Mineralization of nitrogen in long-term pasture soils: effects of management

A field incubation technique with acetylene to inhibit nitrification was used to estimate net N mineralization rates in some grassland soils through an annual cycle. Measurements were made on previously long-term grazed pastures on a silty clay loam soil in S.W. England which had background managements of +/– drainage and +/– fertilizer (200 kg N ha^–1 yr^–1). The effect of fertilizer addition on mineralization during the year of measurement was also determined. Small plots with animals excluded, and with herbage clipped and removed were used as treatment areas and measurements were made using an incubation period of 7 days at intervals of 7 or 14 days through the year. Soil temperature, moisture and mineral N contents were also determined. Mineralization rates fluctuated considerably in each treatment. Maximum daily rates ranged from 1.01 to 3.19 kg N ha^–1, and there was substantial net release of N through the winter period (representing, on average, 27% of the annual release). Changes in temperature accounted for 35% of the variability but there was little significant effect of soil moisture. Annual net release of N ranged from 135 kg ha^–1 (undrained soil, no previous or current fertilizer) to 376 (drained soil, +200 kg N ha^–1 yr^–1 previous and current fertilizer addition). Addition of fertilizer N to a previously unfertilized sward significantly increased the net release of N but there was no immediate effect of withholding fertilizer on mineralization during the year in which measurements were made.     

Nitrogen cycling in an acid forest ecosystem in the Netherlands under increased atmospheric nitrogen input

Within a long-term research project studying the biogeochemical budget of an oak-beech forest ecosystem in the eastern part of the Netherlands, the nitrogen transformations and solute fluxes were determined in order to trace the fate of atmospherically deposited NH₄ ⁺ and to determine the contribution of nitrogen transformations to soil acidification.The oak-beech forest studied received an annual input of nitrogen via throughfall and stemflow of 45 kg N ha^–1 yr^–1, mainly as NH₄ ⁺, whereas 8 kg N ha^–1 yr^–1 was taken up by the canopy. Due to the specific hydrological regime resulting in periodically occurring high groundwater levels, denitrification was found to be the dominant output flux (35 kg N ha^–1 yr^–1). N₂0 emmission rate measurements indicated that 57% of this gaseous nitrogen loss (20 kg N ha^–1 yr^–1) was as N₂O. The forest lost an annual amount of 11 kg N ha^–1 yr^–1 via streamwater output, mainly as N0₃ ^–.Despite the acid conditions, high nitrification rates were measured. Nitrification occurred mainly in the litter layer and in the organic rich part of the mineral soil and was found to be closely correlated with soil temperature. The large amount of NH₄ ⁺ deposited on the forest floor via atmospheric deposition and produced by mineralization was to a large extent nitrified in the litter layer. Almost no NH₄ ⁺ reached the subsurface soil horizons. The N0₃ ^– was retained, taken up or transformed mainly in the mineral soil. A small amount of N0₃ ^– (9 kg N ha^–1 yr^–1) was removed from the system in streamwater output. A relatively small amount of nitrogen was measured in the soil water as Dissolved Organic Nitrogen.On the basis of these data the proton budget of the system was calculated using two different approaches. In both cases net proton production rates were high in the vegetation and in the litter layer of the forest ecosystem. Nitrogen transformations induced a net proton production rate of 2.4 kmol ha^–1 yr^–1 in the soil compartment.     

Soil organic matter as a potential net nitrogen sink in a fertilized cornfield,South Deerfield,Massachusetts, USA

Net nitrogen (N) mineralization in situ and N mineralization potential (N₀) over one complete year (1986–1987) were examined for a conventionally managed silage cornfield that received at least 235 kg fertilizer N ha^-1. Net N mineralization at the site, measured by sequential in situ polyethylene-bag incubations, totaled –54 kg N ha^-1 yr^-1, and –31 kg N ha^-1 over the May-to-August growing season. Nitrogen mineralization potential of the soil organic matter (SOM), measured by laboratory anaerobic incubations, was positive uniformly and varied with month of sample collection. The soil gained 72 kg inorganic N ha^-1 from April to October, principally because of a fall manuring, only 7 kg N ha^-1 from April to September. The in situ incubations, likely more representative of the balance between N mineralization and immobilization under N-fertilized conditions, suggest that SOM at the site is accumulating N.Contribution from the Department of Forestry and Wildlife Management, University of Massachusetts, Amherst, MA 01003, USA.Contribution from the Department of Forestry and Wildlife Management, University of Massachusetts, Amherst, MA 01003, USA.     

Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem

Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO₃ ^–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH₄ ⁺) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha^–1yr^–1, significant in comparison to wet Ndeposition (530 mol ha^–1 yr^–1) andstream NO₃ ^– export (25 mol ha^–1yr^–1) in this northern forest ecosystem. Soil solution fluxes of P_i from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha^–1 yr^–1;this elevated mobilization of P_i coincidedwith heightened NO₃ ^– leaching. Elevated leaching of P_i from the forestfloor was coupled with enhanced retention ofP_i in the mineral soil Bs horizon. Thequantities of P_i mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha^–1) as well asnet P mineralization rates in the forest floor(180 mol ha^–1 yr^–1). Increased fineroot mortality was likely an important sourceof mobile N and P_i from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication.     

Soil N mineralization and nitrification in relation to nitrogen solution chemistry in a small forested watershed

Spatial variations in soil processes regulating mineral N losses to streams were studied in a small watershed near Toronto, Ontario. Annual net N mineralization in the 0–8 cm soil was measured in adjacent upland and riparian forest stands using in situ soil incubations from April 1985 to 1987. Mean annual rates of soil N mineralization and nitrification were higher in a maple soil (93.8 and 87.0 kg.ha^–1) than in a pine soil (23.3 and 8.2 kg.ha^–1 ). Very low mean rates of mineralization (3.3 kg.ha^–1) and nitrification (3.4 kg.ha^–1) were found in a riparian hemlock stand. Average NO₃-N concentrations in soil solutions were 0.3–1.0 mg.L^–1 in the maple stand and >0.06mg.L^–1 in the pine stand. Concentrations of NO₃–N in shallow ground water and stream water were 3–4× greater in a maple subwatershed than in a pine subwatershed. Rapid N uptake by vegetation was an important mechanism reducing solution losses of NO₃–N in the maple stand. Low rates of nitrification were mainly responsible for negligible NO₃–N solution losses in the pine stand.     

Source of the soybean N credit in maize production

Nitrogen response trials throughout the United States Corn Belt show that economic optimum rates of N fertilization are usually less for maize (Zea mays L.) following soybean (Glycine max L.) than for maize following maize; however, the cause of this rotation effect is not fully understood. The objective of this study was to investigate the source of the apparent N contribution from soybean to maize (soybean N credit) by comparing soil N mineralization rates in field plots of unfertilized maize that had either nodulated soybean, non-nodulated soybean, or maize as the previous crop. Crop yields, plant N accumulation, soil inorganic N, and net soil mineralization were measured. Both grain yield (6.3 vs. 2.8 Mg ha^–1) and above-ground N accumulation (97 vs. 71 kg ha^–1) were greatly increased when maize followed nodulated soybean compared with maize following maize. A partial benefit to yield and N accumulation was also observed for maize following non-nodulated soybean. Cumulative net soil N mineralization following nodulated soybean, non-nodulated soybean, and maize was 112, 92 and 79 kg N ha^–1, respectively. Net mineralization of soil N appeared to be influenced by both quality (C:N ratio) and quantity of residue from the previous crop. In addition to an increase in plant available N from mineralization, the amount of soil inorganic N (especially in soil 5 cm from the row) was greater following nodulated soybean than non-nodulated soybean or maize. Based on these data, the soybean N credit appears to result from a combination of a decrease in net soil mineralization in continuous maize production and an increase in residual soil N from symbiotic fixation.     

Nitrogen mineralization in heathland ecosystems dominated by different plant species

Net N mineralization rates were measured in heathlands still dominated by ericaceous dwarf shrubs (Calluna vulgaris or Erica tetralix) and in heathlands that have become dominated by grasses (Molinia caerulea or Deschampsia flexuosa). Net N mineralization was measuredin situ by sequential soil incubations during the year. In the wet area (gravimetric soil moisture content 74–130%), the net N mineralization rates were 4.4 g N m^–2 yr^–1 in the Erica soil and 7.8 g N m^–2 yr^–1 in the Molinia soil. The net nitrification rate was negligibly slow in either soil. In the dry area (gravimetric soil moisture content 7–38%), net N mineralization rates were 6.2 g N M-2 yr^–1 in the Calluna soil, 10.9 g N m^–2 yr^–1 in the Molinia soil and 12.6 g N m^–2 yr^–1 in the Deschampsia soil. The Calluna soil was consistently drier throughout the year, which may partly explain its slower mineralization rate. Net nitrification was 0.3 g N m^–2 yr^–1 in the Calluna soil, 3.6 g N m^–2 yr^–1 in the Molinia soil and 5.4 g N m^–2 yr^–1 in the Deschampsia soil. The net nitrification rate increased proportionally with the net N mineralization rate suggesting ammonium availability may control nitrification rates in these soils. In the dry area, the faster net N mineralization rates in sites dominated by grasses than in the site dominated by Calluna may be explained by the greater amounts of organic N in the soil of sites dominated by grasses. In both areas, however, the net amount of N mineralized per gram total soil N was greater in sites dominated by Molinia or Deschampsia than in sites dominated by Calluna or Erica. This suggests that in heathlands invaded by grasses the quality of the soil organic matter may be increased resulting in more rapid rates of soil N cycling.     

Nitrogen transformations in two nitrogen saturated forest ecosystems subjected to an experimental decrease in nitrogen deposition

Nitrogen transformations were studied in the forest floor and mineral soil (0–5 cm) of a Douglas fir forest (Pseudotsuga menziesii (Mirb.) Franco.) and a Scots pine forest (Pinus sylvestris L.) in the Netherlands. Curren nitrogen depositions (40 and 56 kg N ha^-1 yr^-1, respectively) were reduced to natural background levels (1–2 kg N ha^-1 yr^-1) by a roof construction. The study concentrated on rates and dynamic properties of nitrogen transformations and their link with the leaching pattern and nitrogen uptake of the vegetation under high and reduced nitrogen deposition levels. Results of an in situ field incubation experiment and laboratory incubations were compared. No effect of the reduced N deposition on nitrogen transformations was found in the Douglas fir forest. In the Scots pine forest, however, during some periods of the year nitrogen transformations were significantly decreased under the low nitrogen deposition level. At low nitrogen inputs a net immobilization occurred during most of the year leading to a very small net mineralization for the whole year. In laboratory and in individual field plots nitrogen transformations were negatively correlated with initial inorganic nitrogen concentrations. Nitrogen budget estimates showed that nitrogen transformations were probably underestimated by the in situ incubation technique. Nevertheless less nitrogen was available for plant uptake and leaching at the low deposition plots.     

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     

Nitrogen supply rate in Scots pine (Pinus sylvestris L.) forests of contrasting slope aspect

We studied Nitrogen (N) transformations in Pinus sylvestris forest stands in the foothills of the SE Pre-Pyrenees (NE Spain). Plots were selected in two contrasting aspects (two plots per aspect) and N supply rate was measured by the resin-core incubation technique once every three months. N leaching through litter layers (L and F horizons) was evaluated by 5 zero-tension lysimeters in each plot. NH₄ ⁺-N, NO₃ ^--N and soluble organic-N were determined in all solutions. N supply rate showed a clear seasonal pattern. Ammonification and nitrification were segregated in space and in time. While ammonification showed a peak in spring, nitrification was higher in summer. There was evidence suggesting that nitrification occurs mostly in A1 horizon. Nitrification rates differed significantly among plots. N supply rate was 12.7–23.5 kg N·ha^-1·yr^-1 but it did not differ between aspects or plots. Inorganic-N leached through litter layers was 14–17 kg N·ha^-1·yr^-1, and represented a high proportion of N supply rate. Organic-N leached through litter layers (27.8–37.0 kg N·ha^-1·yr^-1) was higher than leached inorganic-N. However, in most cases organic-N did not represent a high proportion of changes in soluble organic-N pools in H and A1 horizons (about 240 kg N·ha^-1·yr^-1). This large decrease in soluble organic-N was much greater than the increase in inorganic-N. The possible fate of these large amounts of organic-N is discussed.     

A simple model for estimating the contribution of nitrogen mineralization to the nitrogen supply of crops from a stabilized pool of soil organic matter and recent organic input

A simple model was developed to estimate the contribution of nitrogen (N) mineralization to the N supply of crops. In this model the soil organic matter is divided into active and passive pools. Annual soil mineralization of N is derived from the active pool. The active pool comprises stabilized and labile soil organic N. The stabilized N is built up from accumulated inputs of fresh organic N during a crop rotation but the labile N is a fraction of total N added, which mineralizes faster than the stabilized N. The passive pool is considered to have no participation in the mineralization process. Mineralization rates of labile and stabilized soil organic N from different crop residues decomposing in soil were derived from the literature and were described by the first-order rate equation dN/dt =-K*N, where N is the mineralizable organic N from crop residues andK is a constant. The data were groupedK ₁ by short-term (0–1 year) andK ₂ by long-term (0–10 years) incubation. Because the range of variation inK ₂ was smaller than inK ₁ we felt justified in using an average value to derive N mineralization from the stabilized pool. The use of a constant rate ofK ₁ was avoided so net N mineralization during the first year after addition is derived directly from the labile N in the crop residues. The model was applied to four Chilean agro-ecosystems, using daily averages of soil temperature and moisture. The N losses by leaching were also calculated. The N mineralization varied between 30 and 130 kg N ha^–1 yr^–1 depending on organic N inputs. Nitrogen losses by leaching in a poorly structured soil were estimated to be about 10% of total N mineralized. The model could explain the large differences in N- mineralization as measured by the potential N mineralization at the four sites studied. However, when grassland was present in the crop rotation, the model underestimated the results obtained from potential mineralization.     

Leaching of nitrate and ammonium in heathland and forest ecosystems in Northwest Germany under the influence of enhanced nitrogen deposition

To study the impact of high atmospheric nitrogen deposition on the leaching of NO₃^– and NH₄⁺ beneath forest and heathland vegetation, investigations were carried out in adjacent forest and heathland ecosystems in Northwest Germany. The study area is subjected to high deposition of nitrogen ranging from 15.9 kg ha^–1 yr^–1 in bulk precipitation to 65.3 kg ha^–1 yr^–1 beneath a stand of Pinus sylvestris L. with NH₄–N accounting for 70–80% of the nitrogen deposited. Considerable leaching of nitrogen compounds from the upper horizons of the soil, mostly as nitrate, occurred at most of the forest sites and below a mixed stand of Calluna vulgaris (L.) Hull. and Erica tetralix, but was low in a Betula pubescens Ehrh. swamp forest as well as beneath Erica tetralix L. wet heath and heath dominated by Molinia caerulea(L.) Moench. Ground water concentrations of both NO₃–N and NH₄–N did not exceed 1 mg L^–1 at most of the sites investigated.     

Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N₂ fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha^–1 and received urea at 0, 200 or 400 kg N ha^–1 yr^–1 in 8–10 split applications. A fourth farmlet received 400 kg N ha^–1 yr^–1 and had 4.4 cows ha^–1.There was large variation in annual clover production and total N₂ fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha^–1 yr^–1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years.Fertilizer N application decreased the average proportion of clover N derived from N₂ fixation (P_N; estimated by ¹⁵N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N₂ fixation decreased from 154 kg N ha^–1 yr^–1 to 39–53 kg N ha^–1 yr^–1. This includes N₂ fixation in clover tissue below grazing height estimated at 70% of N₂ fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N₂ fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N₂ fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons.In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease P_N from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N₂ fixation because lower pasture mass reduced grass competition, particularly during spring.     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Forest nitrogen sinks in large eastern U.S. watersheds: estimates from forest inventory and an ecosystem model

The eastern U.S. receives elevated rates of Ndeposition compared to preindustrial times, yetrelatively little of this N is exported indrainage waters. Net uptake of N into forestbiomass and soils could account for asubstantial portion of the difference between Ndeposition and solution exports. We quantifiedforest N sinks in biomass accumulation andharvest export for 16 large river basins in theeastern U.S. with two separate approaches: (1)using growth data from the USDA ForestService's Forest Inventory and Analysis (FIA)program, and (2) using a model of forestnitrogen cycling (PnET-CN) linked to FIAinformation on forest age-class structure. Themodel was also used to quantify N sinks in soiland dead wood, and nitrate losses below therooting zone. Both methods agreed that netgrowth rates were highest in the relativelyyoung forests on the Schuylkill watershed, andlowest in the cool forests of northern Maine. Across the 16 watersheds, wood export removedan average of 2.7 kg N ha^–1 yr^–1(range: 1–5 kg N ha^–1 yr^–1), andstanding stocks increased by 4.0 kg N ha^–1yr^–1 (–3 to 8 kg N ha^–1 yr^–1). Together, these sinks for N in woody biomassamounted to a mean of 6.7 kg N ha^–1yr^–1 (2–9 kg N ha^–1 yr^–1), or73% (15–115%) of atmospheric N deposition. Modeled rates of net N sinks in dead wood andsoil were small; soils were only a significantnet sink for N during simulations ofreforestation of degraded agricultural sites. Predicted losses of nitrate depended on thecombined effects of N deposition, and bothshort- and long-term effects of disturbance. Linking the model with forest inventoryinformation on age-class structure provided auseful step toward incorporating realisticpatterns of forest disturbance status acrossthe landscape.     

N2(C2H2−C2H4) fixation in two species of Ceanothus seedlings in second year postfire chaparral

Nitrogen fixation in excised root nodules of 2-year-old, postfireCeanothus tomentosus andC. leucodermis seedlings was measured over an 8-month period using the acetylene reduction method. High levels of NO₃–N and NH₄–N present in postfire soils were limited to the upper 10 cm and did not inhibit nodulation in these deeper-rooting seedlings. Decreases in acetylene reduction activity occurred with decreased soil moisture and increased soil temperature. Nitrogen gains from these two Ceanothus shrub seedlings totalled 1.6 kg N ha^–1 yr^–1.     

Nutrient losses in runoff from grasslandand shrubland habitats in southern New Mexico: II. Field plots

Losses of dissolved nutrients (N, P, K, Ca, Mg, Na,Cl, and SO₄) in runoff were measured on grasslandand shrubland plots in the Chihuahuan desert ofsouthern New Mexico. Runoff began at a lowerthreshold of rainfall in shrublands than ingrasslands, and the runoff coefficient averaged 18.6%in shrubland plots over a 7-year period. In contrast,grassland plots lost 5.0 to 6.3% of incidentprecipitation in runoff during a 5.5-year period. Nutrient losses from shrubland plots were greater thanfrom grassland plots, with nitrogen losses averaging0.33 kg ha^–1 yr^–1 vs0.15 kg ha^–1 yr^–1, respectively, during a 3-year period. Thegreater nutrient losses in shrublands were due tohigher runoff, rather than higher nutrientconcentrations in runoff. In spite of these nutrientlosses in runoff, all plots showed net accumulationsof most elements due to inputs from atmosphericdeposition. Therefore, loss of soil nutrients byhillslope runoff cannot, by itself, account for thedepletion of soil fertility associated withdesertification in the Chihuahuan desert.     

Nitrogen mineralization in high elevation forests of the Appalachians. I. Regional patterns in southern spruce-fir forests

Annual and seasonal rates of net nitrogen mineralization were determined for 19 sites in the spruce-fir forests of the Southern Appalachian Mountains. These sites included high and low elevation stands of red spruce (Picea rubens Sarg.) and Fraser fir (Abies fraseri (Pursh.) Poir.) on east and west exposures on Whitetop Mountain, Virginia; Mt. Mitchell, North Carolina; and Clingman's Dome in the Great Smoky Mountains National Park. Mineralization rates were determined using in situ soil incubations in PVC tubes with ion exchange resin bags placed in the bottom of the tubes to collect leachate. Throughfall was collected in resin bags placed in the top of the tubes. Average initial NH₄-N + NO₃-N ranged from 0.6 to 4.8 kg N/ha across all plots, and average mineralization rates ranged from 26 to 180 kg-N ha⁻¹ yr⁻¹. Throughfall ranged from 18 to 32 kg-N ha⁻¹ yr⁻¹ with NH₄-N accounting for about two-thirds of the throughfall N across all sites. Throughfall and mineralization rates were not related to elevation or exposure. The high rates of N mineralization and relatively high nitrate concentrations indicate that leaching losses of nitrogen and associated cations could be substantial. Requests for offprints     

Atmospheric deposition and sulphur cycling in chalk grasslandA mechanistic model simulating field observations

Sulphate fluxes in bulk deposition, throughfall and soil solution were monitored during two years, and integrated within a model describing the cycling of S in a chalk grassland ecosystem. Throughfall fluxes were strongly determined by interceptive properties of the grassland canopy. Seasonal variation in Leaf Area Index resulted in dry deposition velocities for SO₂ varying between 0.1 cm.s^–1 (snow cover, almost no aerodynamic resistance) to 0.9–1.8 cm.s^–1 in periods with a fully developed canopy. On an annual basis net canopy exchange (assimilation of SO₂ minus foliar leaching) was estimated to be –15% of net throughfall. Simulated soil solution concentrations, being the result of throughfall input, leaching, adsorption, biomass uptake and mineralization, closely fitted actual values (r > 0.92; p > 0.001). Actual and simulated leaching were 1.74 ± 0.03 and 2.00 keq.-ha^–1.yr^–1, respectively. Sulphur budgets for the soil showed net accumulation from April to October and net losses from October to April. Annual budgets for the ecosystem showed atmospheric input (2.02keq.ha^–1.yr^–1) and actual output (2.05keq.ha^–1.yr^–1) to be almost balanced. Apart from increased soil solution concentrations, additional input of sulphate (3.55 keq.ha^–1.yr^–1) to experimental plots resulted in additional accumulation in the ecosystem of 0.62 keq.ha^–1.yr^–1     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.