Vernal nitrogen and phosphorus retention by forest understory vegetation and soil microbes

Northern hardwood forests experience annual maximal loss of nutrients during spring. The vernal dam hypothesis predicts that spring ephemeral herbs in northern hardwood forests serve as sinks for nutrients during this season and reduce the loss of nutrients from the terrestrial ecosystem. Soil microbes of northern hardwood forests also sequester nutrients during spring. We compared the vernal nutrient acquisition ability of a soil microbial community and an understory plant community with species of mixed leaf phenology. We monitored nitrogen and phosphorus pool sizes in understory vegetation and soil microbes during spring from 1999 through 2001 in a northern hardwood forest in the Catskill Mountains, New York. Vegetation nutrient content increased during two spring seasons by an average of 3.07 g N m^–2 and 0.19 g P m^–2 and decreased during one spring by 0.81 g N m^–2 and 0.10 g P m^–2. Evergreen, wintergreen, and deciduous plant species were able to sequester nutrients during spring. Soil microbial nutrient content decreased during one spring by 1.29 g N m^–2 and remained constant during the other two springs. Streamwater nitrogen losses were not correlated with biotic nutrient uptake suggesting a temporal disconnect between the two processes. We conclude that understory vegetation is a larger potential sink for vernal nutrients than are soil microbes in this northern hardwood forest and understory and species representing multiple phenologies are capable of vernal nutrient uptake.     

Ungulate stimulation of nitrogen cycling and retention in Yellowstone Park grasslands

We studied how ungulates and a large variation in site conditions influenced grassland nitrogen (N) dynamics in Yellowstone National Park. In contrast to most grassland N studies that have examined one or two soil N processes, we investigated four rates, net N mineralization, nitrification, denitrification, and inorganic N leaching, at seven paired sites inside and outside long-term (33+ year) exclosures. Our focus was how N fluxes were related to one another among highly variable grasslands and how grazers influenced those relationships. In addition, we examined variation in soil δ¹⁵N among grasslands and the relationships between soil ¹⁵N abundance and N processes. Previously, ungulates were reported to facilitate net N mineralization across variable Yellowstone grasslands and denitrification at mesic sites. In this study, we found that herbivores also promoted nitrification among diverse grasslands. Furthermore, net N mineralization, nitrification, and denitrification (kg N ha^–1 year^–1, each variable) were postively and linearly related to one another among all grasslands (grazed and fenced), and grazers reduced the nitrification/net N mineralization and denitrification/net N mineralization ratios, indicating that ungulates inhibited the proportion of available NH₄ ⁺ that was nitrified and denitrified. There was no relationship between net N mineralization or nitrification with leaching (indexed by inorganic N adsorbed to resin buried at the bottom of rooting zones) and leaching was unaffected by grazers. Soil δ¹⁵N was positively and linearly related to in situ net N mineralization and nitrification in ungrazed grasslands; however, there was no relationship between isotopic composition of N and those rates among grazed grasslands. The results suggested that grazers simultaneously increased N availability (stimulated net N mineralization and nitrification per unit area) and N conservation (reduced N loss from the soil per unit net N mineralization) in Yellowstone grasslands. Grazers promoted N retention by stimulating microbial productivity, probably caused by herbivores promoting labile soil C. Process-level evidence for N retention by grazers was supported by soil δ¹⁵N data. Grazed grassland with high rates of N cycling had substantially lower soil δ¹⁵N relative to values expected for ungrazed grassland with comparable net N mineralization and nitrification rates. These soil ¹⁵N results suggest that ungulates inhibited N loss at those sites. Such documented evidence for consumer control of N availability to plants, microbial productivity, and N retention in Yellowstone Park is further testimony for the widespread regulation of grassland processes by large herbivores. Received: 5 May 1999 / Accepted: 1 November 1999     

Net nitrogen mineralization and net nitrification rates in soils following deforestation for pasture across the southwestern Brazilian Amazon Basin landscape

Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the original forest. In this study, we sought to determine the generality of these patterns by examining soil inorganic N concentrations, net mineralization and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of soil textures and spanned a 700-km geographical range in the southwestern Brazilian Amazon Basin state of Rondônia. We sampled each site during October-November and April-May. Forest soils had higher extractable NO₃ ^?-N and total inorganic N concentrations than pasture soils, but substantial NO₃ ^?-N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils. Greater concentrations of soil organic matter in finer textured soils were associated with greater rates of net N mineralization and net nitrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil type or texture. Net N mineralization and net nitrification rates per unit of total soil organic matter showed no pattern across the different forest sites, suggesting that controls of net N mineralization may be broadly similar across a wide range of soil types. Similar reductions in rates of net N transformations in pastures 3 years old or older across a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in extent. Lower net N mineralization and net nitrification rates in established pastures suggest that annual N losses from largely deforested landscapes may be lower than losses from the original forest. Total ecosystem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately following forest clearing.     

The effects of gap disturbance on nitrogen cycling and retention in late-successional northern hardwood–hemlock forests

Late-successional forests in the upper Great Lakes region are susceptible to nitrogen (N) saturation and subsequent nitrate (NO₃⁻) leaching loss. Endemic wind disturbances (i.e., treefall gaps) alter tree uptake and soil N dynamics; and, gaps are particular susceptible to NO₃⁻ leaching loss. Inorganic N was measured throughout two snow-free periods in throughfall, forest floor leachates, and mineral soil leachates in gaps (300–2,000 m², 6–9 years old), gap-edges, and closed forest plots in late-successional northern hardwood, hemlock, and northern hardwood–hemlock stands. Differences in forest water inorganic N among gaps, edges, and closed forest plots were consistent across these cover types: NO₃⁻ inputs in throughfall were significantly greater in undisturbed forest plots compared with gaps and edges; forest floor leachate NO₃⁻ was significantly greater in gaps compared to edges and closed forest plots; and soil leachate NO₃⁻ was significantly greater in gaps compared to the closed forest. Significant differences in forest water ammonium and pH were not detected. Compared to suspected N-saturated forests with high soil NO₃⁻ leaching, undisturbed forest plots in these late-successional forests are not losing NO₃⁻ (net annual gain of 2.8 kg ha⁻¹) and are likely not N-saturated. Net annual NO₃⁻ losses were observed in gaps (1.3 kg ha⁻¹) and gap-edges (0.2 kg ha⁻¹), but we suspect these N leaching losses are a result of decreased plant uptake and increased soil N mineralization associated with disturbance, and not N-saturation.     

Combined effects of atmospheric CO<Subscript>2</Subscript> and N availability on the belowground carbon and nitrogen dynamics of aspen mesocosms

It is uncertain whether elevated atmospheric CO₂ will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO₂ may alter microbial activities that regulate soil N availability by changing the amount or composition of organic substrates produced by roots. Our objective was to determine the potential for elevated CO₂ to change N availability in an experimental plant-soil system by affecting the acquisition of root-derived C by soil microbes. We grew Populus tremuloides (trembling aspen) cuttings for 2 years under two levels of atmospheric CO₂ (36.7 and 71.5 Pa) and at two levels of soil N (210 and 970 μg N g^–1). Ambient and twice-ambient CO₂ concentrations were applied using open-top chambers, and soil N availability was manipulated by mixing soils differing in organic N content. From June to October of the second growing season, we measured midday rates of soil respiration. In August, we pulse-labeled plants with ¹⁴CO₂ and measured soil ¹⁴CO₂ respiration and the ¹⁴C contents of plants, soils, and microorganisms after a 6-day chase period. In conjunction with the August radio-labeling and again in October, we used ¹⁵N pool dilution techniques to measure in situ rates of gross N mineralization, N immobilization by microbes, and plant N uptake. At both levels of soil N availability, elevated CO₂ significantly increased whole-plant and root biomass, and marginally increased whole-plant N capital. Significant increases in soil respiration were closely linked to increases in root biomass under elevated CO₂. CO₂ enrichment had no significant effect on the allometric distribution of biomass or ¹⁴C among plant components, total ¹⁴C allocation belowground, or cumulative (6-day) ¹⁴CO₂ soil respiration. Elevated CO₂ significantly increased microbial ¹⁴C contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbial ¹⁴C contents at elevated CO₂ was a relatively small quantitative change in the belowground C cycle of our experimental system, but represents an ecologically significant effect on the dynamics of microbial growth. Rates of plant N uptake during both 6-day periods in August and October were significantly greater at elevated CO₂, and were closely related to fine-root biomass. Gross N mineralization was not affected by elevated CO₂. Despite significantly greater rates of N immobilization under elevated CO₂, standing pools of microbial N were not affected by elevated CO₂, suggesting that N was cycling through microbes more rapidly. Our results contained elements of both positive and negative feedback hypotheses, and may be most relevant to young, aggrading ecosystems, where soil resources are not yet fully exploited by plant roots. If the turnover of microbial N increases, higher rates of N immobilization may not decrease N availability to plants under elevated CO₂. Received: 12 February 1999 / Accepted: 2 March 2000     

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. Responsible Editor: Bernard Nicolardet     

Relationships between soil microbial properties and aboveground stand characteristics of conifer forests in Oregon

Eight forest sites representing a large range of climate, vegetation, and productivity were sampled in a transect across Oregon to study the relationships between aboveground stand characteristics and soil microbial properties. These sites had a range in leaf area index of 0.6 to 16 m² m^–2 and net primary productivity of 0.3 to 14 Mg ha^–1 yr^–1.Measurements of soil and forest floor inorganic N concentrations and in situ net N mineralization, nitrification, denitrification, and soil respiration were made monthly for one year. Microbial biomass C and anaerobic N mineralization, an index of N availability, were also measured. Annual mean concentrations of NH ₄ ⁺ ranged from 37 to 96 mg N kg^–1 in the forest floor and from 1.7 to 10.7 mg N kg^–1 in the mineral soil. Concentrations of NO ₃ ^– were low ( < 1 mg N kg^–1) at all sites. Net N mineralization and nitrification, as measured by the buried bag technique, were low on most sites and denitrification was not detected at any site. Available N varied from 17 to 101 mg N kg^–1, microbial biomass C ranged from 190 to 1230 mg Ckg^–1, and soil respiration rates varied from 1.3 to 49 mg C kg^–1 day^–1 across these sites. Seasonal peaks in NH ₄ ⁺ concentrations and soil respiration rates were usually observed in the spring and fall.The soils data were positively correlated with several aboveground variables, including leaf area index and net primary productivity, and the near infrared-to-red reflectance ratio obtained from the airborne simulator of the Thematic Mapper satellite. The data suggest that close relationships between aboveground productivity and soil microbial processes exist in forests approaching semi-equilibrium conditions.Abbreviations IR infrared - LAI leaf area index - k _c proportion of microbial biomass C mineralized to CO₂ - NPP net primary productivity - TM Thematic Mapper     

Spatial Heterogeneity and Soil Nitrogen Dynamics in a Burned Black Spruce Forest Stand: Distinct Controls at Different Scales

We evaluated spatial patterns of soil N and C mineralization, microbial community composition (phospholipid fatty acids), and local site characteristics (plant/forest floor cover, soil pH, soil %C and %N) in a 0.25-ha burned black spruce forest stand in interior Alaska. Results indicated that factors governing soil N and C mineralization varied at two different scales. In situ net N mineralization was autocorrelated with microbial community composition at relatively broad scales (∼ ∼8 m) and with local site characteristics (`site' axis 1 of non-metric scaling ordination) at relatively fine scales (2–4 m). At the scale of the individual core, soil moisture was the best predictor of in situ net N mineralization and laboratory C mineralization, explaining between 47 and 67% of the variation (p < 0.001). Ordination of microbial lipid data showed that bacteria were more common in severely burned microsites, whereas fungi were more common in low fire severity microsites. We conclude that C and N mineralization rates in this burned black spruce stand were related to different variables depending on the scale of analysis, suggesting the importance of considering multiple scales of variability among key drivers of C and N transformations.     

Decline of soil nitrogen mineralization and nitrification during forest conversion of evergreen broad-leaved forest to plantations in the subtropical area of Eastern China

We examined soil nitrogen (N) mineralization and nitrification rates, and soil and forest floor properties in one native forest: evergreen broad-leaved forest (EBLF), one secondary shrubs (SS), and three adjacent plantation forests: Chinese fir plantation (CFP), bamboo plantation (BP) and waxberry groves (WG) in Tiantong National Forest Park, Eastern China. All forests showed seasonal dynamics of N mineralization and nitrification rates. Soil N mineralization rate was highest in EBLF (1.6 ± 0.3 mg-N kg⁻¹ yr⁻¹) and lowest in CFP (0.4 ± 0.1 mg-N kg⁻¹ yr⁻¹). Soil nitrification rate was also highest in EBLF (0.6 ± 0.1 mg-N kg⁻¹ yr⁻¹), but lowest in SS (0.02 ± 0.01 mg-N kg⁻¹ yr⁻¹). During forest conversion of EBLF to SS, CFP, BP and WG, soil N mineralization rate (10.7%, 73%, 40.3% and 69.8%, respectively), soil nitrification rate (94.9%, 32.2%, 33.9% and 39%, respectively), and soil N concentration (50%, 65.4%, 78.9% and 51.9%, respectively) declined significantly. Annual soil N mineralization was positively correlated with total C and N concentrations of surface soil and total N concentration of forest floor, and negatively correlated with soil bulk density, soil pH and C:N ratio of forest floor across the five forests. Annual soil nitrification was positively correlated with total C concentration of surface soil and N concentration of forest floor, and negatively correlated with soil bulk density and forest floor mass. In contrast, annual soil nitrification was not correlated to pH value, total N concentration, C:N ratio of surface soil and total C concentration and C:N ratio of forest floor.     

Microbial Cycling of C and N in Northern Hardwood Forests Receiving Chronic Atmospheric NO 3 − Deposition

Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic atmospheric NO₃⁻ deposition. Experimental NO₃⁻ deposition (3 g NO₃⁻ N m⁻² y⁻¹) has significantly reduced soil respiration and increased the export of DOC/DON and NO₃⁻ across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for these ecosystem-level responses to NO₃⁻ deposition. To test this alternative, we measured microbial biomass, respiration, and N transformations in the mineral soil of four northern hardwood stands that have received 9 years of experimental NO₃⁻ deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed by NO₃⁻ deposition. We also observed no effect of NO₃⁻ deposition on annual rates of net N mineralization. However, NO₃⁻ deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO₃⁻ assimilation, the subsequent turnover of NH₄⁺, and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient to produce the 10-fold observed increase in NO₃⁻ export, suggesting that much of the exported NO₃⁻ resulted directly from the NO₃⁻ deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed to NO₃⁻-induced physiological changes in mineral soil microbial activity. Given the lack of response we have observed in mineral soil, our results point to the potential importance of microbial communities in forest floor, including both saprotrophs and mycorrhizae, in mediating ecosystem-level responses to chronic NO₃⁻ deposition in Lake States northern hardwood forests.     

Differential effects of sugar maple, red oak, and hemlock tannins on carbon and nitrogen cycling in temperate forest soils

Tannins are abundant secondary chemicals in leaf litter that are hypothesized to slow the rate of soil-N cycling by binding protein into recalcitrant polyphenol–protein complexes (PPCs). We studied the effects of tannins purified from sugar maple, red oak, and eastern hemlock leaf litter on microbial activity and N cycling in soils from northern hardwood–conifer forests of the northeastern US. To create ecologically relevant conditions, we applied tannins to soil at a concentration (up to 2 mg g⁻¹ soil) typical of mineral soil horizons. Sugar maple tannins increased microbial respiration significantly more than red oak or hemlock tannins. The addition of sugar maple tannins also decreased gross N mineralization by 130% and, depending upon the rate of application, decreased net rates of N mineralization by 50–290%. At low concentrations, the decrease in mineralization appeared to be driven by greater microbial-N immobilization, while at higher concentrations the decrease in mineralization was consistent with the formation of recalcitrant PPCs. Low concentrations of red oak and hemlock tannins stimulated microbial respiration only slightly, and did not significantly affect fluxes of inorganic N in the soil. When applied to soils containing elevated levels of protein, red oak and hemlock tannins decreased N mineralization without affecting rates of microbial respiration, suggesting that PPC formation decreased substrate availability for microbial immobilization. Our results indicate that tannins from all three species form recalcitrant PPCs, but that the degree of PPC formation and its attendant effect on soil-N cycling depends on tannin concentration and the pool size of available protein in the soil.     

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.     

The effect of soil freezing on N cycling: comparison of two headwater subcatchments with different vegetation and snowpack conditions in the northern Hokkaido Island of Japan

Climate change models predict that the snowpacks of temperate forests will develop later and be shallower resulting in a higher propensity for soil freezing. In the northern most island of Japan, Hokkaido, snowpack depth decreases from west to east. This snowpack depth gradient provided a unique opportunity to test the effects of variable snowpack and soil freezing on N biogeochemistry. The Shibecha Northern Catchment in Shibecha Experimental Forest, eastern Hokkaido had deciduous trees and a mean annual snowpack of 0.7 m while the M3 catchment in Uryu Experimental Forest, western Hokkaido had mixed deciduous and coniferous tree species and a mean annual snowpack of 2.0 m. We conducted a field study (October 2004–April 2005) to determine if differences in Shibecha and Uryu soil extractable N, N mineralization, and nitrification were controlled by the variability in soil freezing during winter or tree species composition that affected the quality of the forest floor. The mixed deciduous and coniferous trees forming the Uryu forest floor had a higher C:N ratio (25.0 vs. 22.4 at Shibecha), higher lignin:N ratio (15 vs. 8.8), and higher lignin concentrations (0.28 vs. 0.18 g lignin g⁻¹). These differences in forest floor quality contributed to higher net N mineralization and nitrification in Shibecha compared to Uryu. In Shibecha, soil remained frozen for the entire study. For Uryu, except for an early period with cold temperatures and no snow, the soil generally remained unfrozen. As a result of the early winter cold period and soil freezing, extractable soil NH₄⁺ did not change but NO₃⁻ increased. Reciprocal 0–5 cm mineral soil transplants made between Shibecha and Uryu and incubated during winter at 0, 5, and 30 cm suggested that soil freezing resulted in greater net N mineralization yet lower nitrification regardless of the soil origin. The effect of soil freezing should be considered when evaluating differences in N dynamics between temperate ecosystems having a propensity for soil freezing.     

Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana)

Canopy CO₂ concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO₂]) throughout the canopy (0.02–38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO₂] in the upper and middle canopy decreased on average 7–10 μmol mol⁻¹ below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO₂] did not differ between the wet and dry seasons. In contrast, [CO₂] below 0.7 m were generally higher during the dry season, resulting in larger [CO₂] gradients. Supporting this observation, soil CO₂ efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO₂] immediately above the forest floor of about 50␣μmol mol⁻¹. Temporal and spatial variations in [CO₂]_canopy were reflected in changes of δ¹³C_canopy and δ¹⁸O_canopy values. Tight relationships were observed between δ¹³C and δ¹⁸O of canopy CO₂ during both seasons (r ² > 0.86). The most depleted δ¹³C_canopy and δ¹⁸O_canopy values were measured immediately above the forest floor (δ¹³C = −16.4‰; δ¹⁸O = 39.1‰ SMOW). Gradients in the isotope ratios of CO₂ between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δ¹³C, and between 1.0‰ and 3.5‰ for δ¹⁸O. The δ¹³C_leaf and calculated c _i/c _a of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δ¹³C_leaf within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c _i/c _a. Plotting 1/[CO₂] vs. the corresponding δ¹³C ratios resulted in very tight, linear relationships (r ² = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δ¹³C of respired sources were close to the measured δ¹³C of soil respired CO₂ and to the δ¹³C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δ_e, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season. Received: 3 March 1996 / Accepted: 19 October 1996     

Nitrogen mineralization and H+ transfers in a Scots pine (Pinus sylvestris L.) forest soil as affected by liming

H⁺ production due to N uptake in a mature Scots pine stand subjected to high NH₄ ⁺ deposition was previously estimated to amount to approx. 2.2 kmol ha^-1 y^-1. The question whether H⁺ transfers related to N mineralization (ammonification and nitrification) offset or corroborate this proton production is investigated in the present research. To determine N mineralization, soil cores were used of which both ends were closed with layers of ion exchange resin (IER) to prevent influx and efflux of ions. The effect of liming on N mineralization and the resulting H⁺ production was investigated in 7 incubation periods of each ca. 8 wk. Because of its high mobility NO₃ accumulated in both IER layers at the expense of that in the incubated forest floor and mineral soil. Net N mineralization in the soil cores as a whole amounted to 40 and 77 kg N ha^-1 in 384 d in the control and limed plots, respectively. In both treatments ca. 65% of mineralized N was nitrified. H⁺ production due to N mineralization amounted to approx. 1.2 kmol ha^-1 y^-1 in the control and limed plots. Liming reduced the amount of C in the forest floor, but not forest floor mass, because of an increased mixing with mineral particles.     

Differences of Soil Microbial Biomass and Nitrogen Transformation under Two Forest Types in Central Germany

In order to observe the tree species effect on soil N status, soil microbial biomass C and N (C_mic, N_mic), potential N mineralization and potential nitrification (under laboratory incubation conditions, 22 °C) in different subhorizons (LOf₁, Of₂, Oh and mineral soil at 0–10 cm depth) were determined at three forest sites in central Germany. At each site, two contrasting stands (Beech, Norway spruce or Scots pine) were selected, where the initial soil conditions were similar. Three sampling dates that represented different stages of tree growth were selected: growing season - August, dormant season - November, after budbreak – April. In organic layers, C_mic-to-total C (C_t) ratios under beech and under conifer were 0.72–4.74% and 0.34–2.11%, respectively. N_mic-to-total N (N_t) ratios were 2.47–11.61% and 0.71–5.77%, respectively. Both concentrations of C_mic and N_mic were significantly affected by the stand type and sampling time. Potential N mineralization rates, ranging from 3.7 to 19.7 mg N kg⁻¹ d⁻¹, showed no clear pattern in relation to stand type. However, potential nitrification rates were mostly significantly higher under beech than under contrasting conifer. In mineral soils, concentrations of C_mic and N_mic showed a clear temporal pattern in the order: August>November>April. The average N_mic and N_mic-to-N_t were higher in soils from beech than conifer, while C_mic and C_mic-to-C_t ratios were similar between the two forest types. In organic layers, the highest values of C_mic-to-N_mic ratio and C_mic were found in November samples, especially under beech. By contrast, in mineral soils the highest value of C_mic-to-N_mic ratios were found in April samples, and at that time the C_mic concentrations were the lowest, especially under conifer. These results revealed the differences in microbial growth form and survival strategy associated with different tree species and soil layers.     

Seasonal variation in nitrogen uptake and turnover in two high-elevation soils: mineralization responses are site-dependent

In arctic and alpine ecosystems, soil nitrogen (N) dynamics can differ markedly between winter and summer months, and nitrogen losses can be measurable during the spring and fall transitions. To explore the effect of seasonality on biogeochemical processes in a temperate alpine environment, we used a combination of field incubations (year-round) and ¹⁵N tracer additions (late fall, early spring, summer) to characterize soil N dynamics in a wet and dry meadow in the Sierra Nevada, California. The snowmelt to early summer season marked a period of high ¹⁵N uptake and turnover in the two soils, coincident with the increase in microbial N pools at the start of snowmelt (wet and dry meadow); an increase in net N mineralization and net nitrification as snowmelt progressed (wet meadow only); and measureable net production of ¹⁵N-NH₄ ⁺ in mid-summer (wet and dry meadow). Whereas fluctuations in microbial biomass were generally synchronous between the wet and dry meadow soils, only wet meadow soils appeared to mineralize N in response to declines in the microbial N pool. Net N mineralization and net nitrification rates in the dry meadow soil were negligible on all but one sampling date, in spite of periodic decreases in biomass of up to 60%. Across both sites, high ¹⁵N recoveries in microbial biomass N, rapid ¹⁵N-NH₄ ⁺ turnover, and low or negative net ¹⁵N-NH₄ ⁺ fluxes suggested tight cycling of N, particularly in the late fall and early spring.     

Transferring soils from high- to low-elevation forests increases nitrogen cycling rates: climate change implications

We assessed the potential impact of global warming resulting from a doubling of preindustrial atmospheric CO₂ on soil net N transformations by transferring intact soil cores (0–15 cm) from a high-elevation old-growth forest to a forest about 800 m lower in elevation in the central Oregon Cascade Mountains, USA. The lower elevation site had mean annual air and soil (10-cm mineral soil depth) temperatures about 2.4 and 3.9 °C higher than the high-elevation site, respectively. Annual rates of soil net N mineralization and nitrification more than doubled in soil transferred to the low-elevation site (17.2–36.0 kg N ha^–1 and 5.0–10.7 kg NO₃^––N ha^–1, respectively). Leaching of inorganic N from the surface soil (in the absence of plant uptake) also increased. The reciprocal treatment (transferring soil cores from the low- to the high-elevation site) resulted in decreases of about 70, 80, and 65% in annual rates of net N mineralization, nitrification, and inorganic N leaching, respectively. Laboratory incubations of soils under conditions of similar temperature and soil water potential suggest that the quality of soil organic matter is higher at the high-elevation site. Similar in situ rates of soil net N transformations between the two sites occurred because the lower temperature counteracts the effects of greater substrate quantity and quality at the high elevation site. Our results support the hypothesis that high-elevation, old-growth forest soils in the central Cascades have higher C and N storage than their low-elevation analogues primarily because low temperatures limit net C and N mineralization rates at higher elevations.     

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ¹⁵N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ¹⁵N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ¹⁵N varied widely, between 0 and –2‰ for nitrogen-fixing species, between 0 and –7‰ for deciduous non-fixing species, and between 0 and –11‰ for coniferous species. Relatively constant δ¹⁵N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ¹⁵N differences among species at individual sites. In fact, the largest variation among plant δ¹⁵N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ¹⁵N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ¹⁵N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T _r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ¹⁵N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ¹⁵N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T _r. Received: 14 December 1998 / Accepted: 16 August 1999     

Contributions to nitrogen leaching and pasture uptake by autumn-applied dairy effluent and ammonium fertilizer labeled with 15N isotope

The objective of this study was to compare the N leaching loss and pasture N uptake from autumn-applied dairy shed effluent and ammonium fertilizer (NH₄Cl) labeled with ¹⁵N, using intact soil lysimeters (80 cm diameter, 120 cm depth). The soil used was a sandy loam, and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The DSE and NH₄Cl were applied twice annually in autumn (May) and late spring (November), each at 200 kg N ha^-1. The N applied in May 1996 was labeled with ¹⁵N. The lysimeters were either spray or flood irrigated during the summer. The autumn-applied DSE resulted in lower N leaching losses compared with NH₄Cl. However, the N applied in the autumn had a higher potential for leaching than N applied in late spring. Between 4.5–8.1% of the ¹⁵N-labeled mineral N in the DSE and 15.1–18.8% of the ¹⁵N-labeled NH₄Cl applied in the autumn were leached within a year of application. Of the annual N leaching losses in the DSE treatments (16.0–26.9 kg N ha^-1), a fifth (20.3–22.9%) was from the mineral N fraction of the DSE applied in the autumn, with the remaining larger proportion from the organic fraction of the DSE, soil N and N applied in spring. In the NH₄Cl treatments, more than half (53.8–64.8%) of the annual N leaching loss (55.9–57.6 kg N ha^-1) was derived from the autumn-applied NH₄Cl. DSE was as effective as NH₄Cl in stimulating pasture production. Since only 4.4–4.5% of the annual herbage N uptake in the DSE treatment and 12.3–13.3% in the NH₄Cl treatment were derived from the autumn-applied mineral N, large proportions of the annual herbage N uptake must have been derived from the N applied in spring, the organic N fraction in the DSE, soil N and N fixed by clover. The recoveries of ¹⁵N in the herbage were similar between the DSE and the NH₄Cl treatments, but those in the leachate were over 50% less from the DSE than from the NH₄Cl treatment. The lower leaching loss of ¹⁵N in the DSE treatment was attributed to the stimulated microbial activities and increased immobilization following the application of DSE. This revised version was published online in June 2006 with corrections to the Cover Date.