Soluble Organic N Research Articles

Vegetation restoration drives dynamics of soil nitrogen content and availability in the subtropics

Nitrogen (N) is an essential element for the functioning of terrestrial ecosystems and is often the primary limiting resource for the productivity of many terrestrial ecosystems. Existing knowledge on the effect of vegetation restoration on soil N dynamics is inconsistent. The aim of this study was to investigate the dynamics of soil N contents and availability with vegetation restoration and other influencing factors. Soil samples were collected to a depth of 40 cm from four distinct plant communities along a restoration gradient from scrub-grassland (4–5 years), shrub (10–12 years), coniferous and broadleaved mixed forest (45–46 years) to the evergreen broadleaved forest (>90 years). Total N (TN), soluble organic N (SON), microbial biomass N (MBN), inorganic N (IN, including ammonium (NH4+), nitrate (NO3–)), IN/SON, NO3–/NH4+, and MBN/IN were compared across vegetation factors, soil physicochemical properties and microbial activities. TN, SON, MBN, and NH4+ contents increased remarkably with vegetation restoration, while NO3– decreased from scrub-grassland to shrub and then increased. The dominant influencing factors of soil N forms were fine root biomass, soil organic carbon, C/N ratio, pH, microbial biomass carbon, and ureases, which indicated that vegetation factors, soil physicochemical properties, and microbial activities collectively control soil N accumulation. IN/SON and NO3–/NH4+ ratios were < 1 and decreased with vegetation restoration, while the MBN/IN ratio increased. The results implied that soil N availability reduced with vegetation restoration and N cycling remained closed, although it has been restored to the broadleaved forest stage. The close correlation between plant diversity index and soil N availability indicated that soil N availability reduces largely due to increasing plant N demands induced by the improvement in plant species diversity. Therefore, vegetation restoration facilitates long-term soil N accumulation and retention in this region.

Impact of Plant-Based Amendments on Water-Soluble Nitrogen Release Dynamics in Cultivated Peatlands

Drained cultivated peatlands have been an essential agricultural resource for many years. To slow and reduce the degradation of these soils, which increases with drainage, the use of plant-based amendments (straw, wood chips, and biochar) has been proposed. Literature on the effects of such amendments in cultivated peatlands is scarce, and questions have been raised regarding the impact of this practice on nutrient cycling, particularly nitrogen (N) dynamics. By means of a six-month incubation experiment, this study assessed the effects of four plant-based amendments (biochar, a forest mix, willow, and miscanthus) on the release kinetics of water-soluble N pools (mineral and organic) in two histosols of differing degrees of decomposition (Haplosaprist and Haplohemist). The amendment rate was set at 15 Mg ha−1 on a dry weight basis. The N release kinetics were significantly impacted by soil type and amendment. Miscanthus and willow were the amendments that most reduced the release of soluble organic N (SON) and mineral N (minN). The addition of plant-based amendments reduced the total amount of released N pools during the incubation (cumulative N pools) by 50.3 to 355.2 mg kg−1, depending on the soil type, the N pool, and the type of amendment. A significant relationship was found between microbial biomass N, urease activity, and the cumulative N at the end of the incubation. The results showed that the input of plant-based amendments in cultivated peatland decreases N release, which could have a beneficial impact by decreasing N leaching; however, it could also restrict crop growth. Further research is needed to fully assess the impact of such amendments used in cultivated peatlands on N and on C fluxes at the soil–plant and soil–atmosphere interfaces to determine if they constitute a long-term solution for more sustainable agriculture.

Controlling organic interference in determination of soil mineral nitrogen

AbstractInorganic nitrogen (N) in the form of exchangeable ammonium (NH4+), nitrate (NO3−), or nitrite (NO2−) is normally extracted by shaking soil with a neutral salt solution and is subject to interference by soluble organic N (SON). After optimizing sequential diffusion methods to expedite recovery of NH4+–N and (NO3− + NO2−)‐N, 15N‐tracer studies were conducted to ascertain whether extraction is quantitative when performed on soils amended with 2 g 15N kg−1 using 0.2, 1, and 2 M potassium chloride (KCl) and can be carried out by a simple leaching method instead of conventional shaking‐filtration. The results verified a significant decrease in SON interference with the optimized diffusion procedures and showed that (a) interference is more serious for NH4+–N than for (NO3− + NO2−)‐N, (b) 2 M KCl is required for quantitative recovery of 15NH4+–N, and (c) leaching virtually eliminates organic interference during diffusion of (NO3− + NO2−)‐N. The leaching‐diffusion approach minimizes the inflating effect of SON on soil inorganic N analyses and will be especially useful in N isotope studies.

Response of the subtropical forest soil N transformations to tannin acid-organic nitrogen complexes

It is well known that the hydrolysable tannin (tannin acid) and condensed tannins in a plant can influence the availability of soil nitrogen (N) through the formation of tannin–organic N complexes. However, the lack of research on the effect of tannin acid–organic N complexes on soil N hinders our understanding of the subsequent role of these complexes. In this line, an incubation experiment was carried out with the addition of tannin acid-arginine and tannin-bovine serum albumin (BSA) complexes, where tannin acid was represented as the hydrolysable tannin. For the necessary comparisons, tannin acid, arginine, or BSA was added to the soil samples. The results showed that tannic acid addition quickly decreased protein contents at the beginning of incubation and decreased soluble organic N (SON) after 3 days to produce the inorganic N in the soil as compared to control. Tannin-arginine complexes increased NH4+–N compared with control and this increased degree was found lower than arginine treatment. Moreover, the addition of tannin–BSA complexes did not significantly increase soil NH4+–N as BSA alone treatment did. NO3−–N and N2O emission increased in each treatment compared with control indicating nitrification was not limited. The increase of NH4+–N was mainly attributed to the decrease of SON after 3 days of incubation. These results suggested that tannin acid quickly formed the complexes with protein to impact soil N. While, the effects of tannin-arginine and tannin–BSA complexes on soil N transformation were quite dissimilar. Thus, this study highlighted the subsequent role of tannin–organic N complexes under the influence of tannin. Tannin and tannin–organic N complexes naturally coexist in the ecosystem to impact soil N dynamics and may interact with each other as well.

Nitrogen addition change soil N pools with litter removal or not in subtropical forest

ABSTRACT There are many nitrogen (N) pools in soil, so their availability and different status can give information about bulk soil response to N deposition. However, the different size of N pools in forest soils and the relationship between them have not been well studied under N deposition when considering the role of litter. Here soil in an N-deposition experiment carried out for 5 years in a broad-leaved forest was used as an object to study the response of N pools to N deposition by stepwise extraction using water or solutions containing 0.5 M K2SO4, 2.5 M H2SO4 (LPI), or 13 M H2SO4 (LPII), and calculation of recalcitrant (RC) N pool. Under N control (CT), soil with the presence of litter had a higher N of 23.8–106.8% in the first four pools, but lower of 80.6% in recalcitrant N pool compared with soil with the absence of litter. In the absence of litter, N addition increased soil N in labile pool but decreased N in the RC pool compared to CT and these impacts were greater at high added N (HN) than low-added N (LN) rates. However, in the presence of litter, LN increased the amount of N in the K2SO4- extracted pool and HN reduced that in the water extracted pool. Additionally, LN and HN increased TN in the RC pool and HN increased the total soluble N (TSN) in the LPI and LPII pool. N changes in the water extraction pool were attributed to inorganic N, whereas they were NH4 + and soluble organic N (SON) in the K2SO4-extracted, LPI, and LPII pools. In the presence of litter, HN increased the SON concentration in the K2SO4, LPI, and LPII extractions; thus, SON may be a potentially important N form for N availability. These results suggested that N additions improve the accumulation of N in RC pool with the presence of litter. The different effects of N additions on soil N pool or N form in each pool depend on litter present or not.

Role of soluble and exchangeable nitrogen pools in N cycling and the impact of nitrogen added in forest soil.

Nitrogen (N) cycle in forest soils is altered by water, salt, or acid solutions, and its internal transfers to and from each existing inorganic pools are not known comprehensively. To evaluate the soluble and exchangeable N pools, bulk soil (B soil), water-extracted soil (W soil), and the 0.5 mol L-1 K2SO4-treated soil (K soil) were incubated for up to 48 days to comprehend the dynamics of inorganic (NH4+ and NO3-) and soluble organic N (SON) in water-soluble, exchangeable, 2.5 mol L-1 H2SO4 (labile pool I, LPI) and 13 mol L-1 H2SO4 (labile pool II, LPII) pools. To test the N deposition effects, additional NH4NO3 solution was added to B, W, and K soils at amount of 40 mg N kg-1 soil. The results showed that though there was more NO3- removed when W soil was prepared, the similar net nitrification rate in W soil to B soil and more than 20 mg N kg-1 water-soluble NO3- were observed in W soil, which indicated that the loss of NO3- would be enhanced. In contrast, there was more water-soluble and exchangeable NH4+ for K soil compared with B soil. The different dynamic of NO3- between W and K soil suggested that nitrifiers might dominate in the soil matrix rather than the soil solution. After incubation, each N form in the LPI decreased, which can be attributed to the allocation of remaining N into the recalcitrant pool, except the increase of NH4+ for B soil and NO3- for K soil, and NO3- in LPII for B soil. Compared with control, N addition increased mineralization of exchangeable SON to promote nitrification regardless of soils, but weakened the immobilization of NO3-. In addition, N in LPI and LPII pools have increased, which might be related to decomposition of recalcitrant organic matter induced by N addition to transform when the water-soluble and exchangeable N was removed. Therefore, the changes of soluble and exchangeable nitrogen pools impact the N cycling. Our findings can give some explanation for whole soil N transformation responses to N deposition.

Effects of rice husk biochar and soil moisture on the accumulation of organic and inorganic nitrogen and nitrous oxide emissions during the decomposition of hairy vetch (Vicia villosa) mulch

ABSTRACT Hairy vetch (Vicia villosa Roth ssp. dasycarpa) mulch is widely used as green manure. This mulch decomposes in the soil and produces nitrous oxide (N2O) emissions. Although rice husk charcoal can be used as a soil conditioner to reduce N2O emissions, the effects of charcoal on the decomposition of vetch mulch and consequent N2O emissions have not been well studied. We conducted an incubation experiment to examine how the decomposition of vetch mulch, charcoal amendments and two soil moisture regimes (field capacity and at a slightly drier moisture regime) affect the mineralisation of N during decomposition. We applied high-performance size-exclusion chromatography and chemiluminescent N detection to quantify soluble organic-N compounds of various molecular sizes. Changes over time in inorganic-N, soluble organic-N fractions and N2O fluxes were measured in soils . We found that vetch mulch induced the accumulation of small-molecular-size soluble organic-N compounds in the early phase during decomposition; however, in wet soils with charcoal, the small-molecular-weight N fraction was depleted earlier than in wet soils without charcoal, suggesting more rapid mineralisation in wet soils. Charcoal in wet soils resulted in earlier and sharper N2O peak emissions and did not reduce cumulative N2O emissions in vetch mulch treatments, suggesting that previously reported N2O emission reductions due to charcoal do not occur in the soils . The rates of N2O emission did not correlate with the concentrations of soluble organic N and inorganic N, indicating that other factors control the N2O emission rates during the decomposition of vetch mulch. Our results indicated that the incorporation of charcoal does not reduce N2O emissions in soils where vetch mulch is applied. The depletion of nitrate-N in wet soils with charcoal demonstrates that adding charcoal can reduce nitrate-N leaching from soils.

Dynamic of inorganic nitrogen and amino sugar to glucosamine addition in forest soils.

Amino sugars (AS) are routinely used as microbial biomarkers to investigate the dynamics of soil carbon (C) and nitrogen (N) under different environments. However, the effect of any AS on soil C and N, or other AS, is not well-defined. In this study, acid soils from Dongbei (D) and Fujian (F) and alkaline soil from Henan (H) were selected to perform an incubation experiment under glucosamine addition for 36 days. In the present study, the dynamics of soil soluble organic C (SOC), NH4+-N, NO3--N, soluble organic N (SON), and four AS: glucosamine (GluN), mannosamine (ManN), galactosamine (GalN), and muramic acid (MurN), were investigated. The results showed that AS was different among the three soils, but had similar dynamics in the same soil. The higher total C and inorganic N in the D and F relative to the H soil were related to the greater AS in two soils. With incubation, AS decreased in D soil and increased in F soil before 1 week, while after 1 week, the inverse dynamics were observed, which suggest that SOC or SOC combined with inorganic N may be a mechanism to adjust the dynamics of C from AS. Overall, glucosamine addition did not significantly affect AS in D, while the reverse was true for F and H soils. Glucosamine addition decreased AS at day 0 for D soil and at day 3 for F and H soils, and increased SOC. The lowered NH4+-N and AS in D soil, but the higher values of these, were observed in F soil after 1 week of incubation. The increase of SON in D soil with glucosamine addition might be due to the depolymerization of soil organic matter (SOM) into SON. However, the decrease of SON in F soil could be attributed to the mineralization of SON.

Denitrification and desulphurization of industrial biowastes via hydrothermal modification

In attempt to decrease NOX and SO2 emission from thermochemical utilization, three industrial biowastes (penicillin mycelia waste, sewage sludge and peat waste) contained high nitrogen (N) and sulfur (S) were chosen to investigate the denitrification and desulphurization of hydrothermal modification. The results demonstrated that hydrothermal modification destroyed the structure of N- and S-containing components, thereby altering their existed conformations. Inorganic-N (N-IN) and most of amino-N/polyamide-N (N-A) were enriched by liquid phase in the forms of NH4+-N and soluble organic-N (Org-N), respectively; subsequently, Org-N could further decompose to NH4+-N at higher temperature. Residual N in hydrochars was converted from N-A to heterocyclic-N (pyrrolic-N, pyridinic-N and quaternary-N) via hydrolysis and cyclization. Similarly, over 60% of S was remove form biowastes at 240 °C. In solid phase, part of organic-S was altered to thiophenes-S after modified, while the remainder was transformed to inorganic-S; but the variation of inorganic-S in hydrochars strongly affected by its specific species.

Evaluation and simulation of nitrogen mineralization of paddy soils in Mollisols area of Northeast China under waterlogged incubation.

BackgroundUnderstanding the nitrogen (N) mineralization process and applying appropriate model simulation are key factors in evaluating N mineralization. However, there are few studies of the N mineralization characteristics of paddy soils in Mollisols area of Northeast China.Materials and methodsThe soils were sampled from the counties of Qingan and Huachuan, which were located in Mollisols area of Northeast China. The sample soil was incubated under waterlogged at 30°C in a controlled temperature cabinet for 161 days (a 2: 1 water: soil ratio was maintained during incubation). Three models, i.e. the single first-order kinetics model, the double first-order kinetics model and the mixed first-order and zero-order kinetics model were used to simulate the cumulative mineralised N (NH4+-N and TSN) in the laboratory and waterlogged incubation.Principal resultsDuring 161 days of waterlogged incubation, the average cumulative total soluble N (TSN), ammonium N (NH4+-N), and soluble organic N (SON) was 122.2 mg kg-1, 85.9 mg kg-1, and 36.3 mg kg-1, respectively. Cumulative NH4+-N was significantly (P < 0.05) positively correlated with organic carbon (OC), total N (TN), pH, and exchangeable calcium (Ca), and cumulative TSN was significantly (P < 0.05) positively correlated with OC, TN, and exchangeable Ca, but was not significantly (P > 0.05) correlated with C/N ratio, cation exchange capacity (CEC), extractable iron (Fe), clay, and sand. When the cumulative NH4+-N and TSN were simulated, the single first-order kinetics model provided the least accurate simulation. The parameter of the double first-order kinetics model also did not represent the actual data well, but the mixed first-order and zero-order kinetics model provided the most accurate simulation, as demonstrated by the estimated standard error, F statistic values, parameter accuracy, and fitting effect.ConclusionsOverall, the results showed that SON was involved with N mineralization process, and the mixed first-order and zero-order kinetics model accurately simulates the N mineralization process of paddy soil in Mollisols area of Northeast China under waterlogged incubation.

Accumulation and Distribution of Soil Soluble N in Greenhouse Soil

The amount of soil total soluble N (TSN), NH4+-N, NO3--N and soluble organic N (SON) declined significantly with increasing of sampling depth (P &lt; 0.0001). Compared with CK, application of chicken manure significantly increased the amount of soil TSN and NO3--N by 77.0% and 213.0% in the CM20 treatment, and 176.4% and 766.4% in the CM 30 treatment, respectively (P &lt; 0.0001). The amount of soil NH4+-N was lower in CM20 treatment than CK and CM30 treatments. However, the amount of soil SON was higher in CM20 treatment than CK and CM30 treatments (P &lt; 0.0001). Proportion of NH4+-N to TSN was lowest. Application of chicken manure remarkably increased the proportion of NO3--N to TSN, and decreased that of NH4+-N and SON to TSN (P &lt; 0.0001). Soil SON accounted for the majority of soluble N irrespective of soil layer in CK and CM20 treatments, and followed by NO3--N. However, the trend was reversed in CM30 treatments. This result indicated that application of chicken manure significantly increased the amount of soil soluble N, especially SON and NO3--N, which may extend N leaching risk into deeper soil layer.

Ectomycorrhizal fungal succession coincides with shifts in organic nitrogen availability and canopy closure in post-wildfire jack pine forests

Successional changes in belowground ectomycorrhizal fungal (EMF) communities have been observed with increasing forest stand age; however, mechanisms behind this change remain unclear. It has been hypothesized that declines of inorganic nitrogen (N) and increases of organic N influence changes in EMF taxa over forest development. In a post-wildfire chronosequence of six jack pine (Pinus banksiana) stands ranging in age from 5 to 56 years, we investigated EMF community composition and compared shifts in taxa with detailed soluble inorganic and organic N data. Taxa were identified by internal transcribed spacer rDNA sequencing, and changes in community composition evaluated with non-metric multi-dimensional scaling (NMDS). Dissimilarities in the community data were tested for correlations with N variables. We observed a successional shift along NMDS axis 1 from such taxa as Suillus brevipes and Thelephora terrestris in sites age 5 and 11 to species of Cortinarius and Russula, among others, in the four older sites. This change was positively correlated with soluble organic N (SON) (r(2) = 0.902, P = 0.033) and free amino-acid N (r(2) = 0.945, P = 0.021), but not inorganic N. Overall, our results show a successional shift of EMF communities occurring between stand initiation and canopy closure without a change in species of the dominant plant-host, and associated with SON and free amino-acid N in soil. It is uncertain whether EMF taxa are responding to these organic N forms directly, affecting their availability, or are ultimately responding to changes in other site variables, such as belowground productivity.

Studies of the influences of different N fertilizers and Microbion UNC bacterial fertilizer on the nutrient content of soil

A field experiment was conducted to examine the effects of different nitrogen fertilizers in combination with bacterial fertilizer onnutrient uptake of horseradish and plant available nutrients of the soil. Three different N fertilizers, ammonium-nitrate, urea and calciumnitrate(116 kg ha-1 N) in combination with Microbion UNC bacterial fertilizer (2 kg ha-1) were applied as treatments in a randomizedcomplete block design in three replications. In this paper we presented the results of soil measurements. The soil of the experimental areawas chernozem with medium sufficiency level of N and P and poor level of K.Our main results:The amount of 0.01M CaCl2 soluble inorganic nitrogen fractions, NO3--N and NH4+-N and also the quantity of soluble organic-N werealmost the same in the soil. N fertilizers significantly increased all the soluble N fractions. The amount of NO3--N increased to the greatestextent and the increase of organic N was the slightest. We measured the largest CaCl2 soluble NO3- -N and total-N contents in the plotstreated with ammonium-nitrate, the largest NH4+-N in the plots treated with calcium-nitrate and the largest organic-N fraction in plotstreated with urea.Bacterial inoculation also increased both soluble inorganic nitrogen forms and also total-N content of soil compared to the control. Inthe case of combined (artificial and bacterial fertilizer) treatments we measured lower NO3--N, organic-N and total-N compared to thevalues of plots having only nitrogen fertilizer treatments. On the contrary in the plots with combined treatments the CaCl2 soluble NH4+-Ncontent of soil in more cases were higher than that of values with artificial fertilizer treatment.As a function of calcium-nitrate application increased AL-P2O5 and AL-K2O values were measured compared to control. MicrobionUNC supplement of calcium nitrate yielded also increase in AL-P2O5 and AL-K2O values, till then supplement of ammonium-nitrate fertilizeryielded a decrease in these values compared to the control.All nitrogen fertilizers resulted in a significant decrease in AL-Mg content of soil compared to the control. Nevertheless bacterialfertilizer increased AL-Mg values in any cases.

Effects of ammonium nitrate, wheat straw and Phylazonit MC bacterial fertilizer on the N supply of soil

An incubation experiment was set up to investigate the effects of NPK fertilizers, wheat straw and food waste compost in combination with Phylazonit MC biofertilizer on the changes in 0.01MCaCl2extractable nitrogen forms (NO3+-N, NH4+-N, organic-N fraction) of three types of soils. The quantities and changes in CaCl2-NH4+-N mainly depended on the adsorption capacity of soils. Higher CaCl2extractable NH4+-N values were measured in the calcareous chernozem and sandy soil with the application of NPK treatment, while in case of the chernozem soil with loamy texture no significant differences were found between the NPK and control treatments. NH4+immobilization appeared in pots given straw treatment. The quantity of CaCl2-NH4+-N changed within the incubation period, the direction and extent of the change were dependent of the production rates and consumption processes. In all soils the 0.01MCaCl2extractable NO3--N was the largest pool for plant nutrition. The highest CaCl2-NO3--N values were found in pots given NPK and combined NPK+straw treatments, while the lowest values were measured in straw-treated pots. Wheat straw, with higher C/N ratio caused a temporary immobilization, followed by remineralization. Food waste compost, with lower C/N ratio, proved to have a good N supplying capacity. The contrasted effect of straw and compost was more conspicuous in sandy soil, in which the original NO3--N content was the lowest. Higher CaCl2-organic N values were measured for pots treated with straw and with compost, as compared to the control. During the incubation period the amount of soluble organic-N changed, the direction and extent of the change depended on the soil type and treatments. The effect of Phylazonit MC was on the quantity of CaCl2extractable N fractions was not really expressed, and the significancy of the effect varied and was dependent of the soil type. Biofertilizer amendment increased the extractable NO3--N in sandy soil, decreased the negative effect of wheat straw, and it also might help in decomposing wheat straw and food waste compost. The application of Phylazonit MC in the calcareous chernozem caused a significantly higher amount of organic-N, especially in straw-treated pots.

Nitrogen dynamics of cattle dung and urine patches on intensively managed boreal pasture

SUMMARYThe aim of the present study was to investigate the nitrogen (N) dynamics in soil and N utilization by grass in dung pats and urine patches, and to quantify the amount of soluble organic N (SON) in grassland soil. The experiment was conducted in 2003/04 at MTT Agrifood Research, Maaninka, Finland. The study consisted of three treatments: dung pats, urine patches and a control patch without excreta. The soil (to a depth of 0·6 m) and herbage were sampled on 10 occasions over the course of 1 year. TN and SON in soil averaged 14 400 and 60 kg/ha N, respectively. Changes in soil mineral N were short-lived and a large proportion of excreta N was immobilized. The grass took up 27 kg excretal N/ha from the dung pat and 71 kg N/ha from the urine patch, which equals 0·07 and 0·19 of the excretal N given to each treatment, respectively. As a large proportion of the excretal N is immobilized and accumulates in soil, re-evaluation of the recommendations for N fertilization of pastures older than 2 years is justified.

Analysis and behavior of soluble organic nitrogen in forest soils

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. The SON represents a significant pool of available and mobile N in forest soils. The SON can be sampled using a number of physical (e.g., suction cup, microdialysis) and chemical (e.g., extraction) methods and analyzed by the wet chemistry method (e.g., persulfate oxidation) and combustion. Chemical and physical fractionation, pyrolysis–mass spectrometry, 13C and 15N nuclear magnetic resonance spectroscopy, and 15N, 13C, and 14C isotopic techniques have been used for investigating the nature and dynamics of SON in forest ecosystems. The amount of SON in soil may vary with land use type, forest species, management practices, and analytical procedures used. Statistical figures, based on 116 datasets available in the literature, have shown that concentrations of soil SON can range from 1 mg N kg−1 (dry weight basis) to up to 448 mg N kg−1, with an average of 35 mg N kg−1, representing an average of 48% of total soluble N. Soil SON consists of a mixture of structurally diverse N-containing compounds, possibly with the amide N, amino acids, and amino sugars being predominant. The biodegradability of SON largely depends upon its chemical characteristics (e.g., C-to-N ratio). Direct utilization of added amino acids by a large number of forest species may present a potentially important new pathway for plant N uptake. However, the evidence of direct utilization of soil native SON by forest plants is still lacking, and the extent of the contribution of SON to forest plants is far from certain. Transformation of soil organic matter into SON, rather than the conversion of SON to NH4 +/NO3 −, may be the rate-limiting step, which regulates the overall N cycling in N-limited forest ecosystems. Future work may well focus on the chemical and biological nature of SON in forest soils, microbial transformation of SON, the nutritional role of the SON pools in the N-limited forest ecosystems, and the potential role of SON in global climate change (e.g., N2O emission).

Effects of Soluble Organic N on Evaluating Soil N-Supplying Capacity

It is important to study the soluble organic N (SON) extracted during water-logged incubation for evaluating soil N- supplying capacity. Soil initial SON and mineral N (N min), cumulative soluble organic N and NH 4 +-N in leachates during water-logged incubation, mineralization potentials of both easily decomposable N (N D) and resistant N (N R), and their relationships with N uptake by crop in pot experiment were investigated by using 10 kinds of farmland soils with widely different physical and chemical properties on the Loess Plateau, China, and the effects of SON on evaluating soil N- supplying capacity were studied. The results showed that the average content of initial SON (23.9 mg kg −1) of 10 soils was 28.8% of initial total soluble N and 2.4% of soil total N. The percentage of cumulative SON in leaching total soluble N (118.1 mg kg −1) was 46.4%, higher than the percentage of initial SON (28.8%), and almost close to the percentage of cumulative NH 4 +-N in the leachates. N D had close correlation with total N, and the correlation coefficients were 0.92 ( P < 0.01, excluding SON in estimating N D) and 0.88 ( P < 0.01, including SON in estimating N D), respectively. N mineralization potential and mineralization rate constant were different with the soil types. N D of Los-Orthic Entisols and Ust-Sandiic Entisols were lower than that of Eum-Orthrosols. Mineralization rate constant for the fast decomposable N-fraction (k D) decreased and the mineralization rate constant of resistant materials (k R) increased when SON was taken into account. Cumulative NH 4 +-N was a better evaluation index of soil N-supplying capacity, and it is not only suitable for the first season crops but also for two successive season crops. Cumulative SON alone was not a satisfactory index for the potential of mineralizable N. But it would be more accurate for N D in revealing the potential mineralizable N when SON was taken into account. Cumulative TSN, to some extent, could also be taken as an index for the potential mineralizable N. Cumulative NH 4 +-N, total soluble N, and N D were good indexes for estimating soil potential mineralizable N, especially for soils of two successive season crops. And cumulative total soluble N and N D in evaluating the permanence of soil N-supply is of greater significance when SON was included.

Nitrogen leaching and herbage production on intensively managed grass and grass‐clover pastures on sandy soil in Finland

SummaryThe main aim of this study was to compare the N leaching from grass fertilized with 220 kg N ha−1 and grass‐clover pastures receiving no fertilization during three grazing years and a renewal year, in a 4‐year ley rotation. The other aim was to compare the herbage and milk production of these pastures. The study was conducted on a lysimeter field; five lysimeters (size 10 × 10 m) were assigned to each treatment. Automated drinking water outlets for the cows were located on one of the lysimeters on each treatment. The amount of leachate was recorded and composite samples were analysed for total N, NO3‐N, NH4‐N and soluble organic N (SON). The number of grazing cows was adjusted according to the herbage mass production. The amount of milk was measured. The total input of N to the area was 290 and 215 kg N ha−1 year−1 for grass and grass‐clover treatments, respectively. The total N leaching during grazing years was 17 and 9 kg N ha−1 from grass and grass‐clover treatments, respectively. Renewal of the sward increased N leaching in both treatments, up to 60 and 40 kg total N ha−1 in grass and grass‐clover treatments, respectively. During the grazing years 96% of the leached N was in the form of NO3‐N, but during the renewal year the proportion of NO3‐N was lowered to 89% and the rest was in the form of SON. The total amount of N in the surface runoff was 3–5 kg ha−1 year−1. As high N fertilizer rates per application are a common practice in Finland, short‐term grass‐clover pastures can be considered environmentally beneficial when compared with intensively fertilized grass pastures in comparable circumstances.

Comparison of Soil Nitrogen Availability Indices under Two Temperate Forest Types

To evaluate the validity of different indices in estimating soil readily mineralizable N, soil microbial biomass (N(subscript mjc)), soil active N (SAN), soluble organic N (SON), net N mineralization rate (NNR) and gross N mineralization rate (GNR) in mineral soils (0-10 cm) from six forest stands located in central Germany were determined and compared with two sampling times: April and November. Additionally, soil density fractionation was conducted for incubated soils (with addition of 15NH4-N and glucose, 40 days) to observe the sink of added 15N in different soil fractions. The study showed that N(subscript mic) and NNR in most stands differed significantly (P≤0.05) between the two sampling times, but not GNR, SAN and SON. In November, no close relationships were found between GNR and other N indices, or between N(subscript mjc), SON, and SAN and forest type. However, in April, GNR was significantly correlated (P≤0.05) with N(subscript mjc), SAN, and NNR along with N(subscript mjc) under beech being significantly higher (P≤0.05) than under conifers. Furthermore, density fractionation revealed that the light fraction (LF, 0.063-2 mm, ＞1.7 g cm^(-3)) was not correlated with the other N indices. In contrast, results from the incubation study proved that more was incorporated into the heavy fraction (HF＜0.063 mm, ＞1.7 g cm^(-3)) than into LF, indicaing that more labile N existed in HF than in LF. These findings suggested that attention should be paid to the differences existing in N status between agricultural and forest soils.

Soil nitrogen dynamics as affected by landscape position and nitrogen fertilizer

The influence o f landscape position on the dynamics of N in the soil-plant system has not been adequately studied. Our aim with this study on a predominantly Black Chernozem soil was to evaluate the effect of slope position (upper vs. lower) and N fertilizer application (none vs. 60 kg N ha-1) on soil and wheat (Triticum aestivum L.) N through the growing season. Landscape position had a dominant effect on soil NO3− and soluble organic N (SON) concentrations, especially in the surface 15 cm. These pools of soil N and net N mineralization were greater at the lower than at the upper slope position. The landscape effect is attributed to higher organic matter content (as measured by organic C) and water availability in lower compared with upper slope positions. Nitrogen application had no measurable effect on soil NO3− and SON concentrations. Exchangeable and non-exchangeable NH4+ were little affected by slope position or N fertilization. Nitrogen application increased wheat N uptake; however, its influence was less than that of slope position, especially on N accumulation in wheat heads during grain-filling. Although N application increased wheat yields, landscape position exerted the greater influence: grain yield was less on upper than lower slope positions due to earlier onset of crop maturity. During grain filling, net N mineralization was suppressed at the upper slope position and by N application. The increase in crop yield and N uptake due to N application was not significantly different between slope positions. This study demonstrated that landscape position had a greater influence on N dynamics and availability than the application of typical amounts of fertilizer N and that the two effects were mostly independent of each other. Key words: Available N, landscape position, N uptake, net N mineralization, soluble organic N