Natural 15N and 13C abundance in Andisols influenced by long-term fertilization management in Japan

Two field experiments were conducted on Andisols in Japan to evaluate the changes in the natural ¹⁵N and ¹³C abundance in the soil profile and to determine whether the values of δ¹⁵N could be used as an indicator of fertilizer sources or fertilizer fate. The 6-year experiment conducted at the National Agricultural Research Center (NARC) consisted of the following treatments: application of swine compost (COMPOST), slow-release nitrogen fertilizer (SRNF), readily available nitrogen fertilizer (RANF), and absence of fertilization (CONTROL). Experimental plots located at the Nippon Agricultural Research Institute (NARI) received cattle compost at different rates for 12 years; a forest soil at this site was sampled for comparison. Swine compost application led to a considerable change in the δ¹⁵N distribution pattern in the soil profile, with the highest δ¹⁵N values recorded in the top 20 cm layers of the COMPOST plot, decreasing in the sequence of CONTROL >- RANF > SRNF, mainly due to the relatively high δ¹⁵N value of swine compost and its subsequent decomposition. In contrast, SRNF application resulted in the lowest δ¹⁵N values in soil, indicating the presence of negligible nitrogen losses relative to input and low nitrogen cycling rates. Values of δ¹⁵N increased with compost application rates at NARI. In the leachate collected at 1-m depth, the δ¹⁵N values decreased in the sequence of COMPOST > RANF ≥ CONTROL > SRNF. The δ¹³C values in soil peaked in the 40–60 cm layers for all the fertilizers. The δ¹³C value was lowest in forest soil due to the presence of plant residues in soil organic matter. These results indicated that the δ¹⁵N values in the upper soil layers or leachate may enable to detect pollution sources of organic or inorganic nitrogen qualitatively in Andisols.     

Areal variation in the relationship between natural 15N abundances of rice (Oryza sativa L.) and soil

This study was performed to clarify whether areal variation exists in the relationship between natural ¹⁵N abundances (δ¹⁵N values) of rice (Oryza sativa L.) and soil without an applied nitrogen (N) source, and to explore possible reasons for any areal variation. We investigated the relationships between δ¹⁵N values of rice and those of unamended soil with no applied N source in two locations; Daisen and Ogata, in Akita Prefecture, Japan. The δ¹⁵N values of rice in Daisen were higher than those in Ogata from 2007 to 2009, irrespective of the cropping year. Results demonstrated areal variation in the relationship between δ¹⁵N values of rice and those of unamended soil. The variation might be attributed to variation in the δ¹⁵N of natural N input and to ammonia nitrification and subsequent denitrification. When the relationship between δ¹⁵N values of rice and those of unamended soil is used to discriminate between organic and conventional rice, the areal variation of the relationship in the target area should be taken into account, from the point of the δ¹⁵N value of natural N input and N transformation in the soil.     

The fate of markerAzospirillum lipoferum inoculated into rice and its effect on growth,yield and N2 fixation of plants studied by acetylene reduction,15N2 feeding and15N dilution techniques

Summary A spontaneous mutant ofAzospirillum lipoferum, resistant to streptomycin and rifampicin, was inoculated into the soil immediately before and 10 days after transplanting of rice (Oryza sativa L.). Two rice varieties with high and low nitrogen-fixing supporting traits, Hua-chou-chi-mo-mor (Hua) and OS4, were used for the plant bacterial interaction study. The effect of inoculation on growth and grain and dry matter yields was evaluated in relation to nitrogen fixation, by in situ acetylene reduction assay,¹⁵N₂ feeding and¹⁵N dilution techniques. A survey of the population of marker bacteria at maximum tillering, booting and heading revealed poor effectivety. The population of nativeAzospirillum followed no definite pattern. Acetylene-reducing activity (ARA) did not differ due to inoculation at two early stages but decreased in the inoculated plants at heading. In contrast, inoculation increased tiller number, plant height of Hua and early reproductive growth of both varieties. Grain yield of both varieties significantly increased along with the dry matter. Total N also increased in inoculated plants, which was less compared with dry matter increase.¹⁵N₂ feeding of OS4 at heading showed more¹⁵N₂ incorporation in the control than in the inoculated plants. The ARA,¹⁵N and N balance studies did not provide clear evidence that the promotion of growth and nitrogen uptake was due to higher N₂ fixation.     

Immobilization of avian excreta-derived nutrients and reduced lignin decomposition in needle and twig litter in a temperate coniferous forest

The possible effects of excreta of the Great Cormorant Phalacrocorax carbo on decomposition processes and dynamics of nutrients (N, P, Ca, K, Mg) and organic chemical components (lignin, total carbohydrates) were investigated in a temperate evergreen coniferous forest near Lake Biwa in central Japan. Two-year decomposition processes of needles and twigs of Chamaecyparis obtusa were examined at two sites, control site never colonized by the cormorants (site C) and colonizing site (site 2). Mass loss was faster in needles than in twigs. Mass loss of these litter types was faster at site C than at site 2, which was ascribed to the decreased mass loss rate of acid-insoluble ‘lignin’ at site 2. Net immobilization of N, P, and Ca occurred in needles and twigs at site 2; whereas at site C, mass of these elements decreased without immobilization during decomposition. Duration of immobilization phase of these nutrients at site 2 was estimated to be 1.6 to 2.5 years in needles and 19.6 to 23.5 years in twigs. Immobilization potential (maximum amount of exogenous nutrient immobilized per gram initial material) was similar between needles and twigs for N and Ca but was about 10 times higher in twigs than in needles for P. δ¹³C in needles was relatively constant during the first year and then increased during the second year, whereas δ¹³C in twigs was variable during decomposition. Acid-insoluble fraction was depleted in ¹³C compared to whole needles (1.6-2.1‰) and twigs (2.0-2.5‰). δ¹⁵N of needles and twigs and their acid-insoluble fractions approached to δ¹⁵N of excreta during decomposition at site 2. This result demonstrated the immobilization of excreta-derived N into litter due to the formation of acid-insoluble lignin-like substances complexed with excreta-derived N. No immobilization occurred in K and Mg and their mass decreased during decomposition at both sites. Based on these results of nutrient immobilization during decomposition and on the data of litter fall and excreta amount at site 2, we tentatively calculated stand-level immobilization potential of litter fall and its contribution to total amount of N and P deposited as excreta. Thus, the potential maximum amount immobilized into litter fall (needles and twigs) was estimated to account for 5-7% of total excreta-derived N and P.     

Changes in natural 15N abundance in paddy soils under different,long-term soil management regimes in the Tohoku region of Japan

Abstract

Long-term temporal changes in natural ¹⁵N abundance (δ¹⁵N value) in paddy soils from long-term field experiments with livestock manure and rice straw composts, and in the composts used for the experiments, were investigated. These field experiments using livestock manure and rice straw composts had been conducted since 1973 and 1968, respectively. In both experiments, control plots to which no compost had been applied were also maintained. The δ¹⁵N values of livestock manure compost reflected the composting method. Composting period had no significant effect on the δ¹⁵N value of rice straw compost. The δ¹⁵N values increased in soils to which livestock manure compost was successively applied, and tended to decrease in soils without compost. In soils to which rice straw compost was successively applied, the δ¹⁵N values of the soils remained constant. Conversely, δ¹⁵N values in soils without rice straw compost decreased. The downward trend in δ¹⁵N values observed in soils to which compost and chemical N fertilizer were not applied could be attributed to the natural input of N, which had a lower δ¹⁵N value than the soils. Thus, the transition of the δ¹⁵N values in soils observed in long-term paddy field experiments indicated that the δ¹⁵N values of paddy soils could be affected by natural N input in addition to extraneous N that was applied in the form of chemical N fertilizers and organic materials.     

Natural abundance of 15N in nitrate,ureides, and amino acids from plant tissues

The natural ¹⁵N abundances (δ¹⁵N values) were measured for nitrate and free and bound amino acids from the leaves of field-grown spinach (Spinacia oleracea L.) and komatsuna (Brassica campestris L.), as well as ureides and free and bound amino acids in the leaves and roots of hydroponically grown soybean (Glycine max L.) totally depending on dinitrogen. Nitrate from the spinach and komatsuna leaves and ureides from leaves and roots of soybean showed higher δ¹⁵N values than the total tissue N and N in free or bound amino acid fractions. The δ¹⁵N values of individual free and bound amino acids, determined by GC/C/MS using their acetylpropyl derivatives, were similar in leaf tissues except for proline but varied in soybean root tissues. The order of ¹⁵N enrichment was similar in the four samples: aspartic acid > glutamic acid > threonine, proline, valine > glycine + alanine +serine, γ-amino butyric acid, and phenylalanine.     

Effects of food quality, starvation and life stage on stable isotope fractionation in Collembola

Naturally occurring stable isotopes of carbon and nitrogen are powerful tools to investigate food webs, where the ratio of ¹⁵N/¹⁴N is used to assign trophic levels and of ¹³C/¹²C to determine the food source. A shift in δ¹⁵N value of 3‰ is generally suggested as mean difference between two trophic levels, whereas the carbon isotope composition of a consumer is assumed to reflect the signal of its diet. This study investigates the effects of food quality, starvation and life stage on the stable isotope fractionation in fungal feeding Collembola. The fractionation of nitrogen was strongly affected by food quality, i.e. the C/N ratio of the fungal diet. Collembola showed enrichment in the heavier isotope with increasing N concentration of the food source. Δ¹⁵N varied between 2.4‰, which assigns a shift in one trophic level, and 6.3‰, suggesting a shift in two trophic levels. Starvation up to 4 weeks resulted in an increase in the total δ¹⁵N value from 2.8‰ to 4.0‰. Different life stages significantly affected the isotope discrimination by Collembola with juveniles showing a stronger enrichment (Δ¹⁵N=4.9‰) compared to adults (Δ¹⁵N=3.5‰). Δ¹³C varied between −2.1‰ and −3.3‰ depending on the food quality, mainly due to compensational feeding on low quality diet. During starvation δ¹³C value decreased by 1.1‰, whereas the life stage of Collembola had no significant effect on isotopic ratios. The results indicate that the food resource and the physiological status of the consumer have important impact on stable isotope discrimination. They may cause differences in fractionation rate comparable to trophic level shifts, a fact to consider when analysing food web structure.     

Use of 15N isotope dilution for quantification of N2 fixation associated with roots of kallar grass (Leptochloa fusca (L.))

Summary Leptochloa fusca (L.) Kunth (kallar grass) has previously been found to exhibit high rates of nitrogen fixation. A series of experiments to determine the level of biological nitrogen fixation using ¹⁵N isotopic dilution were carried out in nutrient solution and saline soil. In the nutrient solution, E. coli inoculated plants were taken as non-nitrogen-fixing control. It was observed that nearly 60%–80% of the plant N was derived from atmospheric fixation. Estimations based on the N difference method gave much lower values (18%–35%). In experiments with saline soil which was initially sterilized with chloroform fumigation, a mixed culture of N₂-fixing rhizospheric isolates from kallar grass roots was inoculated and planted to kallar grass. Uninoculated treatments were regarded as controls. The soil was previously labelled with ¹⁵N by adding cellulose and (¹⁵NH₄)₂SO₄. The results of these studies showed fixation values of 6%–32% when estimated by ¹⁵N dilution, whereas by the N difference method 54% of the plant N was estimated to be derived from fixation. This discrepancy is due to the increase in root proliferation due to inoculation, which results in greater uptake of soil N. The distribution of ¹⁵N in different fractions of the soil-N indicted isotopic dilution due to bacterial fixation of atmospheric N₂.     

Soil Total Nitrogen and Natural 15Nitrogen in Response to Long-Term Fertilizer Management of a Maize–Wheat Cropping System in Northern China

The Fengqiu long-term field experiment was established to examine effects of organic manure and mineral fertilizers on soil total nitrogen (N) and natural ¹⁵N abundance. Fertilizer regimes include organic manure (OM), one-half N from organic manure plus one-half N from mineral N fertilizer (1/2OMN), mineral fertilizers [N–phosphorus (P)–potassium (K), NP, NK, PK], and a control. Organic manure (OM and 1/2OMN) significantly increased soil total N and δ¹⁵N, which was expected as a great amount of the N applied remained in soils. Mineral NPK fertilizer and mineral NP fertilizer significantly increased total N and slightly increaed δ¹⁵N. Phosphorus-deficient fertilization (NK) and N-deficient fertilization (PK) had no effect on soil total N. Significantly greater δ¹⁵N was observed in the NK treatment as compared to the control, suggesting that considerable N was lost by ammonia (NH₃) voltalization and denitrification in this P-deficiency fertilization regime.     

Comparison of 15N natural abundance and difference methods for estimating N2 fixation of soybeans grown in a tropical soil

Abstract

A study was carried out to compare the difference or N-yield method with the ¹⁵N natural abundance method for the estimation of the fractional contribution of biological N₂ fixation in the different plant parts of nodulating and non-nodulating isolines of soybeans. The results indicated that the δ¹⁵N values of most plant parts of soybeans were significantly lower (p<0.05) in the nodulating than in the non-nodulating isoline. However, in the case of the root+nodule component, the δ¹⁵N value was higher in the nodulating than in the non-nodulating isoline possibly due to isotopic discrimination of ¹⁵N over ¹⁴N which may have occurred in the nodules. Inoculation of soybeans with the Bradyrhizobium japonicum strain CB 1809 increased significantly (p<0.05) the δ¹⁵N value of the root+nodule component implying that the effectiveness of the soybean-rhizobium symbiosis had increased by inoculation.

Percentage of plant N derived from atmospheric N₂ fixation (%Ndfa) estimated by the ¹⁵N natural abundance method was highly correlated (r=0.762, p<0.01) with that by the difference or N-yield method and the differences between the two methods were not statistically significant. The agreement between the two methods was closer at maturity than at the early reproductive stage.

The %Ndfa obtained by the difference method ranged from 48.4 to 92.6% whereas the %Ndfa obtained by the ¹⁵N natural abundance method ranged from 43.2 to 92.4% in the different plant parts. Based on the ¹⁵N natural abundance method, approximately 15% of the N in pod, shoot, grain, and shell was derived from the soil but in the case of stover, this fraction was about 55%.     

根系互作对苹果生长及15N-尿素吸收、利用和土壤残留的影响

在苹果/白三叶(M1)和苹果/黑麦草(M2)复合系统中,设置根系分隔(完全分隔N1、尼龙网分隔N2、不分隔N3),采用~(15) N同位素示踪技术,研究了根系互作对苹果生长及~(15) N吸收、利用,损失和土壤残留的影响。结果表明:苹果新梢旺长期,在M1中苹果各生长指标均为N3N2N1,在M2中趋势相反。与N1处理相比,M1中N2和N3处理苹果~(15) N利用率分别增加了11.91%和18.96%,M2中分别降低了5.76%和8.99%,苹果全氮量和~(15) N吸收量趋势相同。苹果根区土壤~(15) N丰度、总氮含量和~(15) N残留率均以N1处理最高,N3处理最低;苹果落叶期,两种复合体系中均以N3处理的苹果各生长指标最大,N1处理最低。在M1中N2和N3处理苹果根区土壤~(15) N丰度分别比N1处理增加了22.33%和34.15%,在M2中增幅分别为13.73%和21.44%,土壤总氮含量呈相同趋势。M1和M2中苹果全氮量、~(15) N吸收量和各器官Ndff值差异显著,均为N3N2N1。与N1处理相比,M1中N2和N3处理下苹果~(15) N利用率分别增加了19.11%和42.66%,而~(15) N损失率分别降低了13.55%和27.12%,在M2中趋势相同。苹果生长前期,黑麦草和苹果以负相竞争为主,白三叶对其促进效果亦不显著。而至苹果生长后期,两种牧草和苹果根系互作降低了苹果根区氮素损失,促进了苹果的氮素吸收利用和营养生长,且以间作白三叶效果最好。     

Dual inoculation of peanut with Glomus sp. and Bradyrhizobium sp. enhanced the symbiotic nitrogen fixation as assessed by 15N-Technique

Abstract

The response of peanut (Arachis hypogaea L.) to inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi (Glomus etunicatum) and Bradyrhizobiurn sp. was studied in pots by the acetylene reduction activity (ARA) and ‘A-value’ methods. The soil used was a Light-coloured Andosol and the treatments consisted of the inoculation of VAM fungi only, inoculation of Bradyrhizobium only, dual inoculation of VAM fungi and Bradyrhizobium and control, under non-sterilized and sterilized soil conditions.

In the non-sterilized soil the ARA and nitrogen fixation determined by the ‘A-value’ method increased significantly only by dual inoculation of VAM fungi and Bradyrhizobium at 100 days after planting (DAP), but no significant difference was observed at 70 DAP. In the case of dual inoculation, 75% of the nitrogen of the plant was derived from fixation whereas the plants inoculated only with Bradyrhizobium derived 68% of their nitrogen from fixation and the control plants, 64%. Amount of P in plant increased significantly only by dual inoculation with VAM fungi and Bradyrhizobium.

In the sterilized soil a highly significant increase in the ARA was observed of the dual inoculation at all the sampling times. Nitrogen fixation determined by the A-value technique and N and P contents in plant also increased significantly by dual inoculation. Results obtained by the A-value method showed that plants with dual inoculation derived 68% of their nitrogen from fixation while the plants inoculated only with Bradyrhizobium, 38%.

From our this study we conclude that nitrogen fixation as well as N and P contents in peanut increased significantly only by dual inoculation with VAM fungi and Bradyrhizobium.     

Interactions between mycorrhizal fungi and mycorrhizosphere bacteria during mineral weathering: Budget analysis and bacterial quantification

The impact of ectomycorrhizal fungi or rhizosphere bacteria on tree seedling growth and nutrient uptake is well known. However, few studies have combined those microorganisms in one experiment to clarify their relative contribution and interactions in nutrient acquisition. Here, we monitored the respective contributions of pine roots, two ubiquitous forest ectomycorrhizal fungi Scleroderma citrinum and Laccaria bicolor, and two S. citrinum-mycorrhizosphere bacterial strains of Burkholderia glathei and Collimonas sp., on mineral weathering, nutrient uptake, and plant growth. Pinus sylvestris plants were grown on quartz–biotite substrate and inoculated or not with combinations of mycorrhizal fungi and/or bacterial strains. Magnesium and potassium fluxes were measured and nutrient budgets were calculated. Both ectomycorrhizal fungi significantly increased Mg plant uptake. No significant effects of the two bacterial strains were detected on the K and Mg budgets, but co-inoculating the mycorrhizal fungus S. citrinum and the efficient mineral-weathering B. glathei bacterial strain significantly improved the Mg budget. Similarly, co-inoculating S. citrinum with the Collimonas sp. bacterial strain significantly improved the pine biomass compared to non-inoculated pine plants. In order to better understand this process, we monitored the survival of the inoculated bacterial strains in the quartz–biotite substrate, the pine rhizosphere, and the mycorrhizal niche. The results showed that the two bacterial strains harboured different colonization behaviours both of which depended on the presence of the ectomycorrhizal partner. The populations of the Burkholderia strain were maintained in all these environments with a significantly higher density in the mycorrhizal niche, especially of S. citrinum. In contrast the population of the Collimonas strain reached the detection level except in the treatment inoculated with S. citrinum. These results highlight the need for taking into account the ecology of the microorganisms, and more specifically the fungal–bacterial interactions, when studying mineral weathering and plant nutrition.     

Saprotrophic fungi transform organic phosphorus from spruce needle litter

Fungal decomposition of and phosphorus transformation from spruce litter needles (Picea abies) were simulated in systems containing litter needles inoculated with individual saprotrophic fungal strains and their mixtures. Fungal strains of Setulipes androsaceus (L.) Antonín, Chalara longipes (Preus) Cooke, Ceuthospora pinastri (Fr.) Höhn., Mollisia minutella (Sacc.) Rehm, Scleroconidioma sphagnicola Tsuneda, Currah & Thormann and an unknown strain NK11 were used as representatives of autochthonous mycoflora. Systems were incubated for 5.5 months in laboratory conditions. Fungal colonization in systems and competition among strains were assessed using the reisolation of fungi from individual needles. After incubation, needles were extracted with NaOH and extracts were analysed using ³¹P nuclear magnetic resonance spectroscopy (NMR). Needle decomposition was determined based on the decrease in C:N ratio. Systems inoculated with the basidiomycete S. androsaceus revealed substantial decrease in C:N ratio (from 25.8 to 11.3) while the effect of ascomycetes on the C:N ratio was negligible. We suppose that tested strains of saprotrophic ascomycetes did not participate substantially in litter decomposition, but were directly involved in phosphorus transformation and together with S. androsaceus could transform orthophosphate monoesters and diesters from spruce litter needles into diphosphates, polyphosphates and phosphonates. These transformations seem to be typical for saprotrophic fungi involved in litter needle decomposition, although the proportion of individual phosphorus forms differed among studied fungal strains. Phosphonate presence in needles after fungal inoculation is of special interest because no previous investigation recorded phosphonate synthesis and accumulation by fungi. Our results confirmed that the ³¹P NMR spectroscopy is an excellent instrumental method for studying transformations of soil organic phosphorus during plant litter decomposition. We suggest that polyphosphate production by S. androsaceus may contribute to the phosphorus cycle in forest ecosystems because this fungus is a frequent litter colonizer that substantially participates in decomposition.     

华北平原玉米种植中施入氮肥的去向研究

为了定量研究玉米氮肥利用特性以及肥料氮的去向,设计了~(15) N标记微区控制试验,设置3个施氮水平:不施氮肥(对照)、低氮处理(120kg N/hm~2)和高氮处理(240kg N/hm~2)。结果表明:土壤中残留~(15) N量随施氮量增加而显著增加(P0.05)。在空间分布上,总体呈现出随土壤深度先下降后上升的趋势,高氮处理和低氮处理~(15) N累积量均以40—60cm和60—80cm土层最多,这两层残留~(15) N总量分别占总投入量的37.55%和18.99%。与对照相比,施氮处理均显著提高了玉米地上、地下生物量和籽粒产量以及各部分吸氮量。虽然高氮处理较低氮处理施氮量增加了1倍,但籽粒产量仅增加0.14倍。氮肥农学效率与氮肥表观利用率随着施氮量增加而显著降低。高氮处理和低氮处理中玉米对~(15) N标记氮肥的利用率分别为28.86%和31.15%,土壤氮残留率分别为50.42%和36.52%,当季进入地下水的比率分别为4.27%和0.68%,其他损失率分别为16.45%和32.33%。研究结果表明,施氮量为120kg/hm~2可有效增加玉米产量,同时提高氮肥利用率,减少土壤氮累积,减小氮肥施用产生的环境污染风险。     

Patterns of natural N in soils and plants from chemically and organically fertilized uplands

Diagnostic tests for organic production of crops would be useful. In this study, the difference in natural ¹⁵N abundances (δ¹⁵N) of soils and plants between fertilizer-applied upland (FU) and compost-applied upland (CU) fields was investigated to study using δ¹⁵N as a marker of organic produce. Twenty samples each of soils and plants were collected from each field in early summer after applying fertilizer or compost. The δ¹⁵N of fertilizers and composts was −1.6±1.5‰ (n=8) and 17.4±1.2‰ (n=10), respectively. The δ¹⁵N of total soil-N was significantly (P<0.05) higher in CU fields (8.8±2.0‰) than in FU fields (5.9±0.7‰) due to long-term continuous application of ¹⁵N-enriched compost, as indicated by a positive correlation (r=0.62) between N content and δ¹⁵N of total soil-N. The NO₃⁻ pool of CU soils (11.6±4.5‰) was also significantly (P<0.05) enriched in ¹⁵N compared to FU soils (4.7±1.1‰), while the ¹⁵N contents of NH₄⁺ pool were not different between both soils. Compost application resulted in ¹⁵N enrichment of plants; the δ¹⁵N values were 14.6±3.3‰ for CU and 4.1±1.7‰ for FU fields. These results showed that long-term application of compost resulted in a significant ¹⁵N-enrichment of soils and plants relative to fertilizer. Therefore, this study suggested that δ¹⁵N could serve as promising indicators of organic fertilizers application when used with other independent evidence. However, further studies under many conditions should be conducted to prepare reliable δ¹⁵N guidelines for organic produce, since the δ¹⁵N of inorganic soil-N and plant-N are influenced by various factors such as soil type, plant species, the rate of N application, and processes such as mineralization, nitrification, and denitrifcation.     

Seasonal Changes of Shoot Nitrogen Concentrations and 15N/14N Ratios in Common Reed in a Constructed Wetland

Abstract

Nitrogen (N) concentrations and stable N isotope abundances (δ¹⁵N) of common reed (Phragmites australis) planted in a constructed wetland were measured periodically between July 2001 and May 2002 to examine their seasonal variations in relation to N uptake and N translocation within common reed. Nitrogen concentrations in P. australis shoots were higher in the growing stage (7.5 to 24.8 g N kg^?1) than in the senescence stage (4.2 to 6.8 g N kg^?1), indicating N translocation from shoots to rhizomes. Meanwhile, the corresponding δ¹⁵N values were higher in the senescence stage (+12.2 to +22.4‰) than in the growing stage (+5.1 to +11.3‰). Coupled with the negative correlation (R²=0.24, P<0.05, n=18) between N concentrations and δ¹⁵N values of shoots in the senescence stage, our results suggested that shoot N became enriched in ¹⁵N due to N isotopic fractionation (with an isotopic fractionation factor, α_s/p, of 1.012) during N translocation to rhizomes. However, the positive correlation between N concentrations and δ¹⁵N values in the growing stage (R²=0.19, P<0.001, n=54) suggested that P. australis relies on N re‐translocated from rhizome in the early growing stage and on mineral N in the sediment during the active growing stage. Therefore, seasonal δ¹⁵N variations provide N‐isotopic evidence of N translocation within and N uptake from external N sources by common reed.     

Soil Amendment Improves Tree Growth and Soil Carbon and Nitrogen Pools in Mongolian Pine Plantations on Post‐Mining Land in Northeast China

Mining activities leave large areas of post‐mining lands to be reclaimed. Increases in soil C and N pools and N availability are important to successfully reestablish trees on post‐mining land. In this study, we determined C and N concentrations and natural stable isotope of ¹³C and ¹⁵N in soil and plant in Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) plantations 8 years after establishment on non‐mined land (NM), post‐mining land without soil amendment (NAM), and post‐mining land with soil amendment (AM) in a placer gold mining region of Northeast China. We found that the growth of Mongolian pine was significantly slower on NAM as compared with NM (decreasing by 73% in tree height and 63% in basal diameter), but tree growth improved on AM. Soil C and N concentrations, plant N concentration, and soil δ¹⁵N value decreased in the order of NM > AM > NAM, implying that soil N availability decreased in post‐mining land, but soil amendment could increase soil N availability. However, the values of δ¹⁵N in plant tissues of Mongolian pine were higher on NAM than on NM and AM, suggesting that soil inorganic N form absorbed by trees might be changed when trees were directly planted on post‐mining land with lower soil N availability. In addition, the values of δ¹³C in 1‐ and 2‐year‐old leaves of Mongolian pine were lowest when planted on NAM, indicating a decrease in intrinsic water‐use efficiency of Mongolian pine. Our results suggest that soil amendment helps us establish forests successfully on post‐mining lands. Copyright © 2015 John Wiley & Sons, Ltd.     

Isotopic fractionation by saprotrophic microfungi: Effects of species, temperature and the age of colonies

Saprotrophic fungi represent an important resource for a number of fungivorous and omnivorous soil animals, but little is known about the patterns of isotopic fractionation by soil fungi. We grew five common species of saprotrophic microfungi in laboratory cultures on simple artificial substrate based on carbohydrates derived either from C3 or C4 plants. Fungal cultures were kept at 15, 20 or 25 °C. Isotopic composition of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) in bulk fungal tissue was determined after 11, 21 and 32 days. The fractionation of carbon and nitrogen stable isotopes was species-specific, but generally did not differ in C3- and C4-based growth media. The Zygomycete Mucor plumbeus did not differ in δ¹³C from the carbon source used, though Ascomycetes (Alternaria alternata, Cladosporium cladosporioides, Trichoderma harzianum and Ulocladium botrytis) were depleted in heavy carbon relative to the carbon source by 0.5-0.9‰. Three species were significantly depleted in ¹⁵N relative to the sodium nitrate that was used as a single source of nitrogen. In all species, δ¹⁵N but not δ¹³C tended to increase with the age of fungal colonies. The effect of temperature on δ¹⁵N was weak and inconsistent in different species. In contrast, all fungi except T. harzianum accumulated more ¹³С at 25 °C than at 15 °C. The overall variation in the isotopic signatures of saprotrophic fungi growing in identical conditions reached 8‰ for δ¹⁵N and 2.5‰ for δ¹³C due to species-specific differences in the isotopic fractionation and the age of individual fungal colonies. This variation should be incorporated into the interpretation of the isotopic composition of fungivorous soil animals.     

Variable impacts of enchytraeid worms and ectomycorrhizal fungi on plant growth in raw humus soil treated with wood ash

An increasing amount of evidence shows the context dependent nature of various biotic interactions across terrestrial and aquatic ecosystems. We established a laboratory experiment to study whether the effects of Cognettia sphagnetorum (Enchytraeidae) and ectomycorrhizal fungi on Scots pine (Pinus sylvestris) seedling growth are influenced by wood ash application. Acidic coniferous forest soil was treated with wood ash at 5000 kg ha⁻¹ or left as ash-free control and inoculated with soil saprotrophic microbes and nematodes. The microcosms were destructively sampled 26 and 51 weeks after initiation of the experiment. We measured enchytraeid and pine seedling biomass, abundance of nematodes and leaching of NH₄⁺-N and NO₃⁻-N at both samplings, and root length and N concentration of pine needles at the end of the experiment. On average enchytraeids and mycorrhizal fungi enhanced pine biomass production in the ash-free control soils, however, their impact was most pronounced when these organisms were alone in the systems. In fact, mycorrhizas tended to have a negative impact on the seedlings in the presence of enchytraeids. Wood ash had a clear negative impact on enchytraeid populations. Wood ash decreased pine growth when enchytraeids and EM-fungi were alone in the systems, but when together they apparently compensated for the negative effects of wood ash on the seedlings. It is concluded that interactions between soil fauna, mycorrhizal fungi and plants are context dependent, thereby rendering predictions of the outcome of species interactions in soil food webs a demanding task.