Evaluating the effect of liming on N<Subscript>2</Subscript>O fluxes from denitrification in an Andosol using the acetylene inhibition and <Superscript>15</Superscript>N isotope tracer methods

We assessed the effect of liming on (1) N₂O production by denitrification under aerobic conditions using the ¹⁵N tracer method (experiment 1); and (2) the reduction of N₂O to N₂ under anaerobic conditions using the acetylene inhibition method (experiment 2). A Mollic Andosol with three lime treatments (unlimed soil, 4 and 20 mg CaCO₃ kg^?1) was incubated at 15 and 25 °C for 22 days at 50% and then 80% WFPS with or without 200 mg N kg^?1 added as ¹⁵N enriched KNO₃ in experiment 1. In experiment 2, the limed and unlimed soils were incubated under completely anaerobic conditions for 44 h (with or without 100 mg N kg^?1 as KNO₃). In experiment 1, limed treatments increased N₂O fluxes at 50% WFPS but decreased these fluxes at 80% WFPS. At 25 °C, cumulative N₂O and ¹⁵N₂O emissions in the high lime treatment were the lowest (with at least 30% less ¹⁵N₂O and total N₂O than the unlimed soil). Under anaerobic conditions, the high lime treatment showed at least 50% less N₂O than the unlimed treatment at both temperatures with or without KNO₃ addition but showed enhanced N₂ production. Our results suggest that the positive effect of liming on the mitigation of N₂O evolution from soil was influenced by soil temperature and moisture conditions.     

Effects of soil type and nitrate concentration on denitrification products (N2O and N2) under flooded conditions in laboratory microcosms

Abstract

Denitrification products nitrous oxide ((N₂O) and nitrogen (N₂)) were measured in three flooded soils (paddy soil from Vietnam, PV; mangrove soil from Vietnam, MV; paddy soil from Japan, PJ) with different nitrate (NO₃^–) concentrations. Closed incubation experiments were conducted in 100-mL bottles for 7 d at 25°C. Each bottle contained 2 g of air-dried soil and 25 mL solution with NO₃^– (concentration 0, 5 or 10 mg N L^?1) with or without acetylene (C₂H₂). The N₂O + N₂ emissions were estimated by the C₂H₂ inhibition method. Results showed that N₂O + N₂ emissions for 7 d were positively correlated with those of NO₃^– removal from solution with C₂H₂ (R² = 0.9872), indicating that most removed NO₃^– was transformed to N₂O and N₂ by denitrification. In PJ soil, N₂O and N₂ emissions were increased significantly (P < 0.05) by the addition of greater NO₃^– concentrations. However, N₂O and N₂ emissions from PV and MV soils were increased by the addition of 0 to 5 mg N L^?1, but not by 5 to 10 mg N L^?1. At 10 mg N L^?1, N₂ emissions for 7 d were greater in PJ soil (pH 7.0) than in PV (pH 5.8) or MV (pH 4.3) soils, while N₂O emissions were higher in PV and MV soils than in PJ soil. In MV soil, N₂O was the main product throughout the experiment. In conclusion, NO₃^– concentration and soil pH affected N₂O and N₂ emissions from three flooded soils.     

Gross nitrogen transformations and related N2O emissions in uncultivated and cultivated black soil

A better understanding of the nitrogen (N) cycle in agricultural soils is crucial for developing sustainable and environmentally friendly N fertilizer management and to propose effective nitrous oxide (N₂O) mitigation strategies. This laboratory study quantified gross nitrogen transformation rates in uncultivated and cultivated black soils in Northeast China. It also elucidated the contribution made by nitrification and denitrification to the emissions of N₂O. In the laboratory, soil samples adjusted to 60 % water holding capacity (WHC) were spiked with ¹⁵NH₄NO₃ and NH₄ ¹⁵NO₃ and incubated at 25 °C for 7 days. The size and ¹⁵N enrichment of the mineral N pools and the N₂O emission rates were determined between 0 and 7 days. The results showed that the average N₂O emission rate was 21.6 ng N₂O-N kg^?1 h^?1 in cultivated soil, significantly higher than in the uncultivated soil (11.6 ng N₂O-N kg^?1 h^?1). Denitrification was found to be responsible for 32.1 % of the N₂O emission in uncultivated soil, and the ratio increased significantly to 43.2 % in cultivated soil, due to the decrease in soil pH. Most of the increase in net N₂O-N emissions observed in the cultivated soil was resulting from the increased production of N₂O through denitrification. Gross nitrification rate was significantly higher in the cultivated soil than in the uncultivated soil, and the ratio of gross nitrification rate/ammonium immobilization rate was 6.87 in cultivated soil, much larger than the uncultivated soil, indicating that nitrification was the dominant NH₄ ⁺ consuming process in cultivated soil, and this will lead to the increased production of nitrate, whereas the increased contribution of denitrification to N₂O emission promoted the larger emission of N₂O. This double impact explains why the risk of N loss to the environment is increased by long-term cultivation and fertilization of native prairie sites, and controlling nitrification maybe effective to abate the negative environmental effects.     

N<Subscript>2</Subscript>O production pathways relate to land use type in acidic soils in subtropical China

Purpose

Agricultural practises impact soil properties and N transformation rate, and have a greater effect on N₂O production pathways in agricultural soils compared with natural woodland soils. However, whether agricultural land use affects N₂O production pathways in acidic soils in subtropical regions remains unknown.

Materials and methods

In this study, we collected natural woodland soil (WD) and three types of agricultural soils, namely upland agricultural (UA), tea plantation (TP) and bamboo plantation (BP) soils. We performed paired ¹⁵N-tracing experiment to investigate the effects of land use types on N₂O production pathways in acidic soils in subtropical regions in China.

Results and discussion

The results revealed that heterotrophic nitrification is the dominant pathway of N₂O production in WD, accounting for 44.6 % of N₂O emissions, whereas heterotrophic nitrification contributed less than 2.7 % in all three agricultural soils, due to a lower organic C content and soil C/N ratio. In contrast, denitrification dominated N₂O production in agricultural soils, accounting for 54.5, 72.8 and 77.1 % in UA, TP and BP, respectively. Nitrate (NO₃ ^?) predominantly affected the contribution from denitrification in soils under different land use types. Autotrophic nitrification increased after the conversion of woodland to agricultural lands, peaking at 42.8 % in UA compared with only 21.5 % in WD, and was positively correlated with soil pH. Our data suggest that pH plays a great role in controlling N₂O emissions through autotrophic nitrification following conversion of woodland to agricultural lands.

Conclusions

Our results demonstrate the variability in N₂O production pathways in soils of different land use types. Soil pH, the quantity and quality of organic C and NO₃ ^? content primarily determined N₂O emissions. These results will likely assist modelling and mitigation of N₂O emissions from different land use types in subtropical acidic soils in China and elsewhere.

     

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.     

The effects of different irrigation regimes on nitrous oxide emissions and influencing factors in greenhouse tomato fields

Purpose

Intensive agricultural practices have enhanced problems associated with the competing use of limited water resources. Nitrous oxide (N₂O) is a major contributor to global warming. It is important for researchers to ascertain the relationship between irrigation and soil N₂O emissions in order to identify mitigation strategies to reduce nitrous oxide emissions. Different irrigation amounts affect soil water dynamics and nitrogen turnover. The effect of three lower limits of irrigation on soil N₂O emissions, influencing factors, and abundance of genes involved in nitrification and denitrification were investigated in tomato irrigated in a greenhouse.

Materials and methods

Observations were performed between April and August 2015 in a long-term irrigated field subjected to different lower limits of irrigation: 20 kPa (D₂₀), 30 kPa (D₃₀), and 40 kPa (D₄₀) from greenhouse soil during the tomato crop season. Soil N₂O fluxes were monitored using the static chamber-gas chromatograph method. Copy numbers of genes were determined using the real-time quantitative polymerase chain reaction (real-time PCR) technique. Characteristics of soil N₂O emissions were analyzed, and differences between irrigation regimes were determined. The effects of influencing factors on soil N₂O emissions were analyzed, including soil temperature, soil moisture, soil pH, and soil mineral nitrogen, as well as changes in the abundance of soil ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) based on amoA genes and denitrifier genes (nosZ, nirK, and cnorB).

Results and discussion

Our results showed that peaks in N₂O emissions occurred 1–5 days after each irrigation. During the whole tomato growth period, soil N₂O fluxes were lowest under D₃₀ treatment compared with those under D₂₀ and D₄₀ treatments. Soil NO₃ ^?-N concentrations were significantly higher than NH₄ ⁺-N concentrations. Soil N₂O fluxes were significantly related to soil moisture, NH₄ ⁺-N concentrations (P < 0.01), soil pH, and AOA copy numbers (P < 0.05). There was no consistent correlation between soil N₂O emissions, soil temperature, and soil NO₃ ^?-N concentrations. Different irrigation regimes significantly affected AOA copy numbers but did not affect the expression of other genes. AOA copy numbers were higher than those of AOB. Soil N₂O fluxes significantly affected the AOA copy numbers and potential nitrification rates (P < 0.05).

Conclusions

Soil moisture, pH, and NH₄ ⁺-N concentration were important factors affecting soil N₂O emissions. Compared with other genes associated with nitrification and denitrification, AOA plays an important role in N₂O emissions from greenhouse soils. Selecting a lower limit of irrigation of 30 kPa could effectively reduce N₂O emissions from vegetable soils.

     

农业土壤中的氧化亚氮排放: 为减排综述时空变化

This short review deals with soils as an important source of the greenhouse gas N2O. The production and consumption of N2O in soils mainly involve biotic processes: the anaerobic process of denitrification and the aerobic process of nitrification. The factors that significantly influence agricultural N2O emissions mainly concern the agricultural practices (N application rate, crop type, fertilizer type) and soil conditions (soil moisture, soil organic C content, soil pH and texture). Large variability of N2O fluxes is known to occur both at different spatial and temporal scales. Currently new techniques could help to improve the capture of the spatial variability. Continuous measurement systems with automatic chambers could also help to capture temporal variability and consequently to improve quantification of N2O emissions by soils. Some attempts for mitigating soil N2O emissions, either by modifying agricultural practices or by managing soil microbial functioning taking into account the origin of the soil N2O emission variability, are reviewed.     

Effects of carbon and phosphorus addition on microbial respiration,N<Subscript>2</Subscript>O emission,and gross nitrogen mineralization in a phosphorus-limited grassland soil

Soil microbes are frequently limited by carbon (C), but also have a high phosphorus (P) requirement. Little is known about the effect of P availability relative to the availability of C on soil microbial activity. In two separate experiments, we assessed the effect of P addition (20 mg P kg^?1 soil) with and without glucose addition (500 mg C kg^?1 soil) on gross nitrogen (N) mineralization (¹⁵N pool dilution method), microbial respiration, and nitrous oxide (N₂O) emission in a grassland soil. In the first experiment, soils were incubated for 13 days at 90% water holding capacity (WHC) with addition of NO₃^? (99 mg N kg^?1 soil) to support denitrification. Addition of C and P had no effect on gross N mineralization. Initially, N₂O emission significantly increased with glucose, but it decreased at later stages of the incubation, suggesting a shift from C to NO₃^? limitation of denitrifiers. P addition increased the N₂O/CO₂ ratio without glucose but decreased it with glucose addition. Furthermore, the ¹⁵N recovery was lowest with glucose and without P addition, suggesting a glucose by P interaction on the denitrifying community. In the second experiment, soils were incubated for 2 days at 75% WHC without N addition. Glucose addition increased soil ¹⁵N recovery, but had no effect on gross N mineralization. Possibly, glucose addition increased short-term microbial N immobilization, thereby reducing N-substrates for nitrification and denitrification under more aerobic conditions. Our results indicate that both C and P affect N transformations in this grassland soil.     

Time-lagged induction of N2O emission and its trade-off with NO emission from a nitrogen fertilized Andisol

Abstract

To understand the influence of basal application of N fertilizer on nitrification potential and N₂O and NO emissions, four soil samples were collected from an upland Andisol field just before (sample 1) and 4 (sample 2), 36 (sample 3) and 72 (sample 4) days after the basal application of N fertilizer during the Chinese cabbage growing season from 12 September to 30 November 2005. The potentials of N₂O production and nitrification of the soils were determined using a ¹⁵N tracer technique and the soils were incubated for 25 days at 25°C and 60% water-filled pore space (WFPS). The results revealed that as much as 84–97% N₂O and almost all NO were produced by nitrification. The ¹⁵N₂O emission peak occurred approximately 350 h after the beginning of incubation for samples 1 and 2, but just 48 h later in samples 3 and 4. Total ¹⁵N₂O emission during the 25-day incubation of samples 3 and 4 ranged from 190 to 198 µg N kg^?1 soil, which was significantly higher than the 99–108 µg N kg^?1 soil recorded in samples 1 and 2. Basal application of N fertilizer did not immediately increase the nitrification potential and the ratio of N₂O to N added, but did dramatically increase the nitrification potential and the ratio of N₂O to N added as (¹⁵NH₄)₂SO₄ 36–72 days after the basal N fertilizer was added. In contrast, NO emission was negatively correlated with nitrification potential and total N₂O emission. As a result, a trade-off relationship between total NO and N₂O emissions was identified. The results indicated that there was a time-lagged induction of the change of N turnover in the soil, which was possibly caused by slow population growth of the nitrifiers and/or a slow shift in the microbial community in the soil.     

Nitrous oxide emissions are greater in silt loam soils with a legacy of manure application than without

Soil carbon (C) content, often found at elevated levels in manured soils, can play a critical role in regulating nitrous oxide emissions. Nitrate availability and oxygen status are the other primary drivers of emissions, yet the interaction of these three variables and the dynamics of the denitrification process are inadequately known. Emissions of N₂O and N₂ were measured from two New York State soils that were historically managed either with regular cattle manure applications (M) or without manure (NM). For 168 h, repacked soil cores were maintained at 80 % water-filled pore space after the application of 0, 50, 100, and 200 kg ha^?1 of labeled K¹⁵NO₃. Significant differences were found in the N₂O emission profiles between the two treatments with a simultaneous increasing trend in emissions with higher fertilizer applications. The M soil produced 53-, 15.5-, and 8.6-fold increases in N₂O emissions over the NM soil at the 50-, 100-, and 200-kg ha^?1 N rates, respectively. Additionally, the mean ratio of nitrous oxide to total denitrification (N₂O/(N₂O + N₂)) was higher for M soil. It increased to values of 0.17, 0.25, and 0.43 for fertilizer rates of 50, 100, and 200 kg ha^?1, respectively, in contrast to ratios in the NM soil of 0.01, 0.03, and 0.14.     

Increased fungal dominance in N2O emission hotspots along a natural pH gradient in organic forest soil

Drained organic forest soils represent a hotspot for nitrous oxide (N₂O) emissions, which are directly related to soil fertility, with generally higher emissions from N-rich soils. Highest N₂O emissions have been observed in organic forest soils with low pH. The mechanisms for these high emissions are not fully understood. Therefore, the present study was conducted to gain a deeper insight into the underlying mechanisms that drive high N₂O emissions from acid soils. Specifically, we investigated the microbial community structure, by phospholipid fatty acid analysis, along a natural pH gradient in an organic forest soil combined with measurements of physico-chemical soil properties. These were then statistically related to site-specific estimates of annual N₂O emissions along the same natural pH gradient. Our results indicate that acidic locations with high N₂O emissions had a microbial community with an increased fungal dominance. This finding points to the importance of fungi for N₂O emissions from acid soils. This may either be directly via fungal N₂O production or indirectly via the effect of fungi on the N₂O production by other microorganisms (nitrifiers and denitrifiers). The latter may be due to fungal mediated N mineralization, providing substrate for N₂O production, or by creating favourable conditions for the bacterial denitrifier community. Therefore, we conclude that enhanced N₂O emission from acid forest soil is related, in addition to the known inhibitory effect of low pH on bacterial N₂O reduction, to a soil microbial community with increased fungal dominance. Further studies are needed to reveal the exact mechanisms.     

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 7-year application of a nitrification inhibitor, dicyandiamide (DCD), on soil microbial biomass, protease and deaminase activities, and the abundance of bacteria and archaea in pasture soils

Purpose

The nitrification inhibitor dicyandiamide (DCD) has been shown to be highly effective in reducing nitrate (NO₃ ^?) leaching and nitrous oxide (N₂O) emissions when used to treat grazed pasture soils. However, there have been few studies on the possible effects of long-term DCD use on other soil enzyme activities or the abundance of the general soil microbial communities. The objective of this study was to determine possible effects of long-term DCD use on key soil enzyme activities involved in the nitrogen (N) cycle and the abundance of bacteria and archaea in grazed pasture soils.

Materials and methods

Three field sites used for this study had been treated with DCD for 7 years in field plot experiments. The three pasture soils from three different regions across New Zealand were Pukemutu silt loam in Southland in the southern South Island, Horotiu silt loam in the Waikato in the central North Island and Templeton silt loam in Canterbury in the central South Island. Control and DCD-treated plots were sampled to analyse soil pH, microbial biomass C and N, protease and deaminase activity, and the abundance of bacteria and archaea.

Results and discussion

The three soils varied significantly in the microbial biomass C (858 to 542 μg C g^?1 soil) and biomass N (63 to 28 μg N g^?1), protease (361 to 694 μg tyrosine g^?1 soil h^?1) and deaminase (4.3 to 5.6 μg NH₄ ⁺ g^?1 soil h^?1) activity, and bacteria (bacterial 16S rRNA gene copy number: 1.64?×?10⁹ to 2.77?×?10⁹ g^?1 soil) and archaea (archaeal 16S rRNA gene copy number: 2.67?×?10⁷ to 3.01?×?10⁸ g^?1 soil) abundance. However, 7 years of DCD use did not significantly affect these microbial population abundance and enzymatic activities. Soil pH values were also not significantly affected by the long-term DCD use.

Conclusions

These results support the hypothesis that DCD is a specific enzyme inhibitor for ammonia oxidation and does not affect other non-target microbial and enzyme activities. The DCD nitrification inhibitor technology, therefore, appears to be an effective mitigation technology for nitrate leaching and nitrous oxide emissions in grazed pasture soils with no adverse impacts on the abundance of bacteria and archaea and key enzyme activities.     

Phosphorus application reduces N2O emissions from tropical leguminous plantation soil when phosphorus uptake is occurring

We examined the effects of phosphorus (P) fertilization on N₂O emissions from an Acacia mangium plantation in Indonesia. We focused on the roles of microbial and plant root activities using a trenching method to prepare root-excluded and root-including plots. In root-excluded plots, P application did not change the amount of N₂O emissions. By contrast, in root-including plots, P application significantly reduced N₂O emissions (from 71.1?±?20.2 to 19.3?±?5.1 mg N m^?2 106 days^?1). Lower total P, Bray-2 P, and Bio-P (microbial P determined by chloroform fumigation extraction method) contents in the soils of root-including plots as compared to root-excluded plots a few days after P application shows that acacia trees absorbed P fertilizer rapidly. This rapid P uptake probably relieved the P limitation of acacia and might have consistently increased root N uptake. This interpretation is supported by lower inorganic N content in P-applied soils (the average of three sampling times is 8.9 and 11.3 μg N g soil^?1 in P-applied soils and soils without P application, respectively), which in turn decreased N₂O emissions. Our study suggests that P fertilizer suppresses N₂O emissions from tropical leguminous forest plantations.     

Effects of soil moisture on gross N transformations and N2O emission in acid subtropical forest soils

Soil moisture changes, arising from seasonal variation or from global climate changes, could influence soil nitrogen (N) transformation rates and N availability in unfertilized subtropical forests. A ¹⁵?N dilution study was carried out to investigate the effects of soil moisture change (30–90 % water-holding capacity (WHC)) on potential gross N transformation rates and N₂O and NO emissions in two contrasting (broad-leaved vs. coniferous) subtropical forest soils. Gross N mineralization rates were more sensitive to soil moisture change than gross NH₄ ⁺ immobilization rates for both forest soils. Gross nitrification rates gradually increased with increasing soil moisture in both forest soils. Thus, enhanced N availability at higher soil moisture values was attributed to increasing gross N mineralization and nitrification rates over the immobilization rate. The natural N enrichment in humid subtropical forest soils may partially be due to fast N mineralization and nitrification under relatively higher soil moisture. In broad-leaved forest soil, the high N₂O and NO emissions occurred at 30 % WHC, while the reverse was true in coniferous forest soil. Therefore, we propose that there are different mechanisms regulating N₂O and NO emissions between broad-leaved and coniferous forest soils. In coniferous forest soil, nitrification may be the primary process responsible for N₂O and NO emissions, while in broad-leaved forest soil, N₂O and NO emissions may originate from the denitrification process.     

Fungal and bacterial denitrification are differently affected by long-term pH amendment and cultivation of arable soil

Bacteria are considered as playing a predominant role in the production of nitrous oxide (N₂O) in arable soil. Despite the knowledge that fungi are able to denitrify their contribution to denitrifier N₂O production from arable soil is uncertain. Here, we assess the capability of fungi and bacteria to contribute to N₂O emission from arable soil by measuring potential denitrification rates (PDR) as N₂O production, after application of selective inhibitors aimed at distinguishing between fungal and bacterial denitrification, and related PDR to characteristics of the soil microbial community. Soil was sampled from a long-term crop rotation maintained since 1961 at seven different pH levels, ranging in 0.5 increments from pH 4.5 to 7.5, and along a cultivation gradient from freshly ploughed soil to three years under ley grass. Over both pH and cultivation gradients, bacteria contributed up to 54% and fungi contributed to 18% of the PDR. Residual N₂O production that was not targeted by the selective inhibitors and hence could not be attributed to fungi or bacteria might be due to pre-synthesised enzymes or resistant organisms. The PDR of the bacterial community responded positively to increase in soil pH with the lowest PDR at pH 4.2 and the highest around pH 5.9. In contrast, fungal denitrification was not influenced by soil pH. Changes in ester linked fatty acids (ELFA) concentrations showed that whilst total bacterial biomass decreased with increasing pH fungal biomass was not significantly influenced by pH, driving an increase in the ratio of fungal–bacterial biomass. Both fungal biomass and bacterial biomass, and the PDR from the control treatment (no inhibitor application) across the pH gradient were greatest under long-term ley. Concentrations of fatty acids a15:0, 16:1ω7 and 17:1ω8 of microbial origin were positively correlated with the proportion of denitrification activity that was repressed by bacterial inhibitors. This suggests that there is a relationship between organisms that possess the fatty acids a15:0, 16:1ω7 and 17:1ω8, and the function of denitrification. Our results demonstrate that both fungal and bacterial denitrification were occurring in this arable soil. That management for pH and cultivation had differing effects on the potential contribution of fungal and bacterial denitrification to N₂O production has implications for the development of appropriate management practices for mitigation of this greenhouse gas.     

Soil Moisture Controls the Denitrification Loss of Urea Nitrogen from Silty Clay Soil

Relative control of soil moisture [30, 60, and 80 percent water-holding capacity (WHC)] on nitrous oxide (N₂O) emissions from Fargo-Ryan soil, treated with urea at 0, 150, and 250 kg N ha^?1 with and without nitrapyrin [2-chloro-(6-trichloromethyl) pyridine] (NP), was measured under laboratory condition for 140 days. Soil N₂O emissions significantly increased with increasing nitrogen (N) rates and WHC levels. Urea applied at 250 kg N ha^?1 produced the greatest cumulative N₂O emissions and averaged 560, 3919, and 15894 µg kg^?1 at 30, 60, and 80 percent WHC, respectively. At WHC ≤ 60 percent, addition of NP to urea significantly reduced N₂O losses by 2.6- to 4.8-fold. Additions of NP to urea reduced N₂O emission at rates similar to the control (0 N) until 48 days for 30 percent WHC and 35 days for 60 and 80 percent WHC. These results can help devise urea-N fertilizer management strategies in reducing N₂O emissions from silty-clay soils.     

Nitrous oxide emissions under different soil and land management conditions

Nitrous oxide (N₂O) emissions of three different soils – a rendzina on cryoturbed soil, a hydromorphic leached brown soil and a superficial soil on a calcareous plateau – were measured using the chamber method. Each site included four types of land management: bare soil, seeded unfertilized soil, a suboptimally fertilized rapeseed crop and an overfertilized rapeseed crop. Fluxes varied from –1g to 100g N₂O-nitrogen ha^–1 day^–1. The highest rates of N₂O emissions were measured during spring on the hydromorphic leached brown soil which had been fertilized with nitrogen (N); the total emissions during a 5-month period exceeded 3500gNha^–1. Significant fluxes were also observed during the summer. Very marked effects of soil type and management were observed. Two factors – the soil hydraulic behaviour and the ability of the microbial population to reduce N₂O – appear to be essential in determining emissions of N₂O by soils. In fact, the hydromorphic leached brown soil showed the highest emissions, despite having the lowest denitrification potential because of its water-filled pore space and low N₂O reductase activity. Soil management also appears to affect both soil nitrate content and N₂O emissions. Received: 4 April 1997     

Influence of water depth and soil amelioration on greenhouse gas emissions from peat soil columns

Recently, large areas of tropical peatland have been converted into agricultural fields. To be used for agricultural activities, peat soils need to be drained, limed and fertilized due to excess water, low nutrient content and high acidity. Water depth and amelioration have significant effects on greenhouse gas (GHG) production. Twenty-seven soil samples were collected from Jabiren, Central Kalimantan, Indonesia, in 2014 to examine the effect of water depth and amelioration on GHG emissions. Soil columns were formed in the peatland using polyvinyl chloride (PVC) pipe with a diameter of 21 cm and a length of 100 cm. The PVC pipe was inserted vertically into the soil to a depth of 100 cm and carefully pulled up with the soil inside after sealing the bottom. The treatments consisting of three static water depths (15, 35 and 55 cm from the soil surface) and three ameliorants (without ameliorant/control, biochar+compost and steel slag+compost) were arranged using a randomized block design with two factors and three replications. Fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) from the soil columns were measured weekly. There was a linear relationship between water depth and CO₂ emissions. No significant difference was observed in the CH₄ emissions in response to water depth and amelioration. The ameliorations influenced the CO₂ and N₂O emissions from the peat soil. The application of biochar+compost enhanced the CO₂ and N₂O emissions but reduced the CH₄ emission. Moreover, the application of steel slag+compost increased the emissions of all three gases. The highest CO₂ and N₂O emissions occurred in response to the biochar+compost treatment followed by the steel slag-compost treatment and without ameliorant. Soil pH, redox potential (Eh) and temperature influenced the CO₂, CH₄ and N₂O fluxes. Experiments for monitoring water depth and amelioration should be developed using peat soil as well as peat soil–crop systems.     

The short-term effects of liming on organic carbon mineralisation in two acidic soils as affected by different rates and application depths of lime

Two acidic soils (initial pH, 4.6) with contrasting soil organic C (SOC) contents (11.5 and 40 g C kg^?1) were incubated with ¹³C-labelled lime (Ca¹³CO₃) at four different rates (nil, target pH 5, 5.8 and 6.5) and three application depths (0–10, 20–30 and 0–30 cm). We hypothesised that liming would stimulate SOC mineralisation by removing pH constraints on soil microbes and that the increase in mineralisation in limed soil would be greatest in the high-C soil and lowest when the lime was applied in the subsoil. While greater SOC mineralisation was observed during the first 3 days, likely due to lime-induced increases in SOC solubility, this effect was transient. In contrast, SOC mineralisation was lower in limed than in non-limed soils over the 87-day study, although only significant in the Tenosol (70 μg C g^?1 soil, 9.15%). We propose that the decrease in SOC mineralisation following liming in the low-C soil was due to increased microbial C-use efficiency, as soil microbial communities used less energy maintaining intracellular pH or community composition changed. A greater reduction in SOC mineralisation in the Tenosol for low rates of lime (0.3 and 0.5 g column^?1) or when the high lime rate (0.8 g column^?1) was mixed through the entire soil column without changes in microbial biomass C (MBC) could indicate a more pronounced stabilising effect of Ca²⁺ in the Tenosol than the Chromosol with higher clay content and pH buffer capacity. Our study suggests that liming to ameliorate soil acidity constraints on crop productivity may also help to reduce soil C mineralisation in some soils.