Soil N2O Concentration Research Articles

The Occurrence of Pecan Kernel Necrosis

Pecan [Carya illinoinensis (Wangenh.) C. Koch] kernels (cotyledon) of ‘Pawnee’ displayed a consistent malady not described previously that was designated as “kernel necrosis.” The most severe form of the problem was blackened, necrotic tissue engulfing the basal one-half to one-third of the kernel. The mildest form was darkened tissue in the dorsal grove at the basal end of the kernel. The problem was first observable during the gel stage of kernel development. No symptoms of kernel necrosis were visible on the shuck (involucre). Kernel necrosis was more prominent on ‘Pawnee’, ‘Choctaw’, and ‘Oklahoma’ than other cultivars observed. At maturity, nuts with kernel necrosis had a larger volume than nuts with normal kernels. There were few differences in elemental concentrations of normal kernels from a severely affected orchard and an orchard with little kernel necrosis, and none of the differences appeared to be associated with this disorder. ‘Pawnee’ kernels with necrosis had more phosphorus, zinc, and manganese than normal kernels. Basal segments of necrotic kernels had more boron and acetic acid-extractable and water-soluble calcium than distal segments or normal kernels. Higher elemental concentrations in basal segments of necrotic kernels did not appear sufficient to cause tissue damage. Soil from the orchard with severe kernel necrosis had unusually high concentrations of nitrate, expressed as nitrogen (NO3-N), in the soil profile. Groundwater used for irrigation was contaminated with 34 mg·L−1 NO3-N. An experiment on ‘Pawnee’ evaluated three nitrogen (N) rates, 0, 0.8 g·cm2 cross-sectional trunk area applied in March, and 1.6 g + 1.6 g + 1.2 g·cm2 cross-sectional trunk area N applied during the second week in March, first week in June, and first week in September, respectively, on the incidence of kernel necrosis, leaf N concentration, soil NO3 concentration, yield, nut quality, and growth over 5 years. Leaf N was affected by treatment only once during the study. Nitrates accumulated in the soil, increasing 24% in 3 years when no supplemental N was applied, except in the contaminated irrigation water. Kernel necrosis was either unaffected by N treatment or during 1 year, kernel necrosis was highest without supplemental N application. Tree yield, kernel quality, and growth were unaffected by N treatment. Yield fluctuations among years were apparent demonstrating that an abundant N supply did not prevent alternate bearing. Kernel necrosis was a severe problem in one orchard and was identified in several orchards at low frequencies. The cause of kernel necrosis remains unknown.

Estimating the Influence of Nitrogen Transformations on Nitrate Leaching in Soils

It is challenging to estimate N leaching in soils attributable to complicated physical, chemical, and biological processes. In this study, a transfer function model was developed to simulate the outflow concentration of NO3 in the field, considering the influence of transient water flow, input of applied N, initial residual N in the soil, and main N transformations on the NO3–N leaching process. The N transformations in the model included immobilization, mineralization, volatilization, and plant uptake. In the probability density function of NO3–N, a weighting factor was introduced to quantify the leaching contributions from applied and residual N. A field experiment was conducted for 196 d during the growing seasons of winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.). Soil water potential and NO3 concentrations were measured during the study period along two soil profiles to a depth of 2 m: at 0.10‐m intervals from the soil surface to the 1‐m depth, and at 0.20‐m intervals from 1 to 2 m. A comparison between the experimental data and simulated results with the transfer function showed that the model provided reasonable predictions of the N leaching process as well as the total amount leached at the 2‐m depth. Results also indicated that by considering the transient water condition and N transformations, the transfer function significantly increased the estimation accuracy. Compared with the measured data, relative errors of the estimated total N leached were 1 and 20% with and without considering the transient water condition and the N transformations, respectively. The transfer function with the weighting factor can be useful to estimate the contributions from the applied and residual N to the leaching process in the field.

Response of Microsprinkler‐Irrigated Navel Oranges to Fertigated Nitrogen Rate and Frequency

Microsprinklers allow precise control of irrigation water applications and offer the potential for higher efficiency of water and fertilizer use compared with flood irrigation. A field experiment was conducted during 1999–2002 in central Arizona (AZ) to evaluate effects of various N rates and fertigation frequencies on fruit yield and quality, leaf N concentration, and residual soil N of ‘Newhall’ navel oranges (Citrus sinensis) on ‘Carrizo’ citrange (Porcirus trifoliata x Citrus sinensis) rootstock (planted in 1997) grown in a Gilman (coarse‐loamy, mixed, superactive, calcareous, hyperthermic Typic Torrifluvents) fine sandy loam. The experiment included nonfertilized control plots and factorial combinations of three fertigation frequencies (27, 9, and 3 applications annually) and three N rates (68, 136, and 204 g N tree−1 yr−1). Maximum yields occurred at N rates of 105 to 153 g N tree−1 yr−1 for the fourth to the sixth growing seasons. The yield‐maximizing N rates were equivalent to 17 to 34% of currently recommended N rates for citrus grown in AZ. Fruit and juice quality did not show significant response to N rate or fertigation frequency. Leaf N concentrations at yield‐maximizing N rates were above the critical leaf tissue N range of 25 to 27 mg g−1, indicating that this range may be too low for these ‘Newhall’ navel orange trees. During all three seasons, higher residual soil NO3 concentrations resulted from the highest N rate. Our results suggest that optimum N rates for microsprinkler‐irrigated ‘Newhall’ navel oranges in AZ are lower than currently recommended N rates.

Modeling Seasonal Variations in Carbon Dioxide and Nitrous Oxide in the Vadose Zone

Soil CO2 and N2O concentrations were simulated with a model predicting C and N transport in the vadose zone during a 7‐mo field experiment, after maize (Zea mays L.) harvesting and incorporation of maize residues into the soil. The gas transport model was based on the dusty gas theory and combined with the PASTIS model. During the experiment, soil atmosphere (CO2 and N2O), soil solution (NO3− and dissolved organic carbon [DOC]), soil water content and temperature, and potential denitrifying and aerobic respiratory activities were measured in a 2.50‐m‐thick soil profile. Soil gas concentrations were correctly simulated even though the model did not simulate all the biological processes that produced N2O. Nitrous oxide concentration peaks after rain were slightly overestimated, as the WFPS (water‐filled pore space) was not estimated accurately enough to predict local anoxic conditions. To model CO2 concentrations, account had to be taken of DOC adsorption onto soil mineral particles and of zymogenous biomass death during the period when the ground was frozen. The model satisfactorily simulated NO3− concentrations in the top soil profile, notably during major rainfall events, and maize residue dry matter loss during the experiment. The modeling of biological processes needs to be improved to provide a better simulation of C and N transport in the vadose zone. In particular, the use of WFPS was not sufficient to predict anoxic periods; simulations should improve if soil aggregate structure is also taken into account.

Subsoil 15N-N2O Concentrations in a Sandy Soil Profile After Application of 15N-fertilizer

Studies on emissions of nitrous oxide (N2O) from agricultural soils mostly focus on fluxes between the soil and the atmosphere or are limited to the atmosphere in the topsoil. However, in soils with shallow water tables, significant N2O formation may occur closer to the groundwater. The aims of this study were (i) to determine the importance of subsoil N2O formation in a sandy soil; and (ii) to obtain a quantitative insight in the contribution of subsoil N2O to the overall losses of N2O to the environment. We applied 15N labeled fertilizer at a rate of 5.22 kg 15N ha−1; 50% as Ca(NO3)2 and 50% as NH4Cl, on a mesic typic Haplaquod seeded with potatoes (Solanum tuberosum L.), and traced soil N2O concentrations and fluxes over a one-year period. Throughout the year, total N2O and the amount of 15N recovered in soil N2O were highest in the subsoil, with a maximum concentration at 48 cm depth in mid-February of 19900 μl m−3 and 24 μg 15N m−3, respectively. The maximum concentration coincided with the highest water-filled pore space of 71%. The cumulative flux of N2O was 446 g N2O-N ha−1, the recovery of 15N in this flux was 0.06%. During the summer, maximum fluxes followed high soil N2O concentrations. During winter, no such relation was found. We concluded that the formation of N2O was the highest in the subsoil, largely controlled by water-filled pore space rather than NO 3 − concentration or temperature. Although high subsoil N2O concentrations did not lead to high surface fluxes of N2O in the winter, artificial draining may lead to high indirect N2O emissions through supersaturated drainage water.

Laboratory-scale measurements of N2O and CH4emissions from hybrid poplars (Populus deltoides × Populus nigra)

The purpose of this study was to determine whether or not young hybrid poplar (Populus deltoides x Populus nigra) could transport landfill biogas internally from the root zone to the atmosphere, thereby acting as conduits for landfill gas release. Fluxes of methane (CH4) and nitrous oxide (N2O) from the seedlings to the atmosphere were measured under controlled conditions using dynamic flux chambers and a tunable diode laser trace gas analyser (TDLTGA). Nitrous oxide was emitted from the seedlings, but only when extremely high soil N2O concentrations were applied to the root zone. In contrast, no detectable emissions of CH4 were measured in a similar experimental trial. Visible plant morphological responses, characteristic of flood-tolerant trees attempting to cope with the negative effects of soil hypoxia, were observed during the CH4 experiments. Leaf chlorosis, leaf abscission and adventitious roots were all visible plant responses. In addition, seedling survival was observed to be highest in the biogas 'hot spot' areas of a local municipal solid waste landfill involved in this study. Based on the available literature, these observations suggest that CH4 can be transported internally by Populus deltoides x Populus nigra seedlings in trace amounts, although future research is required to fully test this hypothesis.

Atmosphere composition and N2O emissions in soils of contrasting textures fertilized with anhydrous ammonia

Nitrous oxide production and emission in agricultural soils are often influenced by soil physical properties and mineral N content. An experiment was initiated on a commercial farm located in the St. Lawrence Lowlands to measure the effects of recommended (150 kg N ha-1) and excessive (250 kg N ha-1) rates of anhydrous ammonia on atmospheric composition (O2, CO2, CH4 and N2O) and N2O emissions in soils of contrasting textures (sandy loam, clay loam and clay) cropped to corn. N2O emissions and soil temperature, water content and atmospheric composition were measured from post-harvest tillage to the first snowfall during the first year (2000), and from spring thaw to mid-July during the following 2 yr. Episodes of high N2O concentrations and surface emissions coincided with periods of high soil water content shortly following rainfall events when soil O2 concentrations were lowest. The convergence of indicators of restricted soil aeration at the time of highest N2O production suggested that denitrification was a major contributor to N2O emissions even in soils receiving an NH4-based fertilizer. Soil texture had a significant influence on soil N2O concentration and emission rates on several sampling dates. However, the effect was relatively small and it was not consistent, likely because of complex interactions between soil physical properties and N2O production, consumption and diffusion processes. Nitrous oxide emissions during the study were not limited by soil N availability as indicated by similar fluxes at recommended and excessive rates of anhydrous ammonia. Finally, greater N2O emissions in 2001 than in 2002 stress the importance of multiyear studies to evaluate the effect of annual weather conditions on soil N2O dynamics. Key words: Greenhouse gasses, denitrification

Effect of earthworm addition on soil nitrogen availability, microbial biomass and litter decomposition in mesocosms

The aim of the study was to determine the effect of adding two tropical earthworm species, Rhinodrilus contortus and Pontoscolex corethrurus, to mesocosms on the availability of mineral N (NH4+ and NO3−concentrations), soil microbial biomass (bio-N), and the decomposition rates of three contrasting leaf litter species, in a glasshouse experiment. The mesocosms were filled with forest soil and covered with a layer of leaf litter differing in nutritional quality: (1) Hevea brasiliensis (C/N=27); (2) Carapa guianensis (C/N=32); (3) Vismia sp., the dominant tree species in the second growth forest (control, C/N= 42); and, (4) a mixture of the former three leaf species, in equal proportions (C/N=34). At the end of the 97-day experiment, the soil mineral N concentrations, bio-N, and leaf litter weight loss were determined. Both earthworm species showed significant effects on the concentrations of soil NO3−(p<0.01) and NH4+ (p<0.05). Bio-N was always greater in the mesocosms with earthworms (especially with R. contortus) and in the mesocosms with leaf litter of H. brasiliensis (6 µg N g−1 soil), the faster decomposing species, than in the other treatments (0.1–1.6 µg N g−1). Thus, earthworm activity increased soil mineral-N concentrations, possibly due to the consumption of soil microbial biomass, which can speed turnover and mineralization of microbial tissues. No significant differences in decomposition rate were found between the mesocosms with and without earthworms, suggesting that experiments lasting longer are needed to determine the effect of earthworms on litter decomposition rates.

Arbuscular mycorrhizal fungi respond to increasing plant diversity

The effect of plant diversity (1, 2, 8, or 16 species) on arbuscular mycorrhizal fungi (AMF) was assessed at the Cedar Creek Long-Term Ecological Research site at East Bethel, Minnesota, from 1997 to 1999. At each of the five samplings, AMF in 16-species plots produced from 30 to 150% more spores and from 40 to 70% greater spore volumes than AMF in one-species plots. Regressions of spore numbers and volumes with percent plant cover, plant diversity, and soil NO3 as independent variables suggest that midsummer plot soil NO3 was the best single predictor of AMF spore production in these plots. Plant diversity influenced spore volume in four samplings and spore numbers in the first three samplings. Plant cover was predictive of spore volume throughout the experiment but of spore number only in the first year. Sporulation by larger-spored AMF species (Gigaspora spp. and Scutellospora spp.) increased significantly with increasing plant diversity, while sporulation of the smaller-spored species varied in response to host diversity. Spore numbers of several AMF species were consistently negatively correlated and none positively correlated with midseason soil NO3 concentrations, demonstrating the adaptation of these AMF species to nitrogen-limited conditions.Key words: mycorrhiza, community, grassland, sporulation, nitrogen, specificity.

Effects of deep application of urea on NO and N 2 O emissions from an Andisol

A modeling study revealed that the depth of nitric oxide (NO) production in soil is crucial for its flux, while that of nitrous oxide (N2O) is not. To verify this result, laboratory experiments with soil columns classified as Andisol (Hydric Hapludand) were conducted, with changing the depth of urea application, at 0–0.1 or 0.1–0.2 m. All the NO concentration profiles in soil exhibited a sharp peak at each fertilized layer within 5 days of fertilizer application. NO concentration in soil decreased abruptly as the distance from the fertilized layer increased. These findings imply that NO is produced mainly within the fertilized layer, but does not diffuse widely in the soil columns, because of rapid NO uptake within the soil. As a result, the NO flux from soil columns fertilized at 0.1–0.2 m depth over the 48-day study period was reduced to almost the same rate as that of the unfertilized one. The total NO emissions from soil columns unfertilized and fertilized at 0–0.1 and 0.1–0.2 m depth were 0.02, 1.39 (± 0.05) and 0.05 (± 0.03) kg N ha−1, respectively, suggesting that NO emission derived from N fertilizer could be reduced to 2% by shifting the depth of fertilizer application by 0.1 m. On the other hand, soil N2O concentration profiles exhibited a gentler peak, because of the lower uptake by soil. N2O fluxes were affected more by the soil conditions, e.g. soil water content, than the distance between fertilized depth and soil surface. The total N2O emissions from soil columns unfertilized and fertilized at 0–0.1 and 0.1–0.2 m were 0.02, 0.16 (± 0.03) and 0.25 (± 0.04) kg N ha−1, respectively.

Assessment of Microirrigation for Sugarbeet Production

ABSTRACT Sugarbeet (SB) is a major value-crop in many furrow (Fu) irrigated areas of Wyoming where nitrate (NO3) contamination prevails. Microirrigation (M) may be an option for managing such pollution while sustaining SB production. With the goal of evaluating efficacy of M for SB production, in comparison to Fu practice, studies were conducted through field experiments, computer simulations, and economic assessments. The field study with SB consisted of a Fu and three M treatments (M1, M2, and M3 representing a management allowable depletion factor of 0.20, 0.35, and 0.50, respectively) and three fertilizer doses. Water use, post-harvest soil NO3 concentrations, SB yields, and water percolation were measured. Computer simulations were performed using a transport and irrigation model to predict soil NO3 concentrations and the results were compared with the experimental data. The economic feasibility of adopting M for 10 to 40 ha area was evaluated using a capital budgeting analysis. Results of field studies indicated lower water use, higher soil NO3 levels, less water percolation and greater SB yields with M than under Fu practice. The yields increased as management allowable depletion factor and fertilizer rate augmented. Computer simulations predicted that post-harvest soil NO3 concentrations were higher when SB were irrigated with M rather than Fu practice. Simulation results were comparable with the field data. The economic analysis indicated that irrigation costs decreased and SB returns increased with M practice. Results of capital budgeting showed that adoption of M in Wyoming was feasible for small-scale SB production. The profitability of the M system was dependent on the irrigation regime and the conversion area.

Distribution of nitrous oxide and regulators of its production across a tropical rainforest catena in the Luquillo Experimental Forest, Puerto Rico

Understanding of N2O fluxes to the atmosphere is complicated by interactions between chemical and physical controls on both production and movement of the gas. To better understand how N2O production is controlled in the soil, we measured concentrations of N2O and of the proximal controllers on its production in soil water and soil air in a field study in the Rio Icacos basin of the Luquillo Experimental Forest, Puerto Rico. A toposequence (ridge, slope-ridge break, slope, slope-riparian break, riparian, and streambank) was used that has been previously characterized for groundwater chemistry and surface N2O fluxes. The proximal controls on N2O production include NO − ,N H + , DOC, and O2. Nitrous oxide and O2 were measured in soil air and NO − ,N H + , and DO were measured in soil water. Nitrate and DOC disappeared from soil solution at the slope-riparian interface, where soil N2O concentrations increased dramatically. Soil N2O concentrations continued to increase through the flood plain and the streambank. Nitrous oxide concentrations were highest in soil air probes that had intermediate O2 concentrations. Changes in N2O concentrations in groundwater and soil air in different environments along the catena appear to be controlled by O2 concentrations. In general, N processing in the unsaturated and saturated zones differs within each topographic position apparently due to differences in redox status.

Nitrous Oxide Concentrations in the Soil of a Mown Grassland: Comparison of Model Results with Soil Profile Measurements

Nitrous oxide (N2O) concentrations in the soilprofile of a fertilized grassland on the Swiss plateauwere measured at irregular intervals during one year.Air samples were taken from air-permeable tubesinstalled at depths between 2 and 100 cm belowthe surface. Highest concentrations in thetopsoil were observed following precipitationafter fertilization. In the subsoil,concentrations were highest when the soil watercontent fell below about 92% after a wet periodand mineral nitrogen was available from priorfertilization. N2O concentrations in thetopsoil were simulated with the process-basedgrassland ecosystem model PaSim 2.5 (PastureSimulation Model) and compared with measured soilN2O concentrations. The model simulated wellthe concentration peaks after fertilizeradditions and the concentrations in winter. Butthe simulated baseline concentrations during thegrowing season were overestimated. Possiblecauses for this discrepancy are discussed andsuggestions are made to improve the model.

General model for N2O and N2 gas emissions from soils due to dentrification

Observations of N gas loss from incubations of intact and disturbed soil cores were used to model N2O and N2 emissions from soil as a result of denitrification. The model assumes that denitrification rates are controlled by the availability in soil of NO3 (e− acceptor), labile C compounds (e− donor), and O2 (competing e− acceptor). Heterotrophic soil respiration is used as a proxy for labile C availability while O2 availability is a function of soil physical properties that influence gas diffusivity, soil WFPS, and O2 demand. The potential for O2 demand, as indicated by respiration rates, to contribute to soil anoxia varies inversely with a soil gas diffusivity coefficient which is regulated by soil porosity and pore size distribution. Model inputs include soil heterotrophic respiration rate, texture, NO3 concentration, and WFPS. The model selects the minimum of the NO3 and CO2 functions to establish a maximum potential denitrification rate for particular levels of e− acceptor and C substrate and accounts for limitation of O2 availability to estimate daily N2+N2O flux rates. The ratio of soil NO3 concentration to CO2 emission was found to reliably (r2=0.5) model the ratio of N2 to N2O gases emitted from the intact cores after accounting for differences in gas diffusivity among the soils. The output of the ratio function is combined with the estimate of total N gas flux rate to infer N2O emission. The model performed well when comparing observed and simulated values of N2O flux rates with the data used for model building (r2=0.50) and when comparing observed and simulated N2O+N2 gas emission rates from irrigated field soils used for model testing (r2=0.47).

Using Growing Degree Days to Predict Nitrogen Availability from Livestock Manures

Predictive tools are needed to better match N release from manure with crop demand. Growing degree days (GDD) have been successfully used to predict N release from crop residues and other amendments. A 112‐d incubation experiment was conducted at 10, 17, and 24°C to evaluate GDD (0°C base temperature) predictions of N transformations from beef (Bos taurus), dairy, poultry (Gallus gallus), and swine (Sus scrofa) manure. Manure was incorporated at rates estimated to provide 150 kg N ha−1 (or 75 mg N kg−1 soil). Soil NO3 and NH4 concentrations were determined at weekly or biweekly intervals. The rate of NO3 accumulation increased with increasing temperature, and could be predicted across temperature regimes using GDD. This predictive ability could be generalized across dairy, poultry, and swine manures using an exponential equation, , while N was immobilized by incorporation of beef manure. The disappearance of NH4 was a linear function of time and of GDD. A single predictive equation was sufficient for dairy, poultry, and swine manures, in the form , with soil NH4 reaching zero at ≈350 GDD. These laboratory data indicate that GDD can be used for predicting NO3 accumulation and NH4 disappearance from a range of livestock manures. If successfully extended to the field, this predictive capability may allow for improved management of N from animal manures.

Soil Carbon and Nitrogen Dynamics Following Application of Pig Slurry for the 19th Consecutive Year II. Nitrous Oxide Fluxes and Mineral Nitrogen

Agricultural soils often receive annual applications of manure for long periods. Our objective was to quantify the effects of 19 consecutive years of pig (Sus scrofa) slurry (PS) application to a loamy soil (loamy, mixed, frigid Aeric Haplaquept) on N2O emissions. Soil surface N2O fluxes (FN2O) were measured 36 times in 1 yr. Nitrous oxide concentration profiles, soil NH+4‐ and NO−3‐N contents, denitrifying enzyme activity (DEA), and denitrification rate (DR) in soil were also determined to explain the variation in FN2O Long‐term (19 yr) treatments on continuous silage maize (Zea mays L.) were 60 (PS60) and 120 Mg ha−1 yr−1 (PS120) of pig slurry and a control receiving mineral fertilizer at a dose of 150 kg ha−1 each of N, P2O5, and K2O. Denitrifying enzyme activity, soil N2O concentrations, and FN2O (&lt;25 ng m−2 s−1) were low in the control plots receiving mineral fertilizer. Annual applications of PS to the soil for 18 yr had positive residual effects on the DEA compared with the long‐term fertilized control plots. Following PS application, there was a strong and rapid increase of FN2O (up to 350 ng m−2 s−1) on manured plots. The PS‐induced FN2O increased with increasing quantity of PS, probably as the result of a greater availability of NO−3‐N for denitrification. The effects of PS on FN2O were mostly limited to the 30 d following application, with low fluxes (&lt;10 ng m−2 s−1) during the rest of the measurement period. Total N2O–N emissions represented 0.62, 1.23, and 1.65% of total N applied in control, PS60, and PS120 plots, respectively. These emission factors for the PS plots agreed with values previously suggested for N‐fertilized soils (1.25%).

Fertilizer Banding Influence on Spatial and Temporal Distribution of Soil Inorganic Nitrogen in a Corn Field

Fertilizer N is commonly applied as a sidedress band application in corn (Zea mays L.) fields. The objectives of this study were to determine the implications of fertilizer band application on spatial and temporal variations in soil inorganic N and to evaluate different sampling strategies for their suitability in estimating the average soil inorganic N concentration of a field. Treatments were 0, 60, and 120 kg N ha−1 in 1994, and 0, 120, and 240 kg N ha−1 in 1995, replicated three times at each of two sites. Fertilizer was applied as a band of NH4NO3 15 cm on each side of each corn row at the corn six‐leaf stage. Soil samples taken at eight interrow locations 0, 5, 10, 15, 20, 25, 30, and 35 cm from the corn row and at depth increments of 0 to 15, 15 to 30, and 30 to 60 cm and on five sampling dates were analyzed for soil NO3 and NH4 concentrations. Random sampling and five systematic sampling strategies were evaluated with respect to bias in estimation of field soil NO3 and NH4 concentration for three depth increments, and with respect to the number of samples required to achieve a given precision and probability level combination for soil NO3 concentration for the 0‐ to 15‐cm depth. All systematic sampling strategies provided adequate estimates of the true soil inorganic N concentration as estimated based on uniform sampling across all interrow locations. There was no consistent benefit to using any one systematic strategy with respect to the number of cores required to obtain a given level of precision in soil NO3 concentration at a given level of probability. Systematic sampling strategies were at least as good as, or superior to, random sampling, particularly after fertilizer band application and at high N rates. The apparent insensitivity of estimated field soil NO3 concentration to sampling strategy was attributed to the relatively high mobility of NO3 in soil and variation in placement of the fertilizer band relative to the corn row during sidedressing.

Denitrification from Fescue Pastures in the Southeastern USA Fertilized with Broiler Litter

AbstractMany aspects of the denitrification process in pasture ecosystems remain poorly investigated, especially as affected by application of animal manures. Denitrification can be, however, a major pathway of N loss from soil, decreasing efficiency of N fertilizers, as well as contributing to global warming and ozone depletion. We quantified N loss via denitrification from tall fescue (Festuca arundinacea Schreb.) pastures following fertilization with broiler litter. Study sites were located in the Coastal Plain (Alabama), Piedmont (Georgia), and Cumberland Plateau (Tennessee) Major Land Resource Areas of the southeast USA. Litter was applied to supply 70 kg of available N ha−1 based on equations developed by the USDA and the USEPA. Measurements were taken every 2 wk during the course of 2 yr (1995–1996 and 1996–1997) using an in‐situ acetylene‐inhibition technique. Denitrification flux was highly variable, ranging from −20 to 2500 mg N gas m−2 h−1. Total losses of N gas were all &lt;6 kg ha−1 during 150 d after application, representing a loss of &lt;5% of total N applied. Losses from treated plots were not significantly higher than those from control plots. Denitrification rates at these sites were not predictable by any one factor, but seemed to respond only to combinations of various factors such as soil temperature, soil NO3 concentration, and soil water content. Our results indicate that risk of N loss and atmospheric pollution due to denitrification from soils in the southeastern USA is not significantly increased by application of broiler litter at appropriate rates.

Sampling spatial and temporal variation in soil nitrogen availability.

There are few studies in natural ecosystems on how spatial maps of soil attributes change within a growing season. In part, this is due to methodological difficulties associated with sampling the same spatial locations repeatedly over time. We describe the use of ion exchange membrane spikes, a relatively nondestructive way to measure how soil resources at a given point in space fluctuate over time. We used this method to examine spatial patterns of soil ammonium (NH+4) and nitrate (NO-3) availability in a mid-successional coastal dune for four periods of time during the growing season. For a single point in time, we also measured soil NH+4 and NO-3 concentrations from soil cores collected from the mid-successional dune and from an early and a late successional dune. Soil nitrogen concentrations were low and highly variable in dunes of all ages. Mean NH+4 and NO-3 concentrations increased with the age of the dune, whereas coefficients of variation for NH+4 and NO-3 concentrations decreased with the age of the dune. Soil NO-3 concentration showed strong spatial structure, but soil NH+4 concentration was not spatially structured. Plant-available NH+4 and NO-3 showed relatively little spatial structure: only NO-3 availability in the second sampling period had significant patch structure. Spatial maps of NH+4 and NO-3 availability changed greatly over time, and there were few significant correlations among soil nitrogen availability at different points in time. NO-3 availability in the second sampling period was highly correlated (r = 0.90) with the initial soil NO-3 concentrations, providing some evidence that patches of plant-available NO-3 may reappear at the same spatial locations at irregular points in time.

Nitrogen uptake and partitioning under alternate- and every-furrow irrigation

Alternate-furrow irrigation, combined with fertilizer placement in the non-irrigated furrow, has the potential to reduce fertilizer leaching in irrigated corn (Zea mays L.). The potential also exists, however, for reduced N uptake under alternate-furrow irrigation. This study examined the effects of fertilizer placement and irrigation treatment on N uptake, roota→shoot→root circulation, and partitioning between reproductive and vegetative tissues. Rainfall was above average in both years of the study, especially during May and June, so that root growth beneath the non-irrigated furrow was equal to root production beneath the irrigated furrow. Under those conditions, soil NO3 concentration in the fertilized furrow during late-vegetative and reproductive growth was greater in the alternate-furrow compared with the every-furrow treatment, resulting in increased fertilizer N uptake during reproductive growth and increased N partitioning to reproductive tissues under alternate-furrow irrigation. About 80% of the fertilizer N found in roots had first been translocated to the shoot and then returned via the phloem to the root system. Nitrogen cycling from root to shoot to root was not affected by irrigation treatment. Alternate-furrow irrigation successfully increased N uptake and reduced the potential for NO3leaching when environmental conditions allowed adequate root development in the non-irrigated furrow, and when the growing season was long enough to allow the crop to reach physiological maturity.