Influence of N and non-N salts on atmospheric methane oxidation by upland boreal forest and tundra soils

 The short-term (24 h) and medium-term (30 day) influence of N salts (NH₄Cl, NaNO₃ and NaNO₂) and a non-N salt (NaCl) on first-order rate constants, k (h^–1) and thresholds (C_Th) for atmospheric CH₄ oxidation by homogenized composites of upland boreal forest and tundra soils was assessed at salt additions ranging to 20 μmol g^–1 dry weight (dw) soil. Additions of NH₄Cl, NaNO₃ and NaCl to 0.5 μmol g^–1 dw soil did not significantly decrease k relative to watered controls in the short term. Higher concentrations significantly reduced k, with the degree of inhibition increasing with increasing dose. Similar doses of NH₄Cl and NaCl gave comparable decreases in k relative to controls and both soils showed low native concentrations of NH₄ ⁺-N (≤1 μmol g^–1dw soil), suggesting that the reduction in k was due primarily to a salt influence rather than competitive inhibition of CH₄ oxidation by exogenous NH₄ ⁺-N or NH₄ ⁺-N released through cation exchange. The decrease in k was consistently less for NaNO₃ than for NH₄Cl and NaCl at similar doses, pointing to a strong inhibitory effect of the Cl^– counter-anion. Thresholds for CH₄ oxidation were less sensitive to salt addition than k for these three salts, as significant increases in C_Th relative to controls were only observed at concentrations ≥1.0 μmol g^–1 dw soil. Both soils were more sensitive to NaNO₂ than to other salts in the short term, showing a significant decrease in k at an addition of 0.25 μmol NaNO₂ g^–1 dw soil that was clearly attributable to NO₂ ^–. Soils showed no recovery from NaCl, NH₄ ⁺-N or NaNO₃ addition with respect to atmospheric CH₄ oxidation after 30 days. However, soils amended with NaNO₂ to 1.0 μmol NaNO₂ g^–1 dw showed values of k that were not significantly different from controls. Recovery of CH₄-oxidizing ability was due to complete oxidation of NO₂ ^–-N to NO₃ ^–-N. Analysis of soil concentrations of N salts necessary to inhibit atmospheric CH₄ oxidation and regional rates of N deposition suggest that N deposition will not decrease the future sink strength of upland high-latitude soils in the atmospheric CH₄ budget. Received: 30 April 1999     

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     

The influence of land use and pesticides on methane oxidation in some Belgian soils

 In a first experiment, the effect of land use on the uptake rate of atmospheric CH₄ was studied in laboratory incubations of intact soil cores. A soil under deciduous forest showed the highest CH₄ oxidation. Its overall CH₄ uptake during the measuring period (202 days) was 1.03 kg CH₄ ha^–1. Natural grassland showed the second highest CH₄ oxidizing capacity (0.71 kg CH₄ ha^–1). The overall amount of CH₄ uptake by fertilized pasture was 0.33 kg CH₄ ha^–1. CH₄ oxidation in arable soils with different fertilizer treatments varied between 0.34 and 0.37 kg CH₄ ha^–1. Undisturbed soils had a higher CH₄ uptake capacity than agricultural soils. The moisture content of the soil was found to be an important parameter explaining temporal variations of CH₄ oxidation. Different methods of fertilization which had been commenced 10 years previously were not yet reflected in the total CH₄ uptake rate of the arable soil. In a second experiment, a number of frequently used pesticides were screened for their possible effect on CH₄ oxidation. In a sandy arable soil lenacil, mikado and oxadixyl caused significantly reduced CH₄ oxidation compared to the control. Under the same conditions, but in a clayey arable soil, mikado, atrazine and dimethenamid caused a reduction of the CH₄ uptake. In a landfill cover soil, with a 100-fold higher CH₄ oxidation rate, no inhibition of CH₄ oxidation was observed, not even when the application rate of pesticides was tenfold higher than usual. Received: 1 December 1998     

Factors influencing nitrous oxide and methane emissions from minerotrophic fens in northeast Germany

 At two field sites representing northeastern German minerotrophic fens (Rhin-Havelluch, a shallow peat site; Gumnitz, a partially drained peat site) the influence of different factors (N fertilization, groundwater table, temperature) on N₂O and CH₄ emissions was investigated. The degraded fens were sources or sinks of the radiatively active trace gases investigated. The gas fluxes measured were much higher than those found in other terrestrical ecosystems such as forests. Lowering the groundwater table increased the release of N₂O and the oxidation of CH₄. High CH₄ emission rates occurred when the groundwater tables and soil temperatures were high (>12  °C). N fertilization stimulated the release of N₂O only when application rates were very high (480 kg N ha^–1). A moderate N supply (60 or 120 kg N ha^–1) hardly increased the release of N₂O in spite of high soluble soil NO₃ ^– contents. Received: 31 October 1997     

Effect of cropping systems on nitrogen mineralization in soils

 Understanding the effect of cropping systems on N mineralization in soils is crucial for a better assessment of N fertilizer requirements of crops in order to minimize nitrate contamination of surface and groundwater resources. The effects of crop rotations and N fertilization on N mineralization were studied in soils from two long-term field experiments at the Northeast Research Center and the Clarion-Webster Research Center in Iowa that were initiated in 1979 and 1954, respectively. Surface soil samples were taken in 1996 from plots of corn (Zea mays L.), soybean (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha^–1 before corn and an annual application of 20 kg P and 56 kg K ha^–1. N mineralization was studied in leaching columns under aerobic conditions at 30  °C for 24 weeks. The results showed that N mineralization was affected by cover crop at the time of sampling. Continuous soybean decreased, whereas inclusion of meadow increased, the amount of cumulative N mineralized. The mineralizable N pool (N _o) varied considerably among the soil samples studied, ranging from 137 mg N kg^–1 soil under continuous soybean to >500 mg N kg^–1 soil under meadow-based rotations, sampled in meadow. The results suggest that the N _o and/or organic N in soils under meadow-based cropping systems contained a higher proportion of active N fractions. Received: 10 February 1999     

Quantification of nitrogen excretion rates for three lumbricid earthworms using 15N

 Nitrogen excretion rates of ¹⁵N-labeled earthworms and contributions of ¹⁵N excretion products to organic (dissolved organic N) and inorganic (NH₄-N, NO₃-N) soil N pools were determined at 10  °C and 18  °C under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clitellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rubellus (Hoffmeister) were labeled with ¹⁵N by providing earthworms with ¹⁵N-labeled organic substrates for 5–6 weeks. The quantity of ¹⁵N excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were calculated. N excretion rates ranged from 274.4 to 744 μg N g^–1 earthworm fresh weight day^–1, with a daily turnover of 0.3–0.9% of earthworm tissue N. The N excretion rates of juvenile L. terrestris were significantly lower than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, particularly NH₄-N, were greater in soils incubated with earthworms for 48 h than soils incubated without earthworms. Between 13 and 40% of excreted ¹⁵N was found in the ¹⁵N-mineral N (NH₄-N+NO₃-N) pool, and 13–23% was in the ¹⁵N-DON pool. Other fates of excreted ¹⁵N may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N forms, or gaseous N losses. Earthworm excretion rates were combined with earthworm biomass measurements to estimate N flux from earthworm populations through excretion. Annual earthworm excretion was estimated at 41.5 kg N ha^–1 in an inorganically-fertilized corn agroecosystem, and was equivalent to 22% of crop N uptake. Our results suggest that the earthworms could contribute significantly to N cycling in corn agroecosystems through excretion processes. Received: 12 April 1999     

Effect of temperature on reduction of iron and production of carbon dioxide and methane in anoxic wetland rice soils

 Wetland rice soils from Italy (Pavia) and the Philippines (Bugallon, Luisiana, Maligaya) were incubated under anoxic conditions at 31 different temperatures ranging from 4.7  °C to 49.5  °C. Production of CO₂ was most intensive at the beginning of the incubation (0–4 days) and was predominantly coupled to the reduction of free Fe(III). The optimum temperature for these processes was between 32  °C and 41  °C. After 9–16 days, CO₂ production rates had decreased and the available Fe(III) had been completely reduced at the optimum temperatures. However, Fe(III) was still available at temperatures below and above the optimum. Maximum CH₄ production rates were observed after 4–16 days (except in soil from Maligaya) with temperature optima between 32  °C and 41  °C, similar to those for CO₂ production and Fe reduction. Since ongoing Fe reduction is known to suppress CH₄ production, the temperature range of optimum CH₄ production was restricted to those temperatures at which Fe(III) had already been depleted. Nevertheless, the temperature characteristics of both CO₂ and CH₄ production often exhibited two temperature optima at some time during the incubation, suggesting a complex pattern of adaptation of the methanogenic microbial community to temperature. When available Fe(III) was completely depleted by anoxic pre-incubation at 30  °C, CH₄ was produced at a constant rate (steady state conditions) which increased with increasing temperature. Steady state CH₄ production reached a first maximum at about 40  °C, but increased further up to at least 50  °C, suggesting the presence of thermophilic microorganisms whose activity was apparently masked when Fe had not been completely reduced. The apparent activation energy of CH₄ production at steady state ranged between 48 kJ mol^–1 and 65 kJ mol^–1. Received: 26 August 1999     

Influence of soil compaction on carbon and nitrogen mineralization of soil organic matter and crop residues

 We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop residues. Samples of unamended soil, and soil amended with leek residues, at six bulk densities ranging from 1.2 to 1.6 Mg m^–3 and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studied did not influence any measure of microbial activity significantly. A small (but insignificant) decrease in nitrification rate at the highest bulk density was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of the incubation were equal at all bulk densities, but first-order N mineralization rates tended to increase with increasing compaction, although the increase was not significant. Nitrification in the amended soils was more affected by compaction, and NO₃ ^–-N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m^–3 were significantly lower (by about 8% and 16% of total added N, respectively), than those of the less compacted treatments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m^–3, compared with the other treatments. The depression of C mineralization in compacted soils can lead to higher organic matter accumulation. Since N mineralization was not affected by compaction (within the range used here) the accumulated organic matter would have had higher C : N ratios than in the uncompacted soils, and hence would have been of a lower quality. In general, increasing soil compaction in this soil, starting at a bulk density of 1.5 Mg m^–3, will affect some microbially driven processes. Received: 10 June 1999     

Increased moisture and methanogenesis contribute to reduced methane oxidation in elevated CO2 soils

Awareness of global warming has stimulated research on environmental controls of soil methane (CH₄) consumption and the effects of increasing atmospheric carbon dioxide (CO₂) on the terrestrial CH₄ sink. In this study, factors impacting soil CH₄ consumption were investigated using laboratory incubations of soils collected at the Free Air Carbon Transfer and Storage I site in the Duke Forest, NC, where plots have been exposed to ambient (370 μL L⁻¹) or elevated (ambient + 200 μL L⁻¹) CO₂ since August 1996. Over 1 year, nearly 90% of the 360 incubations showed net CH₄ consumption, confirming that CH₄-oxidizing (methanotrophic) bacteria were active. Soil moisture was significantly (p < 0.01) higher in the 25–30 cm layer of elevated CO₂ soils over the length of the study, but soil moisture was equal between CO₂ treatments in shallower soils. The increased soil moisture corresponded to decreased net CH₄ oxidation, as elevated CO₂ soils also oxidized 70% less CH₄ at the 25–30 cm depth compared to ambient CO₂ soils, while CH₄ consumption was equal between treatments in shallower soils. Soil moisture content predicted (p < 0.05) CH₄ consumption in upper layers of ambient CO₂ soils, but this relationship was not significant in elevated CO₂ soils at any depth, suggesting that environmental factors in addition to moisture were influencing net CH₄ oxidation under elevated CO₂. More than 6% of the activity assays showed net CH₄ production, and of these, 80% contained soils from elevated CO₂ plots. In addition, more than 50% of the CH₄-producing flasks from elevated CO₂ sites contained deeper (25–30 cm) soils. These results indicate that subsurface (25 cm+) CH₄ production contributes to decreased net CH₄ consumption under elevated CO₂ in otherwise aerobic soils.     

Influence of nitrogen on cellulose and lignin mineralization in blackwater and redwater forested wetland soils

 Microcosms were used to determine the influence of N additions on active bacterial and active fungal biomass, cellulose degradation and lignin degradation at 5, 10 and 15 weeks in soils from blackwater and redwater wetlands in the northern Florida panhandle. Blackwater streams contain a high dissolved organic C concentration which imparts a dark color to the water and contain low concentrations of nutrients. Redwater streams contain high concentrations of suspended clays and inorganic nutrients, such as N and P, compared to blackwater streams. Active bacterial and fungal biomass was determined by direct microscopy; cellulose and lignin degradation were measured radiometrically. The experimental design was a randomized block. Treatments were: soil type (blackwater or redwater forested wetlands) and N additions (soils amended with the equivalent of 0, 200 or 400 kg N ha^–1 as NH₄NO₃). Redwater soils contained higher concentrations of C, total N, P, K, Ca, Mn, Fe, B and Zn than blackwater soils. After N addition and 15 weeks of incubation, the active bacterial biomass in redwater soils was lower than in blackwater soils; the active bacterial biomass in blackwater soils was lower when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. The active fungal biomass in blackwater soils was higher when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. The active fungal biomass in redwater wetland soils was lower when 200 kg N ha^–1, but not when 400 kg N ha^–1, was added. Cellulose and lignin degradation was higher in redwater than in blackwater soils. After 10 and 15 weeks of incubation, the addition of 200 or 400 kg N as NH₄NO₃ ha^–1 decreased cellulose and lignin degradation in both wetland soils to similar levels. This study indicated that the addition of N may slow organic matter degradation and nutrient mineralization, thereby creating deficiencies of other plant-essential nutrients in wetland forest soils. Received: 7 April 1999     

Influence of nitrogen on atrazine and 2, 4 dichlorophenoxyacetic acid mineralization in blackwater and redwater forested wetland soils

 Microcosms were used to determine the influence of N additions on active bacterial and fungal biomass, atrazine and dichlorophenoxyacetic acid (2,4-D) mineralization at 5, 10 and 15 weeks in soils from blackwater and redwater wetland forest ecosystems in the northern Florida Panhandle. Active bacterial and fungal biomass was determined by staining techniques combined with direct microscopy. Atrazine and 2,4-D mineralization were measured radiometrically. Treatments were: soil type, (blackwater or redwater forested wetland soils) and N additions (soils amended with the equivalent of 0, 200 or 400 kg N ha^–1 as NH₄NO₃). Redwater soils contained higher concentrations of C, total N, P, K, Ca, Mn, Fe, B and Zn than blackwater soils. After N addition and 15 weeks of incubation, active bacterial biomass in redwater soils was lower when N was added. Active bacterial biomass in blackwater soils was lower when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. Active fungal biomass in blackwater soils was higher when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. Active fungal biomass in redwater soils was lower when 200 kg N ha^–1, but not when 400 kg N ha^–1, was added. After 15 weeks of incubation 2,4-D degradation was higher in redwater wetland soils than in blackwater soils. After 10 and 15 weeks of incubation the addition of 200 or 400 kg N ha^–1 decreased both atrazine and 2,4-D degradation in redwater soils. The addition of 400 kg N ha^–1 decreased 2,4-D degradation but not atrazine degradation in blackwater soils after 10 and 15 weeks of incubation. High concentrations of N in surface runoff and groundwater resulting from agricultural operations may have resulted in the accumulation of N in many wetland soils. Large amounts of N accumulating in wetlands may decrease mineralization of toxic agricultural pesticides. Received: 26 June 1998     

Potential contribution of Lumbricus terrestris L. to carbon dioxide, methane and nitrous oxide fluxes from a forest soil

 Potential effects of earthworms (Lumbricus terrestris L.) inoculated into soil on fluxes of CO₂, CH₄ and N₂O were investigated for an untreated and a limed soil under beech in open topsoil columns under field conditions for 120 days. Gas fluxes from L. terrestris, beech litter and mineral soil from soil columns were measured separately in jars at 17  °C. The inoculation with L. terrestris and the application of lime had no effect on cumulative CO₂ emissions from soil. During the first 3–4 weeks earthworms significantly (P<0.05) increased CO₂ emissions by 16% to 28%. In contrast, significantly lower (P<0.05) CO₂ emission rates were measured after 11 weeks. The data suggest that earthworm activity was high during the first weeks due to the creation of burrows and incorporation of beech litter into the mineral soil. Low cumulative CH₄ oxidation rates were found in all soil columns as a result of CH₄ production and oxidation processes. L. terrestris with fresh feces and the beech litter produced CH₄ during the laboratory incubation, whereas the mineral soil oxidised atmospheric CH₄. Inoculation with L. terrestris led to a significant reduction (P<0.02) in the CH₄ oxidation rate of soil, i.e. 53% reduction. Liming had no effect on cumulative CH₄ oxidation rates of soil columns and on CH₄ fluxes during the laboratory incubation. L. terrestris significantly increased (P<0.001) cumulative N₂O emissions of unlimed soil columns by 57%. The separate incubation of L. terrestris with fresh feces resulted in rather high N₂O emissions, but the rate strongly decreased from 54 to 2 μg N kg^–1 (dry weight) h^–1 during the 100 h of incubation. Liming had a marked effect on N₂O formation and significantly (P<0.001) reduced cumulative N₂O emissions by 34%. Although the interaction of liming and L. terrestris was not significant, N₂O emissions of limed soil columns with L. terrestris were 8% lower than those of the control. Received: 2 September 1999     

Extractability of microbial N as influenced by C : N ratio in the flush after drying or fumigation

 Extractability of microbial N was estimated using in situ labelling of the microbial population with ¹⁵N. Four arable soils (one grey forest soil and three chernozems with different long-term fertilization) were amended with (NH₄)₂SO₄ (unlabelled or labelled with ¹⁵N) and d-glucose with a C : N ratio of 10 : 1 or 20 : 1 for the grey forest soil and 50 : 1 for the chernozems. d-glucose and labelled N with a C : N ratio of 20 : 1 did not cause microbial immobilization of unlabelled N. The use of substrates with a C : N ratio of 50 : 1 led to a pronounced priming action on soil N and decreased the extractability of immobilized ¹⁵N. Values of the extractable biomass N fraction (k _EN ) assessed for the fumigation-extraction and rehydration procedures were similar and varied in inverse proportion to the C : N ratio of the flush. The k _EN factor was calculated using values of the C : N ratio in flushes and the fixed C : N ratio of structural cell components, with the assumption that the C : N ratio of the extractable cytoplasmic cell fraction is variable. The ratio between the extractable and non-extractable biomass N fraction (k _EC ) and the C : N ratio of non-extractable cell components were assessed as equation parameters optimized for the measured k _EN and C : N ratio of flush data. Received: 31 October 1997     

Carbon and nitrogen mineralization dynamics in different soils of the tropics amended with legume residues and contrasting soil moisture contents

Seasonal drought in tropical agroecosystems may affect C and N mineralization of organic residues. To understand this effect, C and N mineralization dynamics in three tropical soils (Af, An₁, and An₂) amended with haricot bean (HB; Phaseolus vulgaris L.) and pigeon pea (PP; Cajanus cajan L.) residues (each at 5 mg g⁻¹ dry soil) at two contrasting soil moisture contents (pF2.5 and pF3.9) were investigated under laboratory incubation for 100–135 days. The legume residues markedly enhanced the net cumulative CO₂–C flux and its rate throughout the incubation period. The cumulative CO₂–C fluxes and their rates were lower at pF3.9 than at pF2.5 with control soils and also relatively lower with HB-treated than PP-treated soil samples. After 100 days of incubation, 32–42% of the amended C of residues was recovered as CO₂–C. In one of the three soils (An₁), the results revealed that the decomposition of the recalcitrant fraction was more inhibited by drought stress than easily degradable fraction, suggesting further studies of moisture stress and litter quality interactions. Significantly (p < 0.05) greater NH₄⁺–N and NO₃⁻–N were produced with PP-treated (C/N ratio, 20.4) than HB-treated (C/N ratio, 40.6) soil samples. Greater net N mineralization or lower immobilization was displayed at pF2.5 than at pF3.9 with all soil samples. Strikingly, N was immobilized equivocally in both NH₄⁺–N and NO₃⁻–N forms, challenging the paradigm that ammonium is the preferred N source for microorganisms. The results strongly exhibited altered C/N stoichiometry due to drought stress substantially affecting the active microbial functional groups, fungi being dominant over bacteria. Interestingly, the results showed that legume residues can be potential fertilizer sources for nutrient-depleted tropical soils. In addition, application of plant residue can help to counter the N loss caused by leaching. It can also synchronize crop N uptake and N release from soil by utilizing microbes as an ephemeral nutrient pool during the early crop growth period.     

Methane production in unamended and rice-straw-amended soil at different moisture levels

 CH₄ production in an alluvial soil, unamended or amended with rice straw (1% w/w), was examined under nonflooded [–1.5 MPa, –0.01 MPa and 0 MPa (saturated) and flooded (1 : 1.25 soil to water ratio)] conditions during a 40-day incubation in closed Vacutainer tubes. CH₄ production was negligible at –1.5 MPa, but increased with an increase in the moisture level. Addition of rice straw distinctly increased CH₄ production in the soil at all moisture levels including –1.5 MPa. Evidence, in terms of the drop in redox potential and Fe²⁺ accumulated, suggested that the addition of rice straw hastened the reduction of the soil, even under nonflooded conditions; thus its addition stimulated even the nonflooded soil to produce CH₄ in substantial amounts. Our results indicate that many currently unidentified sources of CH₄, possibly including organic-amended nonflooded soils, may make a significant contribution to the global CH₄ budget. Received: 10 July 1997     

Nitrogen dynamics in different types of pasture in the Austrian Alps

 Soil N dynamics were compared in Alpine pastures on two mountains. N-pool sizes and N fluxes were measured relative to N losses via leaching and denitrification in summer. On each mountain, four types of pasture were studied: (1) forest pastures, (2) recently developed pastures formed by forest clearance ("new pastures"), (3) older established pastures, and (4) pastures planted with clover. At both study sites (Scheuchegg and Teufelstein) we obtained similar results. Compared with forest pasture soils, open pasture soils were found to have greater microbial biomass and faster mineralisation potentials, but net field mineralisation rates were slower. In the forest pastures, highest N losses via denitrification were found. Higher potential leaching of NO₃ ^–, estimated by accumulation of NO₃ ^– on ion-exchange resins, in the forest pasture soils suggests lower N uptake by microbes and herbaceous plants compared with open pastures. N₂O-production rates of the forest pasture soils at the Scheuchegg site (11.54 μg N₂O-N m^–2 h^–1) were of similar magnitude to those reported for spruce forests without pastures, but at Teufelstein (53.75 μg N₂O-N m^–2 h^–1) they were higher. However, if forest pastures are not overgrazed, no elevated N loss through N₂O production and leaching of NO₃ ^– is expected. Denitrification rates in the open pastures (0.83–7.50 μg N₂O-N m^–2 h^–1) were low compared with reports on lowland pastures. In soils of the new pastures, rates of microbial N processes were similar to those in the established pastures, indicating a high capacity of soils to restore their internal N cycle after forest clearance. Received: 19 August 1999     

Effects of nitrogen input and grazing on methane fluxes of extensively and intensively managed grasslands in the Netherlands

 Generally, grasslands are considered as sinks for atmospheric CH₄, and N input as a factor which reduces CH₄ uptake by soils. We aimed to assess the short- and long-term effects of a wide range of N inputs, and of grazing versus mowing, on net CH₄ emissions of grasslands in the Netherlands. These grasslands are mostly intensively managed with a total N input via fertilisation and atmospheric deposition in the range of 300–500 kg N ha^–1 year^–1. Net CH₄ emissions were measured with vented, closed flux chambers at four contrasting sites, which were chosen to represent a range of N inputs. There were no significant effects of grazing versus mowing, stocking density, and withholding N fertilisation for 3–9 years, on net CH₄ emissions. When the ground-water level was close to the soil surface, the injection of cattle slurry resulted in a significant net CH₄ production. The highest atmospheric CH₄ uptake was found at the site with the lowest N input and the lowest ground-water level, with an annual CH₄ uptake of 1.1 kg CH₄ ha^–1 year^–1. This is assumed to be the upper limit of CH₄ uptake by grasslands in the Netherlands. We conclude that grasslands in the Netherlands are a net sink of CH₄, with an estimated CH₄ uptake of 0.5 Gg CH₄ year^–1. At the current rates of total N input, the overall effect of N fertilisation on net CH₄ emissions from grasslands is thought to be small or negligible. Received: 27 January 1998     

Tillage and land use effects on methane oxidation rates and their vertical profiles in soil

 The effect of land use and different soil tillage systems on CH₄ oxidation was tested in a laboratory incubation study. Intact soil cores were collected from the topsoil (0–12 cm) of a field site with ploughed, direct-drilled and set-aside treatments, and from an adjacent undisturbed forest site. CH₄ oxidation rates were 4.5 to 11 times higher in the direct-drilled than in the continuously ploughed treatment, in the set-aside soil they were intermediate. The oxidation rates in the forest soil were 11 times the highest rate measured at the field site, pointing to a distinct land use effect. Vertical profiles of CH₄ oxidation activity revealed a very clear zonation in all treatments. CH₄ oxidation increased significantly below the plough layer (0–25 cm), and showed a subsurface maximum under direct-drilling (5–15 cm) and under forest (5–10 cm). The vertical zonation under set-aside was comparable to that under ploughing. Generally, the maximum CH₄ oxidizing activity was in the zone nearest to the soil surface, unless various constraints prevented this. Received: 1 December 1997     

Chemical and biological characteristics of alkaline saline soils from the former Lake Texcoco as affected by artificial drainage

 Soils from the former Lake Texcoco are alkaline saline and were artificially drained and irrigated with sewage effluents since the late 1980s. Undrained soil and soil drained for 1, 5 and 8 years were sampled, characterized and incubated aerobically for 90 days at 22±1  °C while production of CO₂, available P and concentrations of NH₄ ⁺, NO₂ ^– and NO₃ ^– were monitored. Artificial drainage decreased pH_H2O, water holding capacity, organic C, total N, and Na⁺, K⁺, Mg²⁺, B, Cl^– and SO₄ ^2– concentrations, increased inorganic C and Ca²⁺ concentrations more than 5-fold while total P was not affected. Microbial biomass C decreased with increased length of drainage but bacteria, actinomycetes, denitrifiers and cellulose-utilizing bacteria tended to show opposite trends. CO₂ production was less in soils drained ≥5 years compared to undrained soil but more than in soils drained for 1 year. Emission of NH₃ was negligible and concentrations of NH₄ ⁺ remained constant over time in each soil. Nitrification, as witnessed by increases in NO₃ ^– concentrations, occurred in soil drained for 8 years. NO₂ ^– concentrations decreased in soils drained ≤1 year in the first 7 days of the incubation and remained constant thereafter. It was found that artificial drainage of soils from the former Lake Texcoco profoundly affected soil characteristics. Decreases in pH and Na⁺, K⁺, Cl^– and SO₄ ^2– concentrations made conditions more favourable for plant growth, although low concentrations of inorganic N and available P might be limiting factors. Received: 1 December 1999     

Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N₂O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N₂O emission. Residues were added at the rate of 3 t C ha⁻¹ to soil with, and without, 150 kg urea N ha⁻¹. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO₃⁻–N, which reached minimum values of 2.8 mg N kg⁻¹ for sugarcane (at day 28), 10.3 mg N kg⁻¹ for maize (day 7), and 5.9 mg N kg⁻¹ for sorghum (day 7), compared to 22.7 mg N kg⁻¹ for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg⁻¹ in sugarcane, 34 mg kg⁻¹ in maize, 29 mg kg⁻¹ in sorghum, and 16 mg kg⁻¹ in cotton amended soil compared to soil + N fertilizer, although soil NO₃⁻–N increased by 7 mg kg⁻¹ in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N₂O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N₂O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N₂O emission was significantly and positively correlated with NO₃⁻–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO₂ in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).