Extractable Nitrate Research Articles

Moisture pulses, trace gas emissions and soil C and N in cheatgrass and native grass-dominated sagebrush-steppe in Wyoming, USA

Effects of large-scale weed invasion on the nature and magnitude of moisture-pulse-driven soil processes in semiarid ecosystems are not clearly understood. The objective of this study was to monitor carbon dioxide (CO2) and nitrous oxide (N2O) emissions and changes in soil carbon (C) and nitrogen (N) following the application of a water pulse in Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) communities dominated by the exotic annual grass cheatgrass (Bromus tectorum) and by the native perennial grass western wheatgrass (Pascopyrum smithii). Sampling locations were established in shrub interspaces dominated by B. tectorum and P. smithi and beneath shrub canopies adjacent to interspaces dominated by B. tectorum and P. smithi, where no grass was present. Soils were classified as fine-loamy, mixed, Borollic Haplargids. Soil samples (0–10cm) and air samples were collected at 0, 4, 8, 24, 49, 72, and 216h following additions of 25.4mm of water. Soil samples were analyzed for dissolved organic carbon (DOC), microbial biomass carbon (MBC), extractable ammonia (NH4+), extractable nitrate (NO3−), and dissolved organic nitrogen (DON). Grass species induced differences in soil nitrification, N2O and CO2 emissions, and the quantity and timing of labile C available to microbial populations responding to increased moisture availability. In the first 8-h phase after wetting P. smithii soils had the greatest CO2 emissions compared to other soils but B. tectorum soils had the greatest N2O emissions and the greatest increases in CO2 emissions relative to before wetting. Microbial biomass C in B. tectorum interspace soils increased rapidly but the response was short-lived despite sufficient water availability. After the first 8h of soil response to wetting, the observed MBC declines in B. tectorum interspace coincided with disproportional DOC and DON concentration increases. Similar DOC and DON increases were also observed in B. tectorum soils beneath shrub canopy. In contrast, DOC and DON concentrations in P. smithii soils remained unaffected by soil wetting and small MBC increases observed during the first 8-h phase did not decline as rapidly as in B. tectorum interspace soils. In conclusion, summer drying-wetting cycles that occur frequently in areas invaded by B. tectorum can accelerate rates of nitrification and C mineralization, and contribute significantly to trace gas emissions from sagebrush-steppe grasslands. With frequent summer rainfall events, the negative consequences B. tectorum presence in the ecosystem can be significant.

Denitrification rates in marsh fringes and fens in two boreal peatlands in Alberta, Canada

Western Boreal Plain peatlands can play an important role in the global nitrogen cycle by storing N in peat and potentially releasing large amounts of N to the atmosphere. In this study, biological denitrification rates were measured in marsh and fen vegetation zones in two boreal peatland-pond complexes in northcentral Alberta, Canada. Assuming negligible winter denitrification, we estimated annual denitrification rates of 11 g N·m−2 in marshes and 24 g N·m−2 in fens. Two techniques were employed to measure denitrification: 1) measurements of direct N2-flux were taken from intact cores in gas-tight N-free chambers, and 2) nitrous oxide (N2O) flux was measured in the two fens using in situ chambers. N2 fluxes ranged from 2.14–4.19 mg N·m−2·h−1 in marshes and 6.19–6.81 mg N·m−2·h−1 in fens. N2O release from fen peat ranged from consumption to 0.025 mg N·m−2·h−1. Peat with higher carbon and moisture content was a source of N2O whereas peat with lower carbon and moisture content was a sink. Surface water did not appear to be a major source of nitrate for denitrification. However, denitrification rates were positively correlated with peat extractable nitrate. Combined with mineralization studies, this indicated that soil nitrification provided most of the substrate for denitrification.

Fungal control of nitrous oxide production in semiarid grassland

Fungi are capable of both nitrification and denitrification and dominate the microbial biomass in many soils. Recent work suggests that fungal rather than bacterial pathways dominate N transformation in desert soils. We evaluated this hypothesis by comparing the contributions of bacteria and fungi to N2O production at control and N fertilized sites within a semiarid grassland in central New Mexico (USA). Soil samples were taken from the rhizosphere of blue grama (B. gracilus) and the microbiotic crusts that grow in open areas between the bunch grasses. Soils incubated at 30% or 70% water holding capacity, were exposed to one of three biocide treatments (control, cycloheximide or streptomycin). After 48 h, N2O and CO2 production were quantified along with the activities of several extracellular enzymes. N2O production from N fertilized soils was higher than that of control soils (165 vs. 41 pmol h−1 g−1), was higher for crust soil than for rhizosphere soil (108 vs. 97 pmol h−1 g−1), and increased with soil water content (146 vs. 60 pmol h−1 g−1). On average, fungicide (cycloheximide) addition reduced N2O production by 85% while increasing CO2 production by 69%; bactericide (streptomycin) reduced N2O by 53% with mixed effects on CO2 production. N2O production was significantly correlated with C and N mineralization potential as measured by assays for glycosidic and proteolytic enzymes, and with extractable nitrate and ammonium. Our data indicate that fungal nitrifier denitrification and bacterial autotrophic nitrification dominate N transformation in this ecosystem and that N2O production is highly sensitive to soil cover, N deposition and moisture.

Impacts of forest gaps on soil properties and processes in old growth northern hardwood-hemlock forests

We examined the influence of treefall gaps on soil properties and processes in old growth northern hardwood-hemlock forests in the upper Great Lakes region, USA. We found significantly greater solar radiation, soil moisture contents and soil temperatures in gaps compared to adjacent closed canopy plots. Gaps had significantly less exchangeable base cations (K, Ca, and Mg) compared to forest plots in the upper mineral soil (0–25 cm). Gaps also had significantly more dissolved organic N and extractable nitrate at depth (25–50 cm), indicating increased nutrient leaching in gaps. In-situ N mineralization was significantly greater in gaps and edge plots compared to forest plots. We found significantly greater potential N mineralization (measured in the laboratory at 25°C and 40% water holding capacity) in forest compared to gap plots. Microbial biomass N was significantly greater (ca. two-fold) in the gap edge compared to both gaps and closed forest. Using principal component analyses we found that edge plots were positively correlated with all principal components, indicating increased in-situ and potential N mineralization, microbial biomass N, soil NO3− and NH4+, and soil organic matter. The gap edge may be a region of optimal microclimate and substrate to enhance microbial biomass and activity within these forest ecosystems.

Relation of soil-, surface-, and ground-water distributions of inorganic nitrogen with topographic position in harvested and unharvested portions of an aspen-dominated catchment in the Boreal Plain

Spatial distributions of soil extractable nitrate (NO–3) and ammonium (NH+4) concentrations were related to surface- and ground-water NO–3and NH+4concentrations in harvested and forested sections of a catchment dominated by trembling aspen (Populus tremuloides Michx.) in the subhumid boreal forest of Alberta, Canada. NO–3and NH+4concentrations in soils varied spatially throughout the catchment and were larger in surface soils than in subsurface soils. Spatial distributions of soil inorganic nitrogen (N) concentrations were not explained by the harvested versus the unharvested condition; heterogeneity was instead related to topographic position. NO–3concentrations in both surface and subsurface soils were largest in ephemeral draws and wetlands. NH+4concentrations in subsurface soils were largest in ephemeral draws and wetlands, but this pattern was not apparent for surface soils. Soil NO–3and NH+4availability and surface- and ground-water NO–3and NH+4concentrations reflected soil NO–3and H+4concentrations. N-rich surface soils in both forested and harvested areas have a large potential for releasing N to surface waters. This study indicates that even though topography is subtle in this catchment, topographic position and its soil moisture relations, along with vegetation demand, can influence N transformation and transport in both forested and harvested portions of the Boreal Plain landscape.

Loss of forb diversity in relation to nitrogen deposition in the UK: regional trends and potential controls

AbstractIn this study we investigate the impact of nitrogen (N) deposition on the diversity of three different vegetation functional groups – forbs, grasses and mosses – using a field survey of acid grasslands across Great Britain. Our aim is to identify the vegetation types that are most vulnerable to enhanced N deposition, and to shed light on the mechanisms that may be driving N‐initiated species changes in the UK. Sixty‐eight randomly selected grasslands belonging to the UK National Vegetation Classification group U4 (Festuca ovina–Agrostis capillaris–Galium saxatile grassland) were studied along a gradient of atmospheric N deposition ranging from 6 to 36 kg N ha−1 yr−1. At each site, vegetation was surveyed and samples were taken from the topsoil and subsoil. Aboveground plant material was collected from three species: a forb, grass and moss. Both the species richness and cover of forbs declined strongly with increasing N deposition, from greater than eight species/20% cover per m2 quadrat at low levels of N to fewer than two species/5% cover at the highest N deposition levels. Grasses showed a weak but significant decline in species richness, and a trend toward increasing cover with increasing N input. Mosses showed no trends in either species richness or cover. Most of the decline in plant species richness could be accounted for by the level of ammonium deposition. Soil KCl‐extractable ammonium concentration showed a significant positive correlation with N input, but there was no relationship between N deposition and extractable nitrate. In the soil O/A horizon, there was no relationship between N deposition and %N, and only a very weak positive relationship between the level of N deposition and the C : N ratio. Finally, in the vegetation, there was no relationship between N deposition and either shoot tissue N concentration or N : P ratio for any of the three reference species. Combining our regional survey with the results of published N‐addition experiments provides compelling evidence that there has been a significant decline in the species richness and cover of forbs across Great Britain, and that the primary cause is competition due to an increase in the cover of grasses in response to enhanced deposition of reactive N, primarily NH4+.

Poultry Litter Application to Loblolly Pine Forests

Forestland application of poultry manure offers an alternative to the conventional practice of pastureland application. Before such a practice is considered viable, however, it must be demonstrated that the forest ecosystem is capable of absorbing the nutrients contained in poultry manure, especially nitrogen (N) and phosphorus (P). From the forestry perspective, it must also be demonstrated that tree growth is not diminished. We investigated these questions using loblolly pine (Pinus taeda L.) stands growing in central Mississippi in an area of high poultry production. Stockpiled broiler litter was applied to newly thinned, 8-yr-old stands at 0, 4.6, and 18.6 dry Mg ha-1, supplying 0, 200, and 800 kg N ha-1 and 0, 92, and 370 kg P ha-1, respectively. Levels of nitrate in soil water, monitored at a 50-cm depth with porous cup tension lysimeters, exceeded 10 mg N L-1 during the first two years after application in the 18.6 Mg ha-1 rate but only on two occasions in the first year for the lower rate of application. Phosphate was largely absent from lysimeter water in all treatments. Other macronutrients (K, Ca, Mg, S) were elevated in lysimeter water in proportion to litter application rates. Soil extractable nitrate showed similar trends to lysimeter water, with substantial elevation during the first year following application for the 18.6 Mg ha-1 rate. Mehlich III-extractable phosphate peaked in excess of 100 microg P g-1 soil during the third year of the study for the 18.6 Mg ha-1 rate. The 4.6 Mg ha-1 rate did not affect extractable soil P. Tree growth was increased by the poultry litter. Total stem cross-sectional area, or basal area, was approximately 20% greater after 2 yr for both rates of litter application. Overall, the nutrients supplied by the 4.6 Mg ha-1 rate were contained by the pine forest and resulted in favorable increases in tree growth. The higher rate, by contrast, did pose some risk to water quality through the mobilization of nitrate. These results show that, under the conditions of this study, application of poultry litter at moderate rates of approximately 5 Mg ha-1 to young stands of loblolly pine offers an alternative disposal option with minimal impacts to water quality and potential increases in tree growth.

Specific contributions of decaying alfalfa roots to nitrate leaching in a Kalamazoo loam soil

Abstract Alfalfa ( Medicago sativa L.) contributes 430 million kg N year −1 to the US Corn-Belt soils, according to a 1991 survey. Minimizing leaching losses from these very large N inputs requires a better understanding of the specific root dynamics that relate to the shoot-borne nitrates which have been reported to develop throughout many soil profiles. The objective of the present study was to determine the impact of decaying alfalfa roots on nitrate inputs to soils and on soil hydraulic conductivity properties which affect nitrate leaching. An experiment was initiated in 1994 and data for this report were taken from research on a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalf) at the KBS/LTER (long-term ecological research) site in southwestern Michigan, during the period from 1996 through 1997. Soil extractable nitrate (NO 3 -N) and ammonium (NH 4 -N) were monitored to soil depths of 150 cm and soil soluble NO 3 -N and NH 4 -N were monitored by suction lysimeters to the depth of 65 cm. Saturated hydraulic conductivity ( K sat ) of soil was measured by the double-ring infiltrometer method. Following glyphosate termination of the alfalfa stands, nitrate-N released from mineralized alfalfa roots plus shoots totaled 75 kg ha −1 . Alfalfa roots generated 36 kg ha −1 and alfalfa shoots generated 39 kg ha −1 which accumulated in the Ap horizons from April to July in 1997. The presence of decaying alfalfa roots in the profile quadrupled K sat values as compared to bare fallow soils. Nitrates released from decomposing alfalfa roots combined with root-enhanced hydraulic conductivities dramatically increased NO 3 -N leaching following the termination of alfalfa stands. NO 3 -N leaching to deeper horizons approached 83 kg ha −1 in root treatments and 144 kg ha −1 in the root plus shoot treatments during the period from April to December, following alfalfa termination. Our data suggest that under temperate climate such as that of Michigan, groundwater contamination by nitrates can be reduced by terminating alfalfa stands immediately before spring-planting of the subsequent row crop, which can absorb the large quantities of N leaking from decomposing shoots and roots of the legume.

Urban nitrogen biogeochemistry: status and processes in green retention basins

Nitrogen (N) cycling has been poorly characterized in urban ecosystems. Processes involving N are of specific concern due to increasing anthropogenic inputs from fertilizer uses and fossil fuel combustion in cities. Here we report on a study of N biogeochemistry in city green retention basins and city parks in the Phoenix metropolitan area, Arizona, USA. City retention basins receive N inputs from street runoff, and along with city parks, fertilizer input from management, making these urban patches potential hot spots for biogeochemical cycling. We sampled soils from six retention basins and two non-retention city parks and measured soil organic matter (SOM) content, net N mineralization, net nitrification, denitrification potential, and intact core denitrification flux and nitrate retention. Our results showed significantly higher SOM, extractable nitrate, nitrification rates and potential denitrification rates in surface soils (0–7.5 cm; soil that is directly affected by fertilizer N input, irrigation, and storm runoff) than in deeper soils. We also observed a distinct horizontal trend of decreasing SOM and denitrification potentials from inlet to outlet (dry well) in the retention basins. Denitrification rates, measured both as potential rates with substrate amendment (390–1151 ng N2O-N g−1 soil h−1), and as intact core fluxes (3.3–57.6 mg N m −2 d−1), were comparable to the highest rates reported in literature for other ecosystems. Management practices that affect biogeochemical processes in urban retention basins thus could affect the whole-city N cycling.

Agricultural recycling of the by-product concentrate of livestock wastewater treatment plant processed with VSEP RO and bio-ceramic SBR

One of the problems in a reverse osmosis process for livestock wastewater treatment is disposal of the by-product concentrate. The agricultural recycling of the concentrate is more cost saving than a further treatment. Application of the concentrate reduces the chemical fertilizer amendment. The agricultural recycling of the concentrate from the Kimhae livestock wastewater treatment plant, processed with the VSEP RO and bio-ceramic SBR, was studied. The concentrate includes non-biodegradable humic ubstance and residual inorganic ions (NH4+, NO3-, PO4(3-), K+, etc.). The contents of N, P and K were 1,650, 382 and 2,059 mg L(-1), respectively. The total acidity of humic acids extracted from the concentrate was 5.17 cmol(+) g(-1), composed of 2.38 cmol(+) g(-1) of carboxylic group and 2.79 cmol(+) g(-1) of phenolic hydroxyl group. Coliforms and E. coli were not detected in the concentrate. The yield of rice plant with the concentrate applied to it resulted in similar production to that with chemical fertilizer applied. The water extractable nitrate content of the concentrate-applied land did not exceed that of chemical fertilizer applied, at soil depths of 30 and 60 cm. The percolated amount of nitrate into the water table in arable land with the concentrate applied showed a similar level to that treated with the chemical fertilizer.

Douglas-fir soil C and N properties a decade after termination of urea fertilization

Chemical and microbial soil properties were assessed in paired unfertilized and urea fertilized (>89 g N·m -2 ) plots in 13 second-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands distributed throughout western Washington and Oregon. A decade following the termination of fertilization, fertilized plots averaged 28% higher total N in the O layer than unfertilized plots, 24% higher total N in surface (0-5 cm) mineral soil, and up to four times the amount of extractable ammonium and nitrate. Decreased pH (0.2 pH units) caused by fertilization may have been due to nitrification or enhanced cation uptake. In some soil layers, fertilization decreased cellulase activity and soil respira - tion but increased wood decomposition. There was no effect of fertilization on concentrations of light and heavy frac - tions, labile carbohydrates, and phosphatase and xylanase activities. No increase in soil organic C was detected, although variability precluded observing an increase of less than ∼15%. Lack of a regionwide fertilization influence on soil organic C contrasts with several site-specific forest and agricultural studies that have shown C increases resulting from fertilization. Overall, the results indicate a substantial residual influence on soi l N a decade after urea fertilization but much more limited influence on soil C processes and pools. Resume : Les proprietes chimiques et microbiennes du sol ont ete evaluees dans des paires de parcelles non fertilisees

Negative regulation of nitrate reductase gene expression by glutamine or asparagine accumulating in leaves of sulfur-deprived tobacco.

Tobacco (Nicotiana tabacum L.) plants were subjected to a prolonged period of sulfur-deprivation to characterize molecular and metabolic mechanisms that permit control of primary N-metabolism under these conditions. Prior to the appearance of chlorotic lesions, sulfur-deprived tobacco leaves showed a strong decrease in the sulfate content and changes in foliar enzyme activities, mRNA accumulation and amino-acid pools. The basic amino acids glutamine, asparagine and arginine accumulated in the leaves of sulfur-deprived plants, while the foliar concentrations of aspartate, glutamate, serine or alanine remained fairly unchanged. Maximal extractable nitrate reductase (NR; EC 1.6.6.1) activity decreased strongly in response to sulfur-deprivation. The decrease in maximal extractable NR activity was accompanied by a decline in NR transcripts while the mRNAs of the plastidic glutamine synthetase (EC 6.1.3.2) or the beta-subunit of the mitochondrial ATP synthase were much less affected. Nitrate first accumulated in leaves of tobacco during sulfur-deprivation but then declined. An appreciable amount of nitrate was, however, present in severely sulfur-depleted leaves. The repression of NR gene expression is, therefore, not related to the decrease in the leaf nitrate level. However, glutamine- and/or asparagine-mediated repression of NR gene transcription is a possible mechanism of control in situations when glutamine and asparagine accumulate in leaves and provides a feasible explanation for the reduction in NR activity during sulfur-deprivation. The removal of reduced nitrogen from primary metabolism by redirection and storage as arginine, asparagine or glutamine combined with the down-regulation of nitrate reduction via glutamine- and/or asparagine-mediated repression of NR gene transcription may contribute to maintaining a normal N/S balance during sulfur-deprivation and indicate that the co-ordination of N- and S-metabolism is retained under these conditions.

Stabilization of Soil Nitrate by Reseeding with Perennial Ryegrass following Sudden Turf Death

AbstractTurf death can result in the mineralization of organic nitrogen (N) within the turf‐soil ecosystem. If not reestablished within 12 mo, dead turf can leach 10% of its total soil N as nitrate into ground water. This study investigated how rapidly and completely soil N can be restabilized following reseeding of killed turf. Twelve‐year‐old field plots, comprising four cool‐season turfgrass species growing on an Enfield silt loam (coarse‐silty over sandy‐skeletal, mixed, active, mesic Typic Dystrudepts) at Kingston, RI, were used. In early September 1997, one‐half of the plots were killed by applying glyphosate and three‐fourths of the dead plots were reseeded with ‘Palmer III’ perennial ryegrass (Lolium perenne L.) in late March 1998. Soil moisture, temperature, pH, respiration, and extractable nitrate and ammonium were monitored every 6 to 8 wk. Nitrate levels in subsoil water were monitored at 2‐wk intervals and nitrate leaching estimated for each percolation event. Reseeding enhanced soil respiration, within 6 wk of seeding, to levels comparable with those of soil in healthy plots. Surface soil nitrate levels in killed and reseeded plots declined within 6 wk of reseeding. Nitrate levels in soil water beneath the root zone declined in reseeded plots relative to those of healthy plots (9 mg NO3‐N L−1) within 8 wk of reseeding. Nitrate leaching also declined 3 mo after seeding to rates less than half of those from turf that had been killed but not reseeded. Our results indicate that the potential for increased nitrate leaching following turf death can be eliminated within 2 mo if the site is promptly reseeded.

Container production of tomato with food by‐product compost and mineral fines

Agricultural applications are sought for by‐products from agricultural, municipal, and industrial operations. Incorporation into media for container production of crops is a suggested use of organic and mineral by‐products. Composted food by‐products generated by grocery stores and restaurants and mineral fines from the aggregate industry were assessed in the formulation of media for tomato (Lycopersicon esculentum Mill.) production in containers. The basic medium was compost and perlite (2 compost: 1 perlite, v:v). This medium was mixed with basalt fines or glacial moraine fines added separately at 60 or 120 g L‐1. Nitrogen (N) fertilizer was added weekly to the media except for one treatment that included the basic medium without N. Incorporation of mineral fines into the food by‐product compost produced an early stimulation of vegetative growth, but did not affect fruiting of tomatoes significantly; however, fruiting was heavy in the containers to which N was added, with an average of nearly 23 kg of total fruits per plant. Extractable (acidified sodium acetate solution) calcium (Ca) and manganese (Mn) from the media increased, but other nutrients were unaffected by additions of basalt or glacial fines. In all media, extractable nitrate and potassium (K) were almost depleted during the experiment; phosphorus (P) was moderately depleted, whereas extractable Ca, magnesium (Mg), iron (Fe), zinc (Zn), and copper (Cu) were slightly lower or unchanged during the experiment. Tissue analyses showed that leaf concentrations of N, K, Mn, and Zn were below the range needed for well‐nourished plants whereas Ca exceeded concentrations necessary for adequate nutrition. Tissue concentrations of P and Mg were sufficient for proper plant nutrition. Although fertilization with N and K, were necessary to sustain optimum yields, formulations of food by‐product compost and mineral fines were excellent media for container production of tomato.

Differences in earthworm densities and nitrogen dynamics in soils under exotic and native plant species.

Previous studies of the invasion of two exotic plants – Berberis thunbergii and Microstegium vimineum – in hardwood forests of New Jersey have shown a significant increase of pH in soils under the invasive plants as compared with soils from under native shrubs (Vaccinium spp). We present a further investigation of soil properties under the exotic plants in question. We measured the densities of earthworms in the soil under the two exotics and the native shrubs in three parks in New Jersey. In the same populations we also measured the extractable ammonium and nitrate in the top 5 cm of the soil, as well as the respiration of the soils and the potential rates of mineralization (aerobic lab incubation). In addition, we measured the nitrate reductase activity in leaves of the two exotic plants and several native shrubs and trees. Although there were differences between parks, we observed significantly higher earthworm densities in the soil under the exotic species. The worms were all European species. Soil pH, available nitrate and net potential nitrification were significantly higher in soils under the two exotic species. In contrast, total soil C and N and net ammonification were significantly higher under native vegetation. Nitrate reductase activities were much higher in the leaves of exotic plants than in the leaves of native shrubs and trees. Changes in soil properties, especially the change in nitrogen cycling, associated with the invasion of these two plant species may permit the invasion of other weedy or exotic species. Our results also suggest that even if the two exotic species were removed, the restoration of the native flora might be inhibited by the high nitrate concentrations in the soil.

Persistence of tree related patterns in soil nutrients following slash-and-burn disturbance in the tropics

Individual trees are known to influence soil chemical properties, creating spatial patterns that vary with distance from the stem. The influence of trees on soil chemical properties is commonly viewed as the agronomic basis for low-input agroforestry and shifting cultivation practices, and as an important source of spatial heterogeneity in forest soils. Few studies, however, have examined the persistence of the effects of trees on soil after the pathways responsible for the effects are removed. Here, we present evidence from a Mexican dry forest indicating that stem-related patterns of soil nutrients do persist following slash-and-burn removal of trees and two years of cropping. Pre-disturbance concentrations of resin extractable phosphorus (P), bicarbonate extractable P, NaOH extractable P, total P, total nitrogen (N) and carbon (C), KCl extractable nitrate (NO3 -), and net N mineralization and nitrification rates were higher in stem than dripline soils under two canopy dominant species of large-stemmed trees with contrasting morphologies and phenologies (Caesalpinia eriostachys Benth. and Forchhammeria pallida Liebm.). These stem effects persisted through slash burning and a first growing season for labile inorganic and organic P, NaOH inorganic P, and plant-available P, and through a second growing season for labile organic P, NaOH organic P, and plant-available P. While stem effects for extractable NO3 -, net nitrification rates, total N and C disappeared after felling and slash burning, these stem effects returned after the first growing season. These results support the view that tree-influenced patterns of soil nutrients do persist after tree death, and that trees contribute to the long-term spatial heterogeneity of forest soils.

Nitrous oxide, nitric oxide, and methane fluxes from soils following clearing and burning of tropical secondary forest

Conversion of humid tropical forest to agriculture significantly alters trace gas emissions from soils. We report nitrous oxide (N2O), nitric oxide (NO), and methane (CH4) fluxes from secondary forest soils prior to and during deforestation, and throughout the first agricultural cropping. Annual average nitrogen oxide emissions from forest soils were 1.5 ng N cm−2 h−1 for N2O and 0.9 ng N cm−2 h−1 for NO. Forest clearing increased the level of extractable nitrate in soils and average nitrogen oxides fluxes (2.7 ng N cm−2 h−1 for N2O, and 8.1 ng N cm−2 h−1 for NO). Immediately after biomass burning, short‐term peaks of N2O and NO (123 ng N cm−2 h−1 for N2O, and 41 ng N cm−2 h−1 for NO) were superimposed on generally increased fluxes. Peak emissions declined within 3 days after burning. Postburn fluxes stayed higher than measured on adjacent forest sites for 3–4 months (averages for postburn fluxes were 17.5 ng N cm−2 h−1 for N2O, and 19.2 ng N cm−2 h−1 for NO). Increased N2O and NO emissions after clearing and until cropping were probably due to a combination of increased rates of nitrogen cycling and higher gaseous diffusion in drying soils. Compared to emissions from young pastures in the region, fluxes of nitrogen oxides from unfertilized agricultural areas were low (3.9 ng N cm−2 h−1 for N2O and 3.4 ng N cm−2 h−1 for NO), probably due to nitrogen uptake by fast growing corn plants and losses by leaching with draining soil water in the wet season. Variation in CH4 fluxes was high for all land use periods. Forest soils consumed an average of 1.0 mg CH4 m−2 d−1, which slightly increased in drier soils after clearing (1.2 mg CH4 m−2 d−1). Postburn CH4 consumption by soils was slightly reduced (0.8 mg CH4 m−2 d−1) compared to forest soils. Unfertilized agricultural soils consumed less CH4 than forest soils.

Redistribution of extractable nutrients following disc trenching on Luvisols and Brunisols in Saskatchewan

Disc trenching is an important silvicultural management tool for creating microsites to ensure the survival and growth of newly planted seedlings. Mounded microsites are thought to be a source of nutrients for seedling roots; however, little is known concerning how nutrients are redistributed in the soil after disc trenching. The objective of this study, therefore, was to determine the redistribution of soil horizons and selected soil chemical properties following Delta disc trenching. Three sites were selected on both Luvisol and Brunisol soils in northern Saskatchewan. Soil profiles were excavated and samples collected from 15 × 5 cm grids to a 1-m depth. Samples were analyzed for pH and extractable nitrate (NO3), ammonium (NH4), phosphorus (P), potassium (K) and sulfur (S). Disc trenching concentrated extractable N, P and S in the berm microsite compared to the trench microsites. The redistribution of extractable NO3, NH4 and P in the berms compared to the trench microsites was 20 times greater for Luvisol sites compared to the Brunisol sites, while extractable K and S were similar between both microsites for each soil. Seedlings planted on these mounded microsites therefore should have a large pool of nutrients for plant growth; however, further work is needed to determine patterns of nutrient uptake by seedlings in relation to mounded and trench microsites. Key words: Delta disc trenching, boreal forest, Luvisols, Brunisols, nutrients

Nitrogen uptake and growth of two citrus rootstock seedlings in a sandy soil receiving different controlled-release fertilizer sources

Understanding the fate of different forms of nitrogen (N) fertilizers applied to soils is an important step in enhancing N use efficiency and minimizing N losses. The growth and N uptake of two citrus rootstocks, Swingle citrumelo (SC), and Cleopatra mandarin (CM), seedlings were evaluated in a pot experiment using a Candler fine sand (hyperthermic, uncoated, Typic Quartzipsamments) without N application or with 400 mg N kg–1 applied as urea or controlled-release fertilizers (CRF; either as Meister, Osmocote, or Poly-S). Meister and Osmocote are polyolefin resin-coated urea with longevity of N release for 270 days (at 25°C). Poly-S is a polymer and sulfur-coated urea with release duration considerably shorter than that of either Meister or Osmocote. The concentrations of 2 M KCl extractable nitrate nitrogen (NO3 –-N) and ammonium nitrogen (NH4 +-N) in the soil sampled 180 days and 300 days after planting were greater in the soil with SC than with CM rootstock seedlings. In most cases, the extractable NH4 + and NO3 – concentrations were greater for the Osmocote treatment compared to the other N sources. For the SC rootstock seedlings, dry weight was greater with Meister or Poly-S compared with either Osmocote or urea. At the end of the experiment, ranking of the various N sources, with respect to total N uptake by the seedlings, was: Meister = Osmocote > Poly-S > Urea > no N for CM rootstock, and Meister = Poly-S = Osmocote > Urea > no N for SC rootstock. The study demonstrated that for a given rate of N application the total N uptake by seedlings was greater for the CRF compared to urea treatment. This suggests that various N losses were lower from the CRF source as compared to those from soluble fertilizers.

Impact of Land‐Applied Tertiary‐Treated Effluent on Soil Biochemical Properties

AbstractLand application is increasingly used for the disposal and treatment of effluents. We investigated how irrigation of tertiary‐treated domestic effluent influenced 14 soil biochemical properties in a Monterey pine (Pinus radiata D. Don) forest on volcanic soils. The soils were irrigated with either effluent or water at two loading rates (49 and 74 mm wk−1). Surface soils (0–5 cm) were collected from the effluent‐irrigated and adjacent nonirrigated control sites annually for 3 yr and for 2 yr from the water‐irrigated sites. Effluent irrigation significantly (P < 0.05) increased several soil properties including pH, invertase activity, denitrification, mineralizable N, and extractable nitrate. These increases were not observed in the water‐irrigated soils suggesting that the changes resulted from effluent chemistry rather than additional water loading. Phosphatase activity decreased with both water‐ and effluent‐irrigation. No changes were observed in total N, total C, basal respiration, microbial biomass, sulfatase activity, or extractable ammonium in the effluent‐ and water‐irrigated soils. Both rates of effluent application had the same effect on soil properties indicating that the threshold rate that changed soil properties was ≤49 mm of effluent per week.