Mixtures with grass litter may hasten shrub litter decomposition after shrub encroachment into mountain grasslands

Aims

Shrub encroachment in mesic grasslands alters the identity and quality of litters entering the system. As litter from shrubs and grasses can differ in their quality, this can lead to differences in litter decomposition by the direct effect of quality, but also to litter interaction during decomposition. The objective of this study was to examine the occurrence of non-additive effects of litter mixtures on the decomposition rates of legume shrub litter (poor in P) or conifer shrub litter (poor in N) and grass litter.

Methods

In addition to single litter type litterbags for the three species, we mixed litters of each pair of possible combinations to determine the influence of each species on mass loss. Litterbags were placed in the field and collected after 1, 6, 8, 12 and 24 months. In each collection, litter of each species remaining in mixed bags was separated, dry weighed and analyzed for C, N and P.

Results

With respect to shrub litter decomposing alone, mass loss of shrub litter when mixed with grass showed a 9–10 % increase in decomposition rate for conifer and a 3 % increase for legume litter. These litter mixture effects varied with time and they were detected after a decomposition period of 1 year in legume litter and of 2 years in conifer litter.

Conclusions

Grass litter hastened conifer and legume litter decomposition in leaf litter mixtures, at least during the first stages of the process. The potential consequences of this result to alter litter accumulation patterns and thus carbon sequestration rates after shrub encroachment into grasslands will depend on whether the observed trends are maintained in the advanced decomposition stages.     

Effects of shrub encroachment on vertical changes in soil organic carbon in Mongolian grasslands: using a multi-biomarker approach

Background

This study started from typical replant disease symptoms limited to specific foci within three multi-generation orchards showing homogeneous growth.

Methods

A plant growth assay was conducted using soil from symptomatic and asymptomatic tree root zones along planted rows and from strip-rows. Root colonizing fungal communities were investigated, then the study turned to pathogenicity of Cylindrocarpon-like fungi (Dactylonectria and Ilyonectria spp) and their extracellular exudates.

Results

Growth of apple rootstock plantlets in soil from symptomatic trees was significantly lower than in those observed in soil from asymptomatic trees and from strip-rows. Among the main group of endophytic filamentous fungi isolated from roots, Cylindrocapon-like fungi (Dactylonectria torresensis and, to a lesser extent, Ilyonectria robusta), along with binucleate Rhizoctonia spp., prevailed mostly in plantlets grown in soil collected from planted rows. On the other hand, Fusarium spp. prevailed in plantlets grown in soil from the strip-rows. Cylindrocarpon-like fungi was found to be the most negatively correlated with plant growth. As findings of artificial inoculation with main root colonizing fungal species were not in line with what was observed in native soils, a further investigation was performed on secondary metabolites through which Dactylonectria torresensis exerts pathogenicity; this analysis revealed that both phytotoxins (tentoxin, HC toxin and zearalenone) and cytotoxic compounds (rabelomycin and nidulin) may be involved.

Conclusion

Findings suggest that extracellular compounds released by D. torresensis may have contributed to the severe growth reduction associated with replant disease-like symptoms.

     

Changes to soil organic N dynamics with leguminous woody plant encroachment into grasslands

Encroachment of nitrogen-fixing trees and shrubs into grasslands and savannas is a well-documented land cover change that occurs worldwide. In the Rio Grande Plains region of southern Texas, previous studies have shown woody encroachment by leguminous Prosopis glandulosa (mesquite) trees increases soil C and N, decreases microbial biomass N relative to soil N, and accelerates N mineralization and nitrification. We examined responses of the dominant organic N components in soil (amino acids and amino sugars) and two soil-bound protein-N acquiring enzymes (arylamidase and β-N-acetylglucosaminidase) along a grassland-to-woodland successional chronosequence to determine changes to soil N chemistry and extractability. The proportion of total N held within amino compounds was significantly lower in the woodlands (47 %) relative to the grassland soils (62 %). This increase in non-hydrolysable N was accompanied by increases in plant cell wall derived amino acids (e.g. hydroxyproline, serine) and losses of microbial amino sugars, indicating the woodland organic N pool was altered in composition and potentially in quality, either because it was more structurally protected or difficult to degrade due to polymerization/condensation reactions. Soil carbon-normalized activities of both soil-bound N-acquiring enzymes were significantly higher in woodland soils, consistent with changes in the biochemical composition of organic N. Although soil total N increases following woody encroachment, this additional organic N appears to be less extractable by chemical hydrolysis and thus potentially in more refractory forms, which may limit microbial N accessibility, slow the cycling of soil organic carbon, and contribute to observed soil C and N accrual in these systems.     

How spatial heterogeneity of cover affects patterns of shrub encroachment into mesic grasslands

We used a multi-method approach to analyze the spatial patterns of shrubs and cover types (plant species, litter or bare soil) in grassland-shrubland ecotones. This approach allows us to assess how fine-scale spatial heterogeneity of cover types affects the patterns of Cytisus balansae shrub encroachment into mesic mountain grasslands (Catalan Pyrenees, Spain). Spatial patterns and the spatial associations between juvenile shrubs and different cover types were assessed in mesic grasslands dominated by species with different palatabilities (palatable grass Festuca nigrescens and unpalatable grass Festuca eskia). A new index, called RISES ("Relative Index of Shrub Encroachment Susceptibility"), was proposed to calculate the chances of shrub encroachment into a given grassland, combining the magnitude of the spatial associations and the surface area for each cover type. Overall, juveniles showed positive associations with palatable F. nigrescens and negative associations with unpalatable F. eskia, although these associations shifted with shrub development stage. In F. eskia grasslands, bare soil showed a low scale of pattern and positive associations with juveniles. Although the highest RISES values were found in F. nigrescens plots, the number of juvenile Cytisus was similar in both types of grasslands. However, F. nigrescens grasslands showed the greatest number of juveniles in early development stage (i.e. height<10 cm) whereas F. eskia grasslands showed the greatest number of juveniles in late development stages (i.e. height>30 cm). We concluded that in F. eskia grasslands, where establishment may be constrained by the dominant cover type, the low scale of pattern on bare soil may result in higher chances of shrub establishment and survival. In contrast, although grasslands dominated by the palatable F. nigrescens may be more susceptible to shrub establishment; current grazing rates may reduce juvenile survival.     

Woody plant encroachment may decrease plant carbon storage in grasslands under future drier conditions

木本植物沿海拔/气候梯度广泛分布于中国新疆的各种草地类型。木本植物入侵能引起草地碳储量的变化,并且气候条件会调节这种变化。基于这些研究结果,我们预测,在半干旱草地中,木本植物对半干旱草地的植被碳储量有正向影响,而在干旱草地中,木本植物会负向影响碳储量。我们调查草地类型之间地上和地下碳储量的空间分布并对这一预测进行检验。测定纯草地和木质化草地(木本植物的相对地上生物量>50%)的地上活体生物量(AGC)、凋落物量和地下生物量(BGC),共包括6种草地类型,可代表新疆地区半干旱至干旱状态。从荒漠到山地草甸,地上活体生物量、凋落物量和地下生物量逐渐增加。这一结果可能由年均降水量增加或年均气温降低导致,也表明草地类型代表干旱梯度。相比于纯草地,木本植物对草地植被碳储量的大小和分配均有显著影响。并且,由于气候的调节作用,木本植物影响的方向和强度因草地类型而异,较为湿润的条件可以促进木本植物的正向效应。相比于草本植物,木本植物的AGC高导致草地植被AGC增加。然而,随干旱程度增加,木本植物对草本植物呈现更为明显的负面效应,使得在荒漠、草原化荒漠和荒漠草原中,其木本植物对植被AGC的增加幅度小于较为湿润的草地类型。在较为干旱(MAP较低而MAT较高)的气候条件下,木本植物向根部分配的生物量较少,BGC较低并对草本植物的生产力有负面影响,从而降低荒漠、草原化荒漠和荒漠草原的植被BGC。木本植物对新疆最干旱的草地总植被碳储量有负面影响。因此,我们预测,在未来干旱的条件下,木本植物入侵可能降低而不是增加草地的植被碳储量。     

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.     

Aboveground litter inputs determine carbon storage across soil profiles: a meta-analysis

Plant and Soil - Aboveground plant litter inputs are important sources of soil carbon (C). We aimed to establish how experimentally altered litter inputs affect soil C to 1-m depth across different...     

Integrating plant litter quality,soil organic matter stabilization,and the carbon saturation concept

Labile, ‘high‐quality’, plant litters are hypothesized to promote soil organic matter (SOM) stabilization in mineral soil fractions that are physicochemically protected from rapid mineralization. However, the effect of litter quality on SOM stabilization is inconsistent. High‐quality litters, characterized by high N concentrations, low C/N ratios, and low phenol/lignin concentrations, are not consistently stabilized in SOM with greater efficiency than ‘low‐quality’ litters characterized by low N concentrations, high C/N ratios, and high phenol/lignin concentrations. Here, we attempt to resolve these inconsistent results by developing a new conceptual model that links litter quality to the soil C saturation concept. Our model builds on the Microbial Efficiency‐Matrix Stabilization framework (Cotrufo et al., 2013) by suggesting the effect of litter quality on SOM stabilization is modulated by the extent of soil C saturation such that high‐quality litters are not always stabilized in SOM with greater efficiency than low‐quality litters.     

凋落物对土壤有机碳与微生物功能多样性的影响

森林凋落物是影响土壤微生物群落和有机碳含量的重要因素,但其作用的程度和机制尚不清楚,研究该问题对于分析森林生态系统碳循环和资源管理具有重要意义。研究凋落物去除与添加处理下土壤有机碳含量与土壤微生物对碳源利用的差异,明确凋落物去除与添加对土壤微生物群落代谢功能及其多样性的影响,探究不同处理下SOC含量变化的土壤微生物群落代谢机理。选取承德市雾灵山1405-1435 m海拔范围内核桃楸-蒙古栎混交林的表层土壤,采用室内培养结合Biolog-ECO方法,测定了培养第21天的土壤有机碳（soil organic carbon,SOC）含量及微生物群落的AWCD值、Shannon-Wiener多样性指数、Simpson优势度指数、McIntosh均匀度指数、Pielou丰富度指数,分析培养期内凋落物的不同处理下SOC含量与微生物功能多样性的变化特征。结果表明：1）不同凋落物处理对SOC含量与土壤微生物群落多样性具有显著影响（P<0.05）,DL > HL > NL > CK;2）不同凋落物处理下土壤微生物群落代谢活性和土壤微生物对碳源的利用程度具有显著差异（P<0.05）,碳水化合物类和氨基酸类是土壤微生物的主要碳源;3）不同处理的SOC含量与土壤微生物多样性具有正相关关系。双倍凋落物添加在短期内对土壤微生物多样性影响难以达到显著水平且在一定程度上对土壤微生物的代谢活性具有抑制作用,土壤微生物群落功能多样性对SOC含量具有重要影响。     

A comparison of repeated soil inventory and carbon flux budget to detect soil carbon stock changes after conversion from cropland to grasslands

Assessment of soil carbon (C) stock changes over time is typically based on the application of two methods, namely (i) repeated soil inventory and (ii) determination of the ecosystem C budget or net biome productivity (NBP) by continuous measurement of CO₂ exchange in combination with quantification of other C imports and exports. Here, we applied both methods in parallel to determine C stock changes of two temperate grassland fields previously converted from long‐term cropland. The grasslands differed in management intensity with either intensive management (high fertilization, frequent cutting) or extensive management (no fertilization, less frequent cutting). Soil organic C stocks (0–45 cm depth) were quantified at the beginning (2001) and the end (2006) of a 5 year observational period using the equivalent soil mass approach. For the same period and in both fields, NBP was quantified from net CO₂ fluxes monitored using eddy covariance systems, and measured C import by organic fertilizer and C export by harvest. Both NBP and repeated soil inventories revealed a consistent and significant difference between management systems of 170 ± 48 and 253 ± 182  g C m^?2 a^?1, respectively. For both fields, the inventory method showed a tendency towards higher C loss/smaller C gain than NBP. In the extensive field, a significant C loss was observed by the inventory but not by the NBP approach. Thus neither flux measurements nor repeated soil sampling may be suitable for tracking absolute changes in SOC, but both give similar answers with respect to relative changes.     

Differential responses of litter decomposition in the air and on the soil surface to shrub encroachment in a graminoid-dominated temperate wetland

Plant and Soil - Graminoid-dominated wetlands have been subjected to widespread shrub encroachment, yet the effect of this shift in species composition on litter decomposition remains unclear,...     

The impact of black wattle encroachment of indigenous grasslands on soil carbon,Eastern Cape,South Africa

Black wattle (Acacia mearnsii, De Wild.) is a fast growing tree species introduced into South Africa in the nineteenth century for commercial purposes. While being an important source of timber and firewood for local communities, black wattle is an aggressive invasive species and has pervasive adverse environmental impacts in South Africa. Little is known about the effects of black wattle encroachment on soil carbon, therefore the aim of this study was to investigate the impact of black wattle encroachment of natural grassland on soil carbon stocks and dynamics. Focussing on two sites in the Eastern Cape, South Africa, the study analysed carbon stocks in soil and litter on a chronosequence of black wattle stands of varying ages (up to >50 years) and compared these with adjacent native grassland. The study found that woody encroachment of grassland at one site had an insignificant effect on soil and litter carbon stocks. The second site showed a clear decline in combined soil and litter carbon stocks following wattle encroachment. The lowest stock was in the oldest wattle stand, meaning that carbon stocks are still declining after 50 years of encroachment. The results from the two sites demonstrate the importance of considering changes in soil carbon when evaluating ecosystem effects of invasive species.     

Interactions among shrub cover and the soil microclimate may determine future Arctic carbon budgets

Arctic and Boreal terrestrial ecosystems are important components of the climate system because they contain vast amounts of soil carbon (C). Evidence suggests that deciduous shrubs are increasing in abundance, but the implications for ecosystem C budgets remain uncertain. Using midsummer CO₂ flux data from 21 sites spanning 16° of latitude in the Arctic and Boreal biomes, we show that air temperature explains c. one‐half of the variation in ecosystem respiration (ER) and that ER drives the pattern in net ecosystem CO₂ exchange across ecosystems. Woody sites were slightly stronger C sinks compared with herbaceous communities. However, woody sites with warm soils (> 10 °C) were net sources of CO₂, whereas woody sites with cold soils (< 10 °C) were strong sinks. Our results indicate that transition to a shrub‐dominated Arctic will increase the rate of C cycling, and may lead to net C loss if soil temperatures rise.     

Grazing-induced changes in plant composition affect litter quality and nutrient cycling in flooding Pampa grasslands

Changes in plant community composition induced by vertebrate grazers have been found to either accelerate or slow C and nutrient cycling in soil. This variation may reflect the differential effects of grazing-promoted (G+) plant species on overall litter quality and decomposition processes. Further, site conditions associated with prior grazing history are expected to influence litter decay and nutrient turnover. We studied how grazing-induced changes in plant life forms and species identity modified the quality of litter inputs to soil, decomposition rate and nutrient release in a flooding Pampa grassland, Argentina. Litter from G+ forbs and grasses (two species each) and grazing-reduced (G−) grasses (two species) was incubated in long-term grazed and ungrazed sites. G+ species, overall, showed higher rates of decomposition and N and P release from litter. However, this pattern was primarily driven by the low-growing, high litter-quality forbs included among G+ species. Forbs decomposed and released nutrients faster than either G+ or G− grasses. While no consistent differences between G+ and G− grasses were observed, patterns of grass litter decay and nutrient release corresponded with interspecific differences in phenology and photosynthetic pathway. Litter decomposition, N release and soil N availability were higher in the grazed site, irrespective of species litter type. Our results contradict the notion that grazing, by reducing more palatable species and promoting less palatable ones, should decrease nutrient cycling from litter. Plant tissue quality and palatability may not unequivocally link patterns of grazing resistance and litter decomposability within a community, especially where grazing causes major shifts in life form composition. Thus, plant functional groups defined by species’ “responses” to grazing may only partially overlap with functional groups based on species “effects” on C and nutrient cycling.     

Storage, patterns and controls of soil organic carbon in the Tibetan grasslands

The soils of the Qinghai-Tibetan Plateau store a large amount of organic carbon, but the magnitude, spatial patterns and environmental controls of the storage are little investigated. In this study, using data of soil organic carbon (SOC) in 405 profiles collected from 135 sites across the plateau and a satellite-based dataset of enhanced vegetation index (EVI) during 2001–2004, we estimated storage and spatial patterns of SOC in the alpine grasslands. We also explored the relationships between SOC density (soil carbon storage per area) and climatic variables and soil texture. Our results indicated that SOC storage in the top 1 m in the alpine grasslands was estimated at 7.4 Pg C (1 Pg=10¹⁵ g), with an average density of 6.5 kg m⁻². The density of SOC decreased from the southeastern to the northwestern areas, corresponding to the precipitation gradient. The SOC density increased significantly with soil moisture, clay and silt content, but weakly with mean annual temperature. These variables could together explain about 72% of total variation in SOC density, of which 54% was attributed to soil moisture, suggesting a key role of soil moisture in shaping spatial patterns of SOC density in the alpine grasslands.     

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.     

Future increase in temperature may stimulate litter decomposition in temperate mountain streams: evidence from a stream manipulation experiment

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退化高寒草原土壤有机碳时空变化及其与土壤物理性质的关系

利用网格采样法研究了藏北退化高寒草原土壤有机碳变化及与土壤物理性质的关系.结果表明：0～10和11～20 cm土层有机碳含量、有机碳密度及其土层差异均为：轻度退化草地＞正常草地＞中度退化草地＞严重退化草地;有机碳含量、有机碳密度年变化速率则呈相反趋势,且表层土壤有机碳变幅均明显高于其下层土壤.正常草地、轻度退化草地0～10 cm土层有机碳年累积量为0.018和0.003 g·kg.^-1,分别为11～20 cm土层年累积量的6.0和2.0倍;中度、严重退化草地0～10 cm土层年损失量达0.150和0.231 g·kg^-1,分别为11～20 cm土层年损失量的2.3和2.2倍.中度、严重退化草地有机碳年损失总量为正常草地和轻度退化草地年累积总量的38倍,有机碳年损失总量达7.87×10⁵ t C,且具有较大的潜在退化态势.土壤有机碳与5.0～1.0、1.0～0.5和0.5～0.25 mm团聚体含量,土壤有机碳与土壤容重、土壤含水量间均呈极显著或显著正相关.