Conversion of 15N-ammonium in forest soils

To determine the fate of atmospheric ammonium in forest soils, one calcareous and two acid forest soils were incubated with ¹⁵N ammonium. In the calcareous soil about 65% of the applied ¹⁵N-ammonium was recovered as nitrate after 98 days of incubation, whereas in the acid soils less than 10% of the ¹⁵N-ammonium was converted to nitrate. In all soils a large proportion of the ¹⁵N was incorporated in organic nitrogen compounds. This incorporation limits the use of ¹⁵N tracers for the elucidation of the fate of atmospheric ammonium in soils.     

The effect of harvesting intensity on the fate of applied 15N-ammonium to the organic horizons of a coniferous forest in N. Wales

¹⁵N-ammonium sulphate equivalent to 0.5 kg N/ha was added as a tracer to lysimeters containing the organic horizons of an acid forest soil. The effect of logging debris (brash), vegetation and second rotationPicea sitchensis seedlings on the amount of the¹⁵N found in various soil, vegetation and leachate pools was followed over a period of 60 days. Transformation of¹⁵N-ammonium to nitrate occurred within 24 hours. Although total nitrate leachate losses were high, tracer-derived nitrate represented only 0.4%–4.2% of the applied¹⁵N-ammonium. The atom % excess of the KCI-extractable organic-N pool was initially lower than for the inorganic species but due to the large pool size, consistently represented 3–6% of the applied¹⁵N-ammonium. The similarity of the atom % excess of the ammonium and nitrate pools indicated an autotrophic nitrification pathway.A significant proportion of the¹⁵N-ammonium passed through the microbial biomass which contained between 16 and 48% of the¹⁵N-ammonium 2 days after addition of the¹⁵N-ammonium. This nitrogen was in a readily available form or short-term pool for the first two weeks (with no change in the overall biomass pool), after which the nitrogen appeared to become transformed into more stable compounds representing a long-term pool. Total recovery of the¹⁵N was between 68% and 99% for the different treatments. The presence of brash reduced microbial immobilisation of the¹⁵N-ammonium and total retention in the organic matter. This is suggested to be a consequence of greater nitrification and denitrificatiion rate in organic horizons beneath a brash covering due to different microclimatic conditions.     

The use of different soil nitrogen sources by young Norway spruce plants

 Three-year-old Norway spruce trees were planted into a low-nitrogen mineral forest soil and supplied either with two different levels of mineral nitrogen (NH₄NO₃) or with a slow-release form of organic nitrogen (keratin). Supply of mineral nitrogen increased the concentrations of ammonium and nitrate in the soil solution and in CaCl₂-extracts of the rhizosphere and bulk soil. In the soil solution, in all treatments nitrate concentrations were higher than ammonium concentrations, while in the soil extracts ammonium concentrations were often higher than nitrate concentrations. After 7 months of growth, ¹⁵N labelled ammonium or nitrate was added to the soil. Plants were harvested 2 weeks later. Keratin supply to the soil did not affect growth and nitrogen accumulation of the trees. In contrast, supply of mineral nitrogen increased shoot growth and increased the ratio of above-ground to below-ground growth. The proportion of needle biomass to total above-ground biomass was not increased by mineral N supply. The atom-% ¹⁵N was higher in younger needles than in older needles, and in younger needles higher in plants supplied with ¹⁵N-nitrate than in plants supplied with ¹⁵N-ammonium. The present data show that young Norway spruce plants take up nitrate even under conditions of high plant internal N levels. Received: 1 April 1998 / Accepted: 9 October 1998     

Bulk soil pH and rhizosphere pH of peach trees in calcareous and alkaline soils as affected by the form of nitrogen fertilizers

One-year old nectarine trees [Prunus persica, Batsch var. nectarina (Ait.) Maxim.], cv Nectaross grafted on P.S.B2 peach seedlings [Prunus persica (L.) Batsch] were grown for five months in 4-litre pots filled with two alkaline soils, one of which was also calcareous. Soils were regularly subjected to fertigation with either ammonium sulphate or calcium nitrate providing a total of 550 mg N/tree. Trees were also grown in such soils receiving only deionized water, as controls. Rhizosphere pH, measured by the use of a microelectrode inserted in agar sheet containing a bromocresol purple as pH indicator and placed on selected roots, was decreased by about 2–3 units compared to the bulk soil pH in all treatments. This decrease was slightly less marked when plants were supplied with calcium nitrate rather than ammonium sulphate or control. Measurements conducted during the course of the experiment indicated that ammonium concentration was similar in the solution of soils receiving the two N fertilizers. During the experiment, soil solution nitrate-N averaged 115 mg L^–1 in soil fertilized with calcium nitrate, 68 mg L^–1 in those receiving ammonium sulphate and 1 mg L^–1 in control soils. At the end of the experiment nitrate concentrations were similar in soils receiving the two N sources and bulk soil pH was decreased by about 0.4 units by ammonium sulphate fertigation: these evidences suggest a rapid soil nitriflcation activity of added ammonium. Symptoms of interveinal chlorosis in apical leaves appeared during the course of the experiment in trees planted in the alkaline-calcareous soil when calcium nitrate was added. The slightly higher rhizosphere pH for calcium nitrate-fed plants may have contributed to this. The findings suggest that using ammonium sulphate in a liquid form (e.g. by fertigation) in high-pH soils leads to their acidification and the micronutrient availability may be improved.     

Partitioning of 15N-labeled ammonium and nitrate among soil, litter, below- and above-ground biomass of trees and understory in a 15-year-old Picea abies plantation

The partitioning of nitrogen deposition among soil, litter, below- and above-ground biomass of trees and understory vegetation was investigated in a 15-year-old Picea abies (L.) Karst. plantation in the Fichtelgebirge, Germany, by labeling with 62 mg of¹⁵N tracer per square meter in March 1991. Ammonium and nitrate depositions were simulated on five plots each, by labeling with either¹⁵N-NH₄ ⁺ or¹⁵N-NO₃ ^–, and the¹⁵N pulse was followed during two successive growing seasons (1991 and 1992). Total recovery rates of the¹⁵N tracer in the entire stand ranged between 93 and 102% for both nitrogen forms in 1991, and 82% in June 1992. ⁵ N ratios increased rapidly in all compartments of the ecosystem. Roots and soils (to 65 cm depth) showed significant¹⁵N enrichments for both¹⁵N-treatments compared to reference plots. Newly grown spruce tissues were more enriched than older ones, but the most enriched ¹⁵N values were found in the understory vegetation. Although spruce trees were a much larger pool (1860 g biomass/m²) than understory vegetation (Vaccinium myrtillus 333 g/m², Calluna vulgaris 142 g/m², Deschampsia flexuosa 22 g/m²), the ericaceous shrubs and the perennial grass were a much greater sink for the¹⁵N label. Eight months after labeling, 9% of the ammonium and 15% of the nitrate label were found in the understory. P.abies retained only 3% of the¹⁵N-ammonium and 7% of the¹⁵N-nitrate. The main sink for both¹⁵N tracers was the soil, where 87% of the ammonium and 79% of the nitrate tracer were found. The organic soil horizon (5-0 cm depth) contained 63% of the¹⁵N-ammonium and 46% of the¹⁵N -nitrate suggesting strong immobilization by microorganisms of both N forms. Eight months after tracer application, about 16% of both¹⁵N-tracers was found below 25 cm soil depth. This 16% corresponds well to a 20% decrease in the recovery of both¹⁵N tracers after 15 months and indicates a total loss out of the ecosystem. Highly enriched ¹⁵N values were found in fruit bodies of fungi growing in reference lots (no¹⁵N addition), although soils did not show increased ¹⁵N ratios. No transfer of¹⁵N-tracer between fungi and spruce or understory vegetation was apparent yet.     

Uptake of 15N-labelled alanine,ammonium and nitrate in Pinus sylvestris L. ectomycorrhiza growing in forest soil treated with nitrogen,sulphur or lime

The uptake of ¹⁵N-labelled alanine, ammonium and nitrate was studied in ectomycorrhizal morphotypes of intact Pinus sylvestris seedlings. PCR-RFLP analysis of the ITS-region of fungal rDNA was used to identify the morphotypes. Seedlings were grown in forest soil collected at an experimental site in southern Sweden. The treatments compared were a control, N fertilisation (600 kg N ha^-1 as urea), sulfur application (1200 kg S ha^-1) and lime application (6000 kg CaCO₃ ha^-1). The forest, which had been dominated by Picea abies, was clear-cut two years before the forest soil was sampled. Soil was also collected from an adjacent standing forest. The aim of the present study was to detect changes in the ectomycorrhizal communities in forest soils and relate these changes to the functional parameter of uptake of nitrogen from organic (alanine and protein) and inorganic (ammonium and nitrate) sources.Liming resulted in the detection of a morphotype not found in other samples, and one morphotype was only found in samples from the standing forest (the fungi in these two morphotypes could not be identified). All mycorrhizal root tips showed a higher ¹⁵N concentration after exposure to different nitrogen forms than non-mycorrhizal long roots. Uptake of¹⁵ N from a labelled solution of alanine or ammonium was higher (about tenfold) than uptake from a ¹⁵N-labelled solution of nitrate. Uptake of ammonium and alanine varied between 0.2 and 0.5 mg N g^-1 h^-1 and between 0.1 and 0.33 mg N g^-1 h^-1, respectively, among the different morphotypes.In seedlings grown in the control soil and in soil from standing forest, alanine and ammonium were taken up to a similar degree from a supply solution by all morphotypes, whereas ammonium uptake was higher than alanine uptake in seedlings grown in lime-treated soil (about twofold) and, to a lesser extent, in the nitrogen- and sulfur-treated soils. The higher ammonium uptake by morphotypes from the limed soil was confirmed in pure culture studies. In cases where ammonium was used as the N source, an isolate of the S. variegatus morphotype collected in the limed soil produced more biomass compared with isolates of S. variegatus collected in nitrogen- or sulphur-treated soil. One isolate of a silvery white morphotype produced about equal amounts of biomass on alanine and ammonium, whereas all S. variegatus isolated performed better with ammonium as their N source. Based on the results it is hypothesised that liming can induce a shift in the ectomycorrhizal community, favouring individuals that mainly utilise inorganic nitrogen over those that primarily utilise organic nitrogen.     

Ecological and site historical aspects of N dynamics and current N status in temperate forests

Ecological developments during Holocene age and high atmospheric depositions since industrialization have changed the N dynamics of temperate forest ecosystems. A number of different parameters are used to indicate whether the forests are N‐saturated or not, most common among them is the occurrence of nitrates in the seepage water below the rooting zone. The use of different definitions to describe N saturation implies that the N status of ecosystems is not always appropriately assessed. Data on N dynamics from 53 different German forests were used to classify various development states of forest ecosystems according to the forest ecosystem theory proposed by Ulrich for which N balances of input – (output plus plant N increment) were used. Those systems where N output equals N input minus plant N increment are described as (quasi‐) Steady State Type. Those forests where N output does not equal N input minus plant N increment as in a ‘transient state.’ Forests of the transient state may lose nitrogen from the soil (Degradation Type) or gain nitrogen [e.g., from atmospheric depositions (Accumulation Type)]. Forest ecosystems may occur in four different N states: (a) (quasi‐) Steady State Type with mull type humus, (b) Degradation Type with mull type humus, (c) Accumulation Type with moder type humus, and (d) (quasi‐) Steady State Type with moder type humus. Forests with the (quasi‐) steady state with mull type humus in the forest floor (n= 8) have high‐soil pH values, high N retention by plant increment, high N contents in the mineral soils, and have not undergone large changes in the N status. Forests of the Degradation Type lose nitrogen from the mineral soil (currently degradation is occurring on one site). Most forests that have moder or mor type humus and low‐soil pH values, and low N contents in the mineral soil have gone through the transient state of organic matter loss in the mineral soils. They accumulate organic matter in the forest floor (accumulation phase, currently 21 sites are accumulating 6–21 kg N ha^?1 yr^?1) or have reached a new (quasi‐) steady state with moder/mor type humus (n= 15). N retention in the accumulation phase has significantly increased in soil with N deposition (r²= 0.38), soil acidity (considering thickness of the forest floor as indices of soil acidity, r²= 0.43) and acid deposition (sulfate deposition, r²= 0.39). Retention of N (4–20 kg N ha^?1 yr^?1) by trees decreased and of soils increased with a decrease in the availability of base cations indicating the important role of trees for N retention in less acid soils and those of soils in more acid soils. Ecosystem theory could be successfully applied on the current data to understand the dynamics of N in temperate forest ecosystems.     

Changes in soil properties after afforestation of former intensively managed soils with oak and Norway spruce

In many European countries, surplus agricultural production and ecological problems due to intensive soil cultivation have increased the interest in afforestation of arable soils. Many environmental consequences which might rise from this alternative land-use are only known from forest establishment on less intensively managed or marginal soils. The present study deals with changes in soil properties following afforestation of nutrient-rich arable soils. A chronosequence study was carried out comprising seven Norway spruce (Picea abies (Karst.) L.) and seven oak (Quercus robur L.) stands established from 1969 to 1997 on former horticultural and agricultural soils in the vicinity of Copenhagen, Denmark. For comparison, a permanent pasture and a ca. 200-year-old mixed deciduous forest were included. This paper reports on changes in pH values, base saturation (BS_eff), exchangeable calcium, soil N pools (N_min contents), and C/N ratios in the Ap-horizon (0–25 cm) and the accumulated forest floor. The results suggest that afforestation slowly modifies soil properties of former arable soils. Land-use history seems to influence soil properties more than the selected tree species. An effect of tree species was only found in the forest floor parameters. Soil acidification was the most apparent change along the chronosequence in terms of a pH decrease from 6 to 4 in the upper 5 cm soil. Forest floor pH varied only slightly around 5. Nitrogen storage in the Ap-horizon remained almost constant at 5.5 Mg N ha^–1. This was less than in the mineral soil of the ca. 200-year-old forest. In the permanent pasture, N storage was somewhat higher in 0–15 cm depth than in afforested stands of comparable age. Nitrogen storage in the forest floor of the 0–30-year-old stands increased in connection with the build-up of forest floor mass. The increase was approximately five times greater under spruce than oak. Mineral soil C/N ratios ranged from 10 to 15 in all stands and tended to increase in older stands only in 0–5 cm depth. Forest floor C/N ratios were higher in spruce stands (26.4) as compared to oak stands (22.7). All stands except the youngest within a single tree species had comparable C/N ratios.     

农田和森林土壤中氧化亚氮的产生与还原

采用土壤淤浆方法对丹麦农田和山毛榉森林土壤反硝化过程中Ｎ２Ｏ的产生与还原进行了研究。同时考察了硝酸根和铵离子对反硝化作用的影响。结果表明,森林土壤反硝化活性大于农田土壤,但农田土壤中Ｎ２Ｏ还原活性大于森林土壤,表现在农田和森林土壤中Ｎ２Ｏ／Ｎ２的产生比率分别为０．１１和３．６５。硝酸根和铵离子能促进两种土壤中的Ｎ２Ｏ产生,但可降低农田土壤中的Ｎ２Ｏ还原速率,与农田土壤相比,硝酸根可降低森林土壤Ｎ２     

15N-ammonium and 15N-nitrate uptake of a 15-year-old Picea abies plantation

Throughfall nitrogen of a 15-year-old Picea abies (L.) Karst. (Norway spruce) stand in the Fichtelgebirge, Germany, was labeled with either ¹⁵N-ammonium or ¹⁵N-nitrate and uptake of these two tracers was followed during two successive growing seasons (1991 and 1992). ¹⁵N-labeling (62 mg ¹⁵N m^-2 under conditions of 1.5 g N m^-2 atmospheric nitrogen deposition) did not increase N concentrations in plant tissues. The ¹⁵N recovery within the entire stand (including soils) was 94%±6% of the applied ¹⁵N-ammonium tracer and 100%±6% of the applied ¹⁵N-nitrate tracer during the 1st year of investigation. This decreased to 80%±24% and 83%±20%, respectively, during the 2nd year. After 11 days, the ¹⁵N tracer was detectable in 1-year-old spruce needles and leaves of understory species. After 1 month, tracer was detectable in needle litter fall. At the end of the first growing season, more than 50% of the ¹⁵N taken up by spruce was assimilated in needles, and more than 20% in twigs. The relative distribution of recovered tracer of both ¹⁵N-ammonium and ¹⁵N-nitrate was similar within the different foliage age classes (recent to 11-year-old) and other compartments of the trees. ¹⁵N enrichment generally decreased with increasing tissue age. Roots accounted for up to 20% of the recovered ¹⁵N in spruce; no enrichment could be detected in stem wood. Although ¹⁵N-ammonium and ¹⁵N-nitrate were applied in the same molar quantities (¹⁵NH ₄ ⁺ : ¹⁵NO ₃ ^- =1:1), the tracers were diluted differently in the inorganic soil N pools (¹⁵NH ₄ ⁺ /NH ₄ ⁺ : ¹⁵NO ₃ ^- /NO ₃ ^- =1:9). Therefore the measured ¹⁵N amounts retained by the vegetation do not represent the actual fluxes of ammonium and nitrate in the soil solution. Use of the molar ammonium-to-nitrate ratio of 9:1 in the soil water extract to estimate ¹⁵N uptake from inorganic N pools resulted in a 2–4 times higher ammonium than nitrate uptake by P. abies.     

Nitrogen stable isotopes indicate differences in nitrogen cycling between two contrasting Jamaican montane forests

Background and aims

The aim of this study is to enhance our knowledge of nitrogen (N) cycling and N acquisition in tropical montane forests through analysis of stable N isotopes (δ¹⁵N).

Methods

Leaves from eight common tree species, leaf litter, soils from three depths and roots were sampled from two contrasting montane forest types in Jamaica (mull ridge and mor ridge) and were analysed for δ¹⁵N.

Results

All foliar δ¹⁵N values were negative and varied among the tree species but were significantly more negative in the mor ridge forest (by about 2 ‰). δ¹⁵N of soils and roots were also more negative in mor ridge forests by about 3 ‰. Foliar δ¹⁵N values were closer to that of soil ammonium than soil nitrate suggesting that trees in these forests may have a preference for ammonium; this may explain the high losses of nitrate from similar tropical montane forests. There was no correlation between the rankings of foliar δ¹⁵N in the two forest types suggesting a changing uptake ratio of different N forms between forest types.

Conclusions

These results indicate that N is found at low concentrations in this ecosystem and that there is a tighter N cycle in the mor ridge forest, confirmed by reduced nitrogen availability and lower rates of nitrification. Overall, soil or root δ¹⁵N values are more useful in assessing ecosystem N cycling patterns as different tree species showed differences in foliar δ¹⁵N between the two forest types.     

Amino acid uptake in deciduous and coniferous taiga ecosystems

We measured in situ uptake of amino acids and ammonium across deciduous and coniferous taiga forest ecosystems in interior Alaska to examine the idea that late successional (coniferous) forests rely more heavily on dissolved organic nitrogen (DON), than do early successional (deciduous) ecosystems. We traced ¹⁵N-NH₄⁺ and ¹³C-¹⁵N-amino acids from the soil solution into plant roots and soil pools over a 24 h period in stands of early successional willow and late successional black spruce. Late successional soils have much higher concentrations of amino acid in soil solution and a greater ratio of DON to dissolved inorganic N (DIN) (ammonium plus nitrate) than do early successional soils. Moreover, late successional coniferous forests exhibit higher rates of soil proteolytic activity, but lower rates of inorganic N turnover. Differences in ammonium and amino acid uptake by early successional willow stands were insignificant. By contrast, the in situ uptake of amino acid by late successional black spruce forests were approximately 4-fold greater than ammonium uptake. The relative difference in uptake of ammonium and amino acids in these forests was approximately proportional to the relative difference of these N forms in the soil solution. Thus, we suggest that differences in uptake of different N forms across succession in these boreal forests largely reflect edaphic variation in available soil N (composition), rather than any apparent physiological specialization to absorb particular forms of N. These finding are relevant to our understanding of how taiga ecosystems may respond to increases in temperature, fire frequency, N deposition, and other potential consequences of global change.     

Cycling Dynamics of NH4 + and Amino Acid Nitrogen in Soils of a Deciduous Boreal Forest Ecosystem

Conventional studies of nitrogen (N) cycling in forest ecosystems have focused on inorganic N uptake as the primary source of N for plant metabolism. More recently, however, alternative sources of N for plant nutrition, such as free amino acids, have gained attention, particularly in nutrient-limited systems. Using a multiple stable isotope (¹³C and ¹⁵N) design, that allowed us to simultaneously assess root uptake of ammonium (NH₄ ⁺) and glycine, we compared the cycling dynamics of NH₄ ⁺ and amino acid N within the soils of several interior Alaskan floodplain balsam poplar stands. Our design included multiple sampling periods extending from 45 min to 14 days, which permitted us to study interpool transfers of our carbon (C) and N isotopes over time. Microbial biomass N was the largest sink of both ¹⁵N-ammonium and glycine. Percent recovery of ¹⁵N for this pool was an order of magnitude larger than fine-root ¹⁵N uptake for most sampling periods. Although recovery of ¹⁵N in fine-root biomass was small, amino acid N and NH₄ ⁺ were assimilated at approximately the same rate irrespective of sampling period, and total recovery was still increasing 2 weeks after application. Recovery of ¹⁵N in bulk soil samples did not vary significantly over time for either treatment. However, bulk soil ¹³C declined steadily during the experiment, measuring less than 30% recovery of added label after 14 days. We suspect that the majority of ¹³C lost from our soils was respired. Soil microorganisms strongly outcompeted plants in the short term for both NH₄ ⁺ and amino acid N. However, amino acid N appears to cycle through soil N pools at approximately the same rate as inorganic N forms. The similarity in uptake patterns for inorganic and organic N suggests that these stands are meeting part of their N requirements directly from amino acids.     

Nitrogen cycling in canopy soils of tropical montane forests responds rapidly to indirect N and P fertilization

Although the canopy can play an important role in forest nutrient cycles, canopy‐based processes are often overlooked in studies on nutrient deposition. In areas of nitrogen (N) and phosphorus (P) deposition, canopy soils may retain a significant proportion of atmospheric inputs, and also receive indirect enrichment through root uptake followed by throughfall or recycling of plant litter in the canopy. We measured net and gross rates of N cycling in canopy soils of tropical montane forests along an elevation gradient and assessed indirect effects of elevated nutrient inputs to the forest floor. Net N cycling rates were measured using the buried bag method. Gross N cycling rates were measured using ¹⁵N pool dilution techniques. Measurements took place in the field, in the wet and dry season, using intact cores of canopy soil from three elevations (1000, 2000 and 3000 m). The forest floor had been fertilized biannually with moderate amounts of N and P for 4 years; treatments included control, N, P, and N + P. In control plots, gross rates of NH₄⁺ transformations decreased with increasing elevation; gross rates of NO₃^? transformations did not exhibit a clear elevation trend, but were significantly affected by season. Nutrient‐addition effects were different at each elevation, but combined N + P generally increased N cycling rates at all elevations. Results showed that canopy soils could be a significant N source for epiphytes as well as contributing up to 23% of total (canopy + forest floor) mineral N production in our forests. In contrast to theories that canopy soils are decoupled from nutrient cycling in forest floor soil, N cycling in our canopy soils was sensitive to slight changes in forest floor nutrient availability. Long‐term atmospheric N and P deposition may lead to increased N cycling, but also increased mineral N losses from the canopy soil system.     

N uptake and N status in ponderosa pine as affected by soil compaction and forest floor removal

Soil compaction and forest floor removal influence fundamental soil processes that control forest productivity and sustainability. We investigated effects of soil compaction and forest floor removal on tree growth, N uptake and N status in ponderosa pine. Factorial combinations of soil compaction (non-compacted and compacted) and forest floor removal (forest floor present and no forest floor) were applied to three different surface soil textures. For studying N uptake, four trees from every treatment were ¹⁵N labeled with 130.6 mg m^–2 of ¹⁵N. Tree responses to compaction were dependent on the forest floor removal level. In loam and clay soils, non-compacted+no forest floor was beneficial to tree growth. Tree growth was depressed with compaction+no forest floor in clay soil. In sandy loam soil, compaction+no forest floor showed the best tree growth. No N deficiency was found in any soil type but a graphical method suggested correlation between N status and tree growth. In loam and clay soils, compaction+forest floor present increased N uptake. Nitrogen uptake was explained significantly by potential N mineralization in loam and clay soils. In sandy loam soil, the effects of compaction and forest floor removal were more complex, with the N uptake improved in the compaction+no forest floor treatment and reduced under compaction+forest floor present. Soil compaction may have influenced N tracer uptake because of improved unsaturated flow and root-soil contact. However, N immobilization may have restricted N uptake in compaction+forest floor present in the sandy loam soil. The study illustrates how soil properties and site preparation can potentially interact to affect N dynamics and forest productivity.     

In situ gross nitrogen transformations differ between temperate deciduous and coniferous forest soils

Despite long-term enhanced nitrogen (N) inputs, forests can retain considerable amounts of N. While rates of N inputs via throughfall and N leaching are increased in coniferous stands relative to deciduous stands at comparable sites, N leaching below coniferous stands is disproportionally enhanced relative to the N input. A better understanding of factors affecting N retention is needed to assess the impact of changing N deposition on N cycling and N loss of forests. Therefore, gross N transformation pathways were quantified in undisturbed well-drained sandy soils of adjacent equal-aged deciduous (pedunculate oak (Quercus robur L.)) and coniferous (Scots pine (Pinus sylvestris L.)) planted forest stands located in a region with high N deposition (north Belgium). In situ inorganic ¹⁵N labelling of the mineral topsoil (0–10?cm) combined with numerical data analysis demonstrated that (i) all gross N transformations differed significantly (p?<?0.05) between the two forest soils, (ii) gross N mineralization in the pine soil was less than half the rate in the oak soil, (iii) meaningful N immobilization was only observed for ammonium, (iv) nitrate production via oxidation of organic N occurred three times faster in the pine soil while ammonium oxidation was similar in both soils, and (v) dissimilatory nitrate reduction to ammonium was detected in both soils but was higher in the oak soil. We conclude that the higher gross nitrification (including oxidation of organic N) in the pine soil compared to the oak soil, combined with negligible nitrate immobilization, is in line with the observed higher nitrate leaching under the pine forest.     

Forest floor-mineral soil interactions in the internal nitrogen cycle of an old-growth forest

Seasonal patterns and annual rates of N inputs, outputs, and internal cycling were determined for an old-growth mixed-conifer forest floor in the Sierra Nevada Mountains of California. Rates of net N mineralization within the forest floor, and plant N-uptake and leaching of inorganic N from the forest floor were 13, 10, and 9 kg-N ha^-1 yr^-1, respectively. The Mediterranean-type climate appeared to have a significant effect on N cycling within this forest, such that all N-process and flow rates showed distrinct seasonal patterns. We estimated the forest floor supplies less than one-third of the total aboveground plant N-uptake in this forest. The rate of net nitrification within the forest floor was always low (1 kg-NO₃ ^--N ha^-1 30d^-1). Mean residence times for organic matter and N in the forest floor were 13 and 34 years, respectively, suggesting that this forest floor layer is a site of net N immobilization within this ecosystem. We examined the influence of the forest floor on mineral soil N dynamics by injecting small amounts of¹⁵N-enriched (NH₄)₂SO₄ solutions into the surface mineral soil with the forest floor present (+FF) or removed (-FF). K₂SO₄-extractable NO₃ ^--N, total inorganic-N, and total-N pool sizes in the mineral soil were initially increased after forest floor removal (after 4 months), but NO₃ ^--N and total inorganic-N were not significantly different thereafter. Microbial biomass-N and K₂SO₄-extractable total-N pool sizes were also found to be larger in mineral soils without a forest floor after 1 and 1.3 years, respectively. Total¹⁵N-recovery was greater in the +FF treatment compared to the -FF treatment after 1-year (about 50% and 35%, respectively) but did not differ after 1.3 years (both about 35%), suggesting that the forest floor delays but does not prevent the N-loss from the surface mineral soil of this forest. We estimated using our¹⁵N data that fungal translocation from the mineral soil to the forest floor may be as large as 9 kg-N ha^-1 yr^-1 (similar in magnitude to other N flows in this forest), and may account for all of the observed absolute increase of N in litter during the early stages of decomposition at this site. Our results suggest that the forest floor acts both as a source and sink for N in the mineral soil.     

The importance of autotrophic versus heterotrophic oxidation of atmospheric ammonium in forest ecosystems with acid soil

Abstract The role of autotrophic and heterotrophic nitrifying microorganisms in the oxidation of atmospheric ammonium in two acid and one calcareous location of a Dutch woodland area was investigated. In soil slurries nitrate formation was completely inhibited by acetylene, a specific inhibitor of autotrophic ammonium-oxidizing bacteria. A survey of nitrifiers in the forest soils showed that both autotrophic ammonium- and nitrite-oxidizing bacteria were present in high numbers and evidence was obtained that autotrophic bacteria are able to nitrify below pH 4. These results show that autotrophic nitrifying bacteria may account for most of the nitrification in the examined soils. To assess the contribution of heterotrophic nitrifiers, about 200 strains of heterotrophic bacteria and 21 morphologically distinct fungal strains were isolated from the acid soil locations and tested for their ability to nitrify. Only one Penicillium strain produced nitrate in test media, but its nitrate formation when added to acid soils was poor. These findings indicate that in the investigated soil heterotrophs are of minor importance in the oxidation of atmospheric ammonium.     

Nitrogen assimilation in citrus trees

Assimilation of ¹⁵N-ammonium and ¹⁵N-nitrate was examined in 3-year-old satsuma mandarin (Citrus unshiu Marcovitch) trees. Experiments were designed to establish the time course of incorporation of nitrogen just taken up into amino compounds. In fine roots, absorbed ¹⁵N-ammonium was actively incorporated into glutamine and then into glutamic acid and asparagine. When feeding ¹⁵N-nitrate, glutamic acid and asparagine were actively synthesized, but glutamine synthesis was comparatively low as compared with that in ammonium feeding. In current leaves and fruits, a clear difference in the labelling patterns of amino acids was found between the ammonium and nitrate feedings. The amino acid most markedly labelled was asparagine in the ammonium feeding and glutamine in the nitrate feeding. Considering the most heavily labelled component in leaves and fruits, the main form of the nitrogen components transported upward in the xylem was discussed.     

Nitrogen uptake strategies of edaphically specialized Bornean tree species

The association of tree species with particular soil types contributes to high β diversity in forests, but the mechanisms producing such distributions are still debated. Soil nitrogen (N) often limits growth and occurs in differentially available chemical forms. In a Bornean forest where tree species composition changes dramatically along a soil gradient varying in supplies of different N-forms, we investigated whether tree species’ N-uptake and soil specialization strategies covaried. We analyzed foliar ¹⁵N natural abundance for a total of 216 tree species on clay or sandy loam (the soils at the gradient’s extremes) and conducted a ¹⁵N-tracer experiment with nine specialist and generalist species to test whether species displayed flexible or differential uptake of ammonium and nitrate. Despite variation in ammonium and nitrate supplies and nearly 4 ‰ difference in foliar δ¹⁵N between most soil specialists and populations of generalists on these soils, our ¹⁵N tracer experiment showed little support for the hypothesis that soil specialists vary in N-form use or the ratios in which they use these forms. Instead, our results indicate that these species possess flexible capacities to take up different inorganic N forms. Variation between soil specialists in uptake of different N forms is thus unlikely to cause the soil associations of tree species and high β diversity characteristic of this Bornean rain forest. Flexible uptake strategies would facilitate N-acquisition when supply rates of N-forms exhibit spatiotemporal variation and suggest that these species may be functionally redundant in their responses to N gradients and influences on ecosystem N-cycles.