Effect of management, climate and soil conditions on N2O and NO emissions from an arable crop rotation using high temporal resolution measurements

Quantifying the nitrous oxide (N₂O) and nitric oxide (NO) fluxes emitted from croplands remains a major challenge. Field measurements in different climates, soil and agricultural conditions are still scarce and emissions are generally assessed from a small number of measurements. In this study, we report continuously measured N₂O and NO fluxes with a high temporal resolution over a 2-year crop sequence of barley and maize in northern France. Measurements were carried out using 6 automatic chambers at a rate of 16 mean flux measurements per day. Additional laboratory measurements on soil cores were conducted to study the response of NO and N₂O emissions to environmental conditions.The detection limit of the chamber setup was found to be 3 ng N m⁻² s⁻¹ for N₂O and 0.1 ng N m⁻² s⁻¹ for NO. Nitrous oxide fluxes were higher than the threshold 37% of the time, while they were 72% of the time for NO fluxes.The cumulated annual NO and N₂O emissions were 1.7 kg N₂O-N ha⁻¹ and 0.5 kg NO-N ha⁻¹ in 2007, but 2.9 kg N₂O-N ha⁻¹ and 0.7 kg NO-N ha⁻¹ in 2008. These inter-annual differences were largely related to crop types and to their respective management practices. The forms, amounts and timing of nitrogen applications and the mineralization of organic matter by incorporation of crop residues were found to be the main factor controlling the emissions peaks. The inter-annual variability was also due to different weather conditions encountered in 2007 and 2008. In 2007, the fractioned N inputs applied on barley (54 kg ha⁻¹ in March and in April) did not generate N₂O emissions peaks because of the low rainfall during the spring. However, the significant rainfall observed in the summer and fall of 2007, promoted rapid decomposition of barley residues which caused high levels of N₂O emissions. In 2008, the application of dairy cattle slurry and mineral fertilizer before the emergence of maize (107 kg N_min ha⁻¹ or 130 kg N_tot ha⁻¹ in all) coincided with large rainfalls promoting both NO and N₂O emissions, which remained high until early summer.Laboratory measurements corroborated the field observations: NO fluxes were maximum at a water-filled pore space (WFPS) of around 27% while N₂O fluxes were optimal at 68% WFPS, with a maximum potentially 14 times larger than for NO.     

Seasonal variation and fire effects on CH4, N2O and CO2 exchange in savanna soils of northern Australia

Tropical savanna ecosystems are a major contributor to global CO₂, CH₄ and N₂O greenhouse gas exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact upon savanna soil microbial processes involved in N₂O and CH₄ exchange. We measured soil CO₂, CH₄ and N₂O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniata trees above sorghum grass) at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is a frequent natural and management action in these savanna (every 1-2 years). There was no seasonal change and no fire effect upon soil N₂O exchange. Soil N₂O fluxes were very low, generally between −1.0 and 1.0 μg N m⁻² h⁻¹, and often below the minimum detection limit. There was an increase in soil NH₄⁺ in the months after the 2008 fire event, but no change in soil NO₃⁻. There was considerable nitrification in the early wet season but minimal nitrification at all other times.Savanna soil was generally a net CH₄ sink that equated to between −2.0 and −1.6 kg CH₄ ha⁻¹ y⁻¹ with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry season significantly reduced soil gas diffusion and as a consequence soil CH₄ uptake. There were short periods of soil CH₄ emission, up to 20 μg C m⁻² h⁻¹, likely to have been caused by termite activity in, or beneath, automated chambers. Soil CO₂ fluxes showed a strong bimodal seasonal pattern, increasing fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH₄ fluxes, but a much stronger relationship with soil CO₂ fluxes, explaining up to 70% of the variation in unburnt treatments. Australian savanna soils are a small N₂O source, and possibly even a sink. Annual soil CH₄ flux measurements suggest that the 1.9 million km² of Australian savanna soils may provide a C sink of between −7.7 and −9.4 Tg CO₂-e per year. This sink estimate would offset potentially 10% of Australian transport related CO₂-e emissions. This CH₄ sink estimate does not include concurrent CH₄ emissions from termite mounds or ephemeral wetlands in Australian savannas.     

Methane flux characteristics in forest soils under an East Asian monsoon climate

Methane (CH₄) uptake by soil can possibly be suppressed more in regions with heavy summer precipitation, such as those under the East Asian monsoon climate, as compared to that in regions with a dry summer. In order to determine how precipitation patterns affect seasonal and spatial variations in CH₄ fluxes in temperate forest soils, such fluxes and selected environmental variables were measured on different parts of a hill slope in a cypress forest in central Japan. On the upper and middle parts of the slope, CH₄ uptake was observed throughout the year, and the uptake rates increased slightly with soil temperature and decreased with soil water content. The CH₄ flux predicted using data for the middle and upper parts of the slope ranged from −1.12 to −0.83 kg-CH₄ ha⁻¹ y⁻¹ (i.e. CH₄ uptake by soil) and from −2.30 to −2.04 kg-CH₄ ha⁻¹ y⁻¹, respectively. In contrast, in the relatively wet lower part of the slope near an in-stream wetland, large CH₄ emissions (>2 mg-CH₄ m⁻¹ d⁻¹) were observed during the rainy summer. In this wetter plot, the soil functioned as a net annual CH₄ source in a rainy year. Hence the variation in CH₄ flux with a change in soil water conditions and soil temperature on the lower part of the slope contrasted to that on the upper and middle parts of the slope. The predicted CH₄ flux for this lower plot ranged from −0.45 kg-CH₄ ha⁻¹ y⁻¹ in a dry year to 1.80 kg-CH₄ ha⁻¹ y⁻¹ in a rainy year. Our results suggest that consideration of the soil water conditions across a watershed is important for estimating the CH₄ budgets for entire forest watershed, particularly in regions subject to a wet summer.     

Fluxes of CO2, CH4 and N2O from drained organic soils in deciduous forests

We examined net greenhouse gas exchange at the soil surface in deciduous forests on soils with high organic contents. Fluxes of CO₂, CH₄ and N₂O were measured using dark static chambers for two consecutive years in three different forest types; (i) a drained and medium productivity site dominated by birch, (ii) a drained and highly productive site dominated by alder and (iii) an undrained and highly productive site dominated by alder. Although the drained sites had shallow mean groundwater tables (15 and 18 cm, respectively) their average annual rates of forest floor CO₂ release were almost twice as high compared to the undrained site (1.9±0.4 and 1.7±0.3, compared to 1.0±0.2 kg CO₂ m⁻² yr⁻¹). The average annual CH₄ emission was almost 10 times larger at the undrained site (7.6±3.1 compared to 0.9±0.5 g CH₄ m⁻² yr⁻¹ for the two drained sites). The average annual N₂O emissions at the undrained site (0.1±0.05 g N₂O m⁻² yr⁻¹) were lower than at the drained sites, and the emissions were almost five times higher at the drained alder site than at the drained birch site (0.9±0.35 compared to 0.2±0.11 g N₂O m⁻² yr⁻¹). The temporal variation in forest floor CO₂ release could be explained to a large extent by differences in groundwater table and air temperature, but little of the variation in the CH₄ and N₂O fluxes could be explained by these variables. The measured soil variables were only significant to explain for the within-site spatial variation in CH₄ and N₂O fluxes at the undrained swamp, and dark forest floor CO₂ release was not explained by these variables at any site. The between-site spatial variation was attributed to variations in drainage, groundwater level position, productivity and tree species for all three gases. The results indicate that N₂O emissions are of greater importance for the net greenhouse gas exchange at deciduous drained forest sites than at coniferous drained forest sites.     

CH4 oxidation and emissions of CH4 and N2O from Lolium perenne swards under elevated atmospheric CO2

Emissions of N₂O and CH₄ and CH₄ oxidation rates were measured from Lolium perenne swards in a short-term study under ambient (36 Pa) and elevated (60 Pa) atmospheric CO₂ at the Free Air Carbon dioxide Enrichment experiment, Eschikon, Switzerland. Elevated pCO₂ increased (P<0.05) N₂O emissions from high N fertilised (11.2 g N m⁻²) swards by 69%, but had no significant effect on net emissions of CH₄. Application of ¹³C-CH₄ (11 μl l⁻¹; 11 at.% excess ¹³C) to closed chamber headspaces in microplots enabled determination of rates of ¹³C-CH₄ oxidation even when net CH₄ fluxes from main plots were positive. We found a significant interaction between fertiliser application rate and atmospheric pCO₂ on ¹³C-CH₄ oxidation rates that was attributed to differences in gross nitrification rates and C and N availability. CH₄ oxidation was slower and thought to be temporarily inhibited in the high N ambient pCO₂ sward. The most rapid CH₄ oxidation of 14.6 μg ¹³C-CH₄ m⁻² h⁻¹ was measured in the high fertilised elevated pCO₂ sward, and we concluded that either elevated pCO₂ had a stimulatory effect on CH₄ oxidation or inhibition of oxidation following fertiliser application was lowered under elevated pCO₂. Application of ¹⁴NH₄¹⁵NO₃ and ¹⁵NH₄¹⁵NO₃ (10 at.% excess ¹⁵N) to different replicates enabled determination of the respective contributions of nitrification and denitrification to N₂O emissions. Inhibition of CH₄ oxidation in the high fertilised ambient pCO₂ sward, due to competition between NH₃ and CH₄ for methane monooxygenase enzymes or toxic effects of NH₂OH or NO₂⁻ produced during nitrification, was hypothesised to increase gross nitrification (12.0 mg N kg dry soil⁻¹) and N₂O emissions during nitrification (327 mg ¹⁵N-N₂O m⁻² over 11 d). Our results indicate that increasing atmospheric concentrations of CO₂ may increase emissions of N₂O by denitrification, lower nitrification rates and either increase or decrease the ability of soil to act as a sink for atmospheric CH₄ depending on fertiliser management.     

Fluxes of CO2, CH4, and N2O in an alpine meadow affected by yak excreta on the Qinghai-Tibetan plateau during summer grazing periods

To assess the impacts of yak excreta patches on greenhouse gas (GHG) fluxes in the alpine meadow of the Qinghai-Tibetan plateau, methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) fluxes were measured for the first time from experimental excreta patches placed on the meadow during the summer grazing seasons in 2005 and 2006. Dung patches were CH₄ sources (average 586 μg m⁻² h⁻¹ in 2005 and 199 μg m⁻² h⁻¹ in 2006) during the investigation period of two years, while urine patches (average −31 μg m⁻² h⁻¹ in 2005 and −33 μg m⁻² h⁻¹ in 2006) and control plots (average −28 μg m⁻² h⁻¹ in 2005 and −30 μg m⁻² h⁻¹ in 2006) consumed CH₄. The cumulative CO₂ emission for dung patches was about 36-50% higher than control plots during the experimental period in 2005 and 2006. The cumulative N₂O emissions for both urine and dung patches were 2.1-3.7 and 1.8-3.5 times greater than control plots in 2005 and 2006, respectively. Soil water-filled pore space (WFPS) explained 35% and 36% of CH₄ flux variation for urine patches and control plots, respectively. Soil temperature explained 40-75% of temporal variation of CO₂ emissions for all treatments. Temporal N₂O flux variation in urine patches (34%), dung patches (48%), and control (56%) plots was mainly driven by the simultaneous effect of soil temperature and WFPS. Although yak excreta patches significantly affected GHG fluxes, their contributions to the whole grazing alpine meadow in terms of CO₂ equivalents are limited under the moderate grazing intensity (1.45 yak ha⁻¹). However, the contributions of excreta patches to N₂O emissions are not negligible when estimating N₂O emissions in the grazing meadow. In this study, the N₂O emission factor of yak excreta patches varied with year (about 0.9-1.0%, and 0.1-0.2% in 2005 and 2006, respectively), which was lower than IPCC default value of 2%.     

CH4 oxidation and N2O emissions at varied soil water-filled pore spaces and headspace CH4 concentrations

Emission of N₂O and CH₄ oxidation rates were measured from soils of contrasting (30-75%) water-filled pore space (WFPS). Oxidation rates of ¹³C-CH₄ were determined after application of 10 μl ¹³C-CH₄ l⁻¹ (10 at. % excess ¹³C) to soil headspace and comparisons made with estimates from changes in net CH₄ emission in these treatments and under ambient CH₄ where no ¹³C-CH₄ had been applied. We found a significant effect of soil WFPS on ¹³C-CH₄ oxidation rates and evidence for oxidation of 2.2 μg ¹³C-CH₄ d⁻¹ occurring in the 75% WFPS soil, which may have been either aerobic oxidation occurring in aerobic microsites in this soil or anaerobic CH₄ oxidation. The lowest ¹³C-CH₄ oxidation rate was measured in the 30% WFPS soil and was attributed to inhibition of methanotroph activity in this dry soil. However, oxidation was lowest in the wetter soils when estimated from changes in concentration of ¹²⁺¹³C-CH₄. Thus, both methanogenesis and CH₄ oxidation may have been occurring simultaneously in these wet soils, indicating the advantage of using a stable isotope approach to determine oxidation rates. Application of ¹³C-CH₄ at 10 μl ¹³C-CH₄ l⁻¹ resulted in more rapid oxidation than under ambient CH₄ conditions, suggesting CH₄ oxidation in this soil was substrate limited, particularly in the wetter soils. Application of and (80 mg N kg soil⁻¹; 9.9 at.% excess ¹⁵N) to different replicates enabled determination of the respective contributions of nitrification and denitrification to N₂O emissions. The highest N₂O emission (119 μg ¹⁴⁺¹⁵N-N₂O kg soil⁻¹ over 72 h) was measured from the 75% WFPS soil and was mostly produced during denitrification (18.1 μg ¹⁵N-N₂O kg soil⁻¹; 90% of ¹⁵N-N₂O from this treatment). Strong negative correlations between ¹⁴⁺¹⁵N-N₂O emissions, denitrified ¹⁵N-N₂O emissions and ¹³C-CH₄ concentrations (r=−0.93 to −0.95, N₂O; r=−0.87 to −0.95, denitrified ¹⁵N-N₂O; P<0.05) suggest a close relationship between CH₄ oxidation and denitrification in our soil, the nature of which requires further investigation.     

Rice root-derived carbon input and its effect on decomposition of old soil carbon pool under elevated CO2

Rice (Oryza sativa) was grown in sunlit, semi-closed growth chambers (4×3×2 m, L×W×H) at 650 μl l⁻¹ CO₂ (elevated CO₂) to determine: (1) rice root-derived carbon (C) input into the soil under elevated CO₂ in one growing season, and (2) the effect of the newly input C on decomposition of the more recalcitrant native soil organic C. The initial δ¹³C value of the experimental soil was −25.8‰, which was 6‰ less depleted in ¹³C than the plants grown under elevated CO₂. Significant changes in δ¹³C of the soil organic C were detected after one growing season. The amount of new soil C input was estimated to be 0.9 t ha⁻¹ (or 2.1%) at 30 kg N ha⁻¹ and 1.8 t ha⁻¹ (4.1%) at 90 kg N ha⁻¹. Changes in soil δ¹³C suggested that the surface 5 cm of soil received more C input from plants than soils below. Laboratory incubation (25 °C) of soils from different horizons indicated that increased availability of the labile plant-derived C in the soil reduced decomposition of the native soil organic C. Provided the retardant effect of the new C on old soil organic C holds in the field in the longer-term, paddy soils will likely sequester more C from the atmosphere if more plant C enters the soil under elevated atmospheric CO₂.     

Pasture type and fertilization effects on N2 fixation, N budgets and external energy inputs in western Canada

A grazing experiment was conducted in Brandon, Manitoba, Canada. The objectives were to examine the effects of including alfalfa and fertilizer management on N₂ fixation by alfalfa and plant N dynamics, and to compare N budgets in the four contrasting pasture systems and external energy inputs between fertilizer-N-based and legume-based pasture systems. Estimates of annual amounts of N₂ fixed, based on shoot herbage production in grazed mixed alfalfa/grass pastures, ranged from 40 to 118 kg N ha⁻¹ y⁻¹. The amounts would be in the range of 52-153 kg N ha⁻¹ y⁻¹, if the amounts of fixed N stored in the roots, were included. Compared to grass-only pastures, total amounts of N₂ fixed in the mixed pastures should be sufficient to improve total external N inputs, replace N fertilizer and sustain plant protein for grazing. The reliance of alfalfa (Medicago sativa L.) on N₂ fixation for growth was high (70-95%), and %N derived from the atmosphere by alfalfa (%Ndfa) was not affected by P fertilizer management. Thus, the amounts of N₂ fixed were predominantly regulated by alfalfa dry matter productivity. The data also indicated that alfalfa fixed 27 kg N t⁻¹ dry matter produced. In mixed alfalfa/grass pastures, high soil mineral N uptake by companion grasses, was essential to effectively utilize N that was fixed by alfalfa and returned to soils through the decomposition of alfalfa litter and roots. Compared to grass-only pastures with or without N fertilizer, alfalfa-based pastures could supply sufficient plant protein for grazing animals through N₂ fixation, and at same time, sustain animal productivity with only 28% of the external energy input of the grass-only pasture with N fertilizer.     

Responses of soil organic matter and greenhouse gas fluxes to soil management and land use changes in a humid temperate region of southern Europe

We studied the effects of soil management and changes of land use on soils of three adjacent plots of cropland, pasture and oak (Quercus robur) forest. The pasture and the forest were established in part of the cropland, respectively, 20 and 40 yr before the study began. Soil organic matter (SOM) dynamics, water-filled pore space (WFPS), soil temperature, inorganic N and microbial C, as well as fluxes of CO₂, CH₄ and N₂O were measured in the plots over 25 months. The transformation of the cropland to mowed pasture slightly increased the soil organic and microbial C contents, whereas afforestation significantly increased these variables. The cropland and pasture soils showed low CH₄ uptake rates (<1 kg C ha⁻¹ yr⁻¹) and, coinciding with WFPS values >70%, episodes of CH₄ emission, which could be favoured by soil compaction. In the forest site, possibly because of the changes in soil structure and microbial activity, the soil always acted as a sink for CH₄ (4.7 kg C ha⁻¹ yr⁻¹). The N₂O releases at the cropland and pasture sites (2.7 and 4.8 kg N₂O-N ha⁻¹ yr⁻¹) were, respectively, 3 and 6 times higher than at the forest site (0.8 kg N₂O-N ha⁻¹ yr⁻¹). The highest N₂O emissions in the cultivated soils were related to fertilisation and slurry application, and always occurred when the WFPS >60%. These results show that the changes in soil properties as a consequence of the transformation of cropfield to intensive grassland do not imply substantial changes in SOM or in the dynamics of CH₄ and N₂O. On the contrary, afforestation resulted in increases in SOM content and CH₄ uptake, as well as decreases in N₂O emissions.     

Sources and rates of nitrous oxide emissions from grazed grassland after application of N-labelled mineral fertilizer and slurry

Nitrogen (N) fertilizer application and grazing are known to induce nitrous oxide (N₂O) emissions from grassland soils. In a field study, general information on rates of N₂O emission, the effect of cattle grazing and the type (mineral fertilizer, cattle slurry) and amount of N supply on the flux of N₂O from a sandy soil were investigated. N₂O emissions from permanent grassland managed as a mixed system (two cuts followed by two grazing cycles) were monitored over 11 months during 2001-2002 in northern Germany using the closed chamber method. The field experiment consisted of four regionally relevant fertilizer combinations, i.e. two mineral N application rates (0 and 100 kg N ha⁻¹ yr⁻¹) and two slurry levels (0 and 74 kg N ha⁻¹ yr⁻¹).Mean cumulative N₂O-N loss was 3.0 kg ha⁻¹ yr⁻¹, and the cumulative ¹⁵N-labelled N₂O emissions varied from 0.03% to 0.19% of the ¹⁵N applied. ¹⁵N labelling indicated that more N₂O was emitted from mineral N than from slurry treated plots, and in all treatments the soil N pool was always clearly the major source of N₂O. Regarding the total cumulative N₂O losses, differences among treatments were not significant, which was caused by: (i) a high variance in emissions during and after cattle grazing due to the random distribution of excrements and by (ii) high N₂ fixation of white clover in the 0 kg N ha⁻¹ treatments, which resulted in similar N status of all treatments. However before grazing started, treatments showed significant differences. After cattle grazing in summer, N₂O emission rates were higher than around the time of spring fertilizer application, or in winter. Grazing resulted in N₂O flux rates up to 489 μg N₂O-N m⁻² h⁻¹ and the grazing period contributed 31-57% to the cumulative N₂O emission. During freeze-thaw cycles in winter (December-February) N₂O emission rates of up to 147 μg N₂O-N m⁻² h⁻¹ were measured, which contributed up to 26% to the annual N₂O flux. The results suggest that N fertilizer application and grazing caused only short-term increases of N₂O flux rates whereas the major share of annual N₂O emission emitted from the soil N pool. The significantly increased N₂O fluxes during freeze-thaw cycles show the importance of emission events in winter which need to be covered by measurements for obtaining reliable estimates of annual N₂O emissions.     

Carbon dioxide, nitrous oxide and methane dynamics in boreal organic agricultural soils with different soil characteristics

The annual carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) dynamics were measured with static chambers on two organic agricultural soils with different soil characteristics. Site 1 had a peat layer of 30 cm, with an organic matter (OM) content of 74% in the top 20 cm. Site 2 had a peat layer of 70 cm but an OM content of only 40% in the top 20 cm. On both sites there were plots under barley and grass and also plots where the vegetation was removed. All soils were net sources of CO₂ and N₂O, but they consumed atmospheric CH₄. Soils under barley had higher net CO₂ emissions (830 g CO₂-C m⁻² yr⁻¹) and N₂O emissions (848 mg N₂O-N m⁻² yr⁻¹) than those under grass (395 g CO₂-C m⁻³ yr⁻¹ and 275 mg N₂O-N m⁻² yr⁻¹). Bare soils had the highest N₂O emissions, mean 2350 mg N₂O-N m⁻² yr⁻¹. The mean CH₄ uptake rate from vegetated soils was 100 mg CH₄-C m⁻³ yr⁻¹ and from bare soils 55 mg CH₄-C m⁻² yr⁻¹. The net CO₂ emissions were higher from Site 2, which had a high peat bulk density and a low OM content derived from the addition of mineral soil to the peat during the cultivation history of that site. Despite the differences in soil characteristics, the mean N₂O emissions were similar from vegetated peat soils from both sites. However, bare soils from Site 2 with mineral soil addition had N₂O emissions of 2-9 times greater than those from Site 1. Site 1 consumed atmospheric CH₄ at a higher rate than Site 2 with additional mineral soil. N₂O emissions during winter were an important component of the N₂O budget even though they varied greatly, ranging from 2 to 99% (mean 26%) of the annual emission.     

Impacts of 22-year organic and inorganic N managements on soil organic C fractions in a maize field,northeast China

Impacts of 22-year organic and inorganic N managements on total organic carbon (TOC), water-soluble organic C (WSOC), microbial biomass C (MBC), particulate organic C (POC) and KMnO₄ oxidized organic C (KMnO₄-C) concentrations, C management index (CMI), and C storage in surface soil (0–20 cm) were investigated in a maize (Zea may L.) field experiment, Northeast China. The treatments included, CK: unfertilized control, M: organic manure (135 kg N ha^− 1 year^− 1), N: inorganic N fertilizer (135 kg N ha^− 1 year^− 1) and MN: combination of organic manure (67.5 kg N ha^− 1 year^− 1) and inorganic N fertilizer (67.5 kg N ha^− 1 year^− 1). TOC concentration and C storage were significantly increased under the M and MN treatments, but not under the inorganic N treatment. The organic treatments of M and MN were more effective in increasing WSOC, MBC, POC and KMnO₄-C concentrations and CMI than the N treatment. The M treatment was most effective for sequestrating SOC (10.6 Mg ha^− 1) and showed similar increase in degree of grain yield to the N and MN treatments, therefore it could be the best option for improving soil productivity and C storage in the maize cropping system.     

Pathways and dynamics of NO3 and NH4 applied in a mountain Picea abies forest and in a nearby meadow in central Switzerland

To evaluate the pathways and dynamics of inorganic nitrogen (N) deposition in previously N-limited ecosystems, field additions of ¹⁵N tracers were conducted in two mountain ecosystems, a forest dominated by Norway spruce (Picea abies) and a nearby meadow, at the Alptal research site in central Switzerland. This site is moderately impacted by N from agricultural and combustion sources, with a bulk atmospheric deposition of 12 kg N ha⁻¹ y⁻¹ equally divided between NH₄⁺ and NO₃⁻. Pulses of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were applied separately as tracers on plots of 2.25 m². Several ecosystem pools were sampled at short to longer-term intervals (from a few hours to 1 year), above and belowground biomass (excluding trees), litter layer, soil LF horizon (approx. 5-0 cm), A horizon (approx. 0-5 cm) and gleyic B horizon (5-20 cm). Furthermore, extractable inorganic N, and microbial N pools were analysed in the LF and A horizons. Tracer recovery patterns were quite similar in both ecosystems, with most of the tracer retained in the soil pool. At the short-term (up to 1 week), up to 16% of both tracers remained extractable or entered the microbial biomass. However, up to 30% of the added ¹⁵NO₃⁻ was immobilised just after 1 h, and probably chemically bound to soil organic matter. 16% of the NH₄⁺ tracer was also immobilised within hours, but it is not clear how much was bound to soil organic matter or fixed between layers of illite-type clay. While the extractable and microbial pools lost ¹⁵N over time, a long-term increase in ¹⁵N was measured in the roots. Otherwise, differences in recovery a few hours after labelling and 1 year later were surprisingly small. Overall, more NO₃⁻ tracer than NH₄⁺ tracer was recovered in the soil. This was due to a strong aboveground uptake of the deposited NH₄⁺ by the ground vegetation, especially by mosses.     

Micro-scale CO2 and CH4 dynamics in a peat soil during a water fluctuation and sulfate pulse

We investigated the response of CO₂ and CH₄ production to a water table fluctuation and a SO₄²⁻ pulse in a bog mesocosm. Net gas production rates in the mesocosm were calculated from concentration data by diffusive mass-balances. Incubation experiments were used to quantify the effect of SO₄²⁻ addition and the distribution of potential CO₂ and CH₄ production rates. Flooding of unsaturated peat resulted in rapid depletion of O₂ and complex patterns of net CH₄, CO₂, and H₂S production. Methane production began locally and without a time lag at rates of 3-4 nmol cm⁻³ d⁻¹ deeper in the peat. Similar rates were determined after a time lag of 10-60 days in the surface layers, whereas rates at lower depths declined. Net CO₂ production was largest immediately after the water table position was altered (100-300 nmol cm⁻³ d⁻¹) and declined to −50-50 nmol cm⁻³ d⁻¹ after a few weeks. SO₄²⁻ addition (500 mM) significantly increased potential CH₄ production rates in the surface layer from an average of 132-201 nmol cm⁻³ d⁻¹ and reduced it below from an average of 418-256 nmol cm⁻³ d⁻¹. Our results suggest that deeper in the peat (40-70 cm) under in situ conditions, methanogenic populations are less impaired by unsaturated conditions than in the surface layers, and that at these depths after flooding the substrate availability for CH₄ and DIC production is significantly enhanced. They also suggest that methanogenic and SO₄²⁻-reducing activity were non-competitive in the surface layer, which might explain contradictory findings from field studies.     

Variation in NH4 mineralization and microbial communities with stand age in lodgepole pine (Pinus contorta) forests, Yellowstone National Park (USA)

Soils and vegetation were analyzed in 20 lodgepole pine (Pinus contorta) forest stands, varying in age from 50 to 350 years, that had initiated following stand-replacing fire. Our goal was to determine how nitrogen availability (NH₄⁺-N) and microbial community composition varied with stand age-class and to determine whether differences could be explained by canopy, soil, or understory characteristics. Gross NH₄⁺ mineralization was measured using laboratory isotopic pool dilution, and microbial community composition was evaluated using microbial membrane lipids. The microbial community composition of stands in the 300-350 age class was distinct from stands in younger age classes. Microbial community composition among sites varied with pH, % organic matter, and phosphorus. Gross NH₄⁺ mineralization rates averaged 1.45±0.07 mg NH₄⁺ kg soil⁻¹ d⁻¹ while consumption averaged 1.37±0.20 mg NH₄⁺ kg soil⁻¹ d⁻¹, resulting in low net NH₄⁺ mineralization rates (0.08±0.18 mg NH₄⁺ kg soil⁻¹ d⁻¹), but rates were not significantly different with stand age-class at p<0.05. At p<0.10, net NH₄⁺ mineralization was significantly higher in the 300-350 age class compared to the 125-175 age class. None of the measured variables significantly explained NH₄⁺ consumption and net mineralization patterns. However, gross NH₄⁺ mineralization rates were best explained by information on microbial community structure (i.e. lipids). Variation among stands within a given age-classes was high, indicating that patterns of N cycling across landscapes reflect substantial heterogeneity among mature stands.     

Sulphur in soil and light fraction organic matter as influenced by long-term application of superphosphate

Numerous studies have examined the role of light fraction (LF) organic matter in soil C and N cycling, but there is no published information on the amounts and nature of S in LF. The objective of this work was to characterize the S composition of LF in soils receiving different inputs of fertilizer S. Soils (0-7.5 cm) were taken from a long-term experiment (1952-1999) set up to examine the effects of single superphosphate (SP) (applied at 0, 188, or 376 kg ha⁻¹ yr⁻¹, which equates to 0, 21, and 42 kg SO₄-S ha⁻¹ yr⁻¹) on the productivity of an irrigated, grass-clover pasture grazed by sheep. The S content of LF (separated by flotation on NaI solution with specific gravity 1.7) increased by ∼20-30% in response to SP. The LF was enriched in organic S compared with whole soil (S concentration in LF was ∼1000-1400 mg kg⁻¹ vs ∼400-500 mg kg⁻¹ in whole soil), but LF-S represented only 1.3-4.7% of soil S. Most (∼88%) of the S in LF was C-bonded, reflecting the dominance of this form of S in organic matter returned to the soil in dung and plant residues. Hydriodic acid (HI) reducible-S accounted for only ∼12% of LF-S, compared with 28-35% of whole soil organic S. Superphosphate tended to increase total soil N, due to improved clover growth. There was a strong positive relationship between total N and C-bonded S in whole soil and LF, whereas soil HI-S and N were not associated. Increases in C-bonded S where SP was applied appeared to be driven mainly by increases in soil N, which in turn were due to improved clover growth in response to phosphate supplied by SP. Increases in HI-S due to SP application were probably a direct response to inputs of S. As LF is a small pool of S, with a relatively wide C:S ratio (∼200:1), we concluded that it is unlikely to contribute a significant amount of plant-available S.     

Long-term application of organic manure and nitrogen fertilizer on N2O emissions, soil quality and crop production in a sandy loam soil

A long-term field experiment was established to determine the influence of mineral fertilizer (NPK) or organic manure (composed of wheat straw, oil cake and cottonseed cake) on soil fertility. A tract of calcareous fluvo-aquic soil (aquic inceptisol) in the Fengqiu State Key Experimental Station for Ecological Agriculture (Fengqiu county, Henan province, China) was fertilized beginning in September 1989 and N₂O emissions were examined during the maize and wheat growth seasons of 2002-2003. The study involved seven treatments: organic manure (OM), half-organic manure plus half-fertilizer N (1/2 OMN), fertilizer NPK (NPK), fertilizer NP (NP), fertilizer NK (NK), fertilizer PK (PK) and control (CK). Manured soils had higher organic C and N contents, but lower pH and bulk densities than soils receiving the various mineralized fertilizers especially those lacking P, indicating that long-term application of manures could efficiently prevent the leaching of applied N from and increase N content in the plowed layer. The application of manures and fertilizers at a rate of 300 kg N ha⁻¹ year⁻¹ significantly increased N₂O emissions from 150 g N₂O-N ha⁻¹ year⁻¹ in the CK treatment soil to 856 g N₂O-N ha⁻¹ year⁻¹ in the OM treatment soil; however, there was no significant difference between the effect of fertilizer and manure on N₂O emission. More N₂O was released during the 102-day maize growth season than during the 236-day wheat growth season in the N-fertilized soils but not in N-unfertilized soils. N₂O emission was significantly affected by soil moisture during the maize growth season and by soil temperature during the wheat growth season. In sum, this study showed that manure added to a soil tested did not result in greater N₂O emission than treatment with a N-containing fertilizer, but did confer greater benefits for soil fertility and the environment.     

NO, N2O, CH4 and CO2 fluxes in winter barley field of Japanese Andisol as affected by N fertilizer management

The study was carried out at the experimental station of the Japan International Research Center for Agricultural Sciences to investigate gas fluxes from a Japanese Andisol under different N fertilizer managements: CD, a deep application (8 cm) of the controlled release urea; UD, a deep application (8 cm) of the conventional urea; US, a surface application of the conventional urea; and a control, without any N application. NO, N₂O, CH₄ and CO₂ fluxes were measured simultaneously in a winter barley field under the maize/barley rotation. The fluxes of NO and N₂O from the control were very low, and N fertilization increased the emissions of NO and N₂O. NO and N₂O from N fertilization treatments showed different emission patterns: significant NO emissions but low N₂O emissions in the winter season, and low NO emissions but significant N₂O emissions during the short period of barley growth in the spring season. The controlled release of the N fertilizer decreased the total NO emissions, while a deep application increased the total N₂O emissions. Fertilizer-derived NO-N and N₂O-N from the treatments CD, UD and US accounted for 0.20±0.07%, 0.71±0.15%, 0.62±0.04%, and 0.52±0.04%, 0.50±0.09%, 0.35±0.03%, of the applied N, respectively, during the barley season. CH₄ fluxes from the control were negative on most sampling dates, and its net soil uptake was 33±7.1 mg m⁻² during the barley season. The application of the N fertilizer decreased the uptake of atmospheric CH₄ and resulted in positive emissions from the soil. CO₂ fluxes were very low in the early period of crop growth while higher emissions were observed in the spring season. The N fertilization generally increased the direct CO₂ emissions from the soil. N₂O, CH₄ and CO₂ fluxes were positively correlated (P<0.01) with each other, whereas NO and CO₂ fluxes were negatively correlated (P<0.05). The N fertilization increased soil-derived global warming potential (GWP) significantly in the barley season. The net GWP was calculated by subtracting the plant-fixed atmospheric CO₂ stored in its aboveground parts from the soil-derived GWP in CO₂ equivalent. The net GWP from the CD, UD, US and the control were all negative at −243±30.7, −257±28.4, −227±6.6 and −143±9.7 g C m⁻² in CO₂ equivalent, respectively, in the barley season.     

Surface/atmosphere exchange and chemical interactions of reactive nitrogen compounds above a manured grassland

Manure application to managed grassland is a common agricultural practice. There are, however, limited studies looking at the fluxes and interactions of reactive N compounds and aerosols following fertilisation with manure. In this study, state-of-the-art chemical analysers (GRAEGOR, QCLAS, PTRMS) were used to investigate concentrations, fluxes and chemical interactions of reactive nitrogen containing trace gases (NH₃, HNO₃, HONO) and aerosols (NO₃⁻) above a grassland fertilised with 164 kg N ha⁻¹ of cattle slurry. Emissions of NH₃ peaked at >67 μg m⁻² s⁻¹, based on a 30 min average. The estimated overall loss of total ammoniacal nitrogen (TAN) from the applied slurry through NH₃ emissions in the first 5 days was 33.5%. The average trimethylamine flux in the first 31 h following the first slurry application was 40 ng m⁻² s⁻¹ and amounted to 0.38% of the NH₃-N emissions. Apparent nitrate aerosol emissions were observed following the slurry application peaking at 13.0 ng m⁻² s⁻¹. This suggests formation of NH₄NO₃ from reaction of the emitted NH₃ with atmospheric HNO₃, consistent with the observation of gaseous concentration products exceeding the dissociation constants of ammonium nitrate. Fluxes of total nitrate (HNO₃ + NO₃⁻) were bi-directional and positive during the mid-day period after fertilisation, suggesting that the slurry acted as a net source for these compounds. There is evidence of small HONO emission following fertilisation (up to 1 ng m⁻² s⁻¹), although the production process is currently not identified. By contrast, all compounds showed deposition to the adjacent unfertilised grassland.