CO2 efflux from a Mediterranean semi-arid forest soil. II. Effects of soil fauna and surface stoniness

Many forest soils in the Mediterranean basin areshallow and contain high amounts of gravel in theorganic layers. Recent studies on soil organic matteraccumulation have shown high amounts of organic matteroccurring mainly in soils with high levels ofstoniness at the soil surface. The gravel layer mayaffect the microclimatic conditions of the soilsurface and probably the distribution and activity ofsoil fauna.In order to quantify the combined effects soil fauna(epigeic macrofauna and earthworms) and stoniness onthe release of soil CO₂, we performed a threefactor field experiment by using a series ofreconstructed soil profiles. Factors 1 and 2 consistedof the exclusion/presence of soil epigeic macrofaunaand earthworms, and factor 3 of the presence/absenceof a gravel layer intermingled with the H horizon. Weincubated ¹⁴C straw in the H horizon and carriedout three 40 mm rainfall simulations.Soil respiration primarily depended on the season. Theeffects of soil fauna were generally small and did notcoincide with periods of high faunal activity. Thelargest effects of both earthworms and soil epigeicfauna were found after wetting the soil in summer. Theeffects of the earthworms were concentrated in themineral soil while the effects of the epigeic faunawere concentrated in the H horizon and mainly arosetowards the end of the experiment. This suggests thatthe effects of epigeic fauna may have beenunderestimated due to the length of the experiment.The gravel layer increased the effect of faunaprobably by creating more favorable microclimaticconditions. The accumulation of organic matter insoils with high levels of stoniness cannot beexplained by the effect of gravel on soil microclimatenor by its effect on the activity of soil fauna.     

添加玉米残体对土壤-植物系统中氮素转化的影响

采用盆栽试验和^15N示踪技术对黑土添加玉米残体(秸秆和根茬)土壤-植物系统中氮素转化进行了研究,结果表明,玉米残体还田能够增加土壤氮素含量,减轻因其作为燃烧材料而造成的氮素损失和对大气的污染,玉米残体施入土壤,增加了土壤微生物氮含量,提高土壤氮活性,有利于土壤氮素养分的协调供应,玉米残体配施氮肥与氮肥单施相比,玉米植株氮素累积量相近,但氮素在玉米植株不同器官中的分配比例不同;添加玉米残体能够促进氮素从营养器官向籽粒中转移,提高氮素养分的利用效率,同时,添加玉米残体还可以降低土壤NO^-3-N的累积,减少肥料氮的损失4．7％～5．6％。     

同位素示踪技术在丛枝菌根真菌生态学研究中的应用

丛枝菌根(arbuscular mycorrhizal,AM)真菌是生态系统中重要的土壤微生物之一。AM真菌菌丝体网络是由AM真菌菌丝体在土壤生态系统中连接两株或两株以上植物根系所形成的菌丝体网络。随着菌根学研究的深入,如何直观的揭示AM真菌的生态学功能已经成为相关领域关注的热点问题。研究发现,利用同位素示踪技术可以开展AM真菌与宿主植物对土壤矿质营养的吸收、转运等方面的研究,以及菌丝体网络对不同宿主植物之间营养物质的分配研究和AM真菌在生态系统生态学中的功能研究。基于此,为了阐明同位素示踪技术在AM真菌研究中的价值,围绕菌根学最新研究进展,系统回顾了利用同位素示踪技术探究AM共生体对不同元素吸收和转运的机制、同位素示踪技术在AM真菌菌丝体网络研究中的价值和利用同位素示踪技术研究AM真菌在生态系统中的功能,为AM真菌生态学功能的研究提供理论基础,并对本领域未来的研究方向和应用前景进行展望。     

Net and gross incorporation of nitrogen by marine copepods fed on N-labelled diatoms: Methodology and trophic studies

The stable isotope of nitrogen (¹⁵N) and an appropriate three-compartment model were used in two 24-h lasting feeding experiments to trace the flow of N through the copepod Acartia discaudata and Calanus helgolandicus fed on ¹⁵N-labelled Skeletonema costatum and Thalassiosira weissflogii, respectively. Details of the labelling technique and principles of the computation of N transport rates are given. At the end of a single 24-h feeding period only about one third of the total amount of N ingested by A. discaudata was incorporated into the copepod's body N; we refer to this rate as net incorporation. Most of the N ingested was lost as ammonium (48% of total N ingested), followed by losses in the form of eggs + fecal pellets (13%) and dissolved organic N (DON, 9%). The sum of net incorporation and the latter losses is defined as gross incorporation. Net incorporation by C. helgolandicus and N losses did not vary over time during a 24 h lasting time-series feeding experiment. On average, 79% of total N ingested was actually incorporated by the copepod whereas mean N losses as ammonium, eggs + fecal pellets represented only 12 and 9%, respectively. After a 24-h feeding period only 2% of N ingested was lost as DON. Inspection of individual DON pathways showed that both A. discaudata and C. helgolandicus highly contributed to total DON production via direct excretion (79 and 64%, respectively). The remaining DON appearing in the DON pool was derived from phytoplankton via direct release and/or indirect release (copepod ‘sloppy feeding’).     

Useful tracer parameters to investigate the environmental conditions in areas of the Venice Lagoon

Grain-size and redox potential distributions in sediments were used as tracers to investigate environmental conditions in a shallow water area of the Venice Lagoon subjected to summer anoxic events. Data are presented showing different environmental characteristics within the study area. The results illustrate the reliability of these tracers to acquire a preliminary knowledge of the aquatic ecosystem behavior. Even small differences in morphology and hydrodynamics are observable because of the marked influence they exert on sediment parameters.     

Changes in Canopy Processes Following Whole-Forest Canopy Nitrogen Fertilization of a Mature Spruce-Hemlock Forest

Abstract Most experimental additions of nitrogen to forest ecosystems apply the N to the forest floor, bypassing important processes taking place in the canopy, including canopy retention of N and/or conversion of N from one form to another. To quantify these processes, we carried out a large-scale experiment and determined the fate of nitrogen applied directly to a mature coniferous forest canopy in central Maine (18–20 kg N ha⁻¹ y⁻¹ as NH₄NO₃ applied as a mist using a helicopter). In 2003 and 2004 we measured NO₃ ⁻, NH₄ ⁺, and total dissolved N (TDN) in canopy throughfall (TF) and stemflow (SF) events after each of two growing season applications. Dissolved organic N (DON) was greater than 80% of the TDN under ambient inputs; however NO₃ ⁻ accounted for more than 50% of TF N in the treated plots, followed by NH₄ ⁺ (35%) and DON (15%). Although NO₃ ⁻ was slightly more efficiently retained by the canopy under ambient inputs, canopy retention of NH₄ ⁺as a percent of inputs increased markedly under fertilization. Recovery of less than 30% of the fertilizer N in TF suggested that the forest canopy retained more than 70% of the applied N (>80% when corrected for N which bypassed tree surfaces at the time of fertilizer addition). Results from plots receiving ¹⁵N enriched NO₃ ⁻ and NH₄ ⁺ confirmed bulk N estimations that more NO₃ ⁻ than NH₄ ⁺ was washed from the canopy by wet deposition. The isotope data did not show evidence of canopy nitrification, as has been reported in other spruce forests receiving much higher N inputs. Conversions of fertilizer-N to DON were observed in TF for both ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ additions, and occurred within days of the application. Subsequent rain events were not significantly enriched in ¹⁵N, suggesting that canopy DON formation was a rapid process related to recent N inputs to the canopy. We speculate that DON may arise from lichen and/or microbial N cycling rather than assimilation and re-release by tree tissues in this forest. Canopy retention of experimentally added N may meet and exceed calculated annual forest tree demand, although we do not know what fraction of retained N was actually physiologically assimilated by the plants. The observed retention and transformation of DIN within the canopy demonstrate that the fate and ecosystem consequences of N inputs from atmospheric deposition are likely influenced by forest canopy processes, which should be considered in N addition studies. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Extrinsic and intrinsic sources of calcitonin gene-related peptide immunoreactivity in the lamb ileum: a morphometric and neurochemical investigation

To investigate extrinsic origins of calcitonin gene-related peptide immunoreactive (CGRP-IR) nerve fibres in the sheep ileum, the retrograde fluorescent tracer Fast Blue (FB) was injected into the ileum wall. Sections of thoraco-lumbar dorsal root ganglia (DRG) and distal (nodose) vagal ganglia showing FB-labelled neurons were processed for CGRP immunohistochemistry. The distribution of CGRP-IR in fibres and nerve cell bodies in the ileum was also studied. CGRP-IR enteric neurons were morphometrically analysed in myenteric (MP) and submucosal plexuses (SMP) of lambs (2–4 months). Sensory neurons retrogradely labelled with FB were scattered in T5-L4 DRG but most were located at the upper lumbar levels (L1-L3); only a minor component of the extrinsic afferent innervation of the ileum was derived from nodose ganglia. In the DRG, 57% of retrogradely labelled neurons were also CGRP-IR. In cryostat sections, a dense network of CGRP-IR fibres was observed in the lamina propria beneath the epithelium, around the lacteals and lymphatic follicles (Peyer's platches), and along and around enteric blood vessels. Rare CGRP-IR fibres were also present in both muscle layers. Dense pericellular baskets of CGRP-IR fibres were observed around CGRP-negative somata. The only CGRP-IR nerve cells were well-defined Dogiel type II neurons localised in the MP and in the external and internal components of the SMP. CGRP-IR neurons in the myenteric ganglia were significantly larger than those in the submucosal ganglia (mean profile areas: about 1,400 μm² for myenteric neurons, 750 μm² for submucosal neurons). About 6% of myenteric neurons and 25% of submucosal neurons were CGRP-IR Dogiel type II neurons. The percentages of CGRP-IR neurons that were also tachykinin-IR were about 9% (MP) and 42% (SMP), whereas no CGRP-IR neurons exhibited immunoreactivity for vasoactive intestinal peptide, nitric oxide synthase or tyrosine hydroxylase in either plexus. Thus, CGRP immunoreactivity occurs in the enteric nervous system of the sheep ileum (as in human small intestine and MP of pig ileum) in only one morphologically defined type of neuron, Dogiel type II cells. These are probably intrinsic primary afferent neurons. This work was supported by grants from the Ricerca Fondamentale Orientata (RFO) and Fondazione Del Monte di Bo e Ra.     

52Fe Translocation in Barley as Monitored by a Positron-Emitting Tracer Imaging System (PETIS): Evidence for the Direct Translocation of Fe from Roots to Young Leaves via Phloem

The real-time translocation of iron (Fe) in barley (Hordeumvulgare L. cv. Ehimehadaka no. 1) was visualized using the positron-emittingtracer ⁵²Fe and a positron-emitting tracer imaging system (PETIS).PETIS allowed us to monitor Fe translocation in barley non-destructivelyunder various conditions. In all cases, ⁵²Fe first accumulatedat the basal part of the shoot, suggesting that this regionmay play an important role in Fe distribution in graminaceousplants. Fe-deficient barley showed greater translocation of⁵²Fe from roots to shoots than did Fe-sufficient barley, demonstratingthat Fe deficiency causes enhanced ⁵²Fe uptake and translocationto shoots. In the dark, translocation of ⁵²Fe to the youngestleaf was equivalent to or higher than that under the light condition,while the translocation of ⁵²Fe to the older leaves was decreased,in both Fe-deficient and Fe-sufficient barley. This suggeststhe possibility that the mechanism and/or pathway of Fe translocationto the youngest leaf may be different from that to the olderleaves. When phloem transport in the leaf was blocked by steamtreatment, ⁵²Fe translocation from the roots to older leaveswas not affected, while ⁵²Fe translocation to the youngest leafwas reduced, indicating that Fe is translocated to the youngestleaf via phloem in addition to xylem. We propose a novel modelin which root-absorbed Fe is translocated from the basal partof the shoots and/or roots to the youngest leaf via phloem ingraminaceous plants.     

Interactive effects of warming and increased nitrogen deposition on 15N tracer retention in a temperate old field: seasonal trends

The extent to which increased atmospheric nitrogen (N) deposition will drive changes in plant productivity and species composition over the next century will depend on how other influential global change factors, such as climate warming, affect the N retention of ecosystems. We examined the interactive effects of simulated climate warming and N deposition on the recoveries of ¹⁵N‐labeled ammonium and ¹⁵N‐labeled nitrate tracers added as a pulse to grass‐dominated, temperate old‐field plots at spring thaw. In addition to the year‐round warming treatment, a winter‐only warming treatment was applied to a set of plots to explore the contribution of this component of climate warming to the overall warming effect. By the end of the plant growing season, there was approximately twice as much ¹⁵N enrichment in the plant roots and bulk soil from ¹⁵NH₄⁺‐addition plots than from ¹⁵NO₃^?‐addition plots, but there were no effects of warming or N fertilization on ¹⁵N recovery. Over winter, approximately half of the excess ¹⁵N present in plant shoots was lost, which corresponded with large ¹⁵N losses from bulk soil in N fertilized plots and large ¹⁵N increases in bulk soil in nonfertilized plots. By the next spring, there was decreased ¹⁵N recovery in plants in response to N fertilization, which was largely offset by increases in plant ¹⁵N recovery in response to year‐round warming. However, ¹⁵N retention in bulk soil, where the major part of the ¹⁵N label was recovered, was approximately 40% higher in nonfertilized plots than in N fertilized plots. Overall, our results indicate that climate warming increases plant N sequestration in this system but this effect is overwhelmed by the overall effect of nitrogen deposition on ecosystem N losses.