Simulating soil freeze/thaw cycles typical of winter alpine conditions: Implications for N and P availability

Seasonally snow-covered alpine soils may be subjected to freeze/thaw cycles, particularly during years having little snow and during the late winter and early spring periods. Freeze/thaw cycles can stimulate soil mineralization and could therefore be one factor regulating nitrogen (N) and phosphorus (P) availability and cycling. In this study laboratory incubation experiments using four soils having contrasting properties have been used to characterize the change in N and P forms (microbial and soluble inorganic/organic) that occur after simulated freeze/thaw cycles.Soil samples were collected from locations representing extreme examples of either direct human management (grazed meadow (site M) and extensive grazing beneath larch (site L)) or those disturbed by more natural events (recent avalanche and colonisation by alder (site A)) and from beneath the expected forest climax vegetation beneath fir (site F). Topsoil from these sites, maintained at two different water contents (20 and 30%, w/w), were exposed to either a single (SF) or four sequential (4SF) freeze/thaw cycles. Each cycle consisted of 12 h at −9 °C and 12 h at +4 °C mimicking a diurnal pattern.A SF cycle reduced microbial N for soils from sites F and A and was accompanied by a significant increase in dissolved organic nitrogen (DON) at both moisture contents. In contrast, the microbial N of soils from M and L was not affected by the freeze/thaw cycles, suggesting a particular adaptation of soil microbes to these extremes in temperature. Freeze/thaw cycles resulted in a significant increase in the net ammonification in all soils.Extractable total dissolved N (TDN) and total dissolved P (TDP) increased in all soils after a SF cycle, however, the relative importance of the different N and P forms differed. At the lower soil moisture content, NO₃⁻ concentrations remained constant or slightly decreased in all soils, except that from site M. In all other soils DON appeared to replace NO₃⁻ as the potentially mobile N source after the freeze/thaw cycles. The relative contribution of dissolved organic P to TDP after freeze/thaw remained significant, and greater than 50% in all soils.Freeze/thaw cycles, in seasonally snow covered soils, are likely to have a selective effect on the microbial biomass. Freezing and thawing resulted in a pulse of net ammonification and DON release, which represent an important influence upon N cycling in these alpine systems.     

Leaching of soil organic carbon and nitrogen in sandy soils after cultivating grass-clover swards

Several studies have focused on the formation and losses of dissolved organic matter in forest systems, whereas a limited number have dealt with this aspect in agricultural soils. The purpose of this study was to estimate the leaching of dissolved organic carbon (DOC) and nitrogen (DON), with focus on the period after cultivating grass-clover swards. Grass-clovers were ploughed in the spring prior to sowing cereals followed by either catch crops or bare soil. The concentrations of DOC and DON decreased with soil depth and ranged at 90-cm soil depth between 7 and 21 mg C L⁻¹ and between 1 and 3 mg N L⁻¹, respectively, in a sandy loam soil, and between 16 and 63 mg C L⁻¹ and between 1 and 10 mg N L⁻¹, respectively, in a coarse sandy soil. The resulting DOC/DON ratios were in the range between 2 and 42, with higher values in the coarse sandy soil than in the sandy loam soil. The total percolation was 218 mm in the sandy loam soil and 596–645 mm in the coarse sandy soil, which resulted in an annual leaching of 22–40 kg DOC ha⁻¹ year⁻¹ and 3–4 kg DON ha⁻¹ year⁻¹ in the sandy loam soil, and 174–310 kg DOC ha⁻¹ year⁻¹ and 10–31 kg DON ha⁻¹ year⁻¹ in the coarse sandy soil. It was shown that higher amounts of DOC were lost by leaching under the catch crops than from bare soil, that losses of DON were higher from bare soil than from soils with catch crops and that DON contributed significantly to the total N loss. Thus, DON needs to be taken into account in N-balance calculations.     

Long-term applications of chemical and organic fertilizers on plant-available nitrogen pools and nitrogen management index

The objective of this study was to evaluate plant-available N pools and the role of N management index (NMI) in the surface (0–20 cm) of a fluvo-aquic soil after 18 years of fertilization treatments under a wheat–maize cropping system in the North China Plain. The experiment included seven treatments: (1) NPK, balanced application of chemical fertilizer NPK; (2) OM, application of organic manure; (3) 1/2OMN, application of half organic manure plus chemical fertilizer NPK; (4) NP, application of chemical fertilizer NP; (5) PK, application of chemical fertilizer PK; (6) NK, application of chemical fertilizer NK; and (7) CK, unfertilized control. Total organic N (TON), microbial biomass N (MBN), labile N (LN), inorganic N (ION, including ammonium (NH₄⁺)–N and nitrate (NO₃⁻)–N) contents, net ammonification rate (NAR), net nitrification rate (NNR), net N mineralization rate (NNMR), and NMI in the fertilized treatments were higher than in the unfertilized treatment. Application of chemical fertilizer N (NPK, NP, and NK) increased ION in soils, compared with application of organic N or control. Nitrate N prevailed over exchangeable NH₄⁺–N in all treatments. Nitrogen storage of the OM- and 1/2OMN-treated soils increased by 50.0% and 24.3%, respectively, over the NPK-treated soil, which had 5.4–22.5% more N than NP-, PK-, and NK-treated soils. The MBN, LN, and ION accounted for 1.7–2.4%, 25.7–34.2%, and 1.4–2.9% of TON, respectively, in different fertilization treatments. The surface soils (0–20-cm layer) in all treatments mineralized 43.6–152.9 kg N ha^–1 year^–1 for crop growth. Microbial biomass N was probably the better predictor of N mineralization, as it was correlated significantly (P < 0.01) with NNMR. The OM and 1/2OMN treatments were not an optimal option for farmers when the crop yield and labor cost were taken into consideration but an optimal option for increasing soil N supply capacity and N sequestration in soil. The NPK treatment showed the highest crop yields and increased soil N fractions through crop residues and exudates input, and thus, it may be considered as a sustainable system in the North China Plain.     

Nitrogen mineralization and CO2 and N2O emissions in a sandy soil amended with original or acidified pig slurries or with the relative fractions

The following six pig slurries obtained after acidification and/or solid/liquid separation were used in the research: original (S) and acidified (AS) pig slurry, nonacidified (LF) and acidified (ALF) pig slurry liquid fraction, and nonacidified (SF) and acidified (ASF) pig slurry solid fraction. Laboratory incubations were performed to assess the effect of the application of these slurries on N mineralization and CO₂ and N₂O emissions from a sandy soil. Acidification maintained higher NH₄ ⁺-N contents in soil particularly in the ALF-treated soil where NH₄ ⁺-N contents were two times higher than in LF-treated soil during the 55–171-day interval. At the end of the incubation (171 days), 32.9 and 24.2 mg N kg⁻¹ dry soil were mineralized in the ASF- and SF-treated soils, respectively, but no mineralization occurred in LF- and S-treated soils, although acidification decreased N immobilization in ALF- (−25.3 mg N kg⁻¹ soil) and AS- (−12.7 mg N kg⁻¹ soil) compared to LF- (−34.4 mg N kg⁻¹ soil) and S-treated (−18.6 mg N kg⁻¹ soil) soils, respectively. Most of the dissolved CO₂ was lost during the acidification process. More than 90% of the applied C in the LF-treated soil was lost during the incubation, indicating a high availability of the added organic compounds. Nitrous oxide emissions occurred only after day 12 and at a lower rate in soils treated with acidified than nonacidified slurries. However, during the first 61 days of incubation, 1,157 μg N kg⁻¹ soil was lost as N₂O in the AS-treated soil and only 937 in the S-treated soil.     

Influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol

In this study, the influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol from north east Scotland was investigated. Firstly, soil cores were incubated at 5, 10 and 15°C for up to 8 weeks. Net mineralisation was observed at all temperatures with larger rates observed at higher temperatures. In contrast, water extractable dissolved organic nitrogen (DON) displayed no clear trend with time and showed little response to temperature. Secondly, intact cores of the same soil, with and without vegetation, were leached with artificial rain for 6 weeks at 6.5 and 15°C. Temperature and the presence of vegetation interacted to have a significant (P<0.01) effect on the concentration of NO₃⁻ in leachates; highest concentrations were observed in leachates from cores without vegetation at 15°C, whereas lowest concentrations were observed in leachates from cores with vegetation at 6.5°C. In contrast, concentrations of DON and dissolved organic carbon (DOC) were significantly (P<0.001) higher in leachates from cores with vegetation than without vegetation and were not affected by temperature. The cumulative amounts of DON and DOC leached from the cores with vegetation were 4 and 2.5 times greater, respectively, than those leached from the cores without vegetation. Comparison of soil solution (extracted by centrifugation at 0–5 and 5–10 cm depth) after leaching for 6 weeks, showed that the upper layer contained more than twice the amount of DON than the 5–10 cm layer and that the difference in concentration between the two depths was enhanced in the presence of vegetation. The results indicate that vegetation is an important source of DON and DOC. However, the removal of vegetation did not lead to an increase in the quantity of total dissolved nitrogen (TDN) in soil water, but resulted in a change in the dominant N fraction from DON to NO₃⁻. In addition, the results show that DON, in both the incubated and leached cores, did not change as inorganic N was mineralised. This suggests that if water extractable DON was acting as a source of NH₄⁺ or NO₃⁻, then it was being replenished by, and in equilibrium with, a large reserve of organic N. Evidence of such a pool was indirect in the form of additional DON (equivalent to 2 g N m⁻²) being extracted by 0.5 M K₂SO₄.     

Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N₂O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N₂O emission. Residues were added at the rate of 3 t C ha⁻¹ to soil with, and without, 150 kg urea N ha⁻¹. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO₃⁻–N, which reached minimum values of 2.8 mg N kg⁻¹ for sugarcane (at day 28), 10.3 mg N kg⁻¹ for maize (day 7), and 5.9 mg N kg⁻¹ for sorghum (day 7), compared to 22.7 mg N kg⁻¹ for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg⁻¹ in sugarcane, 34 mg kg⁻¹ in maize, 29 mg kg⁻¹ in sorghum, and 16 mg kg⁻¹ in cotton amended soil compared to soil + N fertilizer, although soil NO₃⁻–N increased by 7 mg kg⁻¹ in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N₂O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N₂O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N₂O emission was significantly and positively correlated with NO₃⁻–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO₂ in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).     

Impact of land-use change on nitrogen mineralization in subalpine grasslands in the Southern Alps

 A field study was conducted to investigate the effect of abandonment of management on net N mineralization (NNM) in subalpine meadows. NNM, soil microbial biomass N (SMBN), fungal biomass and physicochemical characteristics (total C, total N, dissolved organic carbon (DOC) and pH) were determined in surface (0–10 cm) soil from May to October 1997 in an intensively managed and an abandoned meadow at 1770 m a.s.l.. The cumulative NNM was lower in the abandoned area and ranged from 150 to 373 and from 25 to 85 μg N g^–1 soil in the intensively managed and the abandoned areas, respectively. The total organic C increased in the abandoned area, while total N showed no difference between abandoned and managed meadow. SMBN showed no difference between the investigated sites, whereas ergosterol, a measure of fungal biomass, increased significantly at the abandoned site. The cumulative NNM was negatively correlated with total organic C, C : N ratio, DOC and ergosterol content, and positively correlated with the NH₄ ⁺-N content of soil. The decrease in NNM at the abandoned site may be explained by the lower availability of N in substrates characterized by a high C : N ratio which, together with a decrease in pH in the litter layer, may have increased fungal biomass. Received: 8 January 1999     

Evaluation of biological and chemical nitrogen indices for predicting nitrogen-supplying capacity of paddy soils in the Taihu Lake region,China

Simple and rapid chemical indices of soil nitrogen (N)-supplying capacity are necessary for fertilizer recommendations. In this study, pot experiment involving rice, anaerobic incubation, and chemical analysis were conducted for paddy soils collected from nine locations in the Taihu Lake region of China. The paddy soils showed large variability in N-supplying capacity as indicated by the total N uptake (TNU) by rice plants in a pot experiment, which ranged from 639.7 to 1,046.2 mg N pot⁻¹ at maturity stage, representing 5.8% of the total soil N on average. Anaerobic incubation for 3, 14, 28, and 112 days all resulted in a significant (P < 0.01) correlation between cumulative mineral NH₄⁺-N and TNU, but generally better correlations were obtained with increasing incubation time. Soil organic C, total soil N, microbial C, and ultraviolet absorbance of NaHCO₃ extract at 205 and 260 nm revealed no clear relationship with TNU or cumulative mineral NH₄⁺-N. Soil C/N ratio, acid KMnO₄-NH₄⁺-N, alkaline KMnO₄-NH₄⁺-N, phosphate–borate buffer extractable NH₄⁺-N (PB-NH₄⁺-N), phosphate–borate buffer hydrolyzable NH₄⁺-N (PB_HYDR-NH₄⁺-N) and hot KCl extractable NH₄⁺-N (H_KCl−NH₄⁺-N) were all significantly (P < 0.05) related to TNU and cumulative mineral NH₄⁺-N of long-term incubation (>28 days). However, the best chemical index of soil N-supplying capacity was the soil C/N ratio, which showed the highest correlation with TNU at maturity stage (R = −0.929, P < 0.001) and cumulative mineral NH₄⁺-N (R = −0.971, P < 0.001). Acid KMnO₄-NH₄⁺-N plus native soil NH₄⁺-N produced similar, but slightly worse predictions of soil N-supplying capacity than the soil C/N ratio.     

Severe soil frost reduces losses of carbon and nitrogen from the forest floor during simulated snowmelt: A laboratory experiment

Considerable progress has been made in understanding the impacts of soil frost on carbon (C) and nitrogen (N) cycling, but the effects of soil frost on C and N fluxes during snowmelt remain poorly understood. We conducted a laboratory experiment to determine the effects of soil frost on C and N fluxes from forest floor soils during snowmelt. Soil cores were collected from a sugar maple (Acer saccharum)–American beech (Fagus grandifolia) and a red spruce (Picea rubens)–balsam fir (Abies balsamea) forest at the Hubbard Brook Experimental Forest in New Hampshire, U.S.A. Soils were exposed to one of three temperature treatments, including severe (?15 °C), mild (?0.5 °C), and no soil frost (+5 °C) conditions. After one week the soils were incubated at +5 °C and snow was placed on top of the soils to simulate spring snowmelt. NO₃^? losses were up to 5.5 mg N kg^?1 soil greater in the mild soil frost treatment than the severe soil frost treatment. Net losses of NH₄⁺ and DON in leachate were up to 19 and 18 mg N kg^?1 soil greater in the no soil frost and mild soil frost treatments, respectively, than the severe soil frost treatment. In contrast, soil frost did not have a significant impact on dissolved organic C or cumulative gaseous fluxes of C and N throughout the snowmelt period. However, the total cumulative flux of C (i.e. dissolved organic C + CO₂ + CH₄) and N (i.e. dissolved organic N + NH₄ + NO₃ + N₂O) in the severe soil frost treatment were between one quarter and one half that observed in the no soil frost treatment for both forest types. Together, the results of this study show that total fluxes of N in leachate, as well as total cumulative C and N fluxes (gases + leachate), were significantly reduced following severe soil frost. We conclude that the extent to which C and N cycling during snowmelt is altered in response to changes in winter climate depend on both the presence and severity of soil frost.     

An evaluation of plant-available soil nitrogen in selected sandy soils by electro-ultrafiltration, KCl, and CaCl2 extraction methods

 Improving the precision in estimating the nitrogen (N) requirement for citrus trees on sandy soils is important for increasing N efficiency by the trees and minimizing potential losses of N in commercial citrus production areas. In this study, representative Florida soils were sampled from major citrus production areas and the electro-ultrafiltration (EUF) technique was used to measure the concentrations of total EUF-extractable nitrogen (EUF-N_t), ammonium-N (EUF-NH₄ ⁺–N) and nitrate-N (EUF-NO₃ ^––N). Available organic N (N_org) was calculated as: EUF-N_t–(NH₄ ⁺–N+NO₃ ^––N). The N concentrations in the EUF extraction were greater than those by the KCl or CaCl₂ method. The N_org fraction, estimated by the EUF method, varied from 4.4 to 40.8 mg kg^–1 soil, equivalent to 10 to 91 kg N ha^–1 (for the top 15 cm depth soil) and was positively correlated with the total soil N determined by the Kjeldahl method. The presence of appreciable amounts of N_org in these soils indicates that these soils contain high proportions of the total soil N in easily mineralizable N_org forms. This study demonstrates that the EUF-extractable organic bound N must be considered in developing N fertilizer recommendations for citrus. Received: 13 January 1999     

Carbon and nitrogen mineralization dynamics in different soils of the tropics amended with legume residues and contrasting soil moisture contents

Seasonal drought in tropical agroecosystems may affect C and N mineralization of organic residues. To understand this effect, C and N mineralization dynamics in three tropical soils (Af, An₁, and An₂) amended with haricot bean (HB; Phaseolus vulgaris L.) and pigeon pea (PP; Cajanus cajan L.) residues (each at 5 mg g⁻¹ dry soil) at two contrasting soil moisture contents (pF2.5 and pF3.9) were investigated under laboratory incubation for 100–135 days. The legume residues markedly enhanced the net cumulative CO₂–C flux and its rate throughout the incubation period. The cumulative CO₂–C fluxes and their rates were lower at pF3.9 than at pF2.5 with control soils and also relatively lower with HB-treated than PP-treated soil samples. After 100 days of incubation, 32–42% of the amended C of residues was recovered as CO₂–C. In one of the three soils (An₁), the results revealed that the decomposition of the recalcitrant fraction was more inhibited by drought stress than easily degradable fraction, suggesting further studies of moisture stress and litter quality interactions. Significantly (p < 0.05) greater NH₄⁺–N and NO₃⁻–N were produced with PP-treated (C/N ratio, 20.4) than HB-treated (C/N ratio, 40.6) soil samples. Greater net N mineralization or lower immobilization was displayed at pF2.5 than at pF3.9 with all soil samples. Strikingly, N was immobilized equivocally in both NH₄⁺–N and NO₃⁻–N forms, challenging the paradigm that ammonium is the preferred N source for microorganisms. The results strongly exhibited altered C/N stoichiometry due to drought stress substantially affecting the active microbial functional groups, fungi being dominant over bacteria. Interestingly, the results showed that legume residues can be potential fertilizer sources for nutrient-depleted tropical soils. In addition, application of plant residue can help to counter the N loss caused by leaching. It can also synchronize crop N uptake and N release from soil by utilizing microbes as an ephemeral nutrient pool during the early crop growth period.     

Analysis and behavior of soluble organic nitrogen in forest soils

Background, aim, and scope  

A large proportion of soil nitrogen (N; >80%) is present in organic form. Current research on plant N uptake in terrestrial ecosystems has focused mainly on inorganic N such as ammonium (NH₄ ⁺) and nitrate (NO₃ ⁻), while soluble organic N (SON) has received little attention. In recent years, the increasing evidence showing the direct uptake of various amino acids by plants and the predominance of the organic form in N loss by leaching in many forest ecosystems has drawn attention to critically re-examine the nature and the ecological role of soil SON in terrestrial N cycling. However, little is known about the sources and dynamics, chemical nature, and ecological functions of soil SON in forest ecosystems. This paper reviews recent advances in the areas of research on current techniques for characterizing soil SON and the size, nature, and dynamics of soil SON pools in forest ecosystems.     

Effects of nitrogen fertilizer application rates on nitrate nitrogen distribution in saline soil in the Hai River Basin,China

Background and Objectives  

Soil nitrate nitrogen (NO₃ ⁻-N) accumulation is related closely to NO₃ ⁻ leaching, which is an important issue in groundwater pollution, especially in intensive agricultural areas with saline soils where volumes of water are used in irrigation to avoid salt accumulation in the root zone. However, in the saline environment in Hai River Basin, China, the importance of detailed research into NO₃ ⁻-N distribution in the root zone has not been adequately recognized. Considering the impacts of eco-environmental system N and crop production, the present study aimed at contributing to an understanding of the effects of N application rate on soil NO₃ ⁻-N distribution, NO₃ ⁻-N residue, N loss, and maize (Zea mays L.) yield in this region.     

Quantification of nitrogen excretion rates for three lumbricid earthworms using 15N

 Nitrogen excretion rates of ¹⁵N-labeled earthworms and contributions of ¹⁵N excretion products to organic (dissolved organic N) and inorganic (NH₄-N, NO₃-N) soil N pools were determined at 10  °C and 18  °C under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clitellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rubellus (Hoffmeister) were labeled with ¹⁵N by providing earthworms with ¹⁵N-labeled organic substrates for 5–6 weeks. The quantity of ¹⁵N excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were calculated. N excretion rates ranged from 274.4 to 744 μg N g^–1 earthworm fresh weight day^–1, with a daily turnover of 0.3–0.9% of earthworm tissue N. The N excretion rates of juvenile L. terrestris were significantly lower than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, particularly NH₄-N, were greater in soils incubated with earthworms for 48 h than soils incubated without earthworms. Between 13 and 40% of excreted ¹⁵N was found in the ¹⁵N-mineral N (NH₄-N+NO₃-N) pool, and 13–23% was in the ¹⁵N-DON pool. Other fates of excreted ¹⁵N may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N forms, or gaseous N losses. Earthworm excretion rates were combined with earthworm biomass measurements to estimate N flux from earthworm populations through excretion. Annual earthworm excretion was estimated at 41.5 kg N ha^–1 in an inorganically-fertilized corn agroecosystem, and was equivalent to 22% of crop N uptake. Our results suggest that the earthworms could contribute significantly to N cycling in corn agroecosystems through excretion processes. Received: 12 April 1999     

Conversion of grassland to coniferous woodland has limited effects on soil nitrogen cycle processes

In the last century, conversion of native North American grasslands to Juniperus virginiana forests or woodlands has dramatically altered ecosystem structure and significantly increased ecosystem carbon (C) stocks. We compared soils under recently established J. virginiana forests and adjacent native C₄-dominated grassland to assess changes in potential soil nitrogen (N) transformations and plant available N. Over a 2-year period, concentrations of extractable inorganic N were measured in soils from forest and grassland sites. Potential gross N ammonification, nitrification, and consumption rates were determined using ¹⁵N isotope-dilution under laboratory conditions, controlling for soil temperature and moisture content. Potential nitrification rates (V_max) and microbial biomass, as well as soil physical and chemical properties were also assessed. Extractable NH₄⁺ concentrations were significantly greater in grassland soils across the study period (P ≤ 0.01), but analysis by date indicated that differences in extractable inorganic N occurred more frequently in fall and winter, when grasses were senescent but J. virginiana was still active. Laboratory-based rates of gross N mineralization (ammonification) and nitrification were greater in grassland soils (P ≤ 0.05), but only on one of four dates. Potential nitrification rates (V_max) were an order of magnitude greater than gross nitrification rates in both ecosystems, suggesting that nitrification is highly constrained by NH₄⁺ availability. Differences in plant uptake of N, C inputs, and soil microclimate as forests replace grasslands may influence plant available N in the field, as evidenced by seasonal differences in soil extractable NH₄⁺, and total soil C and N accumulation. However, we found few differences in potential soil N transformations under laboratory conditions, suggesting that this grassland-to-forest conversion caused little change in mineralizable organic N pools or potential microbial activity.     

Recovery of soil organic matter,organic matter turnover and nitrogen cycling in a post-mining forest rehabilitation chronosequence

Recovery of soil organic matter, organic matter turnover and mineral nutrient cycling is critical to the success of rehabilitation schemes following major ecosystem disturbance. We investigated successional changes in soil nutrient contents, microbial biomass and activity, C utilisation efficiency and N cycling dynamics in a chronosequence of seven ages (between 0 and 26 years old) of jarrah (Eucalyptus marginata) forest rehabilitation that had been previously mined for bauxite. Recovery was assessed by comparison of rehabilitation soils to non-mined jarrah forest references sites. Mining operations resulted in significant losses of soil total C and N, microbial biomass C and microbial quotients. Organic matter quantity recovered within the rehabilitation chronosequence soils to a level comparable to that of non-mined forest soil. Recovery of soil N was faster than soil C and recovery of microbial and soluble organic C and N fractions was faster than total soil C and N. The recovery of soil organic matter and changes to soil pH displayed distinct spatial heterogeneity due to the surface micro-topography (mounds and furrows) created by contour ripping of rehabilitation sites. Decreases in the metabolic quotient with rehabilitation age conformed to conceptual models of ecosystem energetics during succession but may have been more indicative of decreasing C availability than increased metabolic efficiency. Net ammonification and nitrification rates suggested that the low organic C environment in mound soils may favour autotrophic nitrifier populations, but the production of nitrate (NO₃^?) was limited by the low gross N ammonification rates (≤1 μg N g^?1 d^?1). Gross N transformation rates in furrow soils suggested that the capacity to immobilise N was closely coupled to the capacity to mineralise N, suggesting NO₃^? accumulation in situ is unlikely. The C:N ratio of the older rehabilitation soils was significantly lower than that of the non-mined forest soils. However, variation in ammonification rates was best explained by C and N quantity rather than C:N ratios of whole soil or soluble organic matter fractions. We conclude that the rehabilitated ecosystems are developing a conservative N cycle as displayed by non-mined jarrah forests. However, further investigation into the control of nitrification dynamics, particularly in the event of further ecosystem disturbance, is warranted.     

Short-term effects of ammonium nitrogen in upland pasture soil affected or not affected by cattle

The short-term effects of excessive NH₄⁺-N on selected characteristics of soil unaffected (low annual N inputs) and affected (high annual N inputs) by cattle were investigated under laboratory conditions. The major hypothesis tested was that above a theoretical upper limit of NH₄⁺ concentration, an excess of NH₄⁺-N does not further increase NO₃⁻ formation rate in the soil, but only supports accumulation of NO₂⁻-N and gaseous losses of N as N₂O. Soils were amended with 10 to 500 μg NH₄⁺-N g⁻¹ soil. In both soils, addition of NH₄⁺-N increased production of NO₃⁻-N until some limit. This limit was higher in cattle-affected soil than in unaffected soil. Production of N₂O increased in the whole range of amendments in both soils. At the highest level of NH₄⁺-N addition, NO₂⁻-N accumulated in cattle-affected soil while NO₃⁻-N production decreased in cattle-unaffected soil. Despite being statistically significant, observed effects of high NH₄⁺-N addition were relatively weak. Uptake of mineral N, stimulated by glucose amendment, decreased the mineral N content in both soils, but it also greatly increased production of N₂O.     

Contribution of dissolved organic nitrogen deposition to nitrogen saturation in a forested mountainous watershed in Tsukui, Central Japan

Nitrogen (N) budget was estimated with dissolved inorganic N (DIN) and dissolved organic N (DON) in a forested mountainous watershed in Tsukui, Kanagawa Prefecture, about 50 km west of Tokyo in Central Japan. The forest vegetation in the watershed was dominant by Konara oak (Quercus serrata) and Korean hornbeam (Carpinus tschonoskii), and Japanese cedar (Cryptomeria japonica). Nitrate (NO₃ ^?) concentration in the watershed streamwater was averagely high (98.0 ±± 19 (±± SD, n = 36) μmol L^?1) during 2001–2003. There was no seasonal and annual changes in the stream NO^? ₃ concentration even though the highest N uptake rate presumably occurred during the spring of plant growing season, a fact indicating that N availability was in excess of biotic demands. The DON deposition rates (DON input rates) in open area and forest area were estimated as one of the main N sources, accounting for about 32% of total dissolved N (TDN). It was estimated that a part of the DON input rate contributed to the excessive stream NO^? ₃ output rate under the condition of the rapid mineralization and nitrification rates, which annual DON deposition rates were positively correlated with the stream NO₃ ^? output rates. The DON retention rate in the DON budget had a potential capacity, which contributed to the excessive stream NO^? ₃ output rate without other N contributions (e.g. forest floor N or soil N).     

Contribution of dissolved organic nitrogen deposition to total dissolved nitrogen deposition under intensive agricultural activities

For elucidating the atmospheric deposition contribution of dissolved organic nitrogen (DON) to the total dissolved nitrogen (TDN) deposition rate, dissolved inorganic nitrogen (DIN: NH₄ ⁺ + NO₃ ^–) and DON deposition rates were annually and monthly estimated during 4 and half-yr monitoring period in an experimental multi-farm under intensive agricultural activities of N fertilizer use and animal husbandry in Central Japan. Annual NH₄ ⁺, DON and NO₃ ^– deposition rates in bulk and wet deposition data accounted for 48%, 32% and 20% of TDN deposition, respectively, which indicated that this area is strongly affected by the intensive agricultural activities. The DIN and DON deposition rates were respectively estimated at 21.6 and 10.1 kg N ha^?1 yr^?1, which ranked high in a worldwide regional data set. Consequently, this area has been exposed to a large amount of N deposition including DON with N fertilizer input. The difference between bulk and wet deposition rates (NH₄ ⁺ and DON) is one of important factors controlling the N deposition in this area. Monthly DON deposition showed positive correlations with DIN and NH₄ ⁺ deposition rates, respectively, with a significant linear regression curve. The linear regression curve of our monthly data (n = 127) indicates the same trend as the worldwide annual data set (n = 31).     

Changes in microbial community structure in soil as a result of different amounts of nitrogen fertilization

The changes in size, activity and structure of soil microbial community caused by N fertilization were studied in a laboratory incubation experiment. The rates of N fertiliser applied (KNO₃) were 0 (control), 100 and 2,000 μg N g⁻¹ soil. Despite no extra C sources added, a high percentage of N was immobilized. Whereas no significant increase of microbial C was revealed during incubation period, microbial growth kinetics as determined by the substrate-induced growth-response method demonstrated a significant decrease in the specific growth rate of microbial community in soil treated with 2,000 μg N g⁻¹ soil. Additionally, a shift in microbial community structure resulting in an increase in fungal biomarkers, mainly in the treatment with 2,000 μg N g⁻¹ soil was visible.