Amino acid,peptide and protein mineralization dynamics in a taiga forest soil

The availability of inorganic N has been shown to be one of the major factors limiting primary productivity in high latitude ecosystems. The factors regulating the rate of transformation of organic N to nitrate and ammonium, however, remain poorly understood. The aim of this study was to investigate the nature of the soluble N pool in forest soils and to determine the relative rate of inorganic N production from high and low molecular weight (MW) dissolved organic nitrogen (DON) compounds in black spruce forest soils. DON was found to be the dominant N form in soil solution, however, most of this DON was of high MW of which >75% remained unidentified. Free amino acids constituted less than 5% of the total DON pool. The concentration of NO₃⁻ and NH₄⁺ was low in all soils but significantly greater than the concentration of free amino acids. Incubations of low MW DON with soil indicated a rapid processing of amino acids, di- and tri-peptides to NH₄⁺ followed by a slower transformation of the NH₄⁺ pool to NO₃⁻. The rate of protein transformation to NH₄⁺ was slower than for amino acids and peptides suggesting that the block in N mineralization in taiga forest soils is the transformation of high MW DON to low MW DON and not low MW DON to NH₄⁺ or NH₄⁺ to NO₃⁻. Calculated turnover rates of amino acid-derived C and N immobilized in the soil microbial biomass were similar with a half-life of approximately 30 d indicating congruent C and N mineralization.     

Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems

Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH₄⁺) and nitrate (NO₃⁻). To compare NH₄⁺ availability, competition between nitrifiers and heterotrophic microorganisms for NH₄⁺, and microbial NO₃⁻ assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, ¹⁵N isotope pool dilution and ¹⁵N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH₄⁺ as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO₃⁻ and NH₄⁺ was estimated based on the uptake of ¹⁵N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH₄⁺ were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO₃⁻ assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO₃⁻ than NH₄⁺. Heterotrophic microbes assimilated less NH₄⁺ than NO₃⁻ probably because NH₄⁺ concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH₄⁺ in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO₃⁻ immobilization.     

Microbial activity differentially regulates the vertical mobility of nitrogen compounds in soil

Alongside nitrate, dissolved organic nitrogen (DON) represents a significant N loss pathway in many agroecosystems. To better understand the factors controlling DON leaching in soil we followed the vertical movement of ¹⁵N-labeled NO₃⁻, NH₄⁺, alanine and trialanine in packed soil columns in response to a simulated rainfall event. We show that in autoclaved (sterile) soil where sorption is assumed to be the dominant regulating factor, leaching followed the series NO₃⁻ > trialanine > alanine > NH₄⁺. In the non-sterile packed soil columns, the rapid rate of NO₃⁻ leaching was unaffected whilst the movement of the amino acid, peptide and NH₄⁺ was almost completely prevented due to microbial immobilization. Our results support the view that (1) DON loss from agricultural soils occurs mainly in the form of recalcitrant compounds (e.g. humic DON) rather than in the form of labile low MW DON (e.g. oligopeptides and amino acids), and (2) that although nitrate was bioavailable, it was not a preferred N form for the C-limited microbial biomass.     

Differing N status and N retention processes of soils under old-growth lowland forest in Eastern Amazonia,Caxiuanã, Brazil

Indirect evidence of the nitrogen (N) status of tropical forests strongly suggests that in heavily weathered soils under old-growth lowland tropical forests nitrogen is in relative excess. However, within the lowland forests of the Amazon basin, there is substantial evidence that soil texture influences soil NH₄⁺ and NO₃^? concentrations and hence possibly N availability and retention in the soil. Here, we evaluate the soil N status of two heavily weathered soils which contrast in texture (sandy versus clay Oxisol). Using ¹⁵N pool dilution, we quantified gross rates of soil N cycling and retention. We also measured the δ¹⁵N signatures from the litter layer down to 50-cm depth mineral soil and calculated the overall ¹⁵N enrichment factor (ε) for each soil type. The clay soil showed high gross N mineralization and nitrification rates and a high overall ¹⁵N enrichment factor, signifying high N losses. The sandy soil had low gross rates of N cycling and ¹⁵N enrichment factor, manifesting a conservative soil N cycling. Faster turnover rates of NH₄⁺ compared to NO₃^? indicated that NH₄⁺ cycles faster through microorganisms than NO₃^?, possibly contributing to better retention of NH₄⁺ than NO₃^?. However this was opposite to abiotic retention processes, which showed higher conversion of NO₃^? to the organic N pool than NH₄⁺. Our combined results suggest that clay Oxisol in Amazonian forest have higher N availability than sandy Oxisol, which will have important consequences for changes in soil N cycling and losses when projected increase in anthropogenic N deposition will occur.     

Heterotrophic nitrification is the predominant NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> production mechanism in coniferous but not broad-leaf acid forest soil in subtropical China

A study was carried out to investigate the potential gross nitrogen (N) transformations in natural secondary coniferous and evergreen broad-leaf forest soils in subtropical China. The simultaneously occurring gross N transformations in soil were quantified by a ¹⁵N tracing study. The results showed that N dynamics were dominated by NH₄⁺ turnover in both soils. The total mineralization (from labile and recalcitrant organic N) in the broad-leaf forest was more than twice the rate in the coniferous forest soil. The total rate of mineral N production (NH₄⁺ + NO₃⁻) from the large recalcitrant organic N pool was similar in the two forest soils. However, appreciable NO₃⁻ production was only observed in the coniferous forest soil due to heterotrophic nitrification (i.e. direct oxidation of organic N to NO₃⁻), whereas nitrification in broad-leaf forest was little (or negligible). Thus, a distinct shift occurred from predominantly NH₄⁺ production in the broad-leaf forest soil to a balanced production of NH₄⁺ and NO₃⁻ in the coniferous forest soil. This may be a mechanism to ensure an adequate supply of available mineral N in the coniferous forest soil and most likely reflects differences in microbial community patterns (possibly saprophytic, fungal, activities in coniferous soils). We show for the first time that the high nitrification rate in these soils may be of heterotrophic rather than autotrophic nature. Furthermore, high NO₃⁻ production was only apparent in the coniferous but not in broad-leaf forest soil. This highlights the association of vegetation type with the size and the activity of the SOM pools that ultimately determines whether only NH₄⁺ or also a high NO₃⁻ turnover is present.     

Wetting and drying events determine soil N pools in two Mediterranean ecosystems

To improve our knowledge of how nutrient cycling in Mediterranean environments responds to climate change, we evaluated the effects of the continuous changes in soil nitrogen (N) pools during natural wetting and drying events. We measured soil N pools (microbial biomass [MB-N], dissolved organic nitrogen [DON], NH₄⁺ and NO₃⁻) and N ion exchange resins at weekly intervals for one year in two contrasting Mediterranean ecosystems. All soil N fractions in both ecosystems showed high intraseasonal and interseasonal variability that was greater in inorganic soil fractions than in organic N soil fractions. MB-N, DON and resin-NH₄⁺ showed increased concentrations during wetting events. Only the soil NO₃⁻ and resin-NO₃⁻ showed the opposite trend, suggesting a different response to water pulses compared to the other soil variables. Our results show that N pools are continuously changing, and that this high variability is not associated with the total amount of organic matter and labile soil carbon (C) and N soil fractions found in each ecosystem. The highest variability was found for inorganic N forms, which suggests that organic N forms are more buffered in soils exposed to wetting-drying cycles. Our results suggest that the changes in wetting-drying cycles expected with global climate change may have a significant impact on the availability and turnover of organic and inorganic N.     

Microbial populations immobilizing NH4-N and NO3-N differ in their sensitivity to sodium chloride salinity in soil

An incubation experiment was conducted to study the response to sodium chloride (NaCl) salinity of microbial population immobilizing NH₄⁺- and NO₃⁻-N using glucose as an easily oxidizable C source. Immobilization of NH₄⁺-N was faster than that of NO₃⁻-N and was complete within 12 h of -incubation. Presence of NaCl retarded the process of N immobilization; that of NO₃⁻-N being more affected. Remineralization of immobilized N started within 48 h in case of both NH₄⁺- and NO₃⁻-N and was faster for the latter. Both remineralization and nitrification were significantly delayed in the presence of NaCl; inhibition being more at 4000 mg NaCl kg⁻¹ soil. The inhibitory effect of NaCl on remineralization of N was relatively more for NH₄⁺-treated soil. The results of the study suggested a higher sensitivity to NaCl of microorganisms assimilating NO₃⁻. However, remineralization of N from NO₃⁻-assimilating microbial population was less affected by NaCl salinity compared to NH₄⁺-assimilating population.     

Effects of annual grass senescence on NH4 and N-glycine uptake by blue oak (Quercus douglasii) seedlings and soil microorganisms in California oak woodland

Annual grasses are stronger competitors for available soil N than blue oak seedlings and soil microorganisms. However, little is known about the dynamics of N competition during annual grass senescence. We conducted a field experiment in a California oak woodland to study effects of annual grass senescence on N uptake by grasses, blue oak seedlings and soil microorganisms. Labeled N was applied at the beginning of April, May and of June in the form of ¹⁵NH₄⁺ or ¹⁵N-glycine. Plants and soils were harvested after 5 days (¹⁵NH₄⁺ and ¹⁵N-glycine treatments) and after 26 days (¹⁵NH₄⁺ treatment only). We evaluated effects of N form, season and labeling period on N competition among oak seedlings, annual grasses and soil microorganisms. N forms did not affect competition among grasses, oak seedlings and soil microorganisms, but more ¹⁵N was incorporated into the soil organic N pool in the ¹⁵N-glycine treatments than in the ¹⁵NH₄⁺ treatments. There were no seasonal (May vs June) effects on ¹⁵N recovery in blue oak seedlings and soil microorganisms. Plant samples from April harvest were lost. In June, when grasses were senescing, more ¹⁵N was found in the soil inorganic pool than in May. Extremely dry soils in June may have limited inorganic N availability to oak seedlings and soil microorganisms. After 26-day labeling period, ¹⁵N recovery in blue oak seedlings and the soil organic N pool significantly increased, while ¹⁵N recovery in both the soil microbial and inorganic N pools decreased compared to the 5-day labeling period. Although blue oak seedling biomass changed little from early May to late June, N concentrations in oak roots increased 53%. In contrast, annual grass biomass peaked in May, and then decreased rapidly. Our results suggest that blue oak seedlings and annual grasses have different temporal competitive abilities. Blue oak seedlings appear to have a long-term strategy for N competition. Blue oaks take up N slowly but steadily, increasing N uptake from 5 to 26 days. This extended time period has a greater positive effect on N uptake than does reduced grass uptake caused by senescence.     

Soil acidification used as a management strategy to reduce nitrate losses from agricultural land

pH is known to be a primary regulator of nutrient cycling in soil. Increasing soil acidity in agricultural systems has the potential to slow down N cycling and reduce N losses from leaching thereby enhancing sustainability and reducing pollution. We conducted a field experiment to investigate the impact of acidity on N leaching in arable and grassland agricultural systems. The results showed that nitrate (NO₃⁻) concentrations in soil water were greater under arable than under grassland. Soil acidification significantly lowered NO₃⁻ concentrations in soil water over winter and spring under grassland, whilst in cereal plots a similar effect was only observed in spring. Our results suggest that soil acidification decreased nitrification causing an accumulation of NH₄⁺ which was not subject to leaching. Dissolved organic nitrogen (DON) concentrations in soil water were significantly greater under arable than grassland. Soil acidification lowered concentrations of DON in soil water, usually to a greater extent in grassland than in arable plots. It was concluded that it may be possible to use careful soil pH management as a tool to control NO₃⁻ leaching without compromising the quality of drainage water, and that this may be more effective on grassland than on arable crops.     

Potential importance of bacteria and fungi in nitrate assimilation in soil

Soil microorganisms can use a wide range of N compounds but are thought to prefer NH₄⁺. Nevertheless, ¹⁵N isotope dilution studies have shown that microbial immobilization of NO₃⁻ can be an important process in many soils, particularly relatively undisturbed soils. Our objective was to develop a method for measuring NO₃⁻ immobilization potential so that the relative contributions of bacteria and fungi could be determined. We modified and optimized a soil slurry method that included amendments of KNO₃, glucose, and methionine sulfoximine (an inhibitor of N assimilation) in the presence of two protein synthesis inhibitors: chloramphenicol, which inhibits bacteria, or cycloheximide, which inhibits fungi. By adding ¹⁵N-labeled KNO₃, we were able to measure gross rates of NO₃⁻ production (i.e., gross nitrification) and consumption (i.e., gross NO₃⁻ immobilization). We found that bacteria, not fungi, had the greatest potential for assimilating, or immobilizing, NO₃⁻ in these soils. This is consistent with their growth habit and distribution in the heterogeneous soil matrix.     

Short-term competition between crop plants and soil microbes for inorganic N fertilizer

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH₄⁺), nitrate (NO₃⁻) or a combination thereof are expected to differ in soil N transformation rates and fates of NH₄⁺ and NO₃⁻. Using ¹⁵N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH₄⁺, NO₃⁻ or NH₄NO₃. Within each fertilizer treatment traces of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were added separately. During 8 days of fertilization the fate of fertilizer ¹⁵N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45-80% of initially applied ¹⁵N was recovered in crop plants compared to only 1-10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO₃⁻ than NH₄⁺ independent of initially applied N form. Surprisingly, no inhibitory effect of NH₄⁺ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH₄⁺ uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH₄⁺ in both soils. The rapid conversion of NH₄⁺ to NO₃⁻ and preferential use of NO₃⁻ by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO₃⁻ and shift towards enhanced reliance on NO₃⁻ for their N supply.     

Differences in isotopic fractionation of nitrogen in water-saturated and unsaturated soils

Temporal variations in δ¹⁵N of NH₄⁺ and NO₃⁻ in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ¹⁵N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg⁻¹. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH₄⁺-N concentration decreased from 164.8 to 34.4 mg kg⁻¹, and the δ¹⁵N of NH₄⁺ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO₃⁻-N concentration and lower δ¹⁵N of NO₃⁻ (product) than that of NH₄⁺ (substrate) at each sampling time. In saturated soil, the concentration of NH₄⁺-N decreased gradually from 162.4 to 24.2 mg kg⁻¹, and the δ¹⁵N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO₃⁻ concentration was not observed due to loss of NO₃⁻ through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (α_s/p) associated with decrease in NH₄⁺ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH₄⁺ were relatively more dominant than nitrification under the anaerobic conditions.     

Nitrogen fertilization inhibits soil microbial respiration regardless of the form of nitrogen applied

We tested how amendments of different forms of nitrogen (N) affect microbial respiration rates by adding six different forms of N (NH₄NO₃, (NH₂)₂CO (urea), KNO₃, NH₄Cl, (NH₄)₂SO₄, Ca(NO₃)₂) to three distinct soils. All inorganic N forms led to a net reduction in microbial respiration, and the magnitude of the observed response (up to 60 % reduction) was consistent across all soils and negatively correlated with N concentration. Urea also reduced respiration rates in nearly all cases, but the effect was attenuated by the associated input of labile organic carbon. We observed decreases in respiration regardless of soil type, the specific N counter ion, N added as NH₄⁺ or NO₃⁻, or the effects of N form on soil pH, suggesting that decreases in respiration rates were mainly a direct result of the increase in soil N availability, rather than indirect effects caused by the form of N added.     

Dissolved organic carbon and nitrogen dynamics in temperate coniferous forest plantations

Dissolved organic matter (DOM) plays a central role in driving many chemical and biological processes in soil; however, our understanding of the fluxes and composition of the DOM pool still remains unclear. In this study we investigated the composition and dynamics of dissolved organic carbon (DOC) and nitrogen (DON) in five temperate coniferous forests. We subsequently related our findings to the inputs (litterfall, throughfall, atmospheric deposition) and outputs (leaching, respiration) of C and N from the forest and to plant available sources of N. With the exception of NO₃^?, most of the measured soil solution components (e.g. DOC, DON, NH₄⁺, free amino acids, total phenolics and proteins) progressively declined in concentration with soil depth, particularly in the organic horizons. This decline correlated well with total microbial activity within the soil profile. We calculated that the amount of C lost by soil respiration each day was equivalent to 70% of the DOC pool and 0.06% of the total soil C. The rapid rate of amino acid mineralization and the domination of the low molecular weight soluble N pool by inorganic N suggest that the microbial community is C‐ rather than N‐limited and that C‐limitation increases with soil depth. Further, our results suggest that the forest stands were not N‐limited and were probably more reliant on inorganic N as a primary N source rather than DON. In conclusion, our results show that the size of the DON and DOC pools are small relative to both the amount of C and N passing through the soil each year and the total C and N present in the soil. In addition, high rates of atmospheric N deposition in these forests may have removed competition for N resources between the plant and microbial communities.     

Verhalten von Dünger-NH4 in Böden unterschiedlicher tonmineralischer Zusammensetzung im Inkubationsversuch

Fate of fertilizer ammonium in soils with different composition of clay minerals in an incubation experiment In an incubation experiment with three different soils (gray brown podsolic soil from loess, alluvial gley, and brown earth, derived from basalt) the specific adsorption (fixation) and release of fertilizer NH₄⁺ was investigated. In one treatment 120 mg NH₄–N/kg soil was added, while the other treatment (control) received no nitrogen. Soils samples were taken every ten days and analyzed for nonexchangeable and exchangeable NH₄⁺ and NO₃^?. The experimental results are showing that the specific adsorption of applied NH₄⁺ was related to the type of clay minerals. While the loess soil, rich in illite, and the alluvial soil, rich in expansible clay minerals, bound about 40% of the added NH₄⁺ specifically, the soil derived from basalt with mainly kaolinite bound only about 10 %. From the recently “fixed” fertilizer NH₄⁺ about a half was nitrified during the incubation period of about 9 weeks. In the control there was no significant release of specifically bound NH₄⁺. Obviously this NH₄⁺ is located more deeply in the interlayers of the clay minerals and not available to microorganisms.     

Indices of dissolved organic nitrogen, ammonium and nitrate across productivity gradients of boreal forests

Organic nitrogen (N) uptake, rather than solely inorganic N (DIN), is considered a significant pathway for plant nutrition, especially in arctic, alpine and boreal ecosystems. Assays of plant-available N in these ecosystems might therefore be improved with measures of dissolved organic N (DON). We examined DON and DIN abundance from an in situ 5-week incubation across plant associations that represent the widest range in site potential in southern boreal forests of British Columbia, Canada. The supply of N from forest floors and mineral soils (20 cm depth) was measured separately and then combined (kg ha⁻¹) to facilitate comparisons of sites. DON was the predominant form of extractable N, and was increasingly supplemented, rather than replaced, by NH₄⁺ and NO₃⁻ on productive sites. The amount of DIN produced in the soils was very low, perhaps too small to support forest needs, and the correlation of DIN to asymptotic stand height (a measure of site potential) was significant but nonlinear. The combined amount of DON+DIN was considered a more effective index of plant-available N because it was strongly significant as a linear correlation to stand height and more typical of annual forest N uptake. The relative shift in N forms, from a predominance of DON to progressively greater ratios of DIN:DON, was consistent with the current paradigm of N forms across gradients of N availability, although the actual amounts of DON increased, rather than decreased, with site potential. Based on this, we suggest organic N uptake has the potential to contribute to plant nutrition across the entire productivity gradient of soils in southern boreal forests. Although other N indices were effective in characterizing forest productivity, a combined assay of DON+DIN production could provide new insights into functional differences in plant-available N.     

Dissolved organic nitrogen dynamics in a Mediterranean vineyard soil

Dissolved organic carbon (DOC) and nitrogen (DON) have been hypothesized to play a central role in nutrient cycling in agricultural soils. The aim of this study was to investigate the annual dynamics of DOC and DON in a Greek vineyard soil and to assess the potential role of DON in supplying N to the vines. Our results indicated that significant quantities of DOC and DON existed in soil throughout the year and that peaks in concentration appeared to correlate with discrete agronomic events (e.g. onset of irrigation and plowing). Both field and laboratory experiments showed that free amino acids were rapidly mineralized in soil and that consequently free amino acids represented only a small proportion of the soil's total soluble N. Due to rapid nitrification the soil solution N was dominated by NO₃⁻. Based upon the calculation of a plant-soil N budget and previous studies on N uptake in Vitis vinifera L., it is likely that DON uptake does not directly supply significant amounts of N to the plant. As the soil was not N limited we hypothesize that amino acids are used by the microbial community more as a source of C rather than a source of N. While we conclude that DON constitutes a significant N pool in vineyard soils further work is required to chemically characterize its constituent units and their relative bioavailability so that their overall role in N cycling can be determined.     

Effect of the nitrification inhibitor nitrapyrin on the fate of nitrogen applied to a soil incubated under laboratory conditions

The aim of this study was to examine the effect of the nitrification inhibitor nitrapyrin on the fate and recovery of fertilizer nitrogen (N) and on N mineralization from soil organic sources. Intact soil cores were collected from a grassland field. Diammonium phosphate (DAP) and urea were applied as N sources. Cores were equilibrated at –5 kPa matric potential and incubated at 20 °C for 42 to 56 days. Changes in NH₄⁺‐N, accumulation of NO₃^–‐N, apparent recovery of applied N, and emission of N₂O (acetylene was used to block N₂O reductase) were examined during the study. A significant increase in NH₄⁺‐N released through mineralization was recorded when nitrapyrin was added to the control soil without N fertilizer application. In the soils to which N was added either as urea or DAP, 50–90 % of the applied N disappeared from the NH₄⁺‐N pool. Some of this N (8–16 %) accumulated as NO₃^–‐N, while a small proportion of N (1 %) escaped as N₂O. Addition of nitrapyrin resulted in a decrease and delay of NH₄⁺‐N disappearance, accumulation of much lower soil NO₃^–‐N contents, a substantial reduction in N₂O emissions, and a 30–40 % increase in the apparent recovery of added N. The study indicates that N recovery can be increased by using the nitrification inhibitor nitrapyrin in grassland soils at moisture condition close to field capacity.     

Nitrite transformations in an N-saturated forest soil

Nitrite dynamics could be highly associated with forest N cycles. However, they have often been overlooked mainly because of the experimental difficulties that occur owing to chemical reactive nature of NO₂⁻. We investigated NO₂⁻ dynamics in an N-saturated forest soil with a recently developed method using ¹⁵N. Soils were aerobically incubated for 145 h after ¹⁵NO₂⁻ addition, and changes in ¹⁴N and ¹⁵N concentrations of NO₂⁻, NO₃⁻, NH₄⁺, and dissolved organic N (DON) were monitored. Simultaneous production and consumption of NO₂⁻ were observed. The turnover rate of NO₂⁻ was even faster than that of NH₄⁺ and NO₃⁻ calculated in other studies. Of the added ¹⁵NO₂⁻, 28.5% was oxidized to NO₃⁻ and 17.8% was incorporated into the DON pool within 4 h. The remainder might be emitted as gas or fixed by insoluble soil organic matter. Our results suggested that rapid NO₂⁻ turnover could be a major driving force for N transformations in forest soil.     

Bacterial and fungal contributions to soil nitrogen cycling under Douglas fir and red alder at two sites in Oregon

A study was conducted at two experimental tree plantations in the Pacific Northwest to assess the roles of bacteria and fungi in nitrogen (N) cycling. Soils from red alder (Alnus rubra) and Douglas-fir (Pseudotsuga menziesii) plots in low- (H.J. Andrews) and high- (Cascade Head) productivity stands were sampled in 2005 and 2006. Fungal:bacterial ratios were determined using phospholipid fatty acid (PLFA) profiles and quantitative (Q)-PCR. Ratios from these two molecular methods were highly correlated and showed that microbial biomass varied significantly between the two experimental sites and to a lesser extent between tree types with fungal:bacterial biomass ratios lower in more N-rich plots. ¹⁵N isotope dilution experiments, with ammonium (NH₄⁺) and nitrate (NO₃^?), were paired with antibiotics that blocked bacterial (bronopol) and fungal (cycloheximide) protein synthesis. This modified isotope dilution technique was used to determine the relative contribution of bacteria and fungi to net N mineralization and gross rates of ammonification and nitrification. When bacterial protein synthesis was blocked NH₄⁺ consumption and nitrification rates decreased in all treatments except for NH₄⁺ consumption in the Douglas-fir plots at H.J. Andrews, suggesting that prokaryotic nitrifiers are a major sink for mineral NH₄⁺ in forest soils with higher N availability. Cycloheximide consistently increased NH₄⁺ consumption, however the trend was not statistically significant. Both antibiotics additions also significantly increased gross ammonification, which may have been due to continued activity of extra- and intracellular enzymes involved in producing NH₄⁺ combined with the inhibition of NH₄⁺ assimilation into proteins. The implication of this result is that microorganisms are likely a major sink for soil dissolved organic N (DON) in soils.