杉木人工林土壤活性有机质变化特征

在中国科学院会同森林生态实验站对第一代、第二代杉木林和地带性阔叶林土壤活性有机质主要组分进行了研究.结果表明,土壤活性有机质各组分含量均为杉木林低于阔叶林,而第二代杉木林又低于第一代杉木.第一代杉木林活性有机质总量、微生物生物量碳、水溶性有机碳和碳水化合物含量分别为18.79 g·kg^-1、421.7 mg·kg^-1、22.2 mg·kg^-1和136.3 mg·kg^-1,上述活性有机质组分在第二代杉木林中的含量分别是第一代杉木林的73.6%、87.9%、66.3%和3.2%,地带性阔叶林则分别为22.31 g·kg^-1、800. mg·kg^-1、361.1 mg·kg^-1和220.1 mg·kg^-1.相关性分析结果表明,土壤活性有机质各组分之间具有不同程度的相关性,其中土壤微生物生物量碳与其它活性有机质组分的相关性相对较高.     

Contribution of plant photosynthates to dissolved organic carbon in a flooded rice soil

Dissolved organic C (DOC) plays important roles in nutrient cycling and methane production in flooded rice ecosystem. The microcosm experiment was carried out to measure directly the contribution of photosynthates to DOC by using a ¹³C pulse-chase labeling technique. DOC was operationally divided into water-extractable organic C (WEOC) and salt-extractable organic C (SEOC) by successive extraction firstly with deionized water and then with 0.25 M K₂SO₄. Total WEOC increased with plant growth, whereas SEOC concentration did not change significantly over the growing season. About 0.037–0.36% (mean 0.16%) of the assimilated ¹³C was incorporated into WEOC immediately after ¹³CO₂ assimilation (Day 0), but only 0–0.025% (mean 0.01%) was incorporated into SEOC. At the end of the growing season, the ¹³C amounts of WEOC substantially decreased, while those of SEOC slightly increased. The estimated net plant C contribution was 21 mg C plant^–1 to WEOC and 6 mg C plant^–1 to SEOC, corresponding to 33.8% of total WEOC and 20.2% of total SEOC at the end of the growing season, respectively. The results suggest that the incorporation and decomposition of the photosynthesized C occurred rapidly in rice soil which significantly affected the WEOC dynamics, but SEOC appeared to be in equilibrium with the native soil organic matter, receiving less effect from the plant growth.     

Contribution of plant photosynthates to dissolved organic carbon in a flooded rice soil

Dissolved organic C (DOC) plays important roles in nutrient cycling and methane production in flooded rice ecosystem. The microcosm experiment was carried out to measure directly the contribution of photosynthates to DOC by using a ¹³C pulse-chase labeling technique. DOC was operationally divided into water-extractable organic C (WEOC) and salt-extractable organic C (SEOC) by successive extraction firstly with deionized water and then with 0.25?M K₂SO₄. Total WEOC increased with plant growth, whereas SEOC concentration did not change significantly over the growing season. About 0.037–0.36% (mean 0.16%) of the assimilated ¹³C was incorporated into WEOC immediately after ¹³CO₂ assimilation (Day 0), but only 0–0.025% (mean 0.01%) was incorporated into SEOC. At the end of the growing season, the ¹³C amounts of WEOC substantially decreased, while those of SEOC slightly increased. The estimated net plant C contribution was 21?mg?C?plant^?1 to WEOC and 6?mg?C?plant^?1 to SEOC, corresponding to 33.8% of total WEOC and 20.2% of total SEOC at the end of the growing season, respectively. The results suggest that the incorporation and decomposition of the photosynthesized C occurred rapidly in rice soil which significantly affected the WEOC dynamics, but SEOC appeared to be in equilibrium with the native soil organic matter, receiving less effect from the plant growth.     

Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2‐enrichment

Increased partitioning of carbon (C) to fine roots under elevated [CO₂], especially deep in the soil profile, could alter soil C and nitrogen (N) cycling in forests. After more than 11 years of free‐air CO₂ enrichment in a Liquidambar styraciflua L. (sweetgum) plantation in Oak Ridge, TN, USA, greater inputs of fine roots resulted in the incorporation of new C (i.e., C with a depleted δ¹³C) into root‐derived particulate organic matter (POM) pools to 90‐cm depth. Even though production in the sweetgum stand was limited by soil N availability, soil C and N contents were greater throughout the soil profile under elevated [CO₂] at the conclusion of the experiment. Greater C inputs from fine‐root detritus under elevated [CO₂] did not result in increased net N immobilization or C mineralization rates in long‐term laboratory incubations, possibly because microbial biomass was lower in the CO₂‐enriched plots. Furthermore, the δ¹³CO₂ of the C mineralized from the incubated soil closely tracked the δ¹³C of the labile POM pool in the elevated [CO₂] treatment, especially in shallower soil, and did not indicate significant priming of the decomposition of pre‐experiment soil organic matter (SOM). Although potential C mineralization rates were positively and linearly related to total SOM C content in the top 30 cm of soil, this relationship did not hold in deeper soil. Taken together with an increased mean residence time of C in deeper soil pools, these findings indicate that C inputs from relatively deep roots under elevated [CO₂] may increase the potential for long‐term soil C storage. However, C in deeper soil is likely to take many years to accrue to a significant fraction of total soil C given relatively smaller root inputs at depth. Expanded representation of biogeochemical cycling throughout the soil profile may improve model projections of future forest responses to rising atmospheric [CO₂].     

Changes from pasture to a native tree plantation affect soil organic matter in a tropical soil,Panamá

Background and aims

We examined changes in soil organic matter arising from conversion of a 45-year old pasture to a 10 yr. old native tree plantation in Panamá, to evaluate the effect of monoculture and mixtures.

Methods

We intensively sampled the soil 0–10 cm depth in the pasture in 2001 and in 22 plantation plots in 2011, ranging from 5 monocultures to 3- and 6-species treatments; samples were also taken from an undisturbed forest site. Soil analyses included organic carbon (SOC) and δ¹³C.

Results

Conversion of the pasture to tree plantation resulted in an overall loss of SOC of 0.6 kg m^?2 (18%) in the top 10 cm, but neither tree species nor diversity had a significant effect. End-member δ¹³C values suggested that the contribution of C₃ plants to SOC was increased from 26% in the pasture to 55% after 10 years of plantation and SOC turnover times were calculated to be 21–36 yr.

Conclusions

The magnitude of the loss in soil SOC is smaller than the increases in tree biomass (~3 kg C m^?2) and litter (~0.3 kg C m^?2) in the plantation, but still a significant part of the ecosystem C balance.

     

川西亚高山不同森林生态系统碳氮储量及其分配格局

森林采伐和恢复是影响森林碳氮储量的重要因素。以川西亚高山岷江冷杉原始林、粗枝云杉阔叶林、天然次生林和粗枝云杉人工林为研究对象,采用样地调查和生物量实测的方法,研究了不同森林生态系统各组分碳、氮储量及其分配特征。结果表明岷江冷杉原始林、粗枝云杉阔叶林、天然次生林和粗枝云杉人工林生态系统碳储量分别为611.18、252.31、363.07 tC/hm~2和239.06 tC/hm~2;氮储量分别为16.44、12.11、15.48 tN/hm~2和8.92 tN/hm~2。恢复林分与原始林碳储量在土壤—植被的分配格局发生了变化,而氮储量未发生变化。岷江冷杉原始林以植被碳储量为主,恢复林分以土壤为主,氮储量均以土壤为主。乔木层碳储量分别占生态系统总储量的56.65%、17.63%、13.57%和22.05%,土壤层(0—80 cm)分别占32.03%、69.87%、76.20%和72.12%;土壤层氮储量占生态系统总储量的76.80%—92.58%。植物残体碳氮储量分别占生态系统总储量的4.40%—9.83%和2.94%—7.08%,林下植被所占比例最小。空间格局上,岷江冷杉原始林植被部分具有较高的碳储量,应进行保护。3种恢复林分具有较高的碳汇潜力,且地上/地下碳储量较低,表明其碳汇潜力尤其表现在地上部分。天然次生林利于土壤有机碳的积累,而人工林乔木层碳储量较高。     

亚热带地区阔叶林与杉木林土壤活性有机质比较

通过对亚热带3个地区地带性阔叶林和杉木林土壤活性有机质的比较,分析森林类型变化及杉木连栽对土壤有机碳和养分含量的影响.结果表明：地带性阔叶林转变为杉木林后,土壤总有机碳含量下降27.8%～52.1%、腐殖酸碳下降32.2%～52.8%、胡敏酸下降36.4%～59.0%、富里酸下降29.7%～50.0%;杉木连栽也使土壤总有机碳和腐殖质含量下降.森林类型改变和杉木连栽对土壤活性有机质的影响更明显.杉木林取代阔叶林后,土壤微生物生物量碳、微生物生物量氮、可溶性有机碳和可溶性有机氮含量的最大降幅分别为61.8%、38.2%、43.3%和69.0%;与第1代杉木林相比,第2代杉木林土壤微生物生物量碳、微生物生物量氮、可溶性有机碳和可溶性有机氮含量的最大降幅分别为34.7%、29.3%、30.4%、18.4%.经相关性分析,除冷水浸提有机氮外,土壤活性有机质与养分含量之间具有密切的相关关系.     

Soil organic matter dynamics in tiger bush (Niamey,Niger). Preliminary results

Some typical features of soil organic matter dynamics and soil texture were studied to discuss the particular spatial pattern of tiger bush in Niger and its dynamics. The soil texture through silt and clay contents showed a high variability in the vegetation arc as well as in the bare area. These variations were clearly linked to water/wind erosion and termite activity. Tiger bush soils showed a high capacity to store soil organic matter despite a moderate primary production, even in the bare area where the input of plant debris has been nil for many decades. The carbon content was higher within the vegetation arc (0.93 %) than within the bare area (0.45 %). Additionally, potential carbon mineralisation significantly varied in relation to the total carbon content and thus to primary production. Then, the vegetation arcs can be viewed as `fertility islands' as in many arid ecosystems. The measurements of δ¹³C showed a dominant contribution of C₃ plants to the soil organic matter pool. Nevertheless, the contribution of C₄ plants was not negligible. Two hypotheses could be proposed: a different mineralisation rate between C₃ and C₄ plants; or (ii) a better physical protection of C₄ compounds against biodegradation. The soil variables depending totally or partly on biological factors, such as carbon and nitrogen contents, carbon isotopic composition, carbon potential mineralisation, did not show any symmetry in their variations along the studied transects. It was expected in the vegetation arc because the vegetation cover does not show symmetry in its specific composition and spatial structure. In the bare area, a clear asymmetry was observed on some of the variables: carbon content, fine material content and natural abundance of ¹³C. This supports the hypothesis that the vegetation arcs move upslope, and weakens the hypothesis of the alternance of contraction and spreading periods of the vegetation cover.     

Interpretation of soil δ13C as an indicator of vegetation change in African savannas

Question: The relationship between carbon‐13 in soil organic matter and C₃ and C₄ plant abundance is complicated because of differential productivity, litter fall and decomposition. As a result, applying a mass balance equation to δ¹³C data from soils cannot be used to infer past C₃ and C₄ plant abundance; only the proportion of carbon derived from C₃ and C₄ plants can be estimated. In this paper, we compare δ¹³C of surface soil samples with vegetation data, in order to establish whether the ratio of C₃:C₄ plants (rather than the proportion of carbon from C₃ and C₄ plants) can be inferred from soil δ¹³C. Location: The Tsavo National Park, in southeastern Kenya. Methods: We compare vegetation data with δ¹³C of organic matter in surface soil samples and derive regression equations relating the δ¹³C of soil organic matter to C₃:C₄ plant abundance. We use these equations to interpret δ¹³C data from soil profiles in terms of changes in inferred C₃:C₄ plant ratio. We compare our method of interpretation with that derived from a mass balance approach. Results: There was a statistically significant, linear relationship between the δ¹³C of organic matter in surface soil samples and the natural logarithm of the ratio of C₃:C₄ plants in the 100m² surrounding the soil sample. Conclusions: We suggest that interpretation of δ¹³C data from organic matter in soil profiles can be improved by comparing vegetation surveys with δ¹³C of organic matter in surface soil samples. Our results suggest that past C₃ plant abundance might be under‐estimated if a mass balance approach is used.     

Litter type and soil minerals control temperate forest soil carbon response to climate change

Temperate forest soil organic carbon (C) represents a significant pool of terrestrial C that may be released to the atmosphere as CO₂ with predicted changes in climate. To address potential feedbacks between climate change and terrestrial C turnover, we quantified forest soil C response to litter type and temperature change as a function of soil parent material. We collected soils from three conifer forests dominated by ponderosa pine (PP; Pinus ponderosa Laws.); white fir [WF; Abies concolor (Gord. and Glend.) Lindl.]; and red fir (RF; Abies magnifica A. Murr.) from each of three parent materials, granite (GR), basalt (BS), and andesite (AN) in the Sierra Nevada of California. Field soils were incubated at their mean annual soil temperature (MAST), with addition of native ¹³C‐labeled litter to characterize soil C mineralization under native climate conditions. Further, we incubated WF soils at PP MAST with ¹³C‐labeled PP litter, and RF soils at WF MAST with ¹³C‐labeled WF litter to simulate a migration of MAST and litter type, and associated change in litter quality, up‐elevation in response to predicted climate warming. Results indicated that total CO₂ and percent of CO₂ derived from soil C varied significantly by parent material, following the pattern of GR>BS>AN. Regression analyses indicated interactive control of C mineralization by litter type and soil minerals. Soils with high short‐range‐order (SRO) mineral content exhibited little response to varying litter type, whereas PP litter enriched in acid‐soluble components promoted a substantial increase of extant soil C mineralization in soils of low SRO mineral content. Climate change conditions increased soil C mineralization greater than 200% in WF forest soils. In contrast, little to no change in soil C mineralization was noted for the RF forest soils, suggesting an ecosystem‐specific climate change response. The climate change response varied by parent material, where AN soils exhibited minimal change and GR and BS soils mineralized substantially greater soil C. This study corroborates the varied response in soil C mineralization by parent material and highlights how the soil mineral assemblage and litter type may interact to control conifer forest soil C response to climate change.     

Control of Nitrification by Tree Species in a Common-Garden Experiment

We studied the effect of tree species on nitrification in five young plantations and an old native beech coppice forest at the Breuil experimental site in central France. The potential net nitrification (PNN) of soil was high in beech, Corsican pine, and Douglas fir plantations (high nitrifying stands denoted H) and low in spruce and Nordmann fir plantations as well as in native forest stands (low nitrifying stands denoted L). We hypothesized that tree species would stimulate or inhibit nitrification in transplanted soil cores within a few years after the cores were transplanted between stands. We first initiated a transplant experiment where soil cores were exchanged between all stands. The PNN remained high in soil cores from H transferred to H and low in soil cores from L transferred to L. The PNN increased considerably after 16 months in soil cores transferred from L to H, whereas the transfer of soil cores from H to L decreased the PNN only slightly after 28 months. In a second transplant experiment, forest floor material was exchanged between the Douglas fir (H) and the native forest (L) stand. Six months later, the forest floor from the native forest had increased the PNN of the Douglas fir soil considerably, whereas the forest floor from Douglas fir did not affect the PNN of the soil in the native forest stand. It was concluded that beech, Corsican pine, and Douglas fir rapidly stimulate soil nitrification by either activation of suppressed nitrifier communities and/or colonization by new nitrifier communities. Conversely, the slow and irregular reduction of nitrification in spruce, Nordmann fir, and native forest was probably due to the low and heterogeneously distributed flux of inhibiting substances per volume of soil. Our experiments suggest that the inhibition of nitrification is not tightly connected to forest floor leachates, but that the forest floor both reflects and maintains the major ongoing processes. In the long term, humus build up and the production of inhibiting substances may completely block the nitrification activity.     

Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas-fir terracosms

Although numerous studies indicate that increasing atmospheric CO₂ or temperature stimulate soil CO₂ efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO₂ and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO₂ and elevated temperature on these components of soil CO₂ efflux in Douglas-fir terracosms. We measured both soil CO₂ efflux rates and the ¹³C and ¹⁸O isotopic compositions of soil CO₂ efflux in 12 sun-lit and environmentally controlled terracosms with 4-year-old Douglas fir seedlings and reconstructed forest soils under two CO₂ concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO₂ efflux. In most cases, litter decomposition was the dominant component of soil CO₂ efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO₂ concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO₂, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual-isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.     

Mineral control of organic carbon mineralization in a range of temperate conifer forest soils

Coupled climate–ecosystem models predict significant alteration of temperate forest biome distribution in response to climate warming. Temperate forest biomes contain approximately 10% of global soil carbon (C) stocks and therefore any change in their distribution may have significant impacts on terrestrial C budgets. Using the Sierra Nevada as a model system for temperate forest soils, we examined the effects of temperature and soil mineralogy on soil C mineralization. We incubated soils from three conifer biomes dominated by ponderosa pine (PP), white fir (WF), and red fir (RF) tree species, on granite (GR), basalt (BS), and andesite (AN) parent materials, at three temperatures (12.5°C, 7.5°C, 5.0°C). AN soils were dominated by noncrystalline materials (allophane, Al‐humus complexes), GR soils by crystalline minerals (kaolinite, vermiculite), and BS soils by a mix of crystalline and noncrystalline materials. Soil C mineralization (ranging from 1.9 to 34.6 [mg C (g soil C)^?1] or 0.1 to 2.3 [mg C (g soil)^?1]) differed significantly between parent materials in all biomes with a general pattern of ANδ¹³C values of respired CO₂ suggest greater decomposition of recalcitrant soil C compounds with increasing temperature, indicating a shift in primary C source utilization with temperature. Our results demonstrate that soil mineralogy moderates soil C mineralization and that soil C response to temperature includes shifts in decomposition rates, mineralizable pool size, and primary C source utilization.     

蚂蚁筑巢对西双版纳热带森林土壤有机氮矿化的影响

蚂蚁作为生态系统工程师能够调节土壤微生物及理化环境,进而对热带森林土壤有机氮矿化速率及其时间动态产生显著影响。以西双版纳白背桐热带森林群落为研究对象,采用室内需氧培养法测定土壤有机氮矿化速率,比较蚁巢和非蚁巢土壤有机氮矿化速率的时间动态,揭示蚂蚁筑巢活动引起土壤无机氮库、微生物生物量碳及化学性质改变对有机氮矿化速率时间动态的影响。结果表明：（1）蚂蚁筑巢显著影响土壤有机氮矿化速率（P<0.01）,相较于非蚁巢,蚁巢土壤有机氮矿化速率提高了261%;（2）土壤有机氮矿化速率随月份推移呈明显的单峰型变化趋势,即6月最大（蚁巢1.22 mg kg^-1 d^-1、非蚁巢0.41 mg kg^-1 d^-1）,12月最小（蚁巢0.82 mg kg^-1 d^-1、非蚁巢0.18 mg kg^-1 d^-1）;（3）两因素方差分析表明,不同月份及不同处理对土壤有机氮矿化速率、NH₄-N及NO₃-N产生显著影响（P<0.05）,但对NO₃-N的交互作用不显著;（4）蚂蚁筑巢显著提高了无机氮库（NH₄-N与NO₃-N）、微生物生物量碳、有机质、水解氮、全氮及易氧化有机碳等土壤养分含量,而降低了土壤pH值;（5）回归分析表明,铵态氮和硝态氮对土壤有机氮矿化速率产生显著影响,分别解释87.89%、61.84%的有机氮矿化速率变化;（6）主成份分析表明NH₄-N、微生物生物量碳及有机质是影响有机氮矿化速率时间动态的主要因素,而全氮、NO₃-N、易氧化有机碳、水解氮及pH对土壤有机氮矿化速率的影响次之,且pH与土壤有机氮矿化速率呈显著负相关。总之,蚂蚁筑巢活动主要通过影响土壤NH₄-N、微生物生物量碳及有机质的状况,进而调控西双版纳热带森林土壤有机氮矿化速率的时间动态。研究结果将有助于进一步提高对土壤氮矿化生物调控机制的认识。     

亚热带天然阔叶林转换为杉木人工林对土壤呼吸的影响

采用静态箱-气相色谱法对浙江省临安市玲珑山风景区天然阔叶林和由天然阔叶林改造的杉木人工林的土壤呼吸进行1年的定位监测.结果表明：天然阔叶林和杉木人工林土壤CO₂排放速率均呈现一致的季节性变化规律即夏秋季高、冬春季低;天然阔叶林和杉木人工林土壤CO₂排放速率分别为20.0～111.3和4.1～118.6 mg C·m^-2·h^-1;天然阔叶林土壤CO₂年累积排放通量（16.46 t CO₂·hm^-2·a^-1）显著高于杉木人工林（11.99 t CO₂·hm^-2·a^-1）.天然阔叶林和杉木人工林土壤CO₂排放速率与土壤含水量均没有显著相关性,而与5 cm处土壤温度呈显著指数相关,Q₁₀值分别为1.44和2.97;天然阔叶林土壤CO₂排放速率与土壤水溶性碳（WSOC）含量无显著相关性,杉木人工林土壤CO₂排放速率与WSOC含量呈显著相关.天然阔叶林转换为杉木人工林显著降低了土壤CO₂排放,提高了土壤呼吸对环境因子的敏感性.
     

Relationships between the isotopic composition of dissolved organic carbon and its bioavailability in contrasting Ozark streams

Bioavailability of dissolved organic carbon (DOC) can significantly influence nutrient cycling in small streams yet the factors influencing the bioavailability of DOC remains poorly understood. The isotopic composition of DOC and factors controlling its utilization were studied in two contrasting headwater streams to elucidate the relationship between DOC source and its bioavailability. Water samples were collected monthly from Moore Creek (MC), located in a watershed dominated by fertilized pasture, and from Huey Hollow (HH), located in a deciduous forest watershed. Dissolved organic carbon concentrations ranged from 222 to 1130 μm C and 35 to 289 μm C in MC and HH, respectively. The isotopic composition of DOC (δ¹³C_DOC) was more seasonally variable in HH and ranged from ?33.6 to ?28.0?, as compared with MC where it ranged from ?27.2 to ?24.5 ?. The δ¹³C_DOC in Huey Hollow suggests leaf debris was an important source of DOC throughout most of the year while algae was important in winter and early spring. In MC, the δ¹³C_DOC indicated DOC was largely derived from soil organic matter while algal inputs were small relative to the large pool of refractory DOC. Stream water community respiration (SWCR) rates suggest the existence of a larger pool of refractory DOC in MC relative to HH. The ratio of SWCR (μM C h^?1) to DOC concentration (mM C) was generally higher in HH (1.2–13.2) as compared with MC (0.2–4.2), suggesting that relative bioavailability of DOC was often greater in HH. Nutrient addition experiments indicate that bioavailable C limited SWCR in spring and fall in MC, while bioavailable C was never limiting in HH. The results suggest that elevated nutrient and DOC concentrations supported higher levels of microbial activity that resulted in a large pool of refractory DOC in MC. The lower inorganic nutrient and DOC concentrations reduced microbial activity in HH and likely limited the production of refractory DOC. Results of this study suggest that both organic matter source and nutrient concentration are important determinants of DOC bioavailability in small streams. %     

Relationship between the weathering of clay minerals and the nitrification rate: a rapid tree species effect

We compared the properties of the clay mineral fraction and the composition of soil solutions in a Fagus sylvatica coppice (native forest) and four adjacent plantations of Pseudotsuga menziesii, Pinus nigra, Picea abies and Quercus sessiliflora planted in 1976. The results revealed changes of clay fraction properties due to tree species effect. Clay samples from Douglas fir and pine stands differ when compared to other species. Twenty-eight years after planting, we observed the following changes: a more pronounced swelling after citrate extraction and ethylene glycol solvation, a higher CEC and a smaller poorly crystallised aluminium content. All these changes affecting the clay fraction agreed well with soil solution analyses which revealed high NO₃ ^?, H⁺ and Al concentrations under Douglas fir and pine. These changes were explained by a strong net nitrification under Douglas fir and pine stands when compared with other tree species. The higher NO₃ ^? concentrations in soil solutions should be linked to the presence, type and activity of ammonia-oxiding bacteria which are likely influenced by tree species. The production of NO₃ ^? in excess of biological demand leads to a net production of hydrogen ion and enhances the dissolution of poorly crystallised Al-minerals. Secondary Al-bearing minerals constituted the principal acid-consuming system in these soils. As a consequence, the depletion of interlayer spaces of hydroxyinterlayered minerals increases the number of sites for exchangeable cation fixation and increases CEC of the clay fraction. The dissolution of Al oxy-hydroxides explain the increase in Al concentrations of soil solutions under Douglas fir and pine stands when compared to other species. Nitrate and dissolved aluminium were conjointly leached in the soil solutions. A change in environmental conditions, like an introduction of tree species, enough modifies soil processes to induce significant changes in the soil mineralogical composition even over a period of time as short as some tens of years. Generally, mineral weathering has been considered to be very slow and unlikely to change over tens of years, resulting in few studies capable of detecting changes in mineralogy. This study appears to have detected changes in clay mineralogy during a period of 28 years after the planting of forest species. Our study represents a single location with a limited block design, but causes us to conclude that the observed changes could be widely representative. Where available, archived samples should be utilized and long-term experiments set up so that similar changes can be tested for and detected using more robust designs. The plausible hypothesis we present to explain apparent changes in clay mineralogy has strong relevance to the sustainable management of land.     

Using Soil 13C to Detect the Historic Presence of Schizachyrium scoparium (Little Bluestem) Grasslands on Martha's Vineyard

Abstract We used differences in soil carbon δ¹³C values between forested sites and grasslands dominated by the C₄ grass Schizachyrium scoparium (little bluestem) to detect the presence of former grasslands in the historical landscape of the coastal sand plain of Martha's Vineyard, Massachusetts, U.S.A. Soil δ¹³C was measured at (1) sites with long‐term forest or grassland vegetation and (2) sites with known histories where forest vegetation invaded grassland and where forest converted to grassland. The δ¹³C of soil under long‐term grassland was –24.1‰ at 0 to 2 cm depth and –23.4‰ at 2 to 10 cm and was enriched by 3.4‰ and 2.8‰ compared with soil under long‐term forest. In forests that invaded grasslands dominated by S. scoparium, soil δ¹³C decreased as C derived from trees replaced C from S. scoparium. This decline occurred faster in surface soils and in the light soil organic matter fraction than in the mineral soil. In forests that converted to grasslands, soil δ¹³C increased and the rate of increase was similar in surface and mineral soil and in the different soil organic matter fractions. Rates of change indicated that soil δ¹³C could be used to detect changes in vegetation involving the presence or absence of S. scoparium during the last 150 years. Application of this model to a potential grassland restoration site on Martha's Vineyard where the landscape history was not known indicated that the site was previously unoccupied by S. scoparium during this time. The δ¹³C of surface mineral soil can be useful for detecting the presence of historic S. scoparium grasslands but only in the period well after European settlement of these coastal sand plain landscapes.     

Associations between carbon isotope ratios of ecosystem respiration, water availability and canopy conductance

We tested the hypothesis that the stable carbon isotope signature of ecosystem respiration (δ¹³C_R) was regulated by canopy conductance (G_c) using weekly Keeling plots (n=51) from a semiarid old‐growth ponderosa pine (Pinus ponderosa) forest in Oregon, USA. For a comparison of forests in two contrasting climates we also evaluated trends in δ¹³C_R from a wet 20‐year‐old Douglas‐fir (Pseudotsuga menziesii) plantation located near the Pacific Ocean. Intraannual variability in δ¹³C_R was greater than 8.0‰ at both sites, was highest during autumn, winter, and spring when rainfall was abundant, and lowest during summer drought. The δ¹³C_R of the dry pine forest was consistently more positive than the wetter Douglas‐fir forest (mean annual δ¹³C_R: ?25.41‰ vs. ?26.23‰, respectively, P=0.07). At the Douglas‐fir forest, δ¹³C_R–climate relationships were consistent with predictions based on stomatal regulation of carbon isotope discrimination (Δ). Soil water content (SWC) and vapor pressure deficit (vpd) were the most important factors governing δ¹³C_R in this forest throughout the year. In contrast, δ¹³C_R at the pine forest was relatively insensitive to SWC or vpd, and exhibited a smaller drought‐related enrichment (～2‰) than the enrichment observed during drought at the Douglas‐fir forest (～5‰). Groundwater access at the pine forest may buffer canopy–gas exchange from drought. Despite this potential buffering, δ¹³C_R at the pine forest was significantly but weakly related to canopy conductance (G_c), suggesting that δ¹³C_R remains coupled to canopy–gas exchange despite groundwater access. During drought, δ¹³C_R was strongly correlated with soil temperature at both forests. The hypothesis that canopy‐level physiology is a critical regulator of δ¹³C_R was supported; however, belowground respiration may become more important during rain‐free periods.     

Drought response of five conifer species under contrasting water availability suggests high vulnerability of Norway spruce and European larch

The ability of tree species to cope with anticipated decrease in water availability is still poorly understood. We evaluated the potential of Norway spruce, Scots pine, European larch, black pine, and Douglas‐fir to withstand drought in a drier future climate by analyzing their past growth and physiological responses at a xeric and a mesic site in Central Europe using dendroecological methods. Earlywood, latewood, and total ring width, as well as the δ¹³C and δ¹⁸O in early‐ and latewood were measured and statistically related to a multiscalar soil water deficit index from 1961 to 2009. At the xeric site, δ¹³C values of all species were strongly linked to water deficits that lasted longer than 11 months, indicating a long‐term cumulative effect on the carbon pool. Trees at the xeric site were particularly sensitive to soil water recharge in the preceding autumn and early spring. The native species European larch and Norway spruce, growing close to their dry distribution limit at the xeric site, were found to be the most vulnerable species to soil water deficits. At the mesic site, summer water availability was critical for all species, whereas water availability prior to the growing season was less important. Trees at the mesic were more vulnerable to water deficits of shorter duration than the xeric site. We conclude that if summers become drier, trees growing on mesic sites will undergo significant growth reductions, whereas at their dry distribution limit in the Alps, tree growth of the highly sensitive spruce and larch may collapse, likely inducing dieback and compromising the provision of ecosystem services. However, the magnitude of these changes will be mediated strongly by soil water recharge in winter and thus water availability at the beginning of the growing season.