Effects of forest clear-cutting on the carbon and nitrogen fluxes through podzolic soil horizons

Effects of clear-cutting on the dissolved fluxes of organic C (DOC), organic N (DON), NO₃ ^– and NH₄ ⁺ through surface soil horizons were studied in a Norway spruce dominated mixed boreal forest in eastern Finland. Bulk deposition, total throughfall and soil water from below the organic (including understorey vegetation and, after clear-cutting, also logging residues), eluvial and illuvial horizons were sampled weekly from 1993 to 1999. Clear-cutting was carried out in September 1996. The removal of the tree canopy decreased the deposition of DOC and DON to the forest floor and increased that of NH₄ ⁺ and NO₃ ^– but did not affect the deposition of total N (DTN, <3 kg ha^–1 a^–1). The leaching of DOC and DON from the organic horizon increased over twofold after clear-cutting (fluxes were on an average 168 kg C and 3.3 kg N ha^–1 a^–1), but the increased outputs were effectively retained in the surface mineral soil horizons. Inorganic N deposition was mainly retained by the logging residues and organic horizon indicating microbial immobilization. Increased NO₃ ^– formation reflected as elevated concentrations in the percolate from below the mineral soil horizons were observed especially in the third year after clear-cutting. However, the changes were small and the increased leaching of DTN from below the illuvial horizon remained small (<0.4 kg ha^–1 a^–1) and mainly DON. Effects of clear-cutting on the transport of C and N to surface waters will probably be negligible.     

Short-term increases in relative nitrification rates due to trenching in forest floor and mineral soil horizons of different forest types

The representation of NO₃ ^– dynamics within forest growth simulation models could improve forest management. An extensive literature review revealed an 88% probability of measuring a higher relative nitrification index (i.e. RNI = [NO₃ ^–] ÷ [NO₃ ^– + NH₄ ⁺]) in mineral soil horizons than in forest floors, across a wide range of conifer and hardwood ecosystems. We then hypothesised that humus form and fine root density could be used as two crude variables to predict changes in in situ, potential and relative nitrification rates. Twenty-seven trench plots were established in 1999, across nine contrasting hardwood and coniferous stands in the Eastern Townships of Québec. Forest floor and mineral soil samples were collected from each plot, and from a 1 m radius surrounding each plot, on three dates during summer 2000. In situRNI values increased significantly in trench plots as the season progressed. Potential nitrification rates (i.e. NO₃ ^– concentrations following incubation) were two orders of magnitude higher in forest floor than in mineral soil samples. RNI was significantly higher in mineral soil than in forest floor samples after incubations, but the relative increase in RNI due to trenching was higher in forest floor samples. Indices of available C were significantly higher in forest floor than mineral soil samples, and decreased only in forest floor samples during incubations. Likewise, trenching significantly reduced available C in forest floor samples only. Seasonal changes in soil temperature and fine root growth were the most plausible explanations for seasonal changes in NO₃ ^– dynamics, whereas other factors such soil acidity and moisture appeared less important in determining NO₃ ^– dynamics in this study. We conclude that crude assessments of humus form and fine root density have the potential to be used as calibration parameters for the simulation of NO₃ ^– dynamics in forest growth and yield models.     

Dynamics of viable nitrifier community and nutrient availability in dry tropical forest habitat as affected by cultivation and soil texture

Seasonal dynamics of N-mineralization and the size of the viable community of nitrifying bacteria were studied for a forest site and an adjoining cropland site. The forest site was dominated by Boswellia serrata and Acacia catechu in the tree layer, and by Nyctanthes arbortristis and Zizyphus glaberrima in the shrub layer. Crop sequence on the cropland site was Oryza sativa/Lens culinaris. The soil type in both the sites was ultisol (USDA). The cropland soil had significantly higher bulk density, and clay content but lower organic C, total N and total P than forest soil. The soil moisture content, numbers of ammonia-and nitrite oxidizing bacteria and N-mineralization rates were highest in the wet season and lowest in the dry season, while the size of mineral N and P pools showed a reverse trend in both sites. The numbers of free-living cells of ammonia-and nitrite oxidizing bacteria were significantly related with each other as well as with the soil moisture content and N-mineralization rates. In N-mineralization, NO ₃ ^– was the dominating form in the forest site during rainy season, while in other seasons in this site and in all the seasons in the cropland site, NH ₄ ⁺ -N was predominant. The N-mineralization rate and the number of viable nitrifying cells were consistently higher for the forest soil compared to the clay-rich cropland soil. The combination of low soil organic matter and high clay content suppressed the number of free-living cells of nitrifying bacteria and N-mineralization rates in the cropland site.     

Soil organic sulfur dynamics in a coniferous forest

Sulfate microbial immobilization and the mineralization of organic S were measured in vitro in soil horizons (LFH, Ae, Bhf, Bf and C) of the Lake Laflamme watershed (47°17 N, 71°14 O) using ³⁵SO₄. LFH samples immobilized from 23 to 77% of the added ³⁵SO₄ within 2 to 11 days. The ³⁵SO₄ microbial immobilization increased with temperature and reached an asymptote after a few days. The mineral soil generally immobilized less than 20% of the added ³⁵SO₄, and an asymptote was reached after 2 days. An isotopic equilibrium was rapidly reached in mineral horizons. A two-compartment (SO₄ and organic S) model adequately described ³⁵SO₄ microbial immobilization kinetics. The active organic reservoir in the whole soil profile represented less than 1% of the total organic S. The average concentrations of dissolved organic S (DOS) in the soil solutions leaving the LFH, Bhf and Bf horizons were respectively 334, 282 and 143 µgL^–1. Assuming that the DOS decrease with soil depth corresponded to the quantities adsorbed in the B horizons, we estimated that 12 800 kgha^–1 of organic S could have been formed since the last glaciation, which is about 13 times the size of the actual B horizons reservoirs. Our results suggest that the organic S reservoirs present in mineral forest soils are mostly formed by the DOS adsorption resulting from incomplete litter decomposition in the humus layer. The capability of these horizons to immobilize SO₄ from the soil solution would be restricted to a 1% active fraction composed of microorganisms. Despite their refractory nature, these reservoirs can, however, be slowly decomposed by microorganisms and contribute to the S-SO₄ export from the watershed in the long term.     

Effects of a clearcut on the net rates of nitrification and N mineralization in a northern hardwood forest,Catskill Mountains,New York,USA

The Catskill Mountains of southeastern New York receive among the highest rates of atmospheric nitrogen (N) deposition in eastern North America, and ecosystems in the region may be sensitive to human disturbances that affect the N cycle. We studied the effects of a clearcut in a northern hardwood forest within a 24-ha Catskill watershed on the net rates of N mineralization and nitrification in soil plots during 6 years (1994–1999) that encompassed 3-year pre- and post-harvesting periods. Despite stream NO₃^– concentrations that increased by more than 1400 mol l^–1 within 5 months after the clearcut, and three measures of NO₃^– availability in soil that increased 6- to 8-fold during the 1^st year after harvest, the net rates of N mineralization and nitrification as measured by in situ incubation in the soil remained unchanged. The net N-mineralization rate in O-horizon soil was 1– 2 mg N kg^–1 day^–1 and the net nitrification rate was about 1 mg N kg^–1 day^–1, and rates in B-horizon soil were only one-fifth to one-tenth those of the O-horizon. These rates were obtained in single 625 m² plots in the clearcut watershed and reference area, and were confirmed by rate measurements at 6 plots in 1999 that showed little difference in N-mineralization and nitrification rates between the treatment and reference areas. Soil temperature increased 1 ± 0.8 °C in a clearcut study plot relative to a reference plot during the post-harvest period, and soil moisture in the clearcut plot was indistinguishable from that in the reference plot. These results are contrary to the initial hypothesis that the clearcut would cause net rates of these N-cycling processes to increase sharply. The in situ incubation method used in this study isolated the samples from ambient roots and thereby prevented plant N uptake; therefore, the increases in stream NO₃^– concentrations and export following harvest largely reflect diminished uptake. Changes in temperature and moisture after the clearcut were insufficient to measurably affect the net rates of N mineralization and nitrification in the absence of plant uptake. Soil acidification resulting from the harvest may have acted in part to inhibit the rates of these processes. The US Governments right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Fluxes of nitrous oxide and methane from drained peatlands following forest clear-felling in southern Finland

Drainage of waterlogged sites has been part of the normal forestry practice in Fennoscandia, the Baltic countries, the British Isles and in some parts of Russia since the early 20^th century, and currently, about 15 million hectares of peatlands and other wetlands have been drained for forestry purposes. The rate of forest clear-felling on drained peatlands will undergo a rapid increase in the near future, when a large number of these forests approach their regeneration age. A small-scale pilot survey was performed at two nutrient-rich and old peatland drainage areas in southern Finland to study if forest clear-felling has significant impacts on the exchange of nitrous oxide (N₂O) and methane (CH₄) between soil and atmosphere. The average N₂O emissions from the two drainage areas during three growing seasons following clear-felling were 945 and 246 g m^–2 d^–1. The corresponding CH₄ fluxes were –0.07 and –0.52 mg m^–2 d^–1. Clear-felling had impacts on the environmental factors known to affect the N₂O and CH₄ fluxes of peatlands, i.e. clear-felling raised the water table level and increased the peat temperature. However, no substantial changes in the fluxes of CH₄ following clear-felling were observed. The results concerning N₂O indicated a potential for increased emissions following clear-felling of drained peatland forests, but further studies are needed for a critical evaluation of the impacts of clear-felling on the fluxes of CH₄ and N₂O.     

Vertical distribution of biological and geochemical phosphorus subcycles in two southern Appalachian forest soils

We measured Al, Fe, and P fractions by horizon in two southern Appalachian forest soil profiles, and compared solution PO₄ ^–1 removal in chloroform-sterilized and non-sterilized soils, to determine whether biological and geochemical P subcycles were vertically stratified in these soils. Because organic matter can inhibit Al and Fe oxide crystallization, we hypothesized that concentrations of non-crystalline (oxalate-extractable) Al (Al₀) and Fe (Fe₀), and concomitantly P sorption, would be greatest in near-surface mineral (A) horizons of these soils.Al₀ and Fe₀ reached maximum concentrations in forest floor and near-surface mineral horizons, declined significantly with depth in the mineral soil, and were highly correlated with P sorption capacity. Small pools of readily acid-soluble (AF-extractable) and readily-desorbable P suggested that PO₄ ^3– was tightly bound to Al and Fe hydroxide surfaces. P sorption in CHCl₃-sterilized mineral soils did not differ significantly from P sorption in non-sterilized soils, but CHCl₃ sterilization reduced P sorption 40–80% in the forest floor. CHCl₃ labile (microbial) P also reached maximum concentrations in forest floor and near-surface mineral horizons, comprising 31–35% of forest floor organic P. Combined with previous estimates of plant root distributions, data suggest that biological and geochemical P subcycles are not distinctly vertically stratified in these soils. Plant roots, soil microorganisms, and P sorbing minerals all reach maximum relative concentrations in near-surface mineral horizons, where they are likely to compete strongly for PO₄ ^3– available in solution.     

Nitrogen transformations in limed and nitrogen fertilized soil in Norway spruce stands

Nitrogen transformations in the soil, and the resulting changes in carbon and nitrogen compounds in soil percolate water, were studied in two stands of Norway spruce (Picea abies L.). Over the last 30 years the stands were repeatedly limed (total 6000 kg ha^–1), fertilized with nitrogen (total about 900 kg ha^–1), or both treatments together. Both aerobic incubations of soil samples in the laboratory, and intact soil core incubations in the field showed that in control plots ammonification widely predominated over nitrification. In both experiments nitrogen addition increased the formation of mineral-N. In one experiment separate lime and nitrogen treatments increased nitrification, in the other, only lime and nitrogen addition together had this effect. In one experiment immobilization of nitrogen to soil microbial biomass was lower in soil only treated with nitrogen. Soil percolate water was collected by means of lysimeters placed under the humus layer and 10 cm below in the mineral soil. Total N, NH₄-N and NO₃-N were measured, and dissolved organic nitrogen was fractioned according to molecular weight. NO₃-N concentrations in percolate water, collected under the humus layer, were higher in plots treated with N-fertilizer, especially when lime was also added. The treatments had no effect on the N concentrations in mineral soil. A considerable proportion of nitrogen was leached in organic form.     

Patterns of restoration of soil physciochemical properties and microbial biomass in different landslide sites in the sal forest ecosystem of Nepal Himalaya

The pattern of natural restoration in soil components and processes was documented in five landslide-damaged (1–58-year-old) sites in the moist tropical sal (Shorea robusta) forest ecosystem of Nepal Himalaya. Comparisons were made with an undisturbed forest site in the same region. Concentrations of soil organic C, total N, total P and extractable nutrients (Ca, Mg and K) increased with the age of sites. The 58-year-old site showed concentrations of soil organic C, total N and total P that were 75–89% of concentrations in the undisturbed sal forest. The soil microbial biomass, the active fraction of soil organic matter, showed similar seasonal variations at all sites. The amount of mean microbial biomass (expressed as C, N and P contents) increased 4–5 times at the 58-year-old site relative to the 1-year-old site, and the bulk increase occurred within the initial 15 year. The increase in the C/N ratio of soil microbial biomass with age (9.4–11.6 years) reflected change in its composition. Although the net N-mineralization rate increased consistently until 58 years of age, the proportion of nitrification rate relative to ammonification rate distinctly decreased beyond 40 years. On the other hand, the soil available-N (both NO₃⁻ and NH₄⁺) concentrations increased from 1 to 40 year and then declined; with age the proportion of NH₄⁺ increased, however. Rates of restoration in soil properties were faster in the early successional stages (1–15 year) than late stages. Among different soil properties the restoration of soil microbial biomass (C and N) was faster than soil organic C and total N. Best fit power function models showed that the estimated times for the 58-year-old site to reach the level of the undisturbed, mature sal forest would be about 30–35 year for microbial biomass (C and N) and about 100–150 year for organic C and total N. Higher accumulation of soil microbial biomass and high N-mineralization rate at late successional stages indicated the re-establishment of enriched soil and restitution of nutrient cycling during the course of ecosystem restoration.     

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 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.     

Effects of fertilization and liming on the chemical soil conditions and element distribution in forest soils

Summary The distribution and storage of major elements in acid soils from a spruce and a beech forest was investigated after fertilization of NH₄NO₃ and KCl followed by Ca and Mg fertilization by 2 liming applications. All fertilizers were applied on top of the soil without mixing. Most of the added Ca and Mg was detected in the humus layer, a significant part of it still in carbonatic form. The effect of liming on mineral soil pH is very low, and was only observed in the 0–10 cm layer. However, base saturation of the mineral soil increased. The storage of C and N of the humus layer was not affected. N fertilization increased the N storage of the soil only under beech, but was followed by heavy NO₃-losses with seepage water under spruce. High leaching rates for K were also found in the spruce stand. The amount of K that was not leached increased the pool of exchangeable K in the deeper soil layer.     

Change in water quality during the passage through a tropical montane rain forest in Ecuador

We studied five 20-m transects onthe lower slope under tropical lower montanerain forest at 1900–2200 m above sea level. We collectedsamples of soil and of weekly rainfall,throughfall, litter leachate, and stream waterbetween 14 March 1998 and 30 April 1999 anddetermined the concentrations of Al, totalorganic C (TOC), Ca, Cl^–, Cu, K, Mg, Mn,NH₄ ⁺-N, NO₃ ^–-N, total N (TN), Na, P, S, and Zn. The soils were shallowInceptisols; pH ranged 4.4–6.3 in the Ohorizons and 3.9–5.3 in the A horizons, totalCa (6.3–19.3 mg kg^–1) and Mgconcentrations (1.4–5.4) in the O horizon weresignificantly different between the transects.Annual rainfall was 2193 mm; throughfall variedbetween 43 and 91% of rainfall, cloud waterinputs were 3.3 mm a^–1 except forone transect (203). The volume-weighted mean pHwas 5.3 in rainfall and 6.1–6.7 in throughfall.The median of the pH of litter leachate andstream water was 4.8–6.8 and 6.8, respectively.The concentrations of Ca and Mg in litterleachate and throughfall correlatedsignificantly with those in the soil (r =0.76–0.95). Element concentrations inthroughfall were larger than in rainfallbecause of leaching from the leaves (Al, TOC,Ca, K, Mg), particulate dry deposition (TOC,Cu, Cl^–, NH₄ ⁺-N), and gaseousdry deposition (NO₃ ^–-N, total N, S).Net throughfall (= throughfall-rainfalldeposition) was positive for most elementsexcept for Mn, Na, and Zn. High-flow eventswere associated with elevated Al, TOC, Cu, Mn,and Zn concentrations.     

The relative importance of autotrophic and heterotrophic nitrification in a conifer forest soil as measured by 15N tracer and pool dilution techniques

The importance of heterotrophic nitrification was studied in soil from a mixed-conifer forest. Three sites in the forest were sampled: a clear cut area, a young stand and a mature stand. In the mature stand, the mineral soil (0–10 cm) and the organic layer were sampled separately. Gross rates of N mineralization and nitrification were measured by¹⁵NH ₄ ⁺ and¹⁵NO ₃ ^– isotopic pool dilution, respectively. The rates of autotrophic and heterotrophic nitrification were distinguished by use of acetylene as a specific inhibitor of autotrophic nitrification. In samples supplemented with¹⁵NH ₄ ⁺ and treated with acetylene, no¹⁵NO ₃ ^– was detectable showing that the acetylene treatment effectively blocked the autotrophic nitrification, and that NH ₄ ⁺ was not a substrate for heterotrophic nitrification. In the clear cut area, autotrophic nitrification was the most important NO ₃ ^– generating process with total nitrification (45 ug N kg^–1h^–1) accounting for about one-third of gross N mineralization (140 ug N kg^–1 h^–1). In the young and mature forested sites, gross nitrification rates were largely unaffected by acetylene treatment indicating that heterotrophic nitrification dominated the NO ₃ ^– generating process in these areas. In the mature forest mineral and organic soil, nitrification (heterotrophic) was equal to only about 5% of gross mineralization (gross mineralization rates of 90 ug N kg^–1 h^–1 mineral; 550 ug N kg^–1 h^–1 organic). The gross nitrification rate decreased from the clear cut area to the young forest area to the mineral soil of the mature forest (45; 17; 4.5 ug kg^–1 h^–1 respectively). The¹⁵N isotope pool dilution method, combined with acetylene as an inhibitor of autotrophic nitrification provided an effective technique for assessing the importance of heterotrophic nitrification in the N-cycle of this mixed-conifer ecosystem.     

Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH₄NO₃ (0, 5 and 15 g N m^–2 yr^–1) have been made in two forest stands:Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons.Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased.The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m^–2 yr^–1), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO₃-N> NH₄-N> DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.     

Comparison of initial wetting pattern,nutrient concentrations in soil solution and the fate of15N-labelled urine in sheep and cattle urine patch areas of pasture soil

Three field experiments were carried out to compare cattle and sheep urine patches in relation to (i) initial wetting pattern and volume of soil affected, (ii) soil solution ionic composition and (iii) the fate of¹⁵N-labelled urine in the soil over the winter period. The distribution of Br^– (used as a urine tracer) across the soil surface and down the profile was irregular in all the patches. The pasture area covered by Br^– in the sheep patches was 0.04–0.06 m² and Br^– was detected to a depth of 150 mm. Cattle patches were significantly larger covering a surface area of 0.38–0.42 m² and penetrating to a depth of 400 mm. The rapid downward movement of urine occurred through macropore flow but even so, over half of the applied Br^– was detected in the 0–50 mm soil layer in both sheep and cattle patches. Due to the larger volume of urine added to the cattle patches (2000 mL for cattle and 200 mL for sheep) the effective application rate was about 5 L m^–2 compared with 4 L m^–2 for sheep. Concentrations of extractable mineral N and ionic concentrations in soil solution were higher in cattle than sheep patches particularly near the soil surface. In both sheep and cattle patches, urea was rapidly hydrolysed to NH ₄ ⁺ and nitrification occurred between 14 and 29 days after urine application. Initially the major anions and cations in the soil solution were HCO ₃ ^– , SO ₄ ⁼ , Cl^–, NH ₄ ⁺ , Mg⁺⁺, K⁺ and Na⁺, which were derived from the urine application. Ionic concentrations in the soil solution decreased appreciably over time due to plant uptake and possibly some leaching. As nitrification proceeded, NO ₃ ^– became the dominant anion in soil solution and the major accompanying cation was Ca⁺⁺. The fate of¹⁵N-labelled urine-urea was followed during a 5 month period beginning in late autumn. Greater leaching losses of NO ₃ ^– occurred below cattle patches (equivalent to 60 kg N ha^–1 below 300 mm and 37 kg N ha^–1 below 600 mm) compared with sheep patches (10 kg N ha^–1 below 300 mm and 1 kg N ha^– below 600 mm). While 6% of the applied¹⁵N was leached the amount of N leached was equivalent to 11% of the applied urine-N in cattle patches. This suggests that there was significant immobilsation-mineralisation turnover in urine patch soil with the release of mineral N from native soil organic matter. In both sheep and cattle patches 60% of the¹⁵N was accounted for in plant uptake, remaining in the soil and leaching. About 40% of the applied N was therefore lost through gaseous emission.     

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.     

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.     

Lysimeter and field studies on15N in a tropical soil

Twenty four plots, each 2.0 m² in area, were established on St. Augustine loam soil series as field plots and microplots (containing lysimeters) in a completely randomised block design of four treatments (mulched fertilized, unmulched fertilized microplots; mulched fertilized and unmulched fertilized field plots), replicated three times. Labelled (¹⁵N) and unlabelled (NH₂)₂CO fertilizer were applied at rates of 400 kg N ha^–1 and CaH₂PO₄ and KCl were applied at rates of 100 and 150 g ha^–1 respectively to the field plots and microplots. Mulch (bagasse) was maintained to a depth of two cm and the plots were kept bare with regular applications of gramoxone.The maximum depth of leaching as measured by diffusion of NO ₃ ^– –¹⁵N in both the dry and wet seasons was 30 cm. The potential for downward movement of water and NO ₃ ^– –¹⁵N was low in the wet season because high intensity rainfall followed high soil moisture contents. Effects of mulching, on the mobility of applied N fertilizers were inconclusive. Infiltration rates were significantly (P=0.25) inversely correlated with soil moisture content, supporting the hypothesis that high intensity rainfall on a saturated soil surface is more likely to result in NO ₃ ^– –¹⁵N dispersion than NO ₃ ^– –¹⁵N leaching.     

Soil nitrogen mineralization in plantations ofJuglans nigra interplanted with actinorhizalElaeagnus umbellata orAlnus glutinosa

Nitrogen mineralization rates were estimated in 19-year-old interplantings of black walnut (Juglans nigra L.) with dinitrogen fixing autumn-olive (Elaeagnus umbellata Thunb.) or black alder (Alnus glutinosa L. Gaertn.) and in pure walnut plantings at two locations in Illinois USA. N mineralization rates were measured repeatedly over a one year period usingin situ incubations of soil cores in oxygen-permeable polyethylene bags at 0–10 and 10–20 cm soil depths, and also by burying mixed-bed ion-exchange resin in soil. Mineralization rates were highest in summer and in plots containing actinorhizal Elaeagnus and Alnus in contrast with pure walnut plots. Elaeagnus plots at one location yielded 236 kg of mineral N ha^–1 yr^–1 in the upper 20 cm of soil, a value higher than previously reported for temperate decidous forest soils in North America. The highest mean plot values for N mineralization in soil at a location were 185 kg ha^–1 yr^–1 for Alnus interplantings and 90 kg ha^–1 yr^–1 for pure walnut plots. Plots which had high N mineralization rates also had the largest walnut trees. Despite low pH (4.1 and 6.5) and low extractable P concentrations (1.4 and 0.7 mg kg^–1 dry mass) at the two locations, nitrification occurred in all plots throughout the growing season. NO ₃ ^– –N was the major form of mineralized N in soil in the actinorhizal interplantings, with NH ₄ ⁺ –N being the major form of mineral N in control plots. Walnut size was highly correlated with soil nitrogen mineralization, particularly soil NO ₃ ^– –N production in a plot.