissions were observed in Chigh compared to the other two soil types,whereas the highest N2O emissions were observed in Cmedium.Thus,the results demonstrate the importance of soil type-adapted fertilization with respect to changing soil physical and environmental conditions.     

Changing pH shifts the microbial sourceas well as the magnitude of N2O emission from soil

Here, we examine the effect of long-term pH differences and short-term pH change on N₂O emissions from soil, and the microbial source (ammonia oxidation versus denitrification) of ¹⁵N-N₂O emissions. ¹⁵N-fertiliser (20 g N m^?2; 10 atom% excess ¹⁵N) was applied to (1) a silt loam soil of pH 7 held at 50% and 65% water-filled pore space (WFPS) (experiment 1) and (2) a loamy sand soil maintained at pH 4.5 and pH 7 for over 40 years (experiment 2). Soils were limed with CaCO₃ or acidified with H₂SO₄, and comparisons were made with unadjusted soils. Ammonia oxidation was the main microbial source of ¹⁵N-N₂O in soils limed to pH 7.0–8.1, unadjusted pH 7.1 (Experiment 1) and long-term pH 7 (experiment 2) soils. Eighty percent of ¹⁵N-N₂O from the long-term pH 4.5 soil (experiment 2) was derived from denitrification suggesting a possible inhibition of N₂O reduction. Short-term acidification to pH 5.6 or 4.3 lowered N₂O emissions. Liming of the pH 4.5 soil resulted in over four times greater N₂O emission (11 mg ¹⁴⁺¹⁵N-N₂O m^?2 over 41 days) than from the long-term pH 7.0 soil (experiment 2), with an associated increase in ammonia oxidiser-N₂O and decrease in denitrifier-N₂O production. This is the first report of a pH-induced change in microbial source of N₂O. Our results highlight the importance of distinguishing between short- and long-term effects of pH management when predicting N₂O emissions from soil, as they exhibit predominance of different microbial groups in N₂O production, with likely adaptation of the microbial community.     

Potential short-term effects of yak and Tibetan sheep dung on greenhouse gas emissions in two alpine grassland soils under laboratory conditions

Yak and Tibetan sheep graze extensively on natural grasslands in the Qinghai-Tibetan Plateau, and large amounts of excrement are directly deposited onto alpine grasslands. However, information on greenhouse gas (GHG) emissions from this excrement is limited. This study evaluated the short-term effects of yak and Tibetan sheep dung on nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂) emissions from alpine steppe soil at a water holding capacity (WHC) of 40 or 60 % and from alpine meadow soil at a WHC of 60 or 80 % under laboratory conditions. Cumulative N₂O emissions over a 15-day incubation period at low soil moisture conditions ranged from 111 to 232 μg N₂O–N kg soil^?1 in the yak dung treatments, significantly (P?<?0.01) higher than that of sheep dung treatments (28.7 to 33.7 μg N₂O–N kg soil^?1) and untreated soils (1.04–6.94 μg N₂O–N kg soil^?1). At high soil moisture conditions, N₂O emissions were higher from sheep dung than yak dung and non-treated soils. No significant difference was found between the yak dung and non-treated alpine meadow soil at 80 % WHC. Low N₂O emission in the yak dung treatment from relatively wet soil was probably due to complete denitrification to N₂. Yak dung markedly (P?<?0.001) increased CH₄ and CO₂ emissions, likely being the main source of these two gases. The addition of sheep dung markedly (P?<?0.001) elevated CO₂ emissions. Dung application significantly (P?<?0.01) increased global warming potential, particularly for alpine steppe soil. In conclusion, our findings suggest that yak and Tibetan sheep dung deposited on alpine grassland soils may increase GHG emissions.     

Effect of Stover Management and Nitrogen Fertilization on N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> Emissions from Irrigated Maize in a High Nitrate Mediterranean Soil

A high soil nitrogen (N) content in irrigated areas quite often results in environmental problems. Improving the management practices of intensive agriculture can mitigate greenhouse gas (GHG) emissions. This study compared the effect of maize stover incorporation or removal together with different mineral N fertilizer rates (0, 200 and 300 kg N ha^?1) on the emission of nitrous oxide (N₂O) and carbon dioxide (CO₂) on a sprinkler-irrigated maize (Zea mays L.). The trail was conducted in the Ebro Valley (NE Spain) in a high nitrate-N soil (i.e. 200 g NO₃–N kg^?1). Nitrous oxide and CO₂ emissions were sampled weekly using a semi-static closed chamber and quantified using the photoacoustic technique in 2011 and 2012. Applying sidedress N fertilizer tended to increase N₂O emissions whereas stover incorporation did not have any clear effect. Nitrification was probably the main process leading to N₂O. Denitrification was limited by the low soil moisture content (WFPS <?54%), due to an adequate irrigation management. Emissions ranged from ??0.11 to 0.36% of the N applied, below the IPCC (2007) values. Nitrogen fertilization tended to reduce CO₂ emission, but only in 2011. Stover incorporation increased CO₂ emission. Nitrogen use efficiency decreased with increasing mineral fertilizer supply. The application of N in high N soils of the Ebro Valley is not necessary until the soil restores a normal mineral N content, regardless of stover management. This will combine productivity with keeping N₂O and CO₂ emissions under control provided irrigation is adequately managed. Testing soil NO₃ ^?–N contents before fertilizing would improve N fertilizer recommendations.     

The effects of catena positions on greenhouse gas emissions along a seasonal wetland (dambo) transect in tropical Zimbabwe

Wetlands are major natural sources of greenhouse gases (GHGs). In central and southern Africa, one of the most extensive wetlands are dambos (seasonal wetlands) which occupy 20–25% of land area. However, there are very little data on GHG methane (CH₄), carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions from dambos, and this study presents the first estimates from dambos in Zimbabwe. The objective was to evaluate the effects of catena positions; upland, dambo mid-slope and dambo bottom, on GHG emissions along an undisturbed dambo transect. Methane emissions were ?0.3, 29.5 and ?1.3 mg m^?2 hr^?1, N₂O emission were 40.1, 3.9 and 5.5 µg m² hr^?1, while CO₂ emissions were 2648.9, 896.2 and 590.1 mg m^?2 hr^?1 for upland, mid-slope and bottom catena, respectively. Our results showed that uplands were important sources of N₂O and CO₂, and a sink for CH₄, while the dambo mid-slope position was a major source of CH₄, but a weak source of CO₂ and N₂O. Dambo bottom catena was weak source GHGs. Overall, dambos were major sources of CH₄ and weak sources of N₂O and CO₂.We concluded that, depending on catenal position, dambos can be major or minor sources of GHGs.     

Soybean Residue Effect on Carbon Fractions and Gaseous Nitrogen Losses at Contrasting Moisture Contents

Most published studies related to crop effects on denitrification are not continuous and are based on the growing period. The objective of this work was to evaluate the effect of different amounts of soybean stubble, under different soil moisture contents, on gaseous nitrogen (N) losses by denitrification from an agricultural soil. The following soil moisture treatments were reached by adding distilled water to soil cores of a typic Hapludoll: 50 and 100% of water‐filled porosity space (WFPS). Residue treatments included no application of residues, amendment with 2600 kg ha^?1 of soybean residues, and amendment with 5200 kg ha^?1 of soybean residues. Cumulative nitrous oxide + dinitrogen (N₂O + N₂) emissions displayed great variability, ranging between 0 and 581.91 µg N kg^?1, which represented 0 to 3.93% of the N residue applied. Under 50% WFPS moisture conditions, statistical differences in cumulative N₂O + N₂ emissions between residue treatments were not detected (p = 0.21), whereas at saturation conditions, cumulative N₂O + N₂ emissions decreased with the application of increasing amounts of soybean residues (p = 0.017). Daily and cumulative N₂O + N₂ emissions significantly increased as soil moisture increased, except at soils amended with 5200 kg ha^?1 of soybean residues; this lack of statistical difference was probably due to the immobilization of native mineral N. Under 50% WFPS soil moisture contents, aeration seemed to be the main factor controlling redox conditions, limiting the denitrification process, and preventing differences in N emissions between residue treatments. The application of soybean residues to saturated soils notably decreased N₂O + N₂ emissions by denitrification through a strong mineral N immobilization into organic and nondenitrifiable forms.     

Annual emissions of nitrous oxide and nitric oxide from a wheat–maize cropping system on a silt loam calcareous soil in the North China Plain

Nitrogen amendment followed by flooding irrigation is a general management practice for a wheat–maize rotation in the North China Plain, which may favor nitrification and denitrification. Consequently, high emissions of nitrous oxide (N₂O) and nitric oxide (NO) are hypothesized to occur. To test this hypothesis, we performed year-round field measurements of N₂O and NO fluxes from irrigated wheat–maize fields on a calcareous soil applied with all crop residues using a static, opaque chamber measuring system. To interpret the field data, laboratory experiments using intact soil cores with added carbon (glucose) and nitrogen (nitrate, ammonium) substrates were performed. Our field measurements showed that pulse emissions after fertilization and irrigation/rainfall contributed to 73% and 88% of the annual N₂O and NO emissions, respectively. Soil moisture and mineral nitrogen contents significantly affected the emissions of both gases. Annual emissions from fields fertilized at the conventional rate (600 kg N ha⁻¹ yr⁻¹) totaled 4.0 ± 0.2 and 3.0 ± 0.2 kg N ha⁻¹ yr⁻¹ for N₂O and NO, respectively, while those from unfertilized fields were much lower (0.5 ± 0.02 kg N ha⁻¹ yr⁻¹ and 0.4 ± 0.05 kg N ha⁻¹ yr⁻¹, respectively). Direct emission factors (EF_ds) of N₂O and NO for the fertilizer nitrogen were estimated to be 0.59 ± 0.04% and 0.44 ± 0.04%, respectively. By summarizing the results of our study and others, we recommended specific EF_ds (N₂O: 0.54 ± 0.09%; NO: 0.45 ± 0.04%) for estimating emissions from irrigated croplands on calcareous soils with organic carbon ranging from 5 to 16 g kg⁻¹. Nitrification dominated the processes driving the emissions of both gases following fertilization. It was evident that insufficient available carbon limited microbial denitrification and thus N₂O emission. This implicates that efforts to enhance carbon sink in calcareous soils likely increase their N₂O emissions.