Response of citronella Java (Cymbopogon winterianus Jowitt) to VA mycorrhizal fungi and soil compaction in relation to P supply

Nutrient acquisition and growth of citronella Java (Cymbopogon winterianus Jowitt) was studied in a P-deficient sandy soil to determine the effects of mycorrhizal symbiosis and soil compaction. A pasteurized sandy loam soil was inoculated either with rhizosphere microorganisms excluding VAM fungi (non-mycorrhizal) or with the VAM fungus, Glomus intraradices Schenck and Smith (mycorrhizal) and supplied with 0, 50 or 100 mg P kg^-1 soil. The soil was compacted to a bulk density of 1.2 and 1.4 Mg m^-3 (dry soil basis). G. intraradices substantially increased root and shoot biomass, root length, nutrient (P, Zn and Cu) uptake per unit root length and nutrient concentrations in the plant, compared to inoculation with rhizosphere microorganisms when the soil was at the low bulk density and not amended with P. Little or no plant response to the VAM fungus was observed when the soil was supplied with 50 or 100 mg P kg^-1 soil and/or compacted to the highest bulk density. At higher soil compaction and P supply the VAM fungus significantly reduced root length. Non-mycorrhizal plants at higher soil compaction produced relatively thinner roots and had higher concentrations and uptake of P, Zn and Cu than at lower soil compaction, particularly under conditions of P deficiency. The quality of citronella Java oil measured in terms citronellal and d-citronellol concentration did not vary appreciably due to various soil treatments.     

Saprophytic and pathogenic behaviour of R. solani AG2-1 (ZG-5) in a soil amended with Diplotaxis tenuifolia or Brassica nigra manures and incubated at different temperatures and soil water content

The Brassicaceae species Diplotaxis tenuifolia and Brassica nigra contain high concentrations of glucosinolates, the precursors of isothiocyanates (ITCs) that can have biofumigation effects in amended soils. In a laboratory experiment, incorporation of these plants as green manures into soil was expected to suppress Rhizoctonia solani AG2-1 (ZG5), the causal agent of damping-off in canola (Brassica napus). The manures were incorporated at 1 (1% w/w) or 5 (5% w/w) g fresh material per 100 g dry soil and incubated for 6 months at 10, 20, or 30°C and at soil water contents of 10%, 40%, or 70% of water holding capacity. R. solani survived for up to 6 months as a saprophyte in un-amended soil at all soil water contents and at 10 and 20°C. A temperature of 30°C suppressed R. solani below the level of detection in all treatments after one week. At 1% concentration, the green manures increased the colonisation of the soil by R. solani, which caused severe damping-off of canola subsequently sown in this soil treatment. Soil amendments at 1% temporarily increased soil microbial activity. The addition of B. nigra or D. tenuifolia green manure at 5% concentration suppressed the saprophytic growth of R. solani incubated at 10 or 20°C over all soil water contents and significantly increased the microbial activity at all soil temperatures and water contents. Canola sown into these pots did not succumb to damping-off. The efficiency of hydrolysis of glucosinolates in the 5% treatment in the first week of incubation ranged from 1.6% for 2-propenyl ITC, extracted from soil containing tissues of B. nigra, to 3.4% for 3-butenyl ITC extracted from soil containing tissues of D. tenuifolia. 2-propenyl ITC could not be detected after 7 d of incubation. In the longer term (weeks to months), the increase of microbial activity, caused by adding green manures at 5%, or volatiles from the green manures, most likely played a dominant role in suppressing R. solani. The impact of ITCs, if any, appears to be short-term (days). Responsible Editor: Peter A. H. Bakker     

Effects of cadmium and nickel on in vivo carbon dioxide exchange rate of pigeon pea (Cajanus cajan L.)

Effects of acidic soil factors (Al, H-ion, Mo, and Mn) upon the soybean (Glycine max (L.) Merr. cv. Essex)/Bradyrhizobium japonicum symbiosis were examined in acidified soil. Plants were grown under full sunlight in pots containing N-deficient soil (pH 6.7) or similar soil amended with sufficient Al₂(SO₄)₃ or elemental S to give soil pH values of 4.8 and 4.6, respectively, and water-extractable Al levels of 30 and 14 M, respectively. Other treatments consisted of the addition of inorganic N or inoculation with commercial or locally-isolated B. japonicum. Acidification did not reduce shoot or root weights of plants receiving inorganic N but reduced (P0.05) shoot and root dry weights, nodule dry weights and numbers, shoot N concentrations, and chlorophyll levels of inoculated plants. Shoot dry weights and nodulation of inoculated plants were greater (P0.05) in Al₂(SO₄)₃-amended soil than in S-amended soil. Addition of Mo was not beneficial. It was concluded that reduced plant growth was caused by the effects of acidified soil on nodulation and that H-ion toxicity was probably the most limiting factor. Effects of Al, Mn, or Mo appeared less likely.     

Effects of forest degradation on microbial communities and soil carbon cycling: A global meta‐analysis

Aim

The aim was to explore how conversions of primary or secondary forests to plantations or agricultural systems influence soil microbial communities and soil carbon (C) cycling.

Location

Global.

Time period

1993–2017.

Major taxa studied

Soil microbes.

Methods

A meta‐analysis was conducted to examine effects of forest degradation on soil properties and microbial attributes related to microbial biomass, activity, community composition and diversity based on 408 cases from 119 studies in the world.

Results

Forest degradation decreased the ratios of K‐strategists to r‐strategists (i.e., ratios of fungi to bacteria, Acidobacteria to Proteobacteria, Actinobacteria to Bacteroidetes and Acidobacteria + Actinobacteria to Proteobacteria + Bacteroidetes). The response ratios (RRs) of the K‐strategist to r‐strategist ratios to forest degradation decreased and increased with increased RRs of soil pH and soil C to nitrogen ratio (C:N), respectively. Forest degradation increased the bacterial alpha‐diversity indexes, of which the RRs increased and decreased as the RRs of soil pH and soil C:N increased, respectively. The overall RRs across all the forest degradation types ranked as microbial C (?40.4%) > soil C (?33.3%) > microbial respiration (?18.9%) > microbial C to soil C ratio (qMBC; ?15.9%), leading to the RRs of microbial respiration rate per unit microbial C (qCO₂) and soil C decomposition rate (respiration rate per unit soil C), on average, increasing by +43.2 and +25.0%, respectively. Variances of the RRs of qMBC and qCO₂ were significantly explained by the soil C, soil C:N and mean annual precipitation.

Main conclusions

Forest degradation consistently shifted soil microbial community compositions from K‐strategist dominated to r‐strategist dominated, altered soil properties and stimulated microbial activity and soil C decomposition. These results are important for modelling the soil C cycling under projected global land‐use changes and provide supportive evidence for applying the macroecology theory on ecosystem succession and disturbance in soil microbial ecology.     

缙云山4种林分土壤无机磷与活性铝的含量及分布

为探索不同林分对土壤中无机磷与活性铝的含量及分布的影响,以及无机磷与活性铝之间的相互关系,以在我国西南地区酸性土壤上的农林经营管理提供理论和实践依据。研究以缙云山广泛分布的4种林分类型：山莓、马尾松、楠竹和柑橘林土壤为对象,采用酸性土壤无机磷分级方法和铝试剂比色法测定了土壤剖面A、B和C层中各形态的无机磷与活性铝的含量,分析了不同林分土壤中无机磷和活性铝的含量和分布特征。结果表明,林分类型显著影响土壤中无机磷与活性铝的含量与分布,且铁磷(Fe-P)、交换态铝(Ex-Al)和羟基铝(Hy-Al)的含量和比例还受到土壤层次的影响。4种林分相比,山莓林能促进闭蓄态磷(O-P)的形成,而马尾松、楠竹和柑橘林则有利于Al-P、Fe-P和Ca-P的形成;柑橘林有利于低活性的腐殖酸铝(Al-AH)形成,而山莓、马尾松和楠竹林促进高活性的Ex-Al或Hy-Al溶出;Ex-Al、Al-P和Fe-P在土层的分布上有表层富集现象,而Hy-Al集中分布于B层。此外,土壤Ex-Al和Al-P与Fe-P之间,Hy-Al和Ca-P之间均呈显著(P<0.05)正相关。因此,林分类型显著影响土壤无机磷与活性铝的含...     

陕北黄土区不同林地土壤干燥化效应

退耕还林还草工程虽然取得显著成效,但是加剧了区域土壤水分亏缺与土壤干燥化程度,同时制约着植被构建。选取陕西省吴起县金佛坪流域山杏、油松、刺槐、小叶杨和沙棘5种植被类型,对照荒草地。采用人工土钻法取样,分析不同植被类型0—10 m的土壤湿度特征和土壤干燥化效应。结果表明:各植被类型浅层和深层土壤水分均存在显著差异;与荒草地相比,各类林地均出现土壤水分过度消耗,其中刺槐林过耗量最大,山杏林最小;各林地均出现土壤干层,其中刺槐林、小叶杨林和山杏林土壤干层都已达到10 m以下,植被是深层土壤水分的主要影响因素。5类林地的土壤干燥化指数平均值39.26%,都远高于荒草地-9.57%。土壤干燥化强度为小叶杨(80.19%)刺槐(78.03%)沙棘(55.38%)山杏(37.94%)油松(32.34%),这是树种、林木大小和密度、林分生物量、树龄等生长指标综合作用的结果。在今后植被恢复中,应注意根据当地土壤水分条件,合理选择节水树种和合理设计林分结构,尽量避免或减轻土壤干化,维持较高的植被稳定性。     

The effects of traits of Bacillus megaterium onseed and root colonization and their correlation withthe suppression of Rhizoctonia root rot of soybean

Bacillus megaterium strainB153-2-2 is a potential bacterial biocontrol agentagainst Rhizoctonia solani isolate 2B12(ISG-2B). To study the role of antagonism (Ant),chemotaxis (Che), motility (Mot), and sporulation(Spo) of the biocontrol agent during seed and rootcolonization and the correlation between rootcolonization and the suppression of soybean (Glycine max) root rot caused by R. solani,strain B153-2-2(Che⁺Mot⁺Ant⁺⁺Spo⁺⁺) and the sevenderived mutants with altered antagonism, chemotaxis,motility, and/or sporulation were used. The bacterialcells were introduced into soil separately either asa soybean seed coating or soil application. Two soilmixtures defined as coarse and fine soil were used. The bacterial cell chemotactic response to soybeanroot and seed exudates and antagonism to R.solani were significantly (p = 0.05) correlatedwith root and seed colonization in some but not alltreatments. The sporulation-defective mutants had lowcell populations immediately after application and,therefore, reduced root colonization. The differencesin root colonization diminished among the mutants andstrain B153-2-2 when R. solani was present inthe soil or, as seedlings grew older. Soybean seedlingroots grown in coarse soil had significantly greatercolonization by B153-2-2 or its mutants and a lowerdisease index than that in fine soil. There was asignificant positive correlation (r ² = 0.78)between root colonization by strain B153-2-2 or itsmutants and suppression of Rhizoctonia root rot.     

Effects of antibiotics in soil on the population dynamics of transposon Tn5 carrying Pseudomonas fluorescens

The effects of kanamycin and streptomycin added to soil on the survival of transposon Tn5 modified Pseudomonas fluorescens strain R2f were investigated. Kanamycin in high (180 g g^-1 dry soil) or low (18 g g^-1) concentration or streptomycin in low concentration in Ede loamy sand soil had no noticeable effect on inoculant population dynamics in soil and wheat rhizosphere, whereas streptomycin in high concentration had a consistent significant stimulatory effect, in particular in the wheat rhizosphere. Streptomycin exerted its effect by selecting P. fluorescens with Tn5 insertion whilst suppressing the unmodified sensitive parent strain, as evidenced by comparing the behaviour of these two strains in separate and mixed inoculation studies.Soil textural type influenced the effect of streptomycin on the Tn5 carrying inoculant; the effect was consistently detected in rhizosphere and rhizoplane samples of wheat grown in Ede loamy sand after 7 and 14 days incubation, whereas it was only apparent after 7 days in rhizoplane or rhizosphere (and bulk soil) samples of wheat grown in two silt loam soils. Modification of soil pH by the addition of CaCO₃ or bentonite clay resulted in an enhancement of the selective effect of streptomycin by CaCO₃ and its abolishment by bentonite clay.The addition to soil of malic acid or wheat root exudate, but not of glucose, enhanced the streptomycin selective effect on the Tn5-modified P. fluorescens strain. Neither the streptomycin producer Streptomyces griseus nor two non-inhibiting mutants obtained following UV irradiation affected the dynamics of P. fluorescens (chr::Tn5) in soil and wheat rhizosphere.The effect of streptomycin in soil on inoculant Tn5 carrying bacteria depends on conditions such as soil type, the presence of (wheat) root exudates and the type of available substrate.     

基于DGGE研究喀斯特金橘接种AMF对根际细菌多样性和植株生长的影响

该研究分别以喀斯特地区的酸性土和石灰土为盆栽基质,对金橘分别接种摩西管柄囊霉(Funneliformis mosseae,F.m)或幼套近明囊霉(Claroideoglomus etunicatum,C.e),采用变性梯度凝胶电泳(DGGE)等技术研究AMF对金橘根际细菌群落多样性及植株生长的影响。结果表明:与不接种处理相比,同类型土中分别接种C.e或F.m,金橘根际菌根侵染率相关指标(F、m、v)值均为C.e处理的最高,C.e或F.m处理后金橘根际土壤中蔗糖酶、酸性磷酸酶、蛋白酶及脲酶活性均显著提高。其中,蛋白酶和脲酶活性变化为石灰土中各处理酶活性值高于酸性土,蔗糖酶和磷酸酶活性变化则为酸性土中各处理酶活性值高于石灰土,差异均达显著水平(P0.05);两种土壤中上述四种土壤酶活性均为C.e处理的值最高;接种C.e或F.m后,金橘根际土壤细菌的DGGE图谱中,DNA条带数增多,其细菌丰富度(R)、多样性(H)和均匀度(E)指数均比不接种处理高,三种指数值均以C.e处理的最高。细菌DNA优势条带序列分析结果表明:与优势细菌同源性最高的大部分为不可培养细菌,包括酸酐菌属、变形杆菌属、根瘤菌属和放线菌属等,相似性均大于97%。此外,接种后,金橘整株生物量比不接种处理显著提高,而酸性土中接种C.e处理的金橘生物量最高。综上所述,C.e和F.m均能与金橘建立良好的共生关系,且C.e对金橘根系的侵染效果更好。     

Effect of soil texture and its organic content on the efficacy of Trichoderma harzianum (MIAU 145 C) in controlling Meloidogyne javanica and stimulating the growth of kidney beans

The efficiency of Trichoderma harzianum (MIAU 145 C) in promoting kidney bean (cv. Goli) growth in different soil texture (sandy loam, loam and clay loam) and organic matter content (0.5 and 2% of leaf litter) was assessed in a factorial experiment in the absence of Meloidogyne javanica. In another factorial experiment, the effect of soil texture, soil organic content and control measure (no control, 10?ml of T. harzianum containing 10⁶ spore ml^?1 and 2?mg ai cadusafos kg^?1 soil) was determined on nematode-infected kidney bean’s growth, fungus controlling activity and M. javanica reproduction. Except for the shoot length, the fungus improved plant growth. Clay loam was not a proper soil type for the cultivation of kidney bean plants (even in the soil without nematode), but the plant grew better in sandy loam and loam soil. The presence of leaf litter in the soil enhanced plant growth, increased fungal efficiency and increased nematode reproduction. It seems that T. harzianum can activate the plant defence system in sandy loam soil. T. harzianum was more effective in sandy loam or loam soil containing 2% organic matter (leaf litter) and reduced the reproduction factor of the nematode in the tested soil textures equally to the chemical nematicide treatment.     

Soil organic matter and the indigenous nitrogen supply of intensive irrigated rice systems in the tropics

Soil organic matter (SOM) has been proposed as an index of N supply in paddy soils although field validations are few. We evaluated the relationship between the indigenous N supply (N _i ) of the soil-floodwater system and soil organic carbon (SOC) or total N (N _t ) in surface soil of long-term fertility experiments (LTFEs) at 11 sites, in 42 farmer's fiels with similar soil type, and in the same field in ten consecutive rice (Oryza sativa L.) crops. The N _i was estimated by crop N uptake from plots without applied N (N _o plots) under otherwise favorable growth conditions. There was a tight linear correlation between yields and N uptake in N _o plots and tremendous variation in both parameters among LTFE sites, farmer's fields, and in the same field over time. Correlation between N _i and SOC or N _t explained little of this variation. Factors likely to contribute to the poor correlation were: (1) inputs of N from sources other than N mineralization of SOM in surface soil, (2) degree of congruence between soil N supply and crop demand, which is sensitive to soil drying, length of fallow, crop rotation, and residue management, and (3) differences in SOM quality related to intensive cropping in submerged soil. Better understanding of the processes governing the N _i of tropical lowland rice systems would contribute to the development of crop management practices that optimize utilization of indigenous N resources.     

Stable Nitrogen and Carbon Pools in Grassland Soils of Variable Texture and Carbon Content

Nitrogen (N) inputs to many terrestrial ecosystems are increasing, and most of these inputs are sequestered in soil organic matter within 1–3 years. Rapid (minutes to days) immobilization focused previous N retention research on actively cycling plant, microbial, and inorganic N pools. However, most ecosystem N resides in soil organic matter that is not rapidly cycled. This large, stable soil N pool may be an important sink for elevated N inputs. In this study, we measured the capacity of grassland soils to retain ¹⁵N in a pool that was not mineralized by microorganisms during 1-year laboratory incubations (called “the stable pool”). We added two levels (2.5 and 50 g N m⁻²) of ¹⁵NH₄ ⁺ tracer to 60 field plots on coarse- and fine-textured soils along a soil carbon (C) gradient from Texas to Montana, USA. We hypothesized that stable tracer ¹⁵N retention and stable bulk soil (native + tracer) N pools would be positively correlated with soil clay and C content and stable soil C pools (C not respired during the incubation). Two growing seasons after the ¹⁵N addition, soils (0- to 20-cm depth) contained 71% and 26% of the tracer added to low- and high-N treatments, respectively. In both N treatments, 50% of the tracer retained in soil was stable. Total soil C (r ² = 0.72), stable soil C (r ² = 0.68), and soil clay content (r ² = 0.27) were correlated with stable bulk soil N pools, but not with stable ¹⁵N retention. We conclude that on annual time scales, substantial quantities of N are incorporated into stable organic pools that are not readily susceptible to microbial remineralization or subsequent plant uptake, leaching losses, or gaseous losses. Stable N formation may be an important pathway by which rapid soil N immobilization translates into long-term N retention. Received 2 April 2001; accepted 12 November 2001.     

溶磷细菌和丛枝菌根真菌接种对南方红豆杉生长及根际微生物和土壤酶活性的影响

以南方红豆杉实生苗为材料,采用盆栽实验探讨了高效溶磷细菌草木樨中华根瘤菌(Sinorhizobium meliloti)CHW10B与丛枝菌根真菌缩球囊霉(Glomus constrictum)单独和双接种条件下,其植株生长、根际土壤可培养微生物数量、土壤酶活和土壤微生物功能多样性变化,在微生态水平揭示接种对南方红豆杉生长的影响及其机制。结果表明:(1)各接种处理对南方红豆杉幼苗均有促生长作用,接种处理苗高、地径和生物量均较对照显著增加,并以双接种促进效果最好。(2)各接种处理提高了南方红豆杉根际土壤可培养细菌、真菌和放线菌含量,增加了土壤微生物碳源利用率,改变了土壤中物种的丰富度和均一度,增加了土壤中的生物多样性。(3)各接种处理促进了南方红豆杉根际重要土壤酶(酸性磷酸酶、脱氢酶、转化酶)活力的增加,且双接种的促进作用最为明显。可见,溶磷细菌(草木樨中华根瘤菌CHW10B)和丛枝菌根真菌(缩球囊霉)具有协同作用,两者同时接种可显著提高南方红豆杉根际土壤微生物数量及土壤酶活力,提高土壤微生物碳源利用率和土壤肥力,增加土壤中的生物多样性,从而达到间接促进宿主植物南方红豆杉生长的目的。     

Topographic and climatic controls on soil respiration in six temperate mixed-hardwood forest slopes, Korea

To better understand the effects of local topography and climate on soil respiration, we conducted field measurements and soil incubation experiments to investigate various factors influencing spatial and temporal variations in soil respiration for six mixed‐hardwood forest slopes in the midst of the Korean Peninsula. Soil respiration and soil water content (SWC) were significantly greater (P=0.09 and 0.003, respectively) on north‐facing slopes compared to south‐facing slopes, while soil temperature was not significantly different between slopes (P>0.5). At all sites, soil temperature was the primary factor driving temporal variations in soil respiration (r²=0.84–0.96) followed by SWC, which accounted for 30% of soil respiration spatial and temporal variability. Results from both field measurements and incubation experiments indicate that variations in soil respiration due to aspect can be explained by a convex‐shaped function relating SWC to normalized soil respiration rates. Annual soil respiration estimates (1070–1246 g C m^?2 yr^?1) were not closely related to mean annual air temperatures among sites from different climate regimes. When soils from each site were incubated at similar temperatures in a laboratory, respiration rates for mineral soils from wetter and cooler sites were significantly higher than those for the drier and warmer sites (n=4, P<0.01). Our results indicate that the application of standard temperature‐based Q₁₀ models to estimate soil respiration rates for larger geographic areas covering different aspects or climatic regimes are not adequate unless other factors, such as SWC and total soil nitrogen, are considered in addition to soil temperature.     

EFFECT OF THE NON-MYCORRHIZAL PIONEER PLANT SALSOLA KALI L. (CHENOPODIACEAE) ON VESICULAR-ARBUSCULAR MYCORRHIZAL (VAM) FUNGI

The effects of Salsola kali (Russian thistle) on chlamydospore numbers and viable propagules of the vesicular-arbuscular mycorrhizal fungus Glomus fasciculatum were monitored for 18 wk under laboratory conditions. When S. kali or Agropyron smithii (a mycorrhizal species) were grown for 6 wk, the number of chlamydospores in soil declined to the same number as in the control (non-planted) soil; the number of chlamydospores in the S. kali and control soils continued to decline over the 18 wk period. However, using a Zea mays bioassay, the per cent colonization of corn roots obtained from soil in which S. kali grew for 6, 12, or 18 wk was significantly greater than in the control soil. These results suggest that S. kali may stabilize mycorrhizal infectivity of disturbed soils for at least short periods of time.     

Effect of Rice Straw Incorporation on Phytotoxicity of Isoxaflutole to an Exotic Weed Phalaris minor Retz.

Phalaris minorRetz. is a major exotic annual weed in the wheat (Triticum aestivum L.) crop. Unharvested rice (Oryza sativa L.) straw, unburned and burned, is often incorporated in the field prior to cultivating wheat. Isoxaflutole (Balance), a pre-emergent systemic soil applied herbicide, has potential to control P. minor. Glasshouse experiments were conducted to determine the phytotoxicity of isoxaflutole defined by reductions in relation to shoot length of P. minor when grown in unamended soil or soil amended with unburned or burned rice straw. A 120 g soil was amended with 0, 1, 2 and 4 g of unburned or burned rice straw, and placed in 150 mL styrofoam pots. Appropriate amount of isoxaflutole (75% active ingredient, ai) was added to pots to get final concentration of 0, 7.5, 30, 60 and 120 μg ai/pot. Unamended soil and soil amended with unburned or burned rice straw were analyzed for pH and organic matter; two important determinants of isoxaflutole activity. Results indicate a significant reduction in shoot length of P. minor when grown in soil treated with isoxaflutole at 30, 60 or 120 μg ai/pot. Inhibition in the shoot length of P. minor was observed when soil amended with unburned straw was treated with isoxaflutole at 7.5 and 30 μg ai/pot compared with unamended soil treated with similar amounts of isoxaflutole. No significant change in isoxaflutole toxicity was observed when soil amended with unburned straw was treated with isoxaflutole at 60 and 120 lg ai/pot compared with unamended soil treated with similar amounts of isoxaflutole. Isoxaflutole phytotoxicity to P. minor shoot length was eliminated when soil amended with burned straw was treated with isoxaflutole at 7.5 and 30 μg ai/pot. P. minor shoot length was greater when soil amended with burned straw was treated with isoxaflutole at 60 and 120 μg ai/pot relative to herbicide-treated unamended soils. We conclude that incorporation of burned rice straw greatly reduces the phytotoxicity of isoxaflutole toP. minor.     

Short‐term soil carbon sink potential of oil palm plantations

Oil palm plantations cover ≈14.6 million ha worldwide and the total area under cultivation is expected to increase during the 21st century . Indonesia and Malaysia together account for 87% of global palm oil production and the combined harvested area in these countries has expanded by 6.5 million ha since 1990. Despite this, soil C cycling in oil palm systems is not well quantified but such information is needed for C budget inventories. We quantified soil C storage (root biomass, soil organic matter (SOM) and microbial biomass) and losses [potential soil respiration (R_s) and soil surface CO₂ flux (F_s)] in mineral soils from an oil palm plantation chronosequence (11–34 years since planting) in Selangor, Malaysia. There were no significant effects of plantation age on SOM, microbial biomass, R_s or F_s, implying soil C was in dynamic equilibrium over the chronosequence. However, there was a significant increase in root biomass with plantation age, indicating a short‐term C sink. Across the chronosequence, R_s was driven by soil moisture, soil particle size, root biomass and soil microbial biomass N but not microbial biomass C. This suggests that the nutrient status of the microbial community may be of equal or greater importance for soil CO₂ losses than substrate availability and also raises particular concerns regarding the addition of nitrogenous fertilizer, i.e. increased yields will be associated with increased soil CO₂ emissions. To fully assess the impact of oil palm plantations on soil C storage, initial soil C losses following land conversion (e.g. from native forest or other previous plantations) must be accounted for. If initial soil C losses are large, our data show that there is no accumulation of stable C in the soil as the plantation matures and hence the conversion to oil palm would probably represent a net loss of soil C.     

Effects of soil organic matter and nitrogen supply on competition between Festuca ovina and Deschampsia flexuosa during inland dune succession

We tested the prediction that the successional replacement of plant species during succession on inland sand dunes results from the effects of an increase in nitrogen mineralization on competitive interactions. The growth and competitive strength of Festuca ovina and Deschampsia flexuosa on soil substrates with different amounts of soil organic matter or nitrogen supply were measured. Small tillers of Festuca ovina and Deschampsia flexuosa were grown in monocultures and 1:1-mixtures on soil columns with undisturbed layers of soil organic matter from different successional age. There was (a) no visible soil organic matter, (b) a thin soil organic layer (0.5 cm) and (c) a thicker soil organic layer (6.0 cm) present on the soil columns. The species were also grown on columns with no visible soil organic matter (bare sand) with two different levels of N fertilization to mimic the increased N mineralization in the older successional stages.In monoculture, Festuca produced more biomass on the substrates with a soil organic layer compared to the unfertilized sand substrate. It also produced more biomass on sand substrates with N fertilization. Deschampsia produced more biomass in treatments with a soil organic layer compared to the bare sand treatments, but did not respond to the ammonium-nitrate addition. In competition, Festuca seemed to be the stronger competitor on the unfertilized sand substrate. Festuca was also the better competitor on the N fertilized sand treatments, while on the treatments with a soil organic layer Deschampsia was the winning species. Our results do not support the hypothesis that an increase in N supply is responsible for the replacement of Festuca by Deschampsia that concur with the accumulation of soil organic matter during succession in inland dunes.     

Reciprocal interactions between plants and soil in an upland grassland

Through the production of litter, plants with different life history strategies are predicted to both affect and be affected by the properties of soil. Competitive species are expected to increase the fertility of, and have a positive growth feedback with, soil, whereas stress-tolerant species should decrease fertility but show no growth feedback. We maintained monocultures of competitive (Lolium perenne and Agrostis capillaris) and stress-tolerant (Festuca ovina and Nardus stricta) grasses on an unproductive grassland for six years. The Nardus soil developed significantly greater inorganic nitrogen than the Agrostis and Festuca soil, and significantly greater soil moisture content than the Festuca soil. However, there were no differences in organic matter content, phosphate or bulk density between the soil types. In a greenhouse assay, each species was grown in soil cores from the different monocultures as well as natural turf. There were significant differences in growth between plant species and soil types. As expected, L. perenne produced the greatest amount of biomass. However, plants grown on Nardus soil were twice as large and had a 21% lower root allocation than plants grown on any of the other soil types. Lolium perenne, A. capillaris and F. ovina had significant negative growth feedbacks with their own soil (−0.460, −0.821 and −0.792, respectively) and N. stricta had a significant positive feedback (0.560). This study highlights the difficulties of predicting how plant traits will affect soil properties.     

Consequences of soil compaction for seedling establishment: Implications for natural regeneration and restoration

Abstract Soil compaction can affect seedling root development by decreasing oxygen availability and increasing soil strength. However, little quantitative information is available on the compaction tolerances of non‐crop native species. We investigated the effects of soil compaction on establishment and development of two New Zealand native species commonly used in restoration programmes; Cordyline australis (Agavaceae) (cabbage tree) a fleshy rooted species, and Leptospermum scoparium (Myrtaceae) (manuka) a very finely rooted species. Seedlings were grown in a range of soil compaction levels in growth cabinet experiments. Low levels of soil compaction (0.6 MPa) reduced both the number and speed of C. australis seedlings penetrating the soil surface. In contrast, L. scoparium seedlings showed improved establishment at an intermediate compaction level. Root and shoot growth of both species decreased with increasing soil strength, with L. scoparium seedlings tolerating higher soil strengths than did C. australis. Despite these results, soil strength accounted for only a small amount of variation in root length (R² < 0.25), due to greater variability in growth at low soil strengths. Soil strengths of 0.6 MPa are likely to pose a barrier to C. australis regeneration. This is consistent with adaptation to organic and/or soft, waterlogged soils. Active intervention may be necessary to establish C. australis from seed on many sites previously in farmland.