表层和下层免耕黑土有机碳矿化速率及激发效应

激发效应是调控土壤有机质分解的重要机制之一,而土层与激发效应的关系还不清晰.本研究通过室内培养试验,采用¹³C葡萄糖标记和动态碱液吸收的方法,探究免耕农田黑土表层土壤(0～10 cm)和下层土壤(30～40 cm)有机碳矿化速率及其激发效应.结果表明: 表层与下层土壤以单位有机碳表示的矿化速率并未发现显著差异.添加葡萄糖使表层土壤原有机质分解加快36.7%(正激发),但使下层土壤原有机质分解减慢12.4%(负激发).在整个培养期间(30 d),表层和下层土壤的累积激发碳量分别为3.14和-1.24 mg C·g^-1 SOC,但由于新碳(葡萄糖)的补偿作用,即使在产生显著正激发的表层土壤中,仍表现为有机碳净积累.说明外源碳输入使不同土层土壤有机质分解的幅度甚至方向产生明显差别.这为今后免耕和秸秆还田等保护性耕作措施的实践提供了重要的理论基础.     

水分对武夷山草甸土壤有机碳激发效应的影响

水分是影响土壤有机碳激发效应的重要因子,但水分如何影响山地草甸土有机碳激发效应尚不清楚.本试验以武夷山高海拔(2130 m)山地草甸土为研究对象,通过室内添加13C标记的葡萄糖结合控制土壤水分(30％FWC和60％ FWC,FWC为田间持水量),进行为期126 d的室内培养试验,定期测定CO2浓度和13C-CO2丰度值...     

氮沉降增加情景下植物-土壤-微生物交互对自然生态系统土壤有机碳的调控研究进展

大气氮沉降增加倾向于促进受氮限制陆地生态系统地上生物量,但是对地下碳过程和土壤碳截存的影响结果迥异,导致陆地生态系统“氮促碳汇”的评估存在很大的不确定性。大气氮沉降输入直接影响微生物活性或间接影响底物质量,改变凋落物和土壤有机质(SOM)的分解速率和分解程度,进而影响土壤有机碳(SOC)的积累与损耗过程。过去相关研究主要集中在土壤碳转化过程和碳储量动态方面,缺乏植物-微生物-SOM交互作用的理解,对土壤碳截存调控的生物化学和微生物学机理尚不清楚。本文以地下碳循环过程为主线,分别综述了氮沉降增加对植物地下碳分配、SOC激发效应、微生物群落碳代谢过程的影响,深入分析SOM化学稳定性与微生物群落动态的关系。该领域研究的薄弱环节体现在:(1)增氮倾向于降低根系的生长和周转,对根际沉积碳分配(数量和格局)的影响及驱动因素不明确;(2)虽然认识到氮素有效性影响土壤激发效应的方向和强度,但是氧化态NO-3和还原态NH+4输入对有机质激发效应的差异性影响及潜在机理知之甚少;(3)微生物碳利用效率(CUE)是微生物群落碳代谢的关键表征,能够很好地解释土壤碳的积累与损耗过程;由于缺乏适宜的测定方法,难以准确量化土壤微生物的CUE及微生物生物量的周转时间;(4)增氮会抑制土壤真菌群落及其胞外酶活性,对细菌群落组成的影响尚未定论,有关SOM化学质量与土壤微生物群落活性、组成之间的耦合关系尚不清楚。未来研究应基于长期的氮添加控制实验平台,结合碳氧稳定性同位素示踪、有机质化学、分子生物学和宏基因组学等方法,深入分析植物同化碳的地下分配规律、微生物碳代谢和周转、有机质化学结构与功能微生物群落的耦合关系等关键环节。上述研究将有助于揭示植物-土壤-微生物交互作用对SOC动态的调控机制,完善陆地生态系统碳-氮耦合循环模型,有效降低区域陆地碳汇评估的不确定性,并可为陆地生态系统应对全球变化提供科学依据。     

Rhizosphere priming effect increases the temperature sensitivity of soil organic matter decomposition

The temperature sensitivity of soil organic matter (SOM) decomposition has been a crucial topic in global change research, yet remains highly uncertain. One of the contributing factors to this uncertainty is the lack of understanding about the role of rhizosphere priming effect (RPE) in shaping the temperature sensitivity. Using a novel continuous ¹³C‐labeling method, we investigated the temperature sensitivity of RPE and its impact on the temperature sensitivity of SOM decomposition. We observed an overall positive RPE. The SOM decomposition rates in the planted treatments increased 17–163% above the unplanted treatments in three growth chamber experiments including two plant species, two growth stages, and two warming methods. More importantly, warming by 5 °C increased RPE up to threefold, hence, the overall temperature sensitivity of SOM decomposition was consistently enhanced (Q₁₀ values increased 0.3–0.9) by the presence of active rhizosphere. In addition, the proportional contribution of SOM decomposition to total soil respiration was increased by soil warming, implying a higher temperature sensitivity of SOM decomposition than that of autotrophic respiration. Our results, for the first time, clearly demonstrated that root–soil interactions play a crucial role in shaping the temperature sensitivity of SOM decomposition. Caution is required for interpretation of any previously determined temperature sensitivity of SOM decomposition that omitted rhizosphere effects using either soil incubation or field root‐exclusion. More attention should be paid to RPE in future experimental and modeling studies of SOM decomposition.     

杉木凋落物对土壤有机碳分解及微生物生物量碳的影响

利用¹³C稳定同位素示踪技术,研究了杉木凋落物对杉木人工林表层(0～5 cm)和深层(40～45 cm)土壤有机碳分解、微生物生物量碳和可溶性碳动态的影响.结果表明: 杉木人工林中深层土壤有机碳分解速率显著低于表层土壤,但其激发效应却显著高于表层土壤.杉木凋落物添加使土壤总微生物生物量碳和源于原有土壤的微生物生物量碳均显著增加,但对土壤可溶性碳没有显著影响.深层土壤被翻到林地表层,可能加速杉木人工林土壤中碳的损失.     

易分解有机碳输入量对武夷山不同林型土壤激发效应的影响

土壤有机碳库是陆地生态系统中最大的碳储量库,其微小的变化也能使大气中CO₂浓度发生巨大的改变,植物来源碳的输入能通过激发效应促进或抑制土壤有机碳(SOC)的分解,对SOC的动态平衡产生影响。以武夷山三个林型(阔叶林、马尾松林、针阔混交林)土壤为研究对象,通过向土壤中添加不同量的¹³C标记葡萄糖(0、100、200、400 mg C/kg)研究易分解有机碳输入量对不同林型土壤激发效应的影响,并在此基础上探讨易分解有机碳输入量对土壤激发效应影响的作用机理。结果表明,葡萄糖输入对土壤激发效应的影响与葡萄糖输入量和林型有关。葡萄糖的输入均抑制了三个林型SOC的分解(即,呈现负的激发效应)。阔叶林土壤和针阔混交林土壤激效应强度随着葡萄糖输入量的增加而增加,而马尾松林土壤的激发效应强度对葡萄糖输入量的响应并不明显。然而在马尾松林土壤中由葡萄糖所引起的激发效应强度显著高于其他两种林型土壤。研究结果表明,易分解有机碳的输入可以抑制SOC的矿化,形成负激发效应,阔叶林土壤的激发效应强度与土壤可利用氮、葡萄糖添加量与微生物碳量比值有关,而针阔混交林与马尾松林土壤...     

Elevated CO2, rhizosphere processes,and soil organic matter decomposition

The rhizosphere is one of the key fine-scale components of C cycles. This study was undertaken to improve understanding of the potential effects of atmospheric CO2 increase on rhizosphere processes. Using C isotope techniques, we found that elevated atmospheric CO2 significantly increased wheat plant growth, dry mass accumulation, rhizosphere respiration, and soluble C concentrations in the rhizosphere. When plants were grown under elevated CO2 concentration, soluble C concentration in the rhizosphere increased by approximately 60%. The degree of elevated CO2 enhancement on rhizosphere respiration was much higher than on root biomass. Averaged between the two nitrogen treatments and compared with the ambient CO2 treatment, wheat rhizosphere respiration rate increased 60% and root biomass only increased 26% under the elevated CO2 treatment. These results indicated that elevated atmospheric CO2 in a wheat-soil system significantly increased substrate input to the rhizosphere due to both increased root growth and increased root activities per unit of roots. Nitrogen treatments changed the effect of elevated CO2 on soil organic matter decomposition. Elevated CO2 increased soil organic matter decomposition (22%) in the nitrogen-added treatment but decreased soil organic matter decomposition (18%) without nitrogen addition. Soil nitrogen status was therefore found to be important in determining the directions of the effect of elevated CO2 on soil organic matter decomposition.     

Feedback interactions between needle litter decomposition and rhizosphere activity

The aim of our study was to identify interactions between the decomposition of aboveground litter and rhizosphere activity. The experimental approach combined the placement of labelled litter (¹³C=–37.9) with forest girdling in a 35-year-old Norway spruce stand, resulting in four different treatment combinations: GL (girdled, litter), GNL (girdled, no litter), NGL (not girdled, litter), and NGNL (not girdled, no litter). Monthly sampling of soil CO₂ efflux and ¹³C of soil respired CO₂ between May and October 2002 allowed the partitioning of the flux into that derived from the labelled litter, and that derived from native soil organic matter and roots. The effect of forest girdling on soil CO₂ efflux was detectable from June (girdling took place in April), and resulted in GNL fluxes to be about 50% of NGNL fluxes by late August. The presence of litter resulted in significantly increased fluxes for the first 2 months of the experiment, with significantly greater litter derived fluxes from non-girdled plots and a significant interaction between girdling and litter treatments over the same period. For NGL collars, the additional efflux was found to originate only in part from litter decomposition, but also from the decay of native soil organic matter. In GL collars, this priming effect was not significant, indicating an active role of the rhizosphere in soil priming. The results therefore indicate mutual positive feedbacks between litter decomposition and rhizosphere activity. Soil biological analysis (microbial and fungal biomass) of the organic layers indicated greatest activity below NGL collars, and we suppose that this increase indicates the mechanism of mutual positive feedback between rhizosphere activity and litter decomposition. However, elimination of fresh C input from both above- and belowground (GNL) also resulted in greater fungal abundance than for the NGNL treatment, indicating likely changes in fungal community structure (i.e. a shift from symbiotic to saprotrophic species abundance).     

草原土壤有机碳含量的控制因素

基于374个高寒草原和温带草原土壤样品的测试结果,运用多元逐步回归分析模型定量评估了土壤环境因子对土壤有机碳(SOC)含量的影响.结果表明:高寒草原土壤有机碳含量(20.18 kg C/m2)高于温带草原(9.23 kg C/m2).土壤理化生物学因子对高寒草原和温带草原SOC含量(10 cm)变化的贡献分别是87.84％和75.00％.其中,土壤总氮含量和根系对高寒草原SOC含量变化的贡献均大于对温带草原SOC含量变化的相应贡献.土壤水分是温带草原SOC含量变化的主要限制性因素,其对SOC含量变化的贡献达33.27％.高寒草原土壤C/N比显著高于温带草原土壤的相应值,揭示了青藏高原高寒草原较高的SOC含量是由于较低的土壤微生物活性所导致.     

Crop residue-derived dissolved organic matter accelerates the decomposition of native soil organic carbon in a temperate agricultural ecosystem

Crop residue-derived dissolved organic matter (DOM) plays an important role in soil carbon (C) cycling. To investigate the effects of maize residue-derived DOM and urea additions on the native soil organic carbon (SOC) decomposition and soil net C balance a pot experiment was carried out during the winter wheat growing season in the North China Plain (NCP). The results showed that adding maize residue-derived DOM alone (RDOM) or together with urea (RDOM?+?N) accelerated the decomposition of native SOC and resulted in a net SOC loss. The net loss of SOC was 3.90?±?0.61 and 3.53?±?0.48?g?C?m^?2 in RDOM and RDOM?+?N treatments, respectively. The stimulatory effect of per unit DOM-C addition on the native SOC decomposition was 0.25?±?0.05 and 0.45?±?0.07 for the RDOM and RDOM?+?N treatments, respectively. Increases in the microbial biomass and the activity of β-glucosidase, invertase and cellobiohydrolase as well as soil mineral N content were responsible for a more intense priming effect in DOM-amended soils. The positive relationship between primed soil C and soil available N (R?=?0.76, P?<?0.05) suggested that the stimulation of decomposition of native SOC by DOM addition would be enhanced by nitrogen fertilizer application.     

The effects of heating,rhizosphere, and depth on root litter decomposition are mediated by soil moisture

The breakdown and decomposition of plant inputs are critical for nutrient cycling, soil development, and climate-ecosystem feedbacks, but uncertainties persist in how the rates and products of litter decomposition are affected by soil temperature, rhizosphere, and depth of input. We investigated the effects of soil warming (+ 4 °C), rhizosphere, and depth of litter placement on the decomposition of Avena fatua (wild oat grass) root litter in a Mediterranean grassland ecosystem. Field lysimeters were subjected to three environmental treatments (heating, control, and plant removal) and three ¹³C-labeled root litter addition treatments (to A horizon, to B horizon, and no-addition disturbance control) for each of two harvest time points. We buried root litter in February 2014 and measured loss of ¹³C in CO₂ from the soil surface and in leachate as dissolved organic carbon (DOC) over two growing seasons. At the end of each growing season we recovered the ¹³C remaining in the soil. Loss of root litter C occurred almost entirely via heterotrophic respiration, with an estimated < 2% lost as DOC during the initial decay period. The added roots were broken down and incorporated into bulk soil material very quickly; only ~ 30% of added root was visible after 6 months. In the first growing season, decomposition occurred faster in the B than in the A horizon, the latter having greater moisture limitation. Subsequently, there was almost no further decomposition in the B horizon. After two growing seasons, less than 20% of the added root litter C remained in the A or B horizons of all environmental treatments. Heating did not stimulate decomposition, likely because it exacerbated the moisture limitation. However, while plots without plants dried down more slowly than plots with plants, their decomposition rate was not significantly greater, possibly due to the lack of rhizosphere processes such as priming. We conclude that in this Mediterranean grassland ecosystem, soil moisture, which is affected by season, depth, heating, and rhizosphere, plays a dominant role in mediating the effect of those factors on root litter decomposition, which after two seasons did not differ by depth or by treatment.     

Labile carbon retention compensates for CO2 released by priming in forest soils

Increase of belowground C allocation by plants under global warming or elevated CO₂ may promote decomposition of soil organic carbon (SOC) by priming and strongly affects SOC dynamics. The specific effects by priming of SOC depend on the amount and frequency of C inputs. Most previous priming studies have investigated single C additions, but they are not very representative for litterfall and root exudation in many terrestrial ecosystems. We evaluated effects of ¹³C‐labeled glucose added to soil in three temporal patterns: single, repeated, and continuous on dynamics of CO₂ and priming of SOC decomposition over 6 months. Total and ¹³C labeled CO₂ were monitored to analyze priming dynamics and net C balance between SOC loss caused by priming and the retention of added glucose‐C. Cumulative priming ranged from 1.3 to 5.5 mg C g^?1 SOC in the subtropical, and from ?0.6 to 5.5 mg C g^?1 SOC in the tropical soils. Single addition induced more priming than repeated and continuous inputs. Therefore, single additions of high substrate amounts may overestimate priming effects over the short term. The amount of added glucose C remaining in soil after 6 months (subtropical: 8.1–11.2 mg C g^?1 SOC or 41‐56% of added glucose; tropical: 8.7–15.0 mg C g^?1 SOC or 43–75% of glucose) was substantially higher than the net C loss due to SOC decomposition including priming effect. This overcompensation of C losses was highest with continuous inputs and lowest with single inputs. Therefore, raised labile organic C input to soils by higher plant productivity will increase SOC content even though priming accelerates decomposition of native SOC. Consequently, higher continuous input of C belowground by plants under warming or elevated CO₂ can increase C stocks in soil despite accelerated C cycling by priming in soils.     

Temperature sensitivity of soil organic carbon decomposition increased with mean carbon residence time: Field incubation and data assimilation

Temperature sensitivity of soil organic carbon (SOC) decomposition is one of the major uncertainties in predicting climate‐carbon (C) cycle feedback. Results from previous studies are highly contradictory with old soil C decomposition being more, similarly, or less sensitive to temperature than decomposition of young fractions. The contradictory results are partly from difficulties in distinguishing old from young SOC and their changes over time in the experiments with or without isotopic techniques. In this study, we have conducted a long‐term field incubation experiment with deep soil collars (0–70 cm in depth, 10 cm in diameter of PVC tubes) for excluding root C input to examine apparent temperature sensitivity of SOC decomposition under ambient and warming treatments from 2002 to 2008. The data from the experiment were infused into a multi‐pool soil C model to estimate intrinsic temperature sensitivity of SOC decomposition and C residence times of three SOC fractions (i.e., active, slow, and passive) using a data assimilation (DA) technique. As active SOC with the short C residence time was progressively depleted in the deep soil collars under both ambient and warming treatments, the residences times of the whole SOC became longer over time. Concomitantly, the estimated apparent and intrinsic temperature sensitivity of SOC decomposition also became gradually higher over time as more than 50% of active SOC was depleted. Thus, the temperature sensitivity of soil C decomposition in deep soil collars was positively correlated with the mean C residence times. However, the regression slope of the temperature sensitivity against the residence time was lower under the warming treatment than under ambient temperature, indicating that other processes also regulated temperature sensitivity of SOC decomposition. These results indicate that old SOC decomposition is more sensitive to temperature than young components, making the old C more vulnerable to future warmer climate.     

Aboveground litter quality changes may drive soil organic carbon increase after shrub encroachment into mountain grasslands

Shrub encroachment into grasslands is ubiquitous but its impact on soil organic C (SOC) remains unclear. In previous work we had observed that shrub encroachment into mesic mountain grasslands increased SOC content. Here we sought the mechanisms of this increase. To this end, we assessed aboveground and belowground production for a conifer shrub (Juniperus communis L), a legume shrub (Cytisus balansae ssp. europaeus (G. López & Jarvis) Muñoz Garmendia) and grass (Festuca eskia Ramond ex DC), together with decomposition rates for both aboveground litter and roots. Belowground C net inputs do not clearly explain SOC increase: grass root production was higher than that of either shrub and the decomposition rate of grass roots was the lowest. Aboveground C net inputs were only slightly greater in shrubs than in grass, but the decomposition rate of litter of both shrubs was much lower than that of grass. The decomposition of conifer litter was N-limited, whereas that of legume shrub litter was P-limited. Thus we conclude that the SOC increases after shrub encroachment into mesic grasslands probably as a result of higher recalcitrance of shrub aboveground litter relative to grass litter.     

Spatial distribution of rhizodeposit carbon of maize (Zea mays L.) in soil aggregates assessed by multiple pulse 13C labeling in the field

Background and aims

Rhizosphere effect is controlled by spatial distribution of rhizodeposits, which may be influenced by soil aggregation and soil moisture regime in relation to water uptake by roots. The objectives of this study were to measure soil organic carbon (SOC) concentration and its δ¹³C abundance by aggregate size in the rooted bulk soil and by distance in the root-free soil vertically and horizontally away from roots, and to measure DOC concentration and its δ¹³C abundance in pore water in the rooted bulk soil after a seasonal pulse labelings of ¹³CO₂ to maize (Zea mays L.).

Methods

Pulse labeling was conducted in the field once a week for 11 weeks. Soil cells (50 mm in diameter and 100 mm long) mimicking root-free soils were imbedded vertically and horizontally 25–50 mm away from the main root of a maize crop. The rooted bulk soils were sampled to extract soil pore water at different suctions and to fractionate aggregates by wet sieving. The root-free soil cells were sliced by 1 mm intervals from the root end to 20 mm away. All the sampling was 12 days after the last labeling after the crop was harvested.

Results and discussion

The δ¹³C abundance before and after the continuous labeling was ?24.20?±?0.05?‰ and ?23.80?±?0.05?‰ in the rooted bulk soil. The labeling caused increases in δ¹³C abundance in all the aggregates in the rooted bulk soil and down to 14 mm away from the roots in both the root-free cells. The δ¹³C abundance was enriched in the >2 mm and 1–2 mm aggregates (?23.17?±?0.12?‰ and ?23.26?±?0.05?‰) though the SOC concentration was not different among the >0.25 mm aggregates, indicating that rhizodeposits or their metabolites were protected and distributed widely in whole soil through soil aggregation. The δ¹³C abundance in pore water (?24.0?±?0.01?‰) was much lower than those soil aggregates and greatest from the >2 μm soil pores though the DOC concentration was greater from the <20 μm soil pores. The δ¹³C abundance was in general greater in the horizontal cell than in the vertical cell. The δ¹³C abundance decreased with the increasing distance to the roots in the vertical cell and peaked at the 5 and 6 mm distance to the roots in the horizontal cell (?23.66?±?0.11?‰ and ?23.5?±?0.10?‰), possibly due to the drier condition unfavorable to microbial decomposition in the horizontal cell. The higher δ¹³C abundance in the horizontal cell than in the vertical cell was accompanied by a lower SOC concentration and a lower C: N ratio within 3 mm away from the roots, suggesting a stronger priming effect due to the longer residence time of rhizodeposits in the horizontal cell than in the vertical cell.

Conclusions

Rhizodeposits or their metabolites were protected during soil aggregation and distributed to 14 mm beyond the rhizosphere in the natural soil-plant system. This extension is of significance in regulating the formation of soil structure and the priming of soil organic matter during the whole life cycle of plants, which needs further study.     

Rhizosphere priming effects on the decomposition of soil organic matter in C4 and C3 grassland soils

Using a natural abundance ¹³C method, soil organic matter (SOM) decomposition was studied in a C₃ plant – `C<sub>4</sub> soil' (C<sub>3</sub> plant grown in a soil obtained from a grassland dominated by C<sub>4</sub> grasses) system and a C<sub>4</sub> plant – `C₃ soil' (C₄ plant grown in a soil taken from a pasture dominated by C₃ grasses) system. In C₃ plant – `C<sub>4</sub> soil' system, cumulative soil-derived CO<sub>2</sub>–C were higher in the soils planted with soybean (5499 mg pot<sup>–1</sup>) and sunflower (4484 mg pot<sup>–1</sup>) than that in `C₄ soil' control (3237 mg pot^–1) without plants. In other words, the decomposition of SOM in soils planted with soybean and sunflower were 69.9% and 38.5% faster than `C<sub>4</sub> soil' control. In C<sub>4</sub> plant – `C₃ soil' system, there was an overall negative priming effect of live roots on the decomposition of SOM. The cumulative soil-derived CO₂–C were lower in the soils planted with sorghum (2308 mg pot^–1) and amaranthus (2413 mg pot^–1) than that in `C<sub>3</sub> soil' control (2541 mg pot<sup>–1</sup>). The decomposition of SOM in soils planted with sorghum and amaranthus were 9.2% and 5.1% slower than `C₃ soil' control. Our results also showed that rhizosphere priming effects on SOM decomposition were positive at all developmental stages in C₃ plant – `C<sub>4</sub> soil' system, but the direction of the rhizosphere priming effect changed at different developmental stages in the C<sub>4</sub> plant – `C₃ soil' system. Implications of rhizosphere priming effects on SOM decomposition were discussed.     

An invasive wetland grass primes deep soil carbon pools

Understanding the processes that control deep soil carbon (C) dynamics and accumulation is of key importance, given the relevance of soil organic matter (SOM) as a vast C pool and climate change buffer. Methodological constraints of measuring SOM decomposition in the field prevent the addressing of real‐time rhizosphere effects that regulate nutrient cycling and SOM decomposition. An invasive lineage of Phragmites australis roots deeper than native vegetation (Schoenoplectus americanus and Spartina patens) in coastal marshes of North America and has potential to dramatically alter C cycling and accumulation in these ecosystems. To evaluate the effect of deep rooting on SOM decomposition we designed a mesocosm experiment that differentiates between plant‐derived, surface SOM‐derived (0–40 cm, active root zone of native marsh vegetation), and deep SOM‐derived mineralization (40–80 cm, below active root zone of native vegetation). We found invasive P. australis allocated the highest proportion of roots in deeper soils, differing significantly from the native vegetation in root : shoot ratio and belowground biomass allocation. About half of the CO₂ produced came from plant tissue mineralization in invasive and native communities; the rest of the CO₂ was produced from SOM mineralization (priming). Under P. australis, 35% of the CO₂ was produced from deep SOM priming and 9% from surface SOM. In the native community, 9% was produced from deep SOM priming and 44% from surface SOM. SOM priming in the native community was proportional to belowground biomass, while P. australis showed much higher priming with less belowground biomass. If P. australis deep rooting favors the decomposition of deep‐buried SOM accumulated under native vegetation, P. australis invasion into a wetland could fundamentally change SOM dynamics and lead to the loss of the C pool that was previously sequestered at depth under the native vegetation, thereby altering the function of a wetland as a long‐term C sink.     

Root decomposition of <Emphasis Type="Italic">Artemisia halodendron</Emphasis> and its effect on soil nitrogen and soil organic carbon in the Horqin Sandy Land,northeastern China

Root decomposition is a critical feedback from the plant to the soil, especially in sandy land where strong winds remove aboveground litter. As a pioneer shrub in semi-mobile dunes of the Horqin sandy land, Artemisia halodendron has multiple effects on nutrient capture and the microenvironment. However, its root decomposition has not been studied in terms of its influence on soil organic carbon (SOC) and nitrogen (N). In this study, we buried fine (≤2 mm) and coarse roots in litterbags at a depth of 15 cm below semi-mobile dunes. We measured the masses remaining and the C and N contents at intervals during 434 days of decomposition. The soils below the litterbags were then divided into layers and sampled to measure the SOC and N contents. After rapid initial decomposition, both coarse and fine roots decomposed slowly. After 53 days, 36.2 % of coarse roots and 39.8 % of fine roots had decomposed. In contrast, only 18.4 % of coarse roots and 30.5 % of fine roots decomposed in the following 381 days. Fine roots decomposed significantly faster, and their decomposition rate after the initial rapid decay was strongly related to climate (R ² = 0.716, P < 0.05). Root decomposition increased SOC and N contents below the litterbags, with larger effects for fine roots. The SOC content was more variable between soil layers than the N content. Thus, decomposition of A. halodendron roots cannot be ignored when studying SOC and N feedbacks from plants to the soil, particularly for fine roots.     

Defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter under a sunflower species: Helianthus annuus

In the present study, `natural <sup>13</sup>C tracer method' was used to partition the belowground respiration into rhizosphere respiration and soil microbial respiration to test the hypothesis that defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter (SOM). A C<sub>3</sub> plant species, Helianthus annuus (sunflower), was grown in `C₄' soil in microcosms so that the CO₂ evolved from plant-soil system can be partitioned. Four levels of defoliation intensities were established by manual clipping. CO₂ evolved from plant-soil system was trapped during 0–4 h after defoliation (HAD), 5–22 HAD and 23–46 HAD using a closed circulating system, respectively. We found that both rhizosphere respiration and soil microbial respiration of the clipped plants were either unchanged or significantly enhanced compared to unclipped plants at 45% defoliation level during all sampling intervals. Soil microbial respiration increased significantly at all defoliation levels during 0–4 HAD, however, both rhizosphere respiration and soil microbial respiration decreased significantly during 5–22 HAD or 23–46 HAD when 20% or 74 clearly demonstrated that the defoliation treatments modified the rhizosphere priming effect on SOM decomposition. The total cumulative rhizosphere priming effects on SOM decomposition during 0–46 HAD were 146%, 241%, 204% and 205% when 0%, 20%, 45% and <74%.     

易分解有机碳输入量对武夷山常绿阔叶林不同土层深度土壤激发效应的影响

土壤有机碳(SOC)的矿化在碳、氮循环过程中起着极为重要的作用。易分解有机碳的输入可以通过正(负)激发效应加快(减缓)原有SOC的矿化。然而,先前的研究更多关注易分解有机碳输入量对表层(0～20 cm)土壤激发效应的影响,而较少关注其对深层(>20 cm)土壤激发效应的影响。本研究利用¹³C标记葡萄糖(99 atom%)添加试验,研究葡萄糖添加量对武夷山常绿阔叶林表层(0～20 cm)和深层(30～40 cm)土壤激发效应的影响,并通过分析微生物群落组成的变化以及土壤可利用氮含量的变化探讨土壤激发效应产生的机理。结果表明:葡萄糖的添加抑制了表层和深层SOC的矿化(P<0.05),使SOC的矿化量分别减少了26%～61%与62%～68%,呈现负的激发效应,但激发强度因葡萄糖添加量和土层深度而异。对于表层土壤,激发强度随着葡萄糖添加量的增加而增加;而对于深层土壤,激发强度对葡萄糖添加量的响应并不敏感。而且,葡萄糖的添加并未显著影响表层和深层土壤的微生物量碳氮含量和微生物群落组成(总磷脂脂肪酸含量;细菌、真菌、放线菌磷脂脂肪酸含量以及细菌真菌比)(P>...