Nitrification in two coniferous forest soils after different fertilization treatments

The effects of seven different fertilization treatments on nitrification in the organic horizons of a Myrtillus-type (MT) and a Calluna-type pine forest in southern Finland were studied. No (NO^?₃ + NO^?₂)-N accumulated in unfertilized soils during 6 weeks at 14 or 20°C in the laboratory. Net nitrification was stimulated by urea in both soils (but more in the MT pine forest soil) and to a lesser degree by wood ash but not by ammonium nitrate or nitroform (ureaformaldehyde). Nitrification was not detected in nitroform fertilized soils although ammonium accumulation was high during incubation. In the MT pine forest soil, net nitrification appeared to be stimulated by apatite, biotite and micronutrients. Nitrapyrin inhibited nitrification indicating that it was carried out by autotrophic nitrifiers. In the urea-fertilized MT pine forest soil, nitrification took place at an incubation temperature of 0°C. Accumulation of (N0^?₃ + NO^?₂)-N was highest in soil sampled at < 10°C.     

Persistence of effects of nitrification inhibitors added to soils

Abstract

The persistence of the effects of four nitrification inhibitors (2‐ethynylpyridine, nitrapyrin, etridiazole, 3‐methylpyrazole‐l‐carboxamide) on nitrification in soil was assessed by measuring the ability of two soils to nitrify NH₄ ⁺ [added as (NH₄)₂SO₄] after they had been treated with 5 μg inhibitor g^‐1 soil and incubated at 10, 20, or 30°C for 0, 21, 42, 84, 126, or 168 days. The soils used differed markedly in organic‐matter content (1.2 and 4.2% organic C). The data obtained showed that the persistence of the effects of the inhibitors studied decreased markedly with increase in soil temperature from 10 to 30°C and that, whereas the initial inhibitory effects of the test compounds on nitrification were greatest with the soil having the lower organic‐matter content, the persistence of their effects at 20 or 30°C was greatest with the soil having the higher organic‐matter content. The inhibitory effects of 2‐ethynylpyridine and etridiazole on nitrification were considerably more persistent than those of nitrapyrin or 3‐methylpyrazole‐l‐carboxamide and were significant even after incubation of inhibitor‐treated soil at 20°C for 168 days.     

Coupled nitrification-denitrification associated with liquid manure in a gel-stabilized model system

Summary Following the application of liquid manure to soil, the development of the two processes, nitrification and denitrification, was studied in a two-phase model system. A saturated mixture of manure and soil, stabilized with silica gel, was overlain by an aerobic soil phase. Profiles of the redox potential pH, inorganic N, dissolved organic C, nitrification and denitrification potentials, and phospholipid concentrations for an estimate of microbial biomass were measured during a 20-day period. NH ₄ ⁺ diffusing into the aerobic soil was oxidized within 10 mm of the interface, but with only a small accumulation of NO ₂ ^- and NO ₃ ^- . It was estimated that N equivalent to approximately 70% of the NH ₄ ⁺ originally present in the manure was lost through coupled nitrification-denitrification. The potentials for nitrification and denitrification increased 40-and 20-fold, respectively, around the interface. Maximum values were recorded after 14 days. Within 0–5 mm of the anaerobic zone, apparent generation times for NH ₄ ⁺ -oxidizing bacteria of 1.1–1.8 days were estimated between day 1 and day 7. The phospholipid concentration profiles suggested that the biomass within 2 mm on either side of the interface was stimulated throughout the 20-day period.     

Compared effects of long-term pig slurry applications and mineral fertilization on soil denitrification and its end products (N2O, N2)

The long-term (9 years) effect of pig slurry applications vs mineral fertilization on denitrifying activity, N₂O production and soil organic carbon (C) (extractable C, microbial biomass C and total organic C) was compared at three soil depths of adjacent plots. The denitrifying activities were measured on undisturbed soil cores and on sieved soil samples with acetylene method to estimate denitrification rates under field or potential conditions. Pig slurry applications had a moderate impact on the C pools. Total organic C was increased by +6.5% and microbial biomass C by ≥25%. The potential denitrifying activity on soil suspension was stimulated (×1.8, P<0.05) 12 days after the last slurry application. This stimulation was still apparent, but not significant, 10 months later and, according to both methods of denitrifying activity measurement (r ²=0.916, P<0.01 on sieved soil; r ²=0.845, P<0.001 on soil cores), was associated with an increase in microbial biomass C above a threshold of about 105 mg kg⁻¹. The effect of pig slurry on denitrification and N₂O reduction rates was detected on the surface layer (0–20 cm) only. However, no pig slurry effect could be detected on soil cores at field conditions or after NO₃ ⁻ enrichments at 20°C. Although the potential denitrifying activity in sieved soil samples was stimulated, the N₂O production was lower (P<0.03) in the plot fertilized with pig slurry, indicating a lower N₂O/(N₂O + N₂) ratio of the released gases. The pig-slurry-fertilized plot also showed a higher N₂O reduction activity, which is coherent with the lower N₂O production in anaerobiosis.     

Repeated freeze–thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil

Freezing and thawing may substantially influence the rates of C and N cycling in soils, and soil frost was proposed to induce NO losses with seepage from forest ecosystems. Here, we test the hypothesis that freezing and thawing triggers N and dissolved organic matter (DOM) release from a forest soil after thawing and that low freezing temperatures enhance the effect. Undisturbed soil columns were taken from a soil at a Norway spruce site either comprising only O horizons or O horizons + mineral soil horizons. The columns were subjected to three cycles of freezing and thawing at temperatures of –3°C, –8°C, and –13°C. The control columns were kept at constant +5°C. Following the frost events, the columns were irrigated for 20 d at a rate of 4 mm d^–1. Percolates were analyzed for total N, mineral N, and dissolved organic carbon (DOC). The total amount of mineral N extracted from the O horizons in the control amounted to 8.6 g N m^–2 during the experimental period of 170 d. Frost reduced the amount of mineral N leached from the soil columns with –8°C and –13°C being most effective. In these treatments, only 3.1 and 4.0 g N m^–2 were extracted from the O horizons. Net nitrification was more negatively affected than net ammonification. Severe soil frost increased the release of DOC from the O horizons, but the effect was only observed in the first freeze–thaw cycle. We found no evidence for lysis of microorganisms after soil frost. Our experiment did not confirm the hypothesis that soil frost increases N mineralization after thawing. The total amount of additionally released DOC was rather low in relation to the expected annual fluxes.     

A high soil potential net nitrification was observed at 4 m depth: What are the driving factors?

Studies about nitrogen (N) mineralization and nitrification in deep soil layers are rare because N processes are considered to occur mainly in topsoil that hosts active and diverse microbial communities. This study aimed to measure the soil potential net N mineralization (PNM) and nitrification (PNN) down to 4 m depth and to discuss factors controlling their variability. Twenty-one soil cores were collected at the Restinclières agroforestry experimental site, where 14-year-old hybrid walnut trees were intercropped with durum wheat. Soil cores were incubated in the dark in the laboratory at both 6 and 25°C. The soil was a deep calcic fluvisol with a fluctuating water table. It featured a black layer that was very rich in organic matter and permanently water saturated at depths between 3.0 and 4.0 m. The mean soil mineral N content was 3 mg N kg⁻¹ soil in the upper 0.0–0.2 m layer, decreasing until a depth of 2 m and increasing to the maximum value of 25.8 mg N kg⁻¹ soil in the black layer. While nitrate (NO₃⁻) was the dominant form of mineral N (89%) in the upper 0.0–0.2 m layer, its proportion progressively decreased with depth until ammonium (NH₄⁺) became almost the only form of mineral N (97%) in the saturated black layer. Laboratory soil incubation revealed that PNM and PNN occurred at all depths, although the latter remained low at 6°C. The soil nitrate content in the black layer was multiplied by 48 times after 51 days of incubation at 25°C, whereas it was almost inexistent at the sampling date. While the soil total N, the pH and the incubation temperature explained 84% of the variation in PNM, only 29% of the percent nitrification variance was explained by the incubation temperature (T_inc) and the soil C-to-N ratio. These results point out the necessity to consider soil potential net N mineralization and nitrification of deep soil layers to improve model predictions.     

Temperature effects on ammonification and nitrification in a tropical soil

Ammonification of soil organic N and nitrification of ammonium-N was studied in Tindall clay loam over a range of temperatures from 20–60 C. Nitrification rates at each temperature were constant throughout the 28 day incubation, whereas most of the ammonification occurred in the first 7 days. The optimum for nitrification was close to 35 C. exhibiting a sharp peak at this temperature at which the potential rate was 4.8 μg N/g day^?1, compared with 0.5 μg N/g day^?1 at 20°C and 0.25 μg N/g day^?1 at 60°C. The optimum temperature for ammonification was approximately 50°C at which the rate was 2.8 μg N/g day^?1 in the first 7 days but only 0.5 μg N/g day^?1 between 14 and 28 days.The temperature responses could be described mathematically with functions of the type log_oN = k × 1/T.The results are discussed in relation to daily patterns of N mineralization in the field where temperatures show diurnal fluctuation.     

Interactive effects of ammonium application rates and temperature on nitrous oxide emission from tropical agricultural soil

Emissions of nitrous oxide (N₂O), a potent greenhouse gas, from agricultural soil have been recognized to be affected by nitrogen (N) application and temperature. Most of the previous studies were carried out to determine effects of temperature on N₂O emissions at a fixed N application rate or those of N application rates at a specific temperature. Knowledge about the effects of different ammonium (NH₄⁺) application rates and temperatures on N₂O emissions from tropical agricultural soil and their interactions is limited. Five grams of air-dried sandy loam soil, collected in Central Vietnam, were adjusted to 0, 400, 800 and 1200 mg NH₄-N kg^–1 soil (abbreviated as 0 N, 400 N, 800 N and 1200 N, respectively) at 60% water holding capacity were aerobically incubated at 20°C, 25°C, 30°C or 35°C for 28 days. Mineral N contents and N₂O emission rates were determined on days 1, 3, 5, 7, 14, 21 and 28. Cumulative N₂O emissions for 28 days increased with increasing NH₄⁺ application rates from 0 to 800 mg N kg^–1 and then declined to 1200 mg N kg^–1. Cumulative N₂O emissions increased in the order of 35°C, 20°C, 30°C and 25°C. This lowest emission at 35°C occurred because N₂O production was derived only from autotrophic nitrification while other N₂O production processes, e.g., nitrifier denitrification and coupled nitrification-denitrification occurred at lower temperatures. More specifically, cumulative N₂O emissions peaked at 800 N and 25°C, and the lowest emissions occurred at 1200 N and 35°C. In conclusion, N₂O emissions were not exponentially correlated with NH₄⁺ application rates or temperatures. Higher NH₄⁺ application rates at higher temperatures suppressed N₂O emissions.     

Nitrogen mineralization,N<Subscript>2</Subscript>O production and soil microbiological properties as affected by long-term applications of sewage sludge composts

A field study was conducted to investigate the long-term effect of surface application of sewage sludge composts vs chemical N fertilizer on total N, total C, soluble organic C, pH, EC, microbial biomass C and N, protease activity, deaminase activity, urease activity, gross and net rates of N mineralization and nitrification, CO₂ evolution, and N₂O production. Soil samples were taken from five depths (0–15, 15–20, 20–30, 30–40, and 40–50 cm) of a long-term experiment at the University of Tokyo, Japan. Three fields have been receiving sewage sludge composted with rice husk (RH), sawdust (SD), or mixed chemical fertilizer NPK (CF), applied at the rate of 240 kg N ha^–1 each in split applications in summer and autumn since 1978. Significantly higher amounts of total N and C and soluble organic C were found in the compost than in the CF treatments up to the 40-cm soil depth, indicating improved soil quality in the former. In the CF treatment, soil pH values were significantly lower and electrical conductivity values were significantly higher than those of compost-treated soils of up to 50 cm depth. Soil microbial biomass C and N, CO₂ evolution, protease, deaminase, and urease activities were significantly higher in the compost than in the CF treatments due to greater availability of organic substrates that stimulated microbial activity. Gross N mineralization rates determined by ¹⁵N dilution technique were eight and five times higher in the SD and RH treatments than in the CF treatment, respectively, probably due to high levels of microbial and enzyme activities. Net N mineralization rates were also significantly higher in the compost treatments and were negative in the CF treatment indicating immobilization. Net nitrification rates were higher in compost treatments and negative in the CF treatment. Nitrous oxide productions from compost treatments were higher than the CF treatment due to the greater availability of mineral N as a result of higher mineralization and nitrification rates and soluble organic C in the former. Most of the measured parameters were highest in the surface soil (0–15 cm) and were significantly higher in the SD treatment than in the RH treatment.     

Nitrogen turnover and N2O production in the forest floor of beech stands as influenced by forest management

Effects of forest management (thinning) on gross ammonification, net ammonification, net nitrification, microbial biomass, and N₂O production were studied in the forest floor of adjacent untreated control (“C”) and thinned (“T”) plots in three beech (Fagus sylvatica L.) stands in the Swabian Jura (Southern Germany) during three intensive field campaigns in the year 2004. The investigated sites are located less than 1 km apart on the slopes of a narrow valley. Due to different exposure (southwest, northeast, northwest), the three sites are characterized by warm‐dry microclimate (southwest site, SW) and cool‐moist microclimate (northeast site, NE; and northwest site, NW). Measurements at the NW site covered the second year (13 to 20 months) after thinning, and measurements at the SW and NE sites covered the sixth year (61 to 68 months) after thinning. Mean gross ammonification varied insignificantly across the six plots (range: 37.5 ? 31.2 to 51.0 ? 10.5 mg N (kg dry soil)^–1 d^–1). The SW site was characterized by very low net nitrification and nitrate (NO ) concentrations that were not significantly different between control and thinned plot. In contrast, for the thinned plot at the NE site (NET), significantly increased mean net nitrification (2.3 ? 1.2 mg N (kg dry soil)^–1 d^–1 at the NET plot vs. 0.4 ? 0.2 mg N (kg dry soil)^–1 d^–1 at the NEC plot) and mean extractable NO concentrations (43.9 ? 22.8 mg N (kg dry soil)^–1 at the NET plot vs. 4.1 ? 0.8 mg N (kg dry soil)^–1 at the NEC plot) were observed. The differences in net nitrification and NO concentrations across the research plots were related to differences in the forest‐floor C : N ratios: net nitrification increased exponentially below a threshold C : N value of about 25. The results of this study indicate that the forest floor of the warm‐dry SW site is very resistant to N loss triggered by thinning due to high C : N ratios around 30. Under the cool‐moist microclimate of the NE site, a significantly lower C : N ratio of 22.1 at the thinned plot (control plot: 26.7) coincided with significantly increased net nitrification. Thus, different responses of net nitrification to thinning under different microclimate appear to be triggered by different C : N ratios. Nitrous oxide production was mainly governed by forest‐floor water content, and since differences in water content at adjacent control and thinned plots were low, N₂O production was not significantly different between adjacent control and thinned plots.     

Denitrification potentials: Measurement of seasonal variation using a short-term anaerobic incubation technique

A short-term anaerobic incubation technique using the C₂H₂ inhibition of N₂O-reductase for comparing denitrification potentials of soils is described. Twenty grams of soil with added NO^?1₃ are incubated in the presence of He and 0.1 atm C₂H₂ at 25°C and 0 soil matric potential for 8 h. N₂O evolution is linear within 60 to 120 min. The denitrification potential of soils stored at 4°C decreased markedly over 21 days of storage in accordance with changes in the available C. Denitrification under an anaerobic atmosphere was observed at 4 C. Denitrification potentials were independent of NO^?3₃ concentrations above 25 μg NO^?₃-N g^?1 soil. Biphasic linear rates of N₂O evolution were observed in one soil. Incubation of this soil with chloramphenicol suggested the first linear phase is attributable to the in situ enzyme activity at the time of sampling. The second linear phase is indicative of the dentrification potential and is attributed to the full induction of denitrifying enzymes. The denitrification potential of a soil was maintained at or close to the maximum for 8 months of the year. During midsummer months the denitrification potential decreased markedly and the soil demonstrated a biphasic rate of denitrification suggesting an in situ denitrification activity less than the maximum potential. Results indicate that the maximum denitrification potential of this soil may often be limited not by NO^?₃ but by available C.     

Participation of soluble and oxidizable soil organic compounds in denitrification

Summary The role of soluble organic carbon (SOC) in denitrification in four mineral soils and one organic soil was evaluated in laboratory studies. Denitrification capacities and SOC concentrations were determined by nitrate loss from air-dried flooded soil treated with a solution containing 100 g/ml N0₃ ^–-N, while the rate of consumption was measured by Warburg manometry on 20 g air-dried soils to which 10 ml water had been added. High correlation coefficients (r > 0.93) were obtained between denitrification capacities, SOC, and oxygen consumption in the five soils. A mineral soil was amended with extracts of an organic soil. After incubating for 1 week, denitrification capacity was enhanced and SOC concentrations decreased in that soil. Extracted mineral soil had a lower denitrification capacity than an unextracted one. Decreases in concentrations of SOC were related to color change. Infrared spectra of precipitates from soil extracts indicated that absorption at wave number 1420–1440 cm -1 was also related to the color changes. It was implied that low molecular weight fulvic acid like compounds represented the SOC mineralized in denitrification, and that their supply to soil solution by solubilization of organic matter influenced the denitrification rate in the soil.     

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.     

Nitrogen oxides emission from soils bearing a potato crop as influenced by fertilization with treated pig slurries and composts

Nitrous oxide, nitric oxide and denitrification losses from an irrigated soil amended with organic fertilizers with different soluble organic carbon fractions and ammonium contents were studied in a field study covering the growing season of potato (Solanum tuberosum). Untreated pig slurry (IPS) with and without the nitrification inhibitor dicyandiamide (DCD), digested thin fraction of pig slurry (DTP), composted solid fraction of pig slurry (CP) and composted municipal solid waste (MSW) mixed with urea were applied at a rate of 175 kg available N ha⁻¹, and emissions were compared with those from urea (U) and a control treatment without any added N fertilizer (Control). The cumulative denitrification losses correlated significantly with the soluble carbohydrates, dissolved N and total C added. Added dissolved organic C (DOC) and dissolved N affected the N₂O/N₂ ratio, and a lower ratio was observed for organic fertilizers than from urea or unfertilized controls. The proportion of N₂O produced from nitrification was higher from urea than from organic fertilizers. Accumulated N₂O losses during the crop season ranged from 3.69 to 7.31 kg N₂O-N ha⁻¹ for control and urea, respectively, whereas NO losses ranged from 0.005 to 0.24 kg NO-N ha⁻¹, respectively. Digested thin fraction of pig slurry compared to IPS mitigated the total N₂O emission by 48% and the denitrification rate by 33%, but did not influence NO emissions. Composted pig slurry compared to untreated pig slurry increased the N₂O emission by 40% and NO emission by 55%, but reduced the denitrification losses (34%). DCD partially inhibited nitrification rates and reduced N₂O and NO emissions from pig slurry by at least 83% and 77%, respectively. MSW+U, with a C:N ratio higher than that of the composted pig slurry, produced the largest denitrification losses (33.3 kg N ha⁻¹), although N₂O and NO emissions were lower than for the U and CP treatments.This work has shown that for an irrigated clay loam soil additions of treated organic fertilizers can mitigate the emissions of the atmospheric pollutants NO and N₂O in comparison with urea.     

Relationships between the denitrification capacities of soils and total,water-soluble and readily decomposable soil organic matter

The relationships between the denitrification capacities of 17 surface soils and the amounts of total organic carbon, mineralizable carbon, and water-soluble organic carbon in these soils were investigated. The soils used differed markedly in pH, texture, and organic-matter content. Denitrification capacity was assessed by determining the N evolved as N₂ and N₂O on anaerobic incubation of nitrate-treated soil at 20°C for 7 days, and mineralizable carbon was assessed by determining the C evolved as CO₂ on aerobic incubation of soil at 20°C for 7 days. The denitrification capacities of the soils studied were significantly correlated (r = 0·7771) with total organic carbon and very highly correlated (r = 0·9971) with water-soluble organic carbon or mineralizable carbon. The amount of nitrate N lost on anaerobic incubation of nitrate-treated soils for 7 days was very closely related (r = 0·99971) to the amount of N evolved as N₂ and N₂O.The work reported indicates that denitrification in soils under anaerobic conditions is controlled largely by the supply of readily decomposable organic matter and that analysis of soils for mineralizable carbon or water-soluble organic carbon provides a good index of their capacity for denitrification of nitrate.     

Some factors influencing denitrification and nitrogen immobilization in a clay soil

Nitrate-N, enriched with ¹⁵N, was added to small cores of the 0–10 cm layer of a clay soil. The base of each core was sealed, then water, equivalent to 0, 10, 20 or 30mm of rain, was added to the soil surface. The cores were incubated for 1 week at 10, 20, or 30°C in the presence or absence of wheat straw. The recovery of ¹⁵N in the soil mineral-N and organic-N fractions was then measured.No significant losses of ¹⁵N were detected in the cores which received 0–10 mm of added water, and in which the soil water content was close to 0.56 g g^?1 (?10 kPa). However, ¹⁵N losses, assumed due to denitrification, were rapid from cores receiving 20 or 30 mm of water and incubated at 20–30°C. The onset of denitrification was quite sudden as the amount of added water increased from 10 to 20 mm. In this range, a small increment of added water apparently sealed a relatively large volume of soil from atmospheric O₂ diffusion. This phenomenon was strongly temperature-dependent since no losses were detected from any cores at 10°C even though the 30mm addition of water produced a thin layer of free water across the soil surface.The addition of straw did not promote denitrification in soil at water contents close to 0.56 g g^?1. At high soil water contents, adcling straw increased immobilization of labelled NO₃^? and so reduced denitrification losses. The response of immobilization to changing soil water and temperature conditions was very different from that of denitrification.     

Carbon and net nitrogen mineralisation in two forest soils amended with different concentrations of biuret

Biuret is a known contaminant of urea fertilisers that might be useful as a slow release N fertiliser for forestry. We studied carbon (C), net nitrogen (N) mineralisation and soil microbial biomass C and N dynamics in two forest soils (a sandy loam and a silt loam) during a 16-week long incubation following application of biuret (C 23.3%, N 40.8%, O 30.0% and H 4.9%) at concentrations of 0, 2, 10, 100 and 1000 mg kg⁻¹ (oven-dried) soil to assess the potential of biuret as a slow-release N fertiliser. Lower concentrations of biuret specifically increased C mineralisation and soil microbial biomass C in the sandy loam soil, but not in the silt loam soil. A significant decrease of microbial biomass C was found in both soils at week 16 after biuret was applied at higher concentrations. C mineralisation declined with duration of incubation in both soils due to decreased C availability. Biuret at concentrations from 10 to 100 mg kg⁻¹ soil had a significantly positive priming effect on soil organic N mineralisation in both soils. The causes for the priming effects were related to the stimulation of microbial growth and activity at an early stage of the incubation and/or the death of microbes at a later stage, which was biuret-concentration-dependent. The patterns in NH₄⁺-N accumulation differed markedly between the two soils. Net N mineralisation and nitrification were much greater in the sandy loam soil than in the silt loam soil. However, the onset of net nitrification was earlier in the silt loam soil. Biuret might be a potential slow-release N source in the silt loam soil.     

N2O emissions and product ratios of nitrification and denitrification as affected by freezing and thawing

Agricultural soils contribute significantly to atmospheric nitrous oxide (N₂O). A considerable part of the annual N₂O emission may occur during the cold season, possibly supported by high product ratios in denitrification (N₂O/(N₂+N₂O)) and nitrification (N₂O-N/(NO₃⁻-N+NO₂⁻-N)) at low temperatures and/or in response to freeze-thaw perturbation. Water-soluble organic materials released from frost-sensitive catch crops and green manure may further increase winter emissions. We conducted short-term laboratory incubations under standardized moisture and oxygen (O₂) conditions, using nitrogen (N) tracers (¹⁵N) to determine process rates and sources of emitted N₂O after freeze-thaw treatment of soil or after addition of freeze-thaw extract from clover. Soil respiration and N₂O production was stimulated by freeze-thaw or addition of plant extract. The N₂O emission response was inversely related to O₂ concentration, indicating denitrification as the quantitatively prevailing process. Denitrification product ratios in the two studied soils (pH 4.5 and 7.0) remained largely unaltered by freeze-thaw or freeze-thaw-released plant material, refuting the hypothesis that high winter emissions are due to frost damage of N₂O reductase activity. Nitrification rates estimated by nitrate (NO₃⁻) pool enrichment were 1.5-1.8 μg NO₃-N g⁻¹ dw soil d⁻¹ in freeze-thaw-treated soil when incubated at O₂ concentrations above 2.3 vol% and one order of magnitude lower at 0.8 vol% O₂. Thus, the experiments captured a situation with severely O₂-limited nitrification. As expected, the O₂ stress at 0.8 vol% resulted in a high nitrification product ratio (0.3 g g⁻¹). Despite this high product ratio, only 4.4% of the measured N₂O accumulation originated from nitrification, reaffirming that denitrification was the main N₂O source at the various tested O₂ concentrations in freeze-thaw-affected soil. N₂O emission response to both freeze-thaw and plant extract addition appeared strongly linked to stimulation of carbon (C) respiration, suggesting that freeze-thaw-induced release of decomposable organic C was the major driving force for N₂O emissions in our soils, both by fuelling denitrifiers and by depleting O₂. The soluble C (applied as plant extract) necessary to induce a CO₂ and N₂O production rate comparable with that of freeze-thaw was 20-30 μg C g⁻¹ soil dw. This is in the range of estimates for over-winter soluble C loss from catch crops and green manure plots reported in the literature. Thus, freeze-thaw-released organic C from plants may play a significant role in freeze-thaw-related N₂O emissions.