Carbon availability and microbial biomass in soil under an irrigated wheat-maize cropping system receiving different fertilizer treatments

Seasonal changes in carbon availability and microbial biomass were studied in soil under an irrigated wheat-maize cropping system receiving different fertilizter treatments over the past 10 years. Treatments included N-100 and N-200 (urea at 100 and 200kgNha^–1 year^–1, respectively), FYM-16 and FYM-32 (farmyard manure at 16 and 32tha^–1 year^–1, respectively) and a control (unfertilized). Aerobically mineralizable carbon (AMC; C mineralized after 10 days aerobic incubation at 30°C) increased (13–16%) under wheat at both rates of urea whereas under maize it increased (22%) only with the lower rate of urea. Farmyard manure also increased the content of soil AMC under both crops, the effect being two- to threefold higher under wheat than under maize. Urea application caused an 32–78% increase in the specific respiratory activity (SRA) under wheat but caused an 11–50% decrease during the maize season. Farmyard manure also resulted in a higher SRA under both crops but only at the higher application rate. Under wheat, microbial biomass C (MBC) decreased in urea-treated plots but showed a slight increase at the higher rate of FYM. During the maize season, MBC was higher under both urea (42–46%) and FYM (36–47%) treatments as compared to the control. Microbial biomass turnover rate was highest for FYM-32 (2.08), followed by FYM-16 and urea treatments (1.35–1.49); control plots showed a turnover rate of 0.82. The higher AMC and SRA during the active growth period of wheat than that of maize indicated that root-derived C from wheat was higher in amount and more easily degradable. Received: 16 April 1996     

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     

Comparison of two versions of the acetylene inhibition/soil core method for measuring denitrification loss from an irrigated wheat field

 Two versions of the acetylene inhibition (AI)/soil core method were compared for the measurement of denitrification loss from an irrigated wheat field receiving urea-N at a rate of 100 kg ha^–1. With AI/soil core method A, the denitrification rate was measured by analysing the headspace N₂O, followed by estimation of N₂O dissolved in the solution phase using Bunsen absorption coefficients. With AI/soil core method B, N₂O entrapped in the soil was measured in addition to that released from soil cores into the headspace of incubation vessels. In addition, the two methods were also compared for measurement of the soil respiration rate. Of the total N₂O produced, 6–77% (average 40%) remained entrapped in the soil, whereas for CO₂, the corresponding figures ranged from 12–65% (average 44%). The amount of the entrapped N₂O was significantly correlated with the water-filled pore space (WFPS) and with the N₂O concentration in the headspace, whereas CO₂ entrapment was dependent on the headspace CO₂ concentration but not on the WFPS. Due to the entrapment of N₂O and CO₂ in soil, the denitrification rate on several (18 of the 41) sampling dates, and soil respiration rate on almost all (27 of the 30) sampling dates were significantly higher with method B compared to method A. Averaged across sampling dates, the denitrification rate measured with method B (0.30 kg N ha^–1 day^–1) was twice the rate measured with method A, whereas the soil respiration rate measured with method B (34.9 kg C ha^–1 day^–1) was 1.6 times the rate measured with method A. Results of this study suggest that the N₂O and CO₂ entrapped in soil should also be measured to ensure the recovery of the gaseous products of denitrification by the soil core method. Received: 12 May 1998     

Denitrification and total fertilizer-N losses from an irrigated cotton field

 In a 2-year field study, denitrification loss was measured from an irrigated sandy-clay loam under cotton receiving urea-N at 158–173 kg ha^–1. An acetylene inhibition-soil core method was employed for the direct measurement of denitrification, considering also the N₂O entrapped in the soil. Taking into account the N₂O evolved from soil cores and that entrapped in the soil, a total of 65.7 kg N ha^–1 and 64.4 kg N ha^–1 was lost due to denitrification during the 1995 and 1996 cotton-growing seasons, respectively. Most (>70%) of the denitrification loss occurred during June–August, a period characterized by high soil temperatures and heavy monsoon rains. On average, 35% of the denitrification-N₂O was found entrapped in the soil and the amount of entrapped N₂O was significantly correlated with head space N₂O concentration and with water-filled pore space. ¹⁵N-balance during the 1996 growing season revealed a loss of 71.8 kg N ha^–1. It was concluded that a substantial proportion of the fertilizer-N applied to irrigated cotton is lost under the semiarid subtropical climatic conditions prevailing in the Central Punjab region of Pakistan and that denitrification is the major N loss process under irrigated cotton in this region. Received: 8 March 1999     

Nitrous oxide emissions from an irrigated sandy-clay loam cropped to maize and wheat

 Nitrous oxide (N₂O) emissions were measured from an irrigated sandy-clay loam cropped to maize and wheat, each receiving urea at 100 kg N ha^–1. During the maize season (24 August–26 October), N₂O emissions ranged between –0.94 and 1.53 g N ha^–1 h^–1 with peaks during different irrigation cycles (four) ranging between 0.08 and 1.53 g N ha^–1 h^–1. N₂O sink activity during the maize season was recorded on 10 of the 29 sampling occasions and ranged between 0.18 and 0.94 g N ha^–1 h^–1. N₂O emissions during the wheat season (22 November–20 April) varied between –0.85 and 3.27 g N ha^–1 h^–1, whereas peaks during different irrigation cycles (six) were in the range of 0.05–3.27 g N ha^–1 h^–1. N₂O sink activity was recorded on 14 of the 41 samplings during the wheat season and ranged between 0.01 and 0.87 g N ha^–1 h^–1. Total N₂O emissions were 0.16 and 0.49 kg N ha^–1, whereas the total N₂O sink activity was 0.04 and 0.06 kg N ha^–1 during the maize and wheat seasons, respectively. N₂O emissions under maize were significantly correlated with denitrification rate and soil NO₃ ^–-N but not with soil NH₄ ⁺-N or soil temperature. Under wheat, however, N₂O emissions showed a strong correlation with soil NH₄ ⁺-N, soil NO₃ ^–-N and soil temperature but not with the denitrification rate. Under either crop, N₂O emissions did not show a significant relationship with water-filled pore space or soil respiration. Received: 11 June 1997     

Denitrification with and without maize plants (Zea mays L.) under irrigated field conditions

The study was conducted under irrigated field conditions to examine the effect of maize plants on denitrification. Both planted and unplanted field plots received 150kgNha^–1 as urea. In a third treatment, which was also planted and received urea at 150kgNha^–1, the soil nitrate N content was brought up to equal to that in the unplanted plots by applying additional doses of N as calcium nitrate. Soil cores were collected 24 and 72h after irrigation and the denitrification rate was measured by the acetylene inhibition method. Nitrate-N content, aerobically mineralizable C, microbial biomass carrying capacity and denitrification potential were also studied on field-moist soil. Maize plants grown under field conditions always had the potential to increase denitrification in conditions of both high and low water-filled porosity. When nitrate-N content of the planted soil decreased due to plant uptake, denitrification was reduced in the planted soils. However, when nitrate-N uptake by plants was compensated through additional doses of nitrate fertilizer, denitrification was always higher in planted than unplanted soil. The stimulatory effect of plants on denitrification was observed at both high and low soil nitrate-N concentrations, though it was more pronounced at high nitrate-N levels. The effect of plants on denitrification and related parameters was confined to the root zone. Received: 15 April 1996     

Mineralization and denitrification in upland, nearly saturated and flooded subtropical soil I. Effect of nitrate and ammoniacal nitrogen

 The influence of fertilizer N applied through nitrate and ammoniacal sources on the availability of nitrate, supply of C, and gaseous N losses via denitrification (using acetylene inhibition technique) in a semiarid subtropical soil (Typic Ustochrepts) was investigated in a growth chamber simulating upland [60% water-filled pore space (WFPS)], nearly saturated (90% WFPS), and flooded (120% WFPS) conditions. The rate of denitrification was very low in the upland soil conditions, irrespective of fertilizer N treatments. Increasing water content to nearly saturated and flooded conditions resulted in four- to sixfold higher rates of denitrification within 2 days, suggesting that the denitrifying activity commences quickly. Results of this study reveal that (1) under restricted aeration, these soils could support high rates of denitrification (∼6 mg N kg^–1 day^–1) for short periods when nitrate is present; (2) application of fertilizer N as nitrate enhances N losses via denitrification (∼10 mg N kg^–1 day^–1) – however, the supply of available C determines the intensity and duration of denitrification; (3) when fertilizer N is applied as an ammoniacal form, nitrification proceeds slowly and nitrate availability limits denitrification in flooded soil; (4) the nearly saturated soil, being partially aerobic, supported greater nitrification of applied ammoniacal fertilizer N than flooded soil resulting in higher relative rates of denitrification; and (5) under aerobic soil conditions, 26 mg mineral N kg^–1 accumulated in control soil over a 16-day period, demonstrating a modest capacity of such semiarid subtropical soils, low in organic matter, to supply N to growing plants. Received: 7 June 1999     

N dynamics and sources of N2O production following pig slurry application to a loamy soil

Carbon (C) and Nitrogen dynamics and sources of nitrous oxide (N₂O) production were investigated in a loamy soil amended with pig slurry. Pig slurry (40000kgha^–1) or distilled H₂O was applied to intact soil cores of the upper 5cm of a loamy soil which were incubated under aerobic conditions for 28 days at 25°C. Treatments were with or without acetylene (C₂H₂), which is assumed to inhibit the reduction of N₂O to dinitrogen (N₂), and with or without dicyandiamide (DCD), which is thought to inhibit nitrification. Volatilization of ammonia (NH₃), pH, carbon dioxide (CO₂) and N₂O production, and ammonium (NH₄ ⁺) and nitrate NO₃ ^–) concentrations were monitored. The pH of the pig slurry amended soil increased from an initial value of 7.1 to pH 8.3 within 3 days; it then decreased slowly but was still at a value of 7.4 after 28 days. Twenty percent of the NH₄ ⁺ applied volatilized within 28 days. Sixty percent of the C applied in the pig slurry evolved as CO₂, if no priming effect was assumed, but only 38% evolved when the soil was amended with DCD. Pig slurry significantly increased denitrification and the ratio between its gaseous products, N₂O and N₂, was 0.21. No significant increases in NO₃ ^– concentration occurred, and N₂O produced through nitrification was 0.07mg N₂O-N kg^–1 day^–1 or 33% of the total N₂O produced. C₂H₂ was used as a C substrate by microorganisms and increased the production of N₂O. Received: 12 May 1997     

Influence of tillage system on denitrification in maize-cropped soils

Denitrification losses show an irregular pattern through the year, often being caused by climatic conditions and management practices. The objectives of the present work were to quantify denitrification losses and to determine the influence of tillage system on the factors that control denitrification in fertilized soils. The modal profile of the soil was an Vertic Argiudoll, clay loam texture, located in Buenos Aires province, Argentina. The treatments were: (a) fertilized, (b) incorporated fertilization and (c) without fertilization for both no tillage and conventional tillage systems. Chambers were placed in the field to measure denitrification. In this clayish soil the estimated mean values of accumulated denitrification during the crop cycle (90 days) were 0.190kgNha^–1 for conventional tillage and 0.350kgNha^–1 for no tillage. In treatments with no tillage, losses by denitrification were approximately twice those of conventional tillage. These differences were also evidenced by the number of microorganisms, which were significantly higher (P<>;5%) for no tillage on all dates, except for at flowering. The increase at flowering coincided with the period of highest rainfall and consequently the highest water contents in the soil. The highest denitrification losses, except for sowing, were measured when soil moisture content was more than 30% (v/v). Denitrification increased in conjunction with an increase in the availability of carbon that is consumed by the heterotrophic microorganisms (including the denitrifiers). Received: 30 July 1996     

Effects of long-term mineral fertilization on microbial biomass,microbial activity,and the presence of <Emphasis Type="Italic">r</Emphasis>- and <Emphasis Type="Italic">K</Emphasis>-strategists in soil

Long-term effects of mineral fertilization on microbial biomass C (MBC), basal respiration (R _B), substrate-induced respiration (R _S), β-glucosidase activity, and the r–K-growth strategy of soil microflora were investigated using a field trial on grassland established in 1969. The experimental plots were fertilized at three rates of mineral N (0, 80, and 160 kg ha⁻¹ year⁻¹) with 32 kg P ha⁻¹ year⁻¹ and 100 kg K ha⁻¹ year⁻¹. No fertilizer was applied on the control plots (C). The application of a mineral fertilizer led to lower values of the MBC and R _B, probably as a result of fast mineralization of available substrate after an input of the mineral fertilizer. The application of mineral N decreased the content of C extracted by 0.5 M K₂SO₄ (C _ex). A positive correlation was found between pH and the proportion of active microflora (R _S/MBC). The specific growth rate (μ) of soil heterotrophs was higher in the fertilized than in unfertilized soils, suggesting the stimulation of r-strategists, probably as the result of the presence of available P and rhizodepositions. The cessation of fertilization with 320 kg N ha⁻¹ year⁻¹ (NF) in 1989 also stimulated r-strategists compared to C soil, probably as the result of the higher content of available P in the NF soil than in the C soil.     

Combining field incubation with nitrogen-15 labelling to examine nitrogen transformations in low to high intensity grassland management systems

 The ¹⁵N isotope dilution method was combined with a field incubation technique to provide simultaneous measurements of gross and net rates of N turnover in three long-term swards: unfertilized (Z) or receiving N either from N fixation as clover (C), or as 200 kg fertilizer N ha^–1 year^–1 (F). Uniform N enrichment of soil microplots was achieved with a multi-point soil injector to measure mineralization/immobilization turnover and nitrification over a 4-day incubation. Net rates of mineralization ranged between 0.6 and 2.9 μg N g^–1 day^–1 and in all three treatments were approximately half the gross rates. Nitrification rates (gross) were between 1.0 and 1.6 μg N g^–1 day^–1. In the F treatment, the turnover of NH₄ ⁺-N and NO₃ ^–-N pools was on a 2- and 4-day cycle, respectively, whereas in the N-limited treatments (C and Z) turnover rates were faster, with the NO₃ ^–-N pools turning over twice as fast as the NH₄ ⁺-N pools. Therefore, available N was recycled more efficiently in the C and Z treatments, whereas in the F treatment a higher N pool size was maintained which would be more vulnerable to leakage. A large proportion of the added ¹⁵N was recovered in the soil microbial biomass (SMB), which represented a 4–5 times larger sink for N than the plant biomass. Although the C treatment had a significantly lower SMB than the grass-only treatments, there were no differences in microbial activity. Gross rates of nitrification increased along the gradient of N input intensity (i.e. Z<C<F), and the addition of a nitrification inhibitor (C₂H₂) tended to increase microbial immobilization, but did not influence plant N uptake. In this study, the value of combining different techniques to verify net rates was demonstrated and the improved methodology for ¹⁵N labelling of soil enabled measurements to be obtained from relatively undisturbed soil under natural field conditions. Received: 25 May 1999     

Attenuation of Nitrogen,Phosphorus and <Emphasis Type="Italic">E. coli</Emphasis> Inputs from Pasture Runoff to Surface Waters by a Farm Wetland: the Importance of Wetland Shape and Residence Time

Water quantity and quality were monitored for 3 years in a 360-m-long wetland with riparian fences and plants in a pastoral dairy farming catchment. Concentrations of total nitrogen (TN), total phosphorus (TP) and Escherichia coli were 210–75,200 g N m⁻³, 12–58,200 g P m⁻³ and 2–20,000 most probable number (MPN)/100 ml, respectively. Average retentions (±standard error) for the wetland over 3 years were 5 ± 1%, 93 ± 13% and 65 ± 9% for TN, TP and E. coli, respectively. Retentions for nitrate–N, ammonium–N, filterable reactive P and particulate C were respectively −29 ± 5%, 32 ± 10%, −53 ± 24% and 96 ± 19%. Aerobic conditions within the wetland supported nitrification but not denitrification and it is likely that there was a high conversion rate from dissolved inputs of N and P in groundwater, to particulate N and P and refractory dissolved forms in the wetland. The wetland was notable for its capacity to promote the formation of particulate forms and retain them or to provide conditions suitable for retention (e.g. binding of phosphate to cations). Nitrogen retention was generally low because about 60% was in dissolved forms (DON and NO_X–N) that were not readily trapped or removed. Specific yields for N, P and E. coli were c. 10–11 kg N ha⁻¹ year⁻¹, 0.2 kg P ha⁻¹ year⁻¹ and ≤10⁹ MPN ha⁻¹ year⁻¹, respectively, and generally much less than ranges for typical dairy pasture catchments in New Zealand. Further mitigation of catchment runoff losses might be achieved if the upland wetland was coupled with a downslope wetland in which anoxic conditions would promote denitrification.     

Effect of coffee residues on growth and reproduction of Hyperiodrilus africanus (Oligochaeta, Eudrilidae) in Ivory Coast

Hyperiodrilus africanus (Beddard) is a 12-cm to 16-cm-long earthworm, which is widely distributed in West and Central Africa. It lives in the upper 10–20 cm of the soil, and feeds on a mixture of soil and above-ground litter. Cocoons obtained in the laboratory hatched on average 17 days after deposition and produced two juveniles on average. Paired individuals fed soil amended with 2% coffee residues grew significantly (P<0.05) faster than those in the control soil. Daily individual weight increments were respectively 6.1 mg worm^–1 day^–1 and 1.0 mg worm^–1 day^–1 in supplemented and control soil. The generation time was short, and cocoon production reached 9.6 month^–1 (i.e. 115 cocoons adult^–1 year^–1). When H. africanus collected from the field were raised in the laboratory, they grew slowly, laid fewer cocoons and mortality was high. Demographic parameters indicated an improvement when H. africanus were raised in batches rather than individually. Mating enhanced cocoon production although parthenogenesis was possible. Received: 4 April 1997     

Denitrification in intact subsoil cores

 Intact, unamended subsoil cores taken from two contrasting field sites were incubated in the laboratory at 12  °C under aerobic (21% O₂) and anaerobic (1.1–5.5% O₂) conditions. Denitrification of ¹⁵N-NO₃ ^– diffusing into the cores across a semi-permeable membrane was estimated by: (1) direct observation of ¹⁵N-labelled N₂ and N₂O, and (2) mass balance following sectioning at the end of the experiment. The in situ denitrification potential (rates where the supply of NO₃ ^– is not limited by diffusion) was estimated using a finite-difference approximation to a diffusion reaction equation based on comparison of time and depth profiles of NO₃ ^– and Br^– in the incubated cores. Potentials between 0.2 and 2.5 mg N kg^–1 day^–1 were obtained under low O₂ and natural C concentrations. These results indicate the potential for denitrification in glacial till subsoils to reduce NO₃ ^– leaching to ground or surface waters to levels unlikely to result in a pollution hazard. The major product of NO₃ ^– reduction in these subsoils was observed to be N₂, rather than the greenhouse gas and catalyst of stratospheric O₃ removal, N₂O. Received: 31 October 1997     

Methane fluxes and nitrogen dynamics from a drained fenland peat

 Rates of methane uptake were measured in incubation studies with intact cores from adjacent fenland peats that have been under arable management and woodland management for at least the past 30 years. On two separate occasions the woodland peat showed greater rates of uptake than the arable peat. These rates ranged from 23.1 to 223.3 μg CH₄ m^–2 day^–1 for the woodland peat and from 29.6 to 157.6 μg CH₄ m^–2 day^–1 for the arable peat. When the peats were artificially flooded there was a decrease in the rate of methane oxidation, but neither site showed any net efflux of methane. ¹⁵N isotopic dilution was used to characterise nitrogen cycling within the two peats. Both showed similar rates of gross nitrogen mineralisation (3.58 mg N kg^–1 day^–1, arable peat; 3.54 N kg^–1 day^–1, woodland peat) and ammonium consumption (4.19 arable peat and 4.70 mg N kg^–1 day^–1 woodland peat). There were significant differences in their inorganic ammonium and nitrate pool sizes, and the rate of gross nitrification was significantly higher in the woodland peat (4.90 mg N kg^–1 day^–1) compared to the arable peat (1.90 mg N kg^–1 day^–1). These results are discussed in the light of high atmospheric nitrogen deposition. Received: 1 December 1997     

Soil respiration and carbon balance of gray forest soils as affected by land use

 Soil respiration was measured by closed chamber and gradient methods in soils under forest, sown meadow and crops. Annual total soil respiration determined with the closed chamber method ranged from 180 to 642 g CO₂-C m^–2 year^–1 and from 145 to 382 g CO₂-C m^–2 year^–1 determined with the CO₂ profile method. Soil respiration increased in the order: cropland<sown meadow<forest. The C balance calculated as the difference between net primary production (sink) and respiration of heterotrophs (source) suggested an equilibrium between the input and output of C in the cropland, and sequestration of 135 and 387 g CO₂-C m^–2 year^–1 in the forest and meadow, respectively. Received: 1 December 1997     

Nitrous oxide emission from Swedish forest soils in relation to liming and simulated increased N-deposition

Fluxes of N₂O were studied in a Norway spruce forest in the southwest of Sweden. Three differently treated catchments were compared: Limed (6 t dolomite ha^–1), Nitrex (additional N-deposition corresponding to 35 kg ha^–1 year^–1, in small doses) and Control (used as control site). The N-retention was still high (95%) after 2years of N-addition at the Nitrex site when the flux measurements were performed. Each catchment contained both well-drained and poorly drained soils (covered with Sphagnum sp.). The emissions of N₂O were in general low with both a high spatial and temporal variation for all three sites. The measured emissions were 25, 71 and 96 (gN₂O-N ha^–1 year^–1) for the well-drained Limed, Control and Nitrex sites, respectively. The average emissions of N₂O from the wet areas were significantly higher than the well-drained areas within the catchments. For the wet areas the measured emissions were larger: 90, 118 and 254 (g N₂O-N ha^–1 year^–1) for the Limed, Control and Nitrex sites, respectively. Comparison between treatments showed the wet Nitrex site to have a significantly higher emission than all other sites. The increased N-deposition at the Nitrex site increased the N₂O emissions by 0.2% of the added N for the well-drained soils and about 1% for the wet areas, compared with the control site. Since the wet areas represented only a small part of the forest, their larger emissions did not contribute significantly to the overall emission of the forest. Neither temperature nor water content of the soil was well correlated with the N₂O emissions. Soil gas samples showed that most of the N₂O was produced below a 0.3-m depth in the soil. Received: 27 September 1996     

Short-term effects of nitrogen on methane oxidation in soils

 The short-term effects of N addition on CH₄ oxidation were studied in two soils. Both sites are unfertilized, one has been under long-term arable rotation, the other is a grassland that has been cut for hay for the past 125 years. The sites showed clear differences in their capacity to oxidise CH₄, the arable soil oxidised CH₄ at a rate of 0.013 μg CH₄ kg^–1 h^–1 and the grassland soil approximately an order of magnitude quicker. In both sites the addition of (NH₄)₂SO₄ caused an immediate reduction in the rate of atmospheric CH₄ oxidation approximately in inverse proportion to the amount of NH₄ ⁺ added. The addition of KNO₃ caused no change in the rate of CH₄ oxidation in the arable soil, but in the grassland soil after 9 days the rate of CH₄ oxidation had decreased from 0.22 μg CH₄ kg^–1 h^–1 to 0.13 μg CH₄ kg^–1 h^–1 in soil treated with the equivalent of 192 kg N ha^–1. A ¹⁵N isotopic dilution technique was used to investigate the role of nitrifiers in regulating CH₄ oxidation. The arable soil showed a low rate of gross N mineralisation (0.67 mg N kg^–1 day^–1), but a relatively high proportion of the mineralised N was nitrified. The grassland soil had a high rate of gross N mineralisation (18.28 mg N kg^–1 day^–1), but negligible nitrification activity. It is hypothesised that since there was virtually no nitrification in the grassland soil then CH₄ oxidation at this site must be methanotroph mediated. Received: 31 October 1997     

Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions

 N₂O emissions were periodically measured using the static chamber method over a 1-year period in a cultivated field subjected to different agricultural practices including the type of N fertilizer (NH₄NO₃, (NH₄)₂SO₄, CO(NH₂)₂ or KNO₃ and the type of crop (rapeseed and winter wheat). N₂O emissions exhibited the same seasonal pattern whatever the treatment, with emissions between 1.5 and 15 g N ha^–1 day^–1 during the autumn, 16–56 g N ha^–1 day^–1 in winter after a lengthy period of freezing, 0.5–70 g N ha^–1 day^–1 during the spring and lower emissions during the summer. The type of crop had little impact on the level of N₂O emission. These emissions were a little higher under wheat during the autumn in relation to an higher soil NO₃ ^– content, but the level of emissions was similar over a 7-month period (2163 and 2093 g N ha^–1 for rape and wheat, respectively). The form of N fertilizer affected N₂O emissions during the month following fertilizer application, with higher emissions in the case of NH₄NO₃ and (NH₄)₂SO₄, and a different temporal pattern of emissions after CO(NH₂)₂ application. The proportion of applied N lost as N₂O varied from 0.42% to 0.55% with the form of N applied, suggesting that controlling this agricultural factor would not be an efficient way of limiting N₂O emissions under certain climatic and pedological situations. Received: 1 December 1997     

Gross nitrogen mineralization and nitrification rates and their relationships to enzyme activities and the soil microbial biomass in soils treated with dairy shed effluent and ammonium fertilizer at different water potentials

 Gross N mineralization and nitrification rates and their relationships to microbial biomass C and N and enzyme (protease, deaminase and urease) activities were determined in soils treated with dairy shed effluent (DSE) or NH₄ ⁺ fertilizer (NH₄Cl) at a rate equivalent to 200 kg N ha^–1 at three water potentials (0, –10 and –80 kPa) at 20  °C using a closed incubation technique. After 8, 16, 30, 45, 60 and 90 days of incubation, sub-samples of soil were removed to determine gross N mineralization and nitrification rates, enzyme activities, microbial biomass C and N, and NH₄ ⁺ and NO₃ ^– concentrations. The addition of DSE to the soil resulted in significantly higher gross N mineralization rates (7.0–1.7 μg N g^–1 soil day^–1) than in the control (3.8–1.2 μg N g^–1 soil day^–1), particularly during the first 16 days of incubation. This increase in gross mineralization rate occurred because of the presence of readily mineralizable organic substrates with low C : N ratios, and stimulated soil microbial and enzymatic activities by the organic C and nutrients in the DSE. The addition of NH₄Cl did not increase the gross N mineralization rate, probably because of the lack of readily available organic C and/or a possible adverse effect of the high NH₄ ⁺ concentration on microbial activity. However, nitrification rates were highest in the NH₄Cl-treated soil, followed by DSE-treated soil and then the control. Soil microbial biomass, protease, deaminase and urease activities were significantly increased immediately after the addition of DSE and then declined gradually with time. The increased soil microbial biomass was probably due to the increased available C substrate and nutrients stimulating soil microbial growth, and this in turn resulted in higher enzyme activities. NH₄Cl had a minimal impact on the soil microbial biomass and enzyme activities, possibly because of the lack of readily available C substrates. The optimum soil water potential for gross N mineralization and nitrification rates, microbial and enzyme activities was –10 kPa compared with –80 kPa and 0 kPa. Gross N mineralization rates were positively correlated with soil microbial biomass N and protease and urease activities in the DSE-treated soil, but no such correlations were found in the NH₄Cl-treated soil. The enzyme activities were also positively correlated with each other and with soil microbial biomass C and N. The forms of N and the different water potentials had a significant effect on the correlation coefficients. Stepwise regression analysis showed that protease was the variable that most frequently accounted for the variations of gross N mineralization rate when included in the equation, and has the potential to be used as one of the predictors for N mineralization. Received: 10 March 1998