Differences in isotopic fractionation of nitrogen in water-saturated and unsaturated soils

Temporal variations in δ¹⁵N of NH₄⁺ and NO₃⁻ in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ¹⁵N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg⁻¹. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH₄⁺-N concentration decreased from 164.8 to 34.4 mg kg⁻¹, and the δ¹⁵N of NH₄⁺ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO₃⁻-N concentration and lower δ¹⁵N of NO₃⁻ (product) than that of NH₄⁺ (substrate) at each sampling time. In saturated soil, the concentration of NH₄⁺-N decreased gradually from 162.4 to 24.2 mg kg⁻¹, and the δ¹⁵N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO₃⁻ concentration was not observed due to loss of NO₃⁻ through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (α_s/p) associated with decrease in NH₄⁺ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH₄⁺ were relatively more dominant than nitrification under the anaerobic conditions.     

Nitrate leaching in soil: Tracing the NO3 sources with the help of stable N and O isotopes

Legumes increase the plant-available N pool in soil, but might also increase NO₃⁻ leaching to groundwater. To minimize NO₃⁻ leaching, N-release processes and the contribution of legumes to NO₃⁻ concentrations in soil must be known. Our objectives were (1) to quantify NO₃⁻-N export to >0.3 m soil depth from three legume monocultures (Medicago x varia Martyn, Onobrychis viciifolia Scop., Lathyrus pratensis L.) and from three bare ground plots. Furthermore, we (2) tested if it is possible to apply a mixing model for NO₃⁻ in soil solution based on its dual isotope signals, and (3) estimated the contribution of legume mineralization to NO₃⁻ concentrations in soil solution under field conditions. We collected rainfall and soil solution at 0.3 m soil depth during 1 year, and determined NO₃⁻ concentrations and δ¹⁵N and δ¹⁸O of NO₃⁻ for >11.5 mg NO₃⁻-N l⁻¹. We incubated soil samples to assess potential N release by mineralization and determined δ¹⁵N and δ¹⁸O signals of NO₃⁻ derived from mineralization of non-leguminous and leguminous organic matter.Mean annual N export to >0.3 m soil depth was highest in bare ground plots (9.7 g NO₃⁻-N m⁻²; the SD reflects the spatial variation) followed by Medicago x varia monoculture (6.0 g NO₃⁻-N m⁻²). The O. viciifolia and L. pratensis monocultures had a much lower mean annual N export (0.5 and 0.3 g NO₃⁻-N m⁻²). The averaged NO₃⁻-N leaching during 70 days was not significantly different between field estimates and incubation for the Medicago x varia Martyn monoculture.The δ¹⁵N and δ¹⁸O values in NO₃⁻ of rainfall (δ¹⁵N: 3.3±0.8‰; δ¹⁸O: 30.8±4.7‰), mineralization of non-leguminous SOM (9.3±0.9‰; 6.7±0.8‰), and mineralization of leguminous SOM (1.5±0.6‰; 5.1±0.9‰) were markedly different. Applying a linear mixing model based on these three sources to δ¹⁵N and δ¹⁸O values in NO₃⁻ of soil solution during winter 2003, we calculated 18-41% to originate from rainfall, 38-57% from mineralization of non-leguminous SOM, and 18-40% from mineralization of leguminous SOM.Our results demonstrate that (1) even under legumes NO₃⁻-N leaching was reduced compared to bare ground, (2) the application of a three-end-member mixing model for NO₃⁻ based on its dual isotope signals produced plausible results and suggests that under particular circumstances such models can be used to estimate the contributions of different NO₃⁻ sources in soil solution, and (3) in the 2nd year after establishment of legumes, they contributed approximately one-fourth to NO₃⁻-N loss.     

Differing nitrogen use strategies of two tropical rainforest late successional tree species in French Guiana: Evidence from 15N natural abundance and microbial activities

Previous studies in lowland tropical rainforests of French Guiana showed that, among non-N₂-fixing trees, two groups of late successional species contrasting in their leaf ¹⁵N natural abundance coexist, suggesting two different main ways of nitrogen acquisition. Two abundant late-successional species typically co-occurring in rainforests in French Guiana, namely Eperua falcata and Dicorynia guianensis, were chosen as representative of each group. Stable isotope techniques and measurements of potentials of microbial N transformation were performed to assess to what extent leaf ¹⁵N natural abundance of these species could be related to (i) δ¹⁵N signatures of soil mineral N sources and (ii) the capacity of soil to express nitrification and denitrification (both processes being directly involved in the balance between NH₄⁺ and NO₃^–). Soil δ¹⁵N-NH₄⁺ was roughly similar to leaf δ¹⁵N of D. guianensis (around 3.5‰), suggesting a preferential use of NH₄⁺, whereas in E. falcata, leaf δ¹⁵N values were closer to root δ¹⁵N-NO₃^? values (0.2 and ?2.0‰, respectively), suggesting a preferential use of NO₃^?. These differences in N source utilization were not accompanied by differences in availability in soil NO₃^? or in intensity of microbial functions responsible for soil N mineral evolution. However, (i) under both tree species, these functions showed clear spatial partitioning, with denitrification occurring potentially in soil and nitrification in the litter layer, and (ii) E. falcata fine roots colonized the litter layer much more strongly than D. guianensis fine roots. This strongly suggests that (i) the contrasted leaf δ¹⁵N values found in the two late-successional species reveal distinct N acquisition strategies and (ii) the ability of roots to predominantly exploit the litter layer (E. falcata) or the soil (D. guianensis) may constitute an important explanation of the observed differences. A complementarity between tree species, based on mineral N resource partitioning (itself resulting from a spatially structured location of the microbial functions responsible for the balance between NH₄⁺ and NO₃^–), can thus be supposed.     

Application of (15NH4)2SO4 to study N dynamics in hoop pine plantation and adjacent native forest of subtropical Australia: the effects of injection depth and litter addition

Purpose

Hoop pine (Araucaria cunninghamii) is a nitrogen (N)-demanding native Australian softwood plantation species. Litter quality and its effects on soil mineral N and ¹⁵N transformations have not been well studied in the hoop pine plantation and adjacent native forest. The present study was conducted to determine the impact of ¹⁵N injection depth and litter additions on the dynamics and fate of mineral ¹⁵N and also to compare the difference in litter quality, ¹⁵N dynamics, and fate between the hoop pine plantation (HP) and the adjacent native forest (NF).

Materials and methods

The experiments were done in the Yarraman State Forest (26°52′ S, 151°51′ E), southeastern Queensland. Materials of litter addition were prepared on the basis of ten random samples of litters taken from the NF and HP sites using a 1?×?1-m quadrat. Litter additions were defined as: SL represented the average condition of forest floor in the forest ecosystems and DL represented the double average amount of litters in the forest ecosystem. Experiment 1 covered 2 forest types (NF and HP)?×?3 litter rates (nil litter, SL, and DL)?×?3 ¹⁵N injection depths (0, 2.5, and 5.0 cm). Experiment 2 included 2 forest types (NF and HP)?×?2 litter rates (nil litter and SL)?×?3 injection depths (0, 2.5, and 5.0 cm) of distilled water. The in situ core incubation method was used with an incubation period of 28 days. The isotope ratio of mineral N or/and total N (soil and litter) were analyzed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer (Isoprime-EuroEA 3000).

Results

Total N and δ ¹⁵N were significantly higher, and C/N ratios and δ ¹³C were significantly lower in the NF litters than in the HP litters. The NF litters had significantly lower total ¹⁵N and total ¹⁵N recovery than the HP litters after ¹⁵N addition. Litter addition had no significant effect on mineral ¹⁵N transformations and δ ¹⁵N in the NF soil, but decreased ¹⁵NO ₃ ^? –N, mineral ¹⁵N, and δ ¹⁵N and increased immobilized ¹⁵N in the HP soil. The depth of added ¹⁵NH ₄ ⁺ significantly altered total ¹⁵N, δ ¹⁵N, and total ¹⁵N recovery in the litters, whereas it did not influence ¹⁵NH ₄ ⁺ –N, ¹⁵NO ₃ ^? –N, mineral ¹⁵N, or immobilized ¹⁵N in soils in the two forest ecosystems.

Discussion

The NF litters had significantly higher δ ¹⁵N than the HP litters, indicating that the NF soil had a higher rate of nitrification than the HP soil. Higher litter quality in the NF was an important driving force for N cycling to promote strong N dynamics in the NF soil over the HP soil. The HP litters had significantly higher total ¹⁵N than the NF litters after ¹⁵N addition, implying that soil mineral N was relatively deficient in the HP in comparison with the NF. Litters decreased nitrification and increased immobilization in the HP soil, showing forest litters resulted in more N immobilization to prevent the loss of substantial quantities of NO ₃ ^? through leaching or denitrification. The depth of ¹⁵N injection did not significantly alter concentrations of ¹⁵NH ₄ ⁺ –N, ¹⁵NO ₃ ^? –N, mineral ¹⁵N, and immobilized ¹⁵N in the NF and HP soils, suggesting that the depth of ¹⁵N injection had no significant influence on the evaluation of soil N transformations.

Conclusions

The NF litters had significantly higher total N and δ ¹⁵N and lower C/N ratios and δ ¹³C than the HP litters. Mineral N was relatively insufficient in the HP soil relative to the NF soil. The HP litters facilitated more N immobilization in the soil to reduce the loss of substantial quantities of NO ₃ ^? through leaching or denitrification. The depth of ¹⁵N added did not significantly alter concentrations of ¹⁵NH ₄ ⁺ –N, ¹⁵NO ₃ ^? –N, mineral ¹⁵N, and immobilized ¹⁵N in the NF and HP soils. The application of ¹⁵N solution by uniform sprinkling onto the soil surface can be used to study in situ field N (including mineral ¹⁵N) transformations in the 10-cm depth soils of both forest ecosystems.     

Natural N abundance of plant and soil inorganic-N as evidence for over-fertilization with compost

To test the hypothesis that N isotope composition can be used as evidence of excessive compost application, we measured variation in patterns of N concentrations and corresponding δ¹⁵N values of plants and soil after compost application. To do so, a pot experiment with Chinese cabbage (Brassica campestris L. cv. Maeryok) was conducted for 42 days. Compost was applied at rates of 0 (SC0), 500 (SC1), 1000 (SC2), and 1500 mg N kg⁻¹ soil (SC3). Plant-N uptake linearly increased with compost application (r² = 0.956, P < 0.05) with an uptake efficiency of 76 g N kg⁻¹ of compost-N at 42 days after application, while dry-mass accumulation did not show such linear increases. Net N mineralized from compost-N increased linearly (r² = 0.998, P < 0.01) with a slope of 122 g N kg⁻¹ of compost-N. Plant-δ¹⁵N increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SC3 treatments. The δ¹⁵N of soil inorganic-N (particularly NO₃⁻-N) increased with compost application. We found that plant-δ¹⁵N reflected the N isotope signal of soil NO₃⁻-N at each measurement during plant growth, and that δ¹⁵N of inner leaves and soil NO₃⁻-N was similar when initial NO₃⁻ in the compost was abundant. Therefore, we concluded that δ¹⁵N of whole plant (more obviously in newer plant parts) and soil NO₃⁻-N could reveal whether compost application was excessive, suggesting a possible use of δ¹⁵N in plants and soil as evidence of excess compost application.     

Natural N abundances of inorganic nitrogen in soil treated with fertilizer and compost under changing soil moisture regimes

This study was conducted to examine whether the applications of N-inputs (compost and fertilizer) having different N isotopic compositions (δ¹⁵N) produce isotopically different inorganic-N and to investigate the effect of soil moisture regimes on the temporal variations in the δ¹⁵N of inorganic-N in soils. To do so, the temporal variations in the concentrations and the δ¹⁵N of NH₄⁺ and NO₃⁻ in soils treated with two levels (0 and 150 mg N kg⁻¹) of ammonium sulfate (δ¹⁵N=−2.3‰) and compost (+13.9‰) during a 10-week incubation were compared by changing soil moisture regime after 6 weeks either from saturated to unsaturated conditions or vice versa. Another incubation study using ¹⁵N-labeled ammonium sulfate (3.05 ¹⁵N atom%) was conducted to estimate the rates of nitrification and denitrification with a numerical model FLUAZ. The δ¹⁵N values of NH₄⁺ and NO₃⁻ were greatly affected by the availability of substrate for each of the nitrification and denitrification processes and the soil moisture status that affects the relative predominance between the two processes. Under saturated conditions for 6 weeks, the δ¹⁵N of NH₄⁺ in soils treated with fertilizer progressively increased from +2.9‰ at 0.5 week to +18.9‰ at 6 weeks due to nitrification. During the same period, NO₃⁻ concentrations were consistently low and the corresponding δ¹⁵N increased from +16.3 to +39.2‰ through denitrification. Under subsequent water-unsaturated conditions, the NO₃⁻ concentrations increased through nitrification, which resulted in the decrease in the δ¹⁵N of NO₃⁻. In soils, which were unsaturated for the first 6-weeks incubation, the δ¹⁵N of NH₄⁺ increased sharply at 0.5 week due to fast nitrification. On the other hand, the δ¹⁵N of NO₃⁻ showed the lowest value at 0.5 week due to incomplete nitrification, but after a subsequence increase, they remained stable while nitrification and denitrification were negligible between 1 and 6 weeks. Changing to saturated conditions after the initial 6-weeks incubation, however, increased the δ¹⁵N of NO₃⁻ progressively with a concurrent decrease in NO₃⁻ concentration through denitrification. The differences in δ¹⁵N of NO⁻₃ between compost and fertilizer treatments were consistent throughout the incubation period. The δ¹⁵N of NO₃⁻ increased with the addition of compost (range: +13.0 to +35.4‰), but decreased with the addition of fertilizer (−10.8 to +11.4‰), thus resulting in intermediate values in soils receiving both fertilizer and compost (−3.5 to +20.3‰). Therefore, such differences in δ¹⁵N of NO₃⁻ observed in this study suggest a possibility that the δ¹⁵N of upland-grown plants receiving compost would be higher than those treated with fertilizer because NO₃⁻ is the most abundant N for plant uptake in upland soils.     

Measuring gross nitrogen mineralization, and nitrification by 15 N isotopic pool dilution in intact soil cores

The isotope dilution method for measuring gross rates of N mineralization, immobilization, and nitrification was applied to intact soil cores so that the effects of soil mixing were avoided. Soil cores were injected with solutions of either ¹⁵NH₄⁺ or ¹⁵NO₄^?; gross mineralization rates were calculated from the decline in “N enrichment of the NH: pool during a 24-h incubation; gross nitrification rates were calculated from the decline in ¹⁵N enrichment of the NO^?₃ pool; gross rates of NH₄⁺ and NO₃^? consumption were calculated from disappearance of the ¹⁵N label. The assumptions required for application of this method to intact cores are evaluated. Sensitivity analysis revealed that homogeneous mixing of added “N with ambient pools was not a necessary assumption but that bias in distribution of added label, coincident with a non-random distribution of microbial processes, would cause significant errors in rate estimates. Rate estimates were also sensitive to errors in initial ¹⁵N and ¹⁴N pool size estimates, In a silt loam soil from a grassland site, abiotic processes consumed over 30% of the added ¹⁵NH₄⁺ within minutes of adding the label to sterilized soil. Extracting a subset of soil cores at the beginning of an incubation is recommended for obtaining initial pool size estimates. Gross immobilization is probably stimulated by addition of inorganic ¹⁵N substrate and, therefore, is overestimated by the isotope dilution method. As an alternative method, a non-linear equation is given for calculating the gross immobilization rate from the appearance of ¹⁵N in chloroform-labile microbial biomass; but incomplete extraction of biomass N may result in low estimates. Details of the isotope dilution methodology (injection rates, concentrations, experimental artefacts, etc.) are described and discussed. When care is taken to understand the underlying assumptions and sources of error, the isotope dilution method provides a powerful tool for measuring gross rates of microbial transformations of soil nitrogen in intact soil cores.     

Suspending Nitrogen Fertilization Reduces Residual Soil Nitrates in Dryland Cotton

Nitrogen (N) fertilizer use in cotton (Gossypium hirsutum L.) production is a potential source of nitrate (NO₃ ^?) contamination of soils, groundwater, and streams. The McConnell–Mitchell plots, a long-term study of cotton responses to N-fertilization and irrigation methods, were utilized to determine the NO₃ ^?-N in soil cropped to continuous cotton. The McConnell–Mitchell plots had a split-block experiential design. The main blocks of this test were irrigation methods. Each block of plots was irrigated using a single irrigation method for the entirety of the testing. Nitrogen fertilization rates were tested within each irrigation block. The soil NO₃ ^?-N content of two irrigation blocks, furrow flow (FI) and center pivot (CP), were compared to the dryland (DL) control block. Nitrogen treatments tested within each irrigation block ranged from 0 to 168.0 kg N ha^?1 in 33.6-kg N ha^?1 increments. Nitrogen treatments were tested for 18 years (1982 through 1999), discontinued for 4 years (2000 through 2003), and resumed in 2004. Soil samples were taken in the early spring (2000 and 2004) to a depth of 1.50 m in 0.15 m increments and analyzed for NO₃ ^?-N. Soil samples taken in 2004 were prior to any fertilization treatment. Irrigation method was found to influence the distribution of soil NO₃ ^?-N. Little accumulation of soil NO₃ ^?-N was observed in either irrigation block or under dryland production when N rates were less than 67.2 kg N ha^?1. Distribution of soil NO₃ ^?-N in the FI block was significantly different with sample depth and N treatment but not the interaction of depth and treatment in both 2000 and 2004. Presumably, the small and close values of the means and the greater variability of interactions compared to main effects precluded significant interactions. Differences in soil NO₃ ^?-N in the FI block after suspending N treatments for 4 years were similar to those found in 2000, although the soil NO₃ ^?-N was generally depleted in 2004 compared to 2000. The distribution of soil NO₃ ^?-N in the CP-irrigated block was dependent on the interaction of sample depth with N treatment in both 2000 and 2004. Soil NO₃ ^?-N values and differences tended to be too small to be of discernable or practical importance under CP irrigation. The distribution of soil NO₃ ^?-N in the DL block was dependent on the interaction of sample depth with N treatment in 2000 and 2004. Soil NO₃ ^?-N was minimal in the three lowest N treatments (0, 33.6, and 67.2 kg N ha^?1) in 2000. Greatest amounts of soil NO₃ ^?-N were found in conjunction with the 134.4 and 168.0 kg N ha^?1 treatments both years. Depletion of soil NO₃ ^?-N was evident in the surface 0.45 m of the 100.8, 134.4, and 168.0 kg N ha^?1 treatments under DL conditions in 2004.     

Comparison of Factors Affecting Soil Nitrate Nitrogen and Ammonium Nitrogen Extraction

Extraction of soil nitrate nitrogen (NO₃ ^?-N) and ammonium nitrogen (NH₄ ⁺-N) by chemical reagents and their determinations by continuous flow analysis were used to ascertain factors affecting analysis of soil mineral N. In this study, six factors affecting extraction of soil NO₃ ^?-N and NH₄ ⁺-N were investigated in 10 soils sampled from five arable fields in autumn and spring in northwestern China, with three replications for each soil sample. The six factors were air drying, sieve size (1, 3, and 5 mm), extracting solution [0.01 mol L^?1 calcium chloride (CaCl₂), 1 mol L^?1 potassium chloride (KCl), and 0.5 mol L^?1 potassium sulfate (K₂SO₄)] and concentration (0.5, 1, and 2 mol L^?1 KCl), solution-to-soil ratio (5:1, 10:1, and 20:1), shaking time (30, 60, and 120 min), storage time (2, 4, and 6 weeks), and storage temperature (?18 ^oC, 4 ^oC, and 25 ^oC) of extracted solution. The recovery of soil NO₃ ^?-N and NH₄ ⁺-N was also measured to compare the differences of three extracting reagents (CaCl₂, KCl, and K₂SO₄) for NO₃ ^?-N and NH₄ ⁺-N extraction. Air drying decreased NO₃ ^?-N but increased NH₄ ⁺-N concentration in soil. Soil passed through a 3-mm sieve and shaken for 60 min yielded greater NO₃ ^?-N and NH₄ ⁺-N concentrations compared to other treatments. The concentrations of extracted NO₃ ^?-N and NH₄ ⁺-N in soil were significantly (P < 0.05) affected by extracting reagents. KCl was found to be most suitable for NO₃ ^?-N and NH₄ ⁺-N extraction, as it had better recovery for soil mineral N extraction, which averaged 113.3% for NO₃ ^?-N and 94.9% for NH₄ ⁺-N. K₂SO₄ was not found suitable for NO₃ ^?-N extraction in soil, with an average recovery as high as 137.0%, and the average recovery of CaCl₂ was only 57.3% for NH₄ ⁺-N. For KCl, the concentration of extracting solution played an important role, and 0.5 mol L^?1 KCl could fully extract NO₃ ^?-N. A ratio of 10:1 of solution to soil was adequate for NO₃ ^?-N extraction, whereas the NH₄ ⁺-N concentration was almost doubled when the solution-to-soil ratio was increased from 5:1 to 20:1. Storage of extracted solution at ?18 °C, 4 °C, and 25 °C had no significant effect (P < 0.05) on NO₃ ^?-N concentration, whereas the NH₄ ⁺-N concentration varied greatly with storage temperature. Storing the extracted solution at ?18 ^oC obtained significantly (P < 0.05) similar results with that determined immediately for both NO₃ ^?-N and NH₄ ⁺-N concentrations. Compared with the immediate extraction, the averaged NO₃ ^?-N concentration significantly (P < 0.05) increased after storing 2, 4, and 6 weeks, respectively, whereas NH₄ ⁺-N varied in the two seasons. In conclusion, using fresh soil passed through a 3-mm sieve and extracted by 0.5 mol L^?1 KCl at a solution-to-soil ratio of 10:1 was suitable for extracting NO₃ ^?-N, whereas the concentration of extracted NH₄ ⁺-N varied with KCl concentration and increased with increasing solution-to-soil ratio. The findings also suggest that shaking for 60 min and immediate determination or storage of soil extract at ?18 ^oC could improve the reliability of NO₃ ^?-N and NH₄ ⁺-N results.     

Evidence that fungi can oxidize NH4+ to NO3− in a grassland soil

The contribution of bacteria and fungi to NH₄⁺ and organic N (N_org) oxidation was determined in a grassland soil (pH 6.3) by using the general bacterial inhibitor streptomycin or the fungal inhibitor cycloheximide in a laboratory incubation study at 20°C. Each inhibitor was applied at a rate of 3 mg g^?1 oven‐dry soil. The size and enrichment of the mineral N pools from differentially (NH₄¹⁵NO₃ and ¹⁵NH₄NO₃) and doubly labelled (¹⁵NH₄¹⁵NO₃) NH₄NO₃ were measured at 3, 6, 12, 24, 48, 72, 96 and 120 hours after N addition. Labelled N was applied to each treatment, to supply NH₄⁺‐N and NO₃^?‐N at 3.15 μmol N g^?1 oven‐dry soil. The N treatments were enriched to 60 atom % excess in ¹⁵N and acetate was added at 100 μmol C g^?1 oven‐dry soil, to provide a readily available carbon source. The oxidation rates of NH₄⁺ and N_org were analysed separately for each inhibitor treatment with a ¹⁵N tracing model. In the absence of inhibitors, the rates of NH₄⁺ oxidation and organic N oxidation were 0.0045 μmol N g^?1 hour^?1 and 0.0023 μmol N g^?1 hour^?1, respectively. Streptomycin had no effect on nitrification but cycloheximide inhibited the oxidation of NH₄⁺ by 89% and the oxidation of organic N by more than 30%. The current study provides evidence to suggest that nitrification in grassland soil is carried out by fungi and that they can simultaneously oxidize NH₄⁺ and organic N.     

Leaf litter C and N cycling from a deciduous permanent crop

Our understanding of leaf litter carbon (C) and nitrogen (N) cycling and its effects on N management of deciduous permanent crops is limited. In a 30-day laboratory incubation, we compared soil respiration and changes in mineral N [ammonium (NH₄⁺-N) + nitrate (NO₃^–-N)], microbial biomass nitrogen (MBN), total organic carbon (TOC) and total non-extractable organic nitrogen (TON) between a control soil at ¹⁵N natural abundance (δ¹⁵N = 1.08‰) without leaf litter and a treatment with the same soil, but with almond (Prunus dulcis (Mill.) D.A. Webb) leaf litter that was also enriched in ¹⁵N (δ¹⁵N = 213‰). Furthermore, a two-end member isotope mixing model was used to identify the source of N in mineral N, MBN and TON pools as either soil or leaf litter. Over 30 d, control and treatment TOC pools decreased while the TON pool increased for the treatment and decreased for the control. Greater soil respiration and significantly lower (p < 0.05) mineral N from 3 to 15 d and significantly greater MBN from 10 to 30 d were observed for the treatment compared to the control. After 30 d, soil-sourced mineral N was significantly greater for the treatment compared to the control. Combined mineral N and MBN pools derived from leaf litter followed a positive linear trend (R² = 0.75) at a rate of 1.39 μg N g^?1 soil day^?1. These results suggest early-stage decomposition of leaf litter leads to N immobilization followed by greater N mineralization during later stages of decomposition. Direct observations of leaf litter C and N cycling assists with quantifying soil N retention and availability in orchard N budgets.     

Development of a Soil N Test for Fertilizer Requirements for Corn Production in Quebec

Corn requires high nitrogen (N) fertilizer use, but no soil N test for fertilizer N requirement is yet available in Quebec. Objectives of this research were (1) to determine the effects of soil nitrate (NO₃ ^?)-N, soil ammonium (NH₄ ⁺)-N, and N fertilizer rates on corn yields and (2) to determine soil sampling times and depths most highly correlated with yields and fertilizer N response under Quebec conditions. Soil samples were taken from 0- to 30-cm and 30- to 60-cm depths at seeding and postseeding (when corn height reached 20 cm) to determine soil NH₄ ⁺ and NO₃ ^? in 44 continuous corn sites fertilized with four rates of N in two replications using a quick test (N-Trak) and a laboratory method. The N-Trak method overestimated soil NO₃ ^?-N in comparison with the laboratory method. Greater coefficients of determination were observed for soil NO₃ ^?-N analyses at postseeding compared with seeding.     

DEVELOPMENT OF A SOIL N TEST FOR FERTILIZER REQUIREMENTS FOR WHEAT

Optimal fertilizer nitrogen (N) rates result in economic yield levels and reduced pollution. A soil test for determining optimal fertilizer N rates for wheat has not been developed for Quebec, Canada, or many other parts of the world. Therefore, the objectives were to determine: 1) the relationship among soil nitrate (NO^? ₃)- N, soil ammonium (NH ⁺ ₄)- N and N fertilizer on wheat yields; and 2) the soil sampling times and depths most highly correlated with yield response to soil NO^? ₃-N and NH ⁺ ₄-N. In a three year research work, wet and dried soil samples of 0- to 30- and 30- to 60-cm depths from 20 wheat fields that received four rates of N fertilizer at seeding and postseeding (plants 15 cm tall) were analyzed for NH ⁺ ₄-N and NO^? ₃ -N using a quick-test (N-Trak) and a standard laboratory method. Wheat yield response to N fertilizer was limited, but strong to soil NO^? ₃-N.     

Anionic Exchange Membranes as a Soil Test for Nitrogen Availability

Abstract

A better understanding of nitrogen (N) availability to crops remains an essential key for a productive and safe production system. The main objective of this study was to evaluate the potential of anionic exchange membranes (AEMs) as part of a soil‐testing procedure to predict in situ soil NO₃‐N availability for forage and corn produced in eastern Canada. The AEMs were buried in the surface horizon (0–15 cm) at four experimental sites for forage and at one site for corn. Treatments consisted of five NH₄NO₃ rates (0, 60, 120, 180, and 240 kg N ha^?1) in forage and of six anhydrous ammonia (0, 50, 100, 150, 200, and 250 kg N ha^?1) in corn production. In all sites, NO₃ ^? adsorbed on AEMs (NO_3AEMs) increased significantly with N fertilizer rates, indicating the ability of the AEMs to detect differences between N fertilizer treatments and to predict the soil N availability to crops. The NO_3AEMs fluxes were significantly related to soil NO₃‐N concentration as extracted by water or KCl (0.66≤R²≤0.95). Significant relationships between crop N uptake and NO_3AEMs were obtained (0.52≤R²≤0.94), suggesting that AEMs can be used as an index of soil N availability. Results indicated that AEMs provide a reasonably accurate evaluation of N availability to forage and corn. Because of their low cost, simplicity, and consistency over years, soils, and crops, AEMs could be efficiently used in soil N availability analysis.     

Residual Soil Nitrate: A Comparison between Air-Dried and Field-Moist Soil Samples

In the framework of the European nitrate directive (91/676/EEG), losses of nitrate (NO₃)– nitrogen (N) to both surface and groundwater are limited to 50 mg/l. Because the residual NO₃-N in the soil profile after harvest is considered the main determinant of nitrate leaching during wintertime, the Flemish government imposed a limit value of 90 kg NO₃-N ha^?1 up to a soil depth of 90 cm between 1 October and 15 November. This study compared two different soil sample preparation methodologies. When samples were analyzed immediately upon arrival, no differences in NO₃-N concentration were observed. However, although field-moist samples are maintained at 4 °C, nitrification is not completely stopped, as indicated by the increased NO₃-N concentration in field-moist samples 10 days after storage at 4 °C . In contrast, nitrification in air-dried samples is stopped during the oven drying when 40 °C is reached. Moreover, the reproducibility was significantly greater in air-dried samples as compared to field-moist samples.     

Formation of carcinogenic nitrosamines in soils

The possible formation of carcinogenic nitrosamines in soils was examined. Soil samples amended with NO₂^?-N and dimethylamine incubated for 30 days and analysed every 3 days, showed increasing amounts of dimethylnitrosamine up to 12–15 days. The concentration reached as high as 6.5 parts/10⁶, thereafter, a decline was noted. Most of the nitrosamines disappeared in soils after 30 days. Addition of inorganic N reduced the decomposition of dimethylamine. Soil incubation studies with NO₂^? and trimethylamine showed about 80% reduction in the amount of nitrosamines formed as compared to dimethylamine. Analysis of soil samples from fertilized and polluted areas showed significant amounts of NO^?₃-N but no nitrosamines. Application of 10 parts/10⁶ of dimethylamine to these soil samples resulted in the formation of 0.10 to 0.50 parts/10⁶ of nitrosamines. Autoclaved soil samples incubated with NO₂^? and dimethylamine for 12–15 days produced small amounts of nitrosamines. Addition of glucose to soil samples increased the amounts of nitrosamines formed.     

Effects of weed control and fertilization at early establishment on tree nitrogen and water use in an exotic F1 hybrid pine of subtropical Australia

Purpose

We investigated the effects of weed control and fertilization at early establishment on foliar stable carbon (δ¹³C) and nitrogen (N) isotope (δ¹⁵N) compositions, foliar N concentration, tree growth and biomass, relative weed cover and other physiological traits in a 2-year old F₁ hybrid (Pinus elliottii var. elliottii (Engelm) × Pinus caribaea var. hondurensis (Barr. ex Golf.)) plantation grown on a yellow earth in southeast Queensland of subtropical Australia.

Materials and methods

Treatments included routine weed control, luxury weed control, intermediate weed control, mechanical weed control, nil weed control, and routine and luxury fertilization in a randomised complete block design. Initial soil nutrition and soil fertility parameters included (hot water extractable organic carbon (C) and total nitrogen (N), total C and N, C/N ratio, labile N pools (nitrate (NO₃ ^?) and ammonium (NH₄ ⁺)), extractable potassium (K⁺)), soil δ¹⁵N and δ¹³C. Relative weed cover, foliar N concentrations, tree growth rate and physiological parameters including photosynthesis, stomatal conductance, photosynthetic nitrogen use efficiency, foliar δ¹⁵N and foliar δ¹³C were also measured at early establishment.

Results and discussion

Foliar N concentration at 1.25 years was significantly different amongst the weed control treatments and was negatively correlated to the relative weed cover at 1.1 years. Foliar N concentration was also positively correlated to foliar δ¹⁵N and foliar δ¹³C, tree height, height growth rates and tree biomass. Foliar δ¹⁵N was negatively correlated to the relative weed cover at 0.8 and 1.1 years. The physiological measurements indicated that luxury fertilization and increasing weed competition on these soils decreased leaf xylem pressure potential (Ψ_xpp) when compared to the other treatments.

Conclusions

These results indicate how increasing N resources and weed competition have implications for tree N and water use at establishment in F₁ hybrid plantations of southeast Queensland, Australia. These results suggest the desirability of weed control, in the inter-planting row, in the first year to maximise site N and water resources available for seedling growth. It also showed the need to avoid over-fertilisation, which interfered with the balance between available N and water on these soils.     

Comparative Study on Disturbed and Undisturbed Soil Sample Incubation for Estimating Soil Nitrogen-Supplying Capacity

Application of nitrogen (N) fertilizers without knowing the N-supplying capacity of soils may lead to low N use efficiency, uneconomical crop production, and pollution of the environment. Based on the results from pot experiments treated with soil initial nitrate leaching and native soil, long-term alternate leaching aerobic incubation was conducted to study the disturbed and undisturbed soil N-supplying capacity of surface soil samples in 11 sites with different fertilities on the Loess Plateau. The results indicated that the entire indexes and ryegrass (Lolium perenne) uptake N with soil initial nitrate leaching showed a better correlation than that without soil initial nitrate leaching. Except the correlation coefficients for soil initial nitrate (NO₃ ^?)-N and mineral N extracted by calcium chloride (CaCl₂) before aerobic incubation with ryegrass uptake without soil initial nitrate leaching, the correlation coefficients for soil initial NO₃ ^?-N and mineral N extracted by CaCl₂ before aerobic incubation with ryegrass uptake with soil initial nitrate leaching and those for mineralizable N extracted by aerobic incubation, soil initial mineral N and mineralizable N extracted by aerobic incubation, potentially mineralizable N (N₀) and soil initial mineral N + N₀ with ryegrass uptake N under the two cases in disturbed treatment were all higher than those in undisturbed treatment. We concluded that NO₃ ^?-N in soil extracted by CaCl₂ before aerobic incubation can reflect soil N-supplying capacity but cannot reflect soil potential N-supplying capacity. Without soil initial nitrate leaching, the effect of disturbed and undisturbed soil samples incubated under laboratory conditions for estimating soil N-supplying capacity was not good; however, with soil initial nitrate leaching, this method could give better results for soil N-supplying capacity. Based on the results from pot experiments treated with soil initial nitrate leaching and native soil, the mineralization of disturbed soil samples can give provide better results for predicting soil N-supplying capacity for in situ structure soil conditions on the Loess Plateau than undisturbed soil samples.     

Re-Evaluation of Soil Nitrogen Sampling Strategy Effects on Statistical Power

Soil sampling may be used as a decision-making tool for late-vegetative stage nitrogen (N) fertilizer applications in corn (Zea mays L.). Recommended sampling strategies following banded fertilizer applications commonly suggest taking cores from both on the fertilizer band (B) and off the band (O-B), however we hypothesized that soil nitrate concentrations (NO₃^?ppm) in the O-B were not influenced by N application rate. Analyzing samples from six experiments, we found there was a strong relationship between NO₃^?ppm and applied N rate in the B, but not the O-B position. Power analysis revealed that finding significant differences in applied N rates was only likely when sampling on the B and the difference in N rate was greater than 110 kg N ha^?1. This demonstrates that soil N sampling is not sensitive to small differences in applied N, and that O-B soil cores may only dilute the ability to detect these differences.

Abbreviations: B, on N fertilizer band; O-B, halfway between the corn row and the N fertilizer band; NO₃^?ppm, log-transformed nitrate-N concentration (ppm); NH₄⁺ppm; ammonium-N concentration (ppm); D1, 0–30 cm depth; D2, 30–60 cm depth; C-220, contrast of N rates differing by 220 kg N ha^?1; C-110, contrast of N rates differing by 110 kg N ha^?1; C-55H, contrast of N rates differing by 55 kg N ha^?1 at high N rates; C-55L, contrast of N rates differing by 55 kg N ha^?1 at low N rates; A:N, ratio of non-transformed ammonium-N to nitrate-N concentrations; 0N, unfertilized treatment; CV, coefficient of variation; SE, standard error.     

How weed control and fertilisation influence tree physiological processes and growth at early establishment in an exotic F<Subscript>1</Subscript> hybrid pine plantation of subtropical Australia

Purpose

This study investigated how nitrogen (N) nutrition and key physiological processes varied under changed water and nitrogen competition resulting from different weed control and fertilisation treatments in a 2-year-old F₁ hybrid (Pinus elliottii Engelm var. elliottii?×?P. caribaea var. hondurensis Barr. ex Golf.) plantation on a grey podzolic soil type, in Southeast Queensland.

Materials and methods

The study integrated a range of measures including growth variables (diameter at ground level (DGL), diameter at breast height (DBH) and height (H)), foliar variables (including foliar N concentration, foliar δ¹³C and δ¹⁵N) and physiological variables (including photosynthesis (A_n), stomatal conductance (g_s), transpiration (E), intrinsic water use efficiency (WUE_i) (A/g_s) and xylem pressure potential (Ψ_XPP)) to better understand the mechanisms influencing growth under different weed control and fertilisation treatments. Five levels of weed control were applied: standard (routine), luxury, intermediate, mechanical and nil weed control, all with routine fertilisation plus an additional treatment, routine weed control and luxury fertilisation. Relative weed cover was assessed at 0.8, 1.1 and 1.6 years after plantation establishment to monitor the effectiveness of weed control treatments. Soil investigation included soil ammonium (NH₄ ⁺-N), nitrate (NO₃ ^?-N), potentially mineralizable N (PMN), gravimetric soil moisture content (MC), hot water extractable organic carbon (HWETC), hot water extractable total N (HWETN), total C, total N, stable C isotope composition (δ¹³C), stable N isotope composition (δ¹⁵N), total P and extractable K.

Results and discussion

There were significant relationships between foliar N concentrations and relative weed cover and between tree growth and foliar N concentration or foliar δ¹⁵N, but initial site preparation practices also increased soil N transformations in the planting rows reducing the observable effects of weed control on foliar δ¹⁵N. A positive relationship between foliar N concentration and foliar δ¹³C or photosynthesis indicated that increased N availability to trees positively influenced non-stomatal limitations to photosynthesis. However, trees with increased foliar N concentrations and photosynthesis were negatively related to xylem pressure potential in the afternoons which enhanced stomatal limitations to photosynthesis and WUE_i.

Conclusions

Luxury and intermediate weed control and luxury fertilisation positively influenced growth at early establishment by reducing the competition for water and N resources. This influenced fundamental key physiological processes such as the relationships between foliar N concentration, A _n, E, g_s and Ψ_XPP. Results also confirmed that time from cultivation is an important factor influencing the effectiveness of using foliar δ¹⁵N as an indicator of soil N transformations.