Fate of urine nitrogen on mineral and peat soils in New Zealand

A field lysimeter experiment was conducted over 150 days to examine the fate of synthetic urinary nitrogen (N) applied to peat and mineral soils, with and without a water table. At the start of the winter season, synthetic urine labelled with ¹⁵N, was applied at 500 kg N ha^–1. Plant uptake, leaching losses and nitrous oxide (N₂O) fluxes were monitored. Total plant uptake ranged from 11% to 35% of the urine-N applied depending on soil type and treatment. Plant uptake of applied N was greater in the presence of a water table in the mineral soil. Nitrate-N (NO₃ ^--N) was only detected in leachates from the mineral soil, at concentrations up to 146 g NO₃ ^--N mL^–1. Presence of a water table in the mineral soil reduced leaching losses (as inorganic-N) from 47% to 6%, incrased plant uptake and doubled apparent denitrification losses. In the peat soils leaching losses of applied urine-N as inorganic-N were low (<5%). Losses of N as N₂O were greater in the mineral soil than in the peat soils, with losses of 3% and <1% of N applied respectively after 100 days. Apparent denitrification losses far exceeded N₂O losses and it is postulated that the difference could be due to dinitrogen (N₂) loss and soil entrapment of N₂.     

15N abundance of soils and plants along an experimentally induced forest nitrogen supply gradient

¹⁵N abundances of soils and a grass species (Deschampsia flexuosa (L.) Trin.) were analysed in a forest fertilization experiment 10 years after the last fertilization. Nitrogen had been given as urea, at seven doses, ranging from 0 to 2400 kg N ha^-1. Previously, we have shown that plants in systems experiencing large losses of N become enriched with ¹⁵N. This was explained by the fact that processes leading to loss of N, e.g. ammonia volatilization, nitrification followed by leaching or denitrification and denitrification itself, tend to fractionate against ¹⁵N. In this experiment, ¹⁵N abundance increased with dose of N applied in both grass and soil total-N, but more so in the grass. This was interpreted to be due to the grass sampling small but active pools of N subject to losses. In contrast, soil total-N largely consists of inactive N that does not immediately exchange with pools of N from which fractionating losses occur. Hence, soil total-N shows a large pretreatment ¹⁵N memory effect, and is, therefore, and integrator of the long-term N balance. When short-term changes (years, decades) in N balances are monitored using variations in ¹⁵N abundance, plants are more suitable indicators of such change than is soil total-N.     

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.     

Land-use change effects on soil C and N transformations in soils of high N status: comparisons under indigenous forest, pasture and pine plantation

Globally, land-use change is occurring rapidly, and impacts on biogeochemical cycling may be influenced by previous land uses. We examined differences in soil C and N cycling during long-term laboratory incubations for the following land-use sequence: indigenous forest (soil age = 1800 yr); 70-year-old pasture planted after forest clearance; 22-year-old pine (Pinus radiata) planted into pasture. No N fertilizer had been applied but the pasture contained N-fixing legumes. The sites were adjacent and received 3–6 kg ha^–1 yr^–1volcanic N in rain; NO₃ ^–-N leaching losses to streamwater were 5–21 kg ha^–1 yr^–1, and followed the order forest < pasture = pine. Soil C concentration in 0–10 cm mineral soil followed the order: pasture > pine = forest, and total N: pasture > pine > forest. Nitrogen mineralization followed the order: pasture > pine > forest for mineral soil, and was weakly related to C mineralization. Based on radiocarbon data, the indigenous forest 0–10 cm soil contained more pre-bomb C than the other soils, partly as a result of microbial processing of recent C in the surface litter layer. Heterotrophic activity appeared to be somewhat N limited in the indigenous forest soil, and gross nitrification was delayed. In contrast, the pasture soil was rich in labile N arising from N fixation by clover, and net nitrification occurred readily. Gross N cycling rates in the pine mineral soil (per unit N) were similar to those under pasture, reflecting the legacy of N inputs by the previous pasture. Change in land use from indigenous forest to pasture and pine resulted in increased gross nitrification, net nitrification and thence leaching of NO₃ ^–-N.     

In situ estimates of annual net nitrogen mineralization and nitrification in a subarctic watershed

Summary Annual estimates of surface soil nitrogen transformations were determined using an in situ method in four different subarctic vegetation types within a watershed in southwestern Alaska. The net nitrogen mineralization estimates were 22.5, 0.5, 4.7, and 2.7 kg-N ha^-1 yr^-1 for the alder, dry tundra, moist tundra, and white spruce sites, respectively. Only the soil from the alder site showed net nitrification (about 10 kg-N ha^-1 yr^-1). Annual inogranic nitrogen flux from the overlying organic layer to the mineral soil was almost seven times greater than net N production in the surface mineral soil in the alder site, indicating that the alder forest floor is potentially a substantial source for plant-available N. Rates of mobilization of N from the surface organic layers of the other sites were similar to net N production rates in surface mineral soils. In situ rates of N transformations showed a similar trend among sites as did laboratory estimates conducted in a previous study, suggesting a strong substrate control of N transformations in these soils.     

Recovery of fertilizer-derived inorganic-15N in a vegetable field soil as affected by application of an organic amendment

To examine the effect of organic amendment application on the fate of inorganic-N accumulated in a vegetable field soil during conversion from inorganic to organic input, a pot experiment using ¹⁵N-labeled soil was conducted. The soil was labeled with ¹⁵N through addition of urea-¹⁵N (98 atom % ¹⁵N) and was then incubated for 1 year resulting in inorganic soil-N concentration and ¹⁵N abundance of 211 mg kg^–1 soil and 4.950 atom %, respectively. Chinese cabbage [Brassica campestris (L.) Samjin] plants were grown in the labeled soil for 30 and 60 days after application of organic amendment at the rates of 0 (control), 200, 400, and 600 mg N kg^–1 soil. Although organic amendment application did not show any significant effect on the uptake efficiency of inorganic-N by Chinese cabbage during the first 30 days, it significantly (P<0.05) increased inorganic-N uptake efficiency as well as total-N uptake and dry matter yield at the end of the 60-day growth period. Application of the organic amendment also increased microbial immobilization of inorganic-N in both growth periods. Between 30 and 60 days of growth, however, the amount of immobilized N from the inorganic-¹⁵N pool decreased, indicating re-mineralization of previously immobilized N. Although the amount of inorganic-¹⁵N lost was virtually the same among treatments at day 30, increased immobilization of inorganic-¹⁵N caused by organic amendment application led to the higher retention of inorganic-N in the soil and less loss of N at day 60 as compared to the control. These results indicate that increased immobilization by organic amendment application in the early growth season and the subsequent gradual re-mineralization may play an important role in increasing plant uptake of inorganic-¹⁵N, while minimizing N loss.     

Mineralization-immobilization and plant uptake of nitrogen as influenced by the spatial distribution of cattle slurry in soils of different texture

The effect of incorporating cattle slurry in soil, either by mixing or by simulated injection into a hollow in soil, on the ryegrass uptake of total N and ¹⁵NH₄ ⁺-N was determined in three soils of different texture. The N accumulation in Italian ryegrass (Lolium multiflorum L.) from slurry N and from an equivalent amount of NH₄ ⁺-N in (¹⁵NH₄) SO₄ (control) was measured during 6 months of growth in pots. After this period the total recovery of labelled N in the top soil plus herbage was similar in the slurry and the control treatments. This indicated that gaseous losses from slurry NH₄ ⁺-N were insignificant. Consequently, the availability of slurry N to plants was mainly influenced by the mineralization-immobilization processes. The apparent utilization of slurry NH₄ ⁺-N mixed into soil was 7%, 14% and 24% lower than the utilization of (NH₄)₂SO₄-N in a sand soil, a sandy loam soil and a loam soil, respectively. Thus, the net immobilization of N due to slurry application increased with increasing soil clay content, whereas the recovery in plants of ¹⁵N-labelled NH₄ ⁺-N from slurry was similar on the three soils. A parallel incubation experiment showed that the immobilization of slurry N occurred within the first week after slurry application. The incorporation of slurry N by simulated injection increased the plant uptake of both total and labelled N compared to mixing the slurry into the soil. The apparent utilization of injected slurry NH₄ ⁺-N was 7% higher, 8% lower and 4% higher than the utilization of (NH₄)₂SO₄-N in the sand, the sandy loam and the loam soil, respectively. It is concluded that the spatial distribution of slurry in soil influenced the net mineralization of N to the same degree as did the soil type.     

天山林区不同类型群落土壤氮素对冻融过程的动态响应

季节性冻融过程对北方温带森林土壤氮素的转化与流失具有重要影响,但不同类型群落对冻融过程响应的差异尚不明确。通过在林地、草地、灌丛上设置系列监测样地,采用原位培养的方法,利用林冠遮挡形成的自然雪被厚度差异,监测分析了冻融期天山林区不同群落表层土壤(0—15 cm)的氮素动态及净氮矿化速率间的差异。结果表明:(1)不同类型群落土壤的铵态氮(NH+4-N)含量、微生物量氮(MBN)含量基本与土壤(5 cm)温度呈正相关,深冻期林地土壤铵态氮含量低于其他群落类型而硝态氮含量高于其他群落类型;(2)硝态氮(NO-3-N)为天山林区季节性冻融期间土壤矿质氮的主体,占比达78.4%。灌丛土壤硝态氮流失风险较大,融化末期较融化初期灌丛土壤硝态氮含量下降了64.6%;(3)冻融时期对整体氮素矿化速率影响显著,群落类型对氨化速率影响显著;(4)天山林区土壤氮素在冻结期主要以氮固持为主。通过揭示不同类型群落土壤氮素对冻融格局的响应,能够助益于对北方林区冬季土壤氮素循环的认识。     

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     

Phosphorus budget of a 70-year-old northern hardwood forest

Recent measurements have made it possible to revise and improve the phosphorus budget of the Hubbard Brook Experimental Forest, including partitioning P uptake by vegetation from the forest floor and mineral soil and estimating net P mineralization in the forest floor. Both living biomass and forest floor are accumlating P (at rates of 1.3 and 0.16 kg P ha^-1 yr^-1 respectively) in this 70-yr old northern hardwood forest. About 61% of the P taken up by the vegetation each year comes from the forest floor (5.9 kg P ha^-1 yr^-1 of a total 9.6 kg P ha^-1 yr^-1), even though the P content of this pool is just 5% of that in mineral soil. The turnover rate of P in the forest floor is 7% yr^-1, while that of the mineral soil is 0.3% yr^-1. Recycling of P in the forest floor is very efficient; of the 5.6 kg P ha^-1 yr^-1 net mineralization in the forest floor, only 0.3 kg P ha^-1 leaches into the mineral soil; the rest is taken up by plants. This tight recycling of P is important because P is less readily available in the mineral soil than in the forest floor.     

Turnover of residual 15N-labelled fertilizer N in soil following harvest of oilseed rape (t Brassica napus L.)

We studied the fate of ¹⁵N-labelled fertilizer nitrogen in a sandy loam soil after harvest of winter oilseed rape (Brassica napus L. cv. Ceres) given 100 or 200 kg N ha^-1 in spring, with or without irrigation. Our main objective was to quantify the temporal variations of the soil mineral N, the extractable soil organic N and soil microbial biomass N, and fertilizer derived N in these pools during autumn and winter. Nitrogen use efficiency of the oilseed rape crop varied from 47% of applied N in the 100N, irrigated treatment to 34% in the 200N, non-irrigated treatment. However, only in the latter treatment did we find significantly higher fertilizer derived soil mineral N than in the three other treatments which all had low soil mineral N contents at the first sampling after harvest (8 days after stubble tillage). Between 31% and 42% of the applied N could not be accounted for in the harvested plants or 0-15 cm soil layer at this first sampling. Over the following autumn and winter none of the remaining fertilizer derived soil N was lost from the 0–5 cm depth, but from the 5–15 cm depth a marked proportion of N derived from fertilizer was lost, probably by leaching. Negligible amounts of fertilizer derived extractable soil organic and mineral N (<1 kg N ha^-1, 0-15 cm) were found in all treatments after the first sampling.Soil microbial biomass N was not significantly affected by treatments and showed only small temporal variability (±11% of the mean 76 kg N ha^-1, 0- 15 cm depth). Surprisingly, the average amount of soil microbial biomass N derived from fertilizer was significantly affected by the treatments, with the extremes being 5.5 and 3.1 kg N ha^-1 in the 200N, non-irrigated and 100N, irrigated treatments, respectively. Also, the estimated exponential decay rate of microbial biomass N derived from fertilizer, differed greatly (2 fold) between these two treatments, indicating highly different microbial turnover rates in spite of the similar total microbial biomass N values. In studies utilising ¹⁵N labelling to estimate turnover rates of different soil organic matter pools this finding is of great importance, because it may question the assumption that turnover rates are not affected by the insertion of the label.     

Effects of fire and harvesting on nitrogen transformations and ionic mobility in soils of Eucalyptus regnans forests of south-eastern Australia

Summary Effects of fire and forest harvesting on inorganic-N in the soil, on net N-mineralization, and on the leaching of NO _inf3 ^sup- -N and metallic cations were measured in forests of Eucalyptus regnans following a severe wildfire in 1983. E. regnans regenerates profusely by seed after fire, and this study compared unburnt forest with forests burnt at varying intensities (surface fire and crown fire), and with logged and burnt forest (slash fire). Total inorganic-N in soil (0–5 cm) increased with increasing fire intensity to a maximum of 158 g g^-1 in the slash fire plot (compared with 51 g g^-1 in the unburnt forest) over the first 205 days after fire. Total inorganic-N returned to a concentration equal to that in the unburnt forest after 485 days at the slash fire plot, and after only 205 days at the surface fire plot. Studies of net mineralization in situ and of NO _inf3 ^sup- -N in soil solution support the hypothesis that inorganic-N was immobilized in all of the burnt forests; microbial immobilization after fire is identified as a key process in N-conservation, limiting the substrate available for nitrification and thereby limiting the loss of N from the system by leaching. The concentrations of NO _inf3 ^sup- -N and metallic cations in soil solution increased with increasing fire intensity. For the first 318 days after the fire, [NO _inf3 ^sup- -N] in soil solution at 10 cm averaged 0.6 g ml^-1 in the unburnt forest, 9.7 mg l^-1 in the surface fire plot, 26 mg l^-1 in the crown fire plot, and 70 mg l^-1 in the slash fire plot. The concentration of metallic cations in soil solution was significantly correlated with [NO _inf3 ^sup- -N], the observed order of mobility being Ca²⁺>Mg²⁺>K⁺>Na⁺. Processes which limit the production and persistence of NO _inf3 ^sup- -N in soil solution following disturbance will significantly reduce nutrient losses or redistribution.     

Soil nitrate accumulation explains the nonlinear responses of soil CO2 and CH4 fluxes to nitrogen addition in a temperate needle-broadleaved mixed forest

The responses of soil-atmosphere carbon (C) exchange fluxes to growing atmospheric nitrogen (N) deposition are controversial, leading to large uncertainty in the estimated C sink of global forest ecosystems experiencing substantial N inputs. However, it is challenging to quantify critical load of N input for the alteration of the soil C fluxes, and what factors controlled the changes in soil CO₂ and CH₄ fluxes under N enrichment. Nine levels of urea addition experiment (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha⁻¹ yr⁻¹) were conducted in the needle-broadleaved mixed forest in Changbai Mountain, Northeast China. Soil CO₂ and CH₄ fluxes were monitored weekly using the static chamber and gas chromatograph technique. Environmental variables (soil temperature and moisture in the 0–10 cm depth) and dissolved N (NH₄⁺-N, NO₃⁻-N, total dissolved N (TDN), and dissolved organic N (DON)) in the organic layer and the 0–10 cm mineral soil layer were simultaneously measured. High rates of N addition (≥60 kg N ha⁻¹ yr⁻¹) significantly increased soil NO₃⁻-N contents in the organic layer and the mineral layer by 120%-180% and 56.4%-84.6%, respectively. However, N application did not lead to a significant accumulation of soil NH₄⁺-N contents in the two soil layers except for a few treatments. N addition at a low rate of 10 kg N ha⁻¹ yr⁻¹ significantly stimulated, whereas high rate of N addition (140 kg N ha⁻¹ yr⁻¹) significantly inhibited soil CO₂ emission and CH₄ uptake. Significant negative relationships were observed between changes in soil CO₂ emission and CH₄ uptake and changes in soil NO₃⁻-N and moisture contents under N enrichment. These results suggest that soil nitrification and NO₃⁻-N accumulation could be important regulators of soil CO₂ emission and CH₄ uptake in the temperate needle-broadleaved mixed forest. The nonlinear responses to exogenous N inputs and the critical level of N in terms of soil C fluxes should be considered in the ecological process models and ecosystem management.     

The role of monoterpenes in soil nitrogen cycling processes in ponderosa pine

The effects of select monoterpenes on nitrogen (N) mineralization and nitrification potentials were determined in four separate laboratory bioassays. The effect of increasing monoterpene addition was an initial reduction in NO₃ ^–-N production (nitrification inhibition), followed by a reduction in the sum of NH₄ ⁺-N and NO₃ ^–-N (inhibition of net N mineralization and net immobilization at high monoterpene additions. Monoterpenes could produce this pattern by inhibiting nitrification, reducing net N mineralization, enhancing immobilization of NO₃ ^–-N relative to NH₄ ⁺-N, and/or stimulating overall net immobilization of N by carbon-rich material.Initial monoterpene concentrations in the assay soils were about 5% of the added amount and were below detection after incubation in most samples.Potential N mineralization-immobilization, nitrification, and soil monoterpene concentrations were determined by soil horizon for four collections from a ponderosa pine (Pinus ponderosa) stand in New Mexico. Concentrations of monoterpenes declined exponentially with soil depth and varied greatly within a horizon. Monoterpene content of the forest floor was not correlated with forest floor biomass. Net N mineralization was inversely correlated with total monoterpene content of all sampled horizons. Nitrification was greatest in the mineral soil, intermediate in the F-H horizon, and never occurred in the L horizon. Nitrification in the mineral soil was inversely correlated with the amount of monoterpenes in the L horizon that contain terminal unsaturated carbon-carbon bonds (r ² = 0.37, P 0.01). This pattern in the field corresponded to the pattern shown in the laboratory assays with increasing monoterpene additions.     

Denitrification and total N losses from an irrigated sandy-clay loam under maize–wheat cropping system

Denitrification and total N losses were quantified from an irrigated field cropped to maize and wheat, each receiving urea at 100 kg N ha^-1. During the maize growing season (60 days), the denitrification loss measured directly by acetylene inhibition-soil cover method amounted 2.72 kg N ha^-1 whereas total N loss measured by ¹⁵N balance was 39 kg ha^-1. Most (87%) of the denitrification loss under maize occurred during the first two irrigation cycles. During the wheat growing season (150 days), the denitrification loss directly measured by acetylene inhibition-soil cover and acetylene inhibition-soil core methods was 1.14 and 3.39 kg N ha^-1, respectively in contrast to 33 kg N ha^-1 loss measured by ¹⁵N balance. Most (70-88%) of the denitrification loss under wheat occurred during the first three irrigation cycles. Soil moisture and NO ₃ ^- -N were the major factors limiting denitrification under both crops. Higher N losses measured by ¹⁵N balance than C₂H₂ inhibition method were perhaps due to underestimation of denitrification by C₂H₂ inhibition method and losses other than denitrification, most probably NH₃ volatilization.     

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.     

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.     

Soil conditions and regeneration after clear felling of a Pinus sylvestris L. stand in a nitrogen experiment,Central Sweden

Forest N fertilization is a common practice in areas of Sweden that are not affected by high levels of N deposition. The environmental consequences of high N input to closed forests are fairly well known, but the long-term effects following clear-felling are a lot less well known. Thus, residual effects on soil and planted seedlings of previous N additions at an experimental N gradient 11 years after clear-felling were studied at a naturally nutrient-poor forest site in central Sweden. The experimental N gradient had been established by three repeated applications (in 1967, 1974 and 1981) of six dosages of NH₄NO₃ with increments of 120 kg N ha^–1. Thus, in total, the applied N dose ranged between 0 and 1800 kg N ha^–1. The study examined extractable base cations and P, soil pH, total-N, total-C, net N-mineralization and potential nitrification in four soil horizons (the humus layer, and 0–5, 5–10 and 10–20 cm in the mineral soil). We also measured the survival and growth of planted Pinus sylvestris L. seedlings. The applied N had no effect on the amounts of extractable-P or base cations in the soil. The soil pH decreased with increasing N dose in the deeper soil horizons, while in the humus the pH showed a weak but statistically significant increase due to the N application. Both total-C and total-N increased as a result of the N application, while the C/N ratio decreased. In the humus layer and the uppermost mineral soil layer NH₄ ⁺ was the major inorganic N source, in contrast to the lowest mineral soil horizon where NO₃ ^– dominated. For most of the studied horizons, there was a positive linear relationship between applied N dose and amount of inorganic N. Both net N-mineralization and potential nitrification showed increases with increasing N dose. As for the plants, no difference in survival or growth was found between the different N treatments. For doses generally applied in forest fertilization no significant differences in any of the studied properties were found.     

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.     

Dynamics of Aboveground and Soil Carbon and Nitrogen Stocks and Cycling of Available Nitrogen along a Land-use Gradient in Rondônia,Brazil

High rates of deforestation in the Brazilian Amazon have the potential to alter the storage and cycling of carbon (C) and nitrogen (N) across this region. To investigate the impacts of deforestation, we quantified total aboveground biomass (TAGB), aboveground and soil pools of C and N, and soil N availability along a land-use gradient in Rondônia, Brazil, that included standing primary forest, slashed primary and secondary forest, shifting cultivation, and pasture sites. TAGB decreased substantially with increasing land use, ranging from 311 and 399 Mg ha^–1 (primary forests) to 63 Mg ha^–1 (pasture). Aboveground C and N pools declined in patterns and magnitudes similar to those of TAGB. Unlike aboveground pools, soil C and N concentrations and pools did not show consistent declines in response to land use. Instead, C and N concentrations were strongly related to percent clay content of soils. Concentrations of NO₃-N and NH₄-N generally increased in soils following slash-and-burn events along the land-use gradient and decreased with increasing land use. Increasing land use resulted in marked declines in NO₃-N pools relative to NH₄-N pools. Rates of net nitrification and N-mineralization were also generally higher in postfire treatments relative to prefire treatments along the land-use gradient and declined with increasing land use. Results demonstrate the linked responses of aboveground C and N pools and soil N availability to land use in the Brazilian Amazon; steady reductions in aboveground pools along the land-use gradient were accompanied by declines in inorganic soil N pools and transformation rates.