Land use dynamics derived from colluvial deposits and bogs in the Black Forest,Germany

The Black Forest is considered to be a rather unfavorable area, having a short vegetation period, low mean annual temperatures, high precipitation, and a pronounced relief. These conditions do not favor agricultural land use and thus it is widely accepted that people only began using the land intensively during the Middle Ages. In this integrated study 17 soil profiles, two peat bogs and a database of archaeological finds were used to reconstruct past land use impacts on the environment. AMS–¹⁴C datings of charcoals, luminescence datings of colluvial deposits, archaeological finds and pollen records indicate land use already during the Neolithic. This pre‐medieval land use might be related to seasonal settlements dominated by pastoralism and the use of wood or bedrock to build settlements and infrastructure or as energy supply. There is new evidence of human activity dating back to the Bronze and Iron Age, which is a discrepancy to the absence of archaeological finds in the direct vicinity of the studied sites. With the beginning of the Middle Ages land use practices changed, most likely with the expansion and intensification of agricultural land use, which coincides with the increasing use of natural resources in the Black Forest. Hence, the main phases of colluvial deposition date to the Middle Ages and Modern Times. Increased contents of As, Cr, Cu, Pb, or Zn in medieval colluvial deposits might indicate smelting or mining, even though there are no known archeological sites pointing to such activities nearby. Whereas the pattern of colluvial deposition in the southeastern Black Forest points to distinct, but local land use in pre‐medieval times and to intensified and widespread land use since the Middle Ages, thick and multi‐layered colluvial deposits indicate intensive land use in the neighboring Baar region since the Neolithic. The different land use patterns of these two regions originate from the rather favorable conditions for agriculture in terms of soils, climate, and topography in the Baar region compared to the unfavorable conditions in the Black Forest.     

Reconstruction of the paleotemperature and precipitation in the Pleistocene according to the isotope composition of humus and carbonates in loess on the Russian Plain

The isotope composition of carbon and oxygen in humus and carbonates has been studied in the Pleistocene loess and soil of the Russian Plain in order to reconstruct the paleoclimatic parameters during the time of their formation. It is established that most of the buried soils were formed upon the mean annual temperature exceeding the modern temperature by 1–2°C. The climate aridity (the portion of C4 plants in the ecosystem) varied from 0 to 25%. It is shown that the isotope composition of carbon and oxygen in the humus and carbonates may be a reliable indicator of the paleoclimate (i.e., the paleotemperature, paleoprecipitation, and aridity dynamics). An isotope procedure of culling soil samples unsuitable for paleoclimatic reconstruction is proposed.     

泰国东部土地由森林转化为玉米地引起土壤有机碳流失及其储量的降低

Soil organic carbon(SOC) content and its stable carbon isotopic composition(within the upper 1 m) were measured to determine the e?ect of land-use changes from dry evergreen forest to maize fields in eastern Thailand.Digital land cover maps,derived from aerial photography and satellite images for years 1989,1996,and 2002 were used in association with field surveys and farmer interviews to derive land-use history and to assist in study site selection.Conversion from forest to maize cultivation for the duration of 12 years reduced SOC stocks at the rate of 6.97 Mg C ha-1 year-1.Reduction was most pronounced in the top 10 cm soil layer,which was 47% after 12 years of cultivation.Stable carbon isotope data revealed that the main fraction lost was forest-derived C.Generally low input rates of maize-derived C were not sufficient to maintain SOC at the level prior to forest conversion.After 12 years of continuous maize cultivation,the maize-derived C fraction made up about 20% of total SOC(5 Mg ha-1 of the total 25.31 Mg ha-1).     

Variation of soil respiration at three spatial scales: Components within measurements, intra-site variation and patterns on the landscape

Soil respiration is an important component of terrestrial carbon cycling and can be influenced by many factors that vary spatially. This research aims to determine the extent and causes of spatial variation of soil respiration, and to quantify the importance of scale on measuring and modeling soil respiration within and among common forests of Northern Wisconsin. The potential sources of variation were examined at three scales: [1] variation among the litter, root, and bulk soil respiration components within individual 0.1 m measurement collars, [2] variation between individual soil respiration measurements within a site (<1 m to 10 m), and [3] variation on the landscape caused by topographic influence (100 m to 1000 m). Soil respiration was measured over a two-year period at 12 plots that included four forest types. Root exclusion collars were installed at a subset of the sites, and periodic removal of the litter layer allowed litter and bulk soil contributions to be estimated by subtraction. Soil respiration was also measured at fixed locations in six northern hardwood sites and two aspen sites to examine the stability of variation between individual measurements. These study sites were added to an existing data set where soil respiration was measured in a random, rotating, systematic clustering which allowed the examination of spatial variability from scales of <1 m to 100+ m. The combined data set for this area was also used to examine the influence of topography on soil respiration at scales of over 1000 m by using a temperature and moisture driven soil respiration model and a 4 km² digital elevation model (DEM) to model soil moisture. Results indicate that, although variation of soil respiration and soil moisture is greatest at scales of 100 m or more, variation from locations 1 m or less can be large (standard deviation during summer period of 1.58 and 1.28 μmol CO₂ m⁻² s⁻¹, respectively). At the smallest of scales, the individual contributions of the bulk soil, the roots, and the litter mat changed greatly throughout the season and between forest types, although the data were highly variable within any given site. For scales of 1-10 m, variation between individual measurements could be explained by positive relationships between forest floor mass, root mass, carbon and nitrogen pools, or root nitrogen concentration. Lastly, topography strongly influenced soil moisture and soil properties, and created spatial patterns of soil respiration which changed greatly during a drought event. Integrating soil fluxes over a 4 km² region using an elevation dependent soil respiration model resulted in a drought induced reduction of peak summer flux rates by 37.5%, versus a 31.3% when only plot level data was used. The trends at these important scales may help explain some inter-annual and spatial variability of the net ecosystem exchange of carbon.     

Prolonged drought changes the bacterial growth response to rewetting

Rewetting a dry soil can result in two response patterns of bacterial growth and respiration. In type 1, bacterial growth starts to increase linearly immediately upon rewetting and respiration rates are highest immediately upon rewetting. In type 2, bacterial growth starts to increase exponentially after a lag period with a secondary increase in respiration occurring at the start of the exponential increase in growth. We previously observed that the type 1 response occurred after rewetting 4-day dried soil and type 2 for 1-year dried soil. Here we studied in detail how the duration of drought related to the two types of responses of bacterial growth and respiration to rewetting. Soil was air dried for different time periods from 4 days up to 48 weeks. Upon rewetting, bacterial growth and respiration was measured repeatedly at 17 °C during one week. Drought periods of ≤2 weeks resulted in a type 1 response whereas drought periods of ≥4 weeks resulted in a type 2 response. The lag period increased with drought duration and reached a maximum of ca. 18 h. The bacterial growth response was also affected by incubation of moist soil before drying–rewetting. The lag period increased with duration of moist soil incubation before the 4-day drying–rewetting event and reached also a maximum of ca. 18 h. The exponential growth increase in the type 2 response coincided with a secondary increase in respiration, which increased in magnitude with increasing drought duration. Cumulative respiration increased with drought duration and was ca. 4 times higher after 48 weeks of drought compared to 4 days. Thus, prolonged drought affected the response type of bacterial growth and respiration to rewetting, and also increased lag period, the magnitude of the secondary increase in respiration and total C release. The effect of drought was, however, modified by the lenght of the incubation period of moist soil before drought, suggesting that soil conditions before a drying–rewetting event need consideration when evaluating microbial responses.     

PaleoVertisols of the northwest Caucasus: (Micro)morphological,physical, chemical,and isotopic constraints on early to late Pleistocene climate

Physical and chemical properties, macro‐ and micromorphology, clay mineralogy, and stable‐isotope compositions of paleosols within a pedostratigraphic column (PSC) of early to late Pleistocene age, interstratified paleosols, and loess (NW Caucasus, S Russia) were examined to better understand the evolution of the pedogenic environment over this time period, separating the effects of postpedogenic diagenesis. The column includes eight paleosols and six intercalated loessic horizons. Most of paleosols represent Vertisols or vertic intergrades. Vertic features increase in the middle of the PSC, where the paleosols are more clayey in texture and reddish in color. The morphology of carbonate nodules and soft masses, morphology‐ and depth(age)‐related changes in stable C and O isotope compositions, soil color, redoximorphic features, clay mineralogy, and illuviated clay indicate periods of wetter pedoenvironment in the past and suggest the Pleistocene paleosols are polygenetic and were formed with several wet/dry stages under a climate generally similar to the modern environment in the N Caucasus (mean annual temperature approx. 9°C–12°C). Interpretation of the time sequence of climate/environmental change requires careful separation of pedogenic mineral phases from phases altered by later diagenesis. The early Pleistocene period of paleosol formation appeared to be wetter or more humid, resulting in more significant development of vertic features. The terrestrial ecosystem remained dominated by C3 vegetation throughout the formation of the PSC, with four small periods of change towards a greater proportion of C4 plants or increased moisture stress.     

碳氮稳定同位素检测能力的验证——2013年实验室间比对分析结果的汇总

稳定性同位素示踪技术,特别是碳、氮稳定性同位素已广泛地应用于农业化学、地球化学和环境化学。碳、氮同位素比值的质谱检测结果直接关系到示踪试验的可靠性。在目前缺乏富集碳、氮同位素的标准物质的条件下,通过实验室间的比对可以验证各实验室对碳、氮同位素比值检测能力、检测结果的准确性和可比性。我所在2013年组织和实施了一次由全国14个单位的检测实验室共17台仪器参加的农业、生态和环境样品中碳氮百分含量及其稳定性同位素比值的实验室间比对。我所制备了多种被检测的样品,有含碳氮的化学肥料、土壤和植物样品;在稳定性同位素富集度上,有自然丰度的和不同富集程度的同位素样品。除固体样品外,还有3种不同丰度的N2O和CO2气体样品。在进行检测结果的统计和评价时,采用Z比分衡量各实验室检测结果的可信度。本文汇总了2013年实验室间比对的结果。     

基于改进帕默尔干旱指数的中国气象干旱时空演变分析

近几十年来随着全球变暖的不断加剧,中国多地频繁发生干旱灾害,经济损失也越来越大,因而研究干旱的时空演变对社会经济发展具有重要的现实意义。论文基于改进帕默尔干旱指数(sc PDSI)分析了1961—2009年间年和四季干湿变化,并利用旋转经验正交函数(rotated empirical orthogonal function,REOF)探讨了中国气象干旱的时空演变特征。结果表明:1)近49 a来,整体上中国年和四季均呈显著变湿趋势,且均在20世纪70年代初期发生了由干变湿的突变;2)年及四季均存在2~8 a的振荡周期,其中夏季主周期为4.4 a,其余主周期均为6.2 a;3)根据REOF时空分解的前8个空间模态,将中国划分成8个干湿特征区域,其中大兴安岭地区、东南地区、西北地区和青藏高原南部地区有变湿趋势,后两个区域变湿趋势显著;东北—内蒙古高原区、青藏高原北部地区、中部地区和黄淮海平原地区呈变干趋势,前3个区变干趋势显著,黄淮海平原分区干湿变化不明显;各分区普遍具有2~9 a的振荡周期;4)极涡指数、印度洋偶极子和太平洋涛动与部分地区气象干旱有较好的相关关系。研究结果可为研究区防旱、抗旱与干旱预测提供参考依据。     

Late Pleistocene ‐ Holocene palaeosols in the north of Sonora,Mexico: chronostratigraphy,pedogenesis and implications for environmental history

Red palaeosols of the late Pleistocene‐early Holocene, both buried and non‐buried, were studied recently in Sonora (NW Mexico) to reconstruct their pedogenesis as well as the palaeoenvironmental conditions. The alluvial palaeosol‐sedimentary sequence of the La Playa archaeological site is a key locality for the buried San Rafael palaeosol, which exhibits a 2Ah‐2Bw‐2BCk‐3Bgk profile and was defined as a Chromic Cambisol. Radiocarbon dates from pedogenic carbonates and charcoal set the soil formation interval between > 18 000 and 4300 calibrated years before present (cal. year BP). Micro‐morphological observations together with profile distribution of clay, carbonates, organic carbon, pedogenic iron oxides and rock magnetic properties indicated a strong eluvial‐illuvial redistribution of carbonates, moderate silicate weathering and gleying in the lower horizon. Although this soil was much more developed than the overlying syn‐sedimentary late Holocene Fluvisols, clay mineral composition and stable carbon isotope signatures of humus and carbonates were similar in both soils. We suggest that pedogenesis of the San Rafael palaeosol took place under a slightly more humid climate and relative geomorphic stability. This agrees with the regional palaeoclimate reconstruction, which indicates a moister climate during the Late Wisconsin glaciation (MIS 2). An abrupt termination of the San Rafael pedogenesis marked by disturbance and aridization features in the Ap horizon of the palaeosol could be linked to a global drought around 4200 years cal. year BP. Surface Chromic Cambisols in northern Sonora show similar pedogenetic characteristics to the buried red palaeosols of La Playa. They appear to be a relict component of the present day soil mantle.     

基于植被温度条件指数的克里雅河流域干旱监测研究

干旱是干旱半干旱地区由气候变化和人类生产活动而引起的环境问题之一。区域环境干旱化趋势的研究对该区域生态环境、水资源管理及农业生产有着重要意义。在植被温度条件指数（VTCI）的基础上对克里雅河流域进行干旱监测。结果表明：2005-2010年研究区整体上向干旱化方向发展,由北向南干旱趋势依次增强。其中,南部山区2005年VTCI值为0.696 4,到2010年VTCI值为0.486 4,其变化相差-0.210 0,旱化趋势较为严重;研究区中部的VTCI相差值分别为0.071 3和0.042 6,表现为有轻微干旱程度减弱趋势,变化不明显;北部地区即克里雅河下游地区的VTCI值从2005年的0.121 0减少到2010年的0.075 4,VTCI相差-0.045 6,呈轻微旱化趋势。     

Late Holocene land use at Orstad, Jæren, southwestern Norway, evidence from pollen analysis and soil micromorphology

Pollen analysis was combined with radiocarbon dating, physical, chemical and biological soil analyses and soil micromorphology, to investigate the prehistoric land use at Orstad, in Jæren, southwestern Norway.Orstad is an Early Bronze Age clearance cairn field in a cultural landscape with traces of land use back to the Neolithic. Samples were mainly collected from an 84 m long excavated trench with mainly podzolic soils. Although pollen preservation was poor, it was possible to differentiate five phases in the local vegetation and land use history. Human impact on the site could be traces back to about 4400–4000 BP uncal./3045–2600 cal. BC. Wheat and barley have been cultivated at Orstad during a period from ca. 3600 BP uncal./1945–1750 cal. BC until the Older Iron Age, when heather vegetation began to spread on the site.Soil micromorphology revealed different soil management practices for this time period: The soils seem to have been cleared by fire, prior to cultivation until ca. 3200 BP uncal./1610–1455 cal. BC. Then, new cultivation fields were laid out on higher levels, and seem to have been improved by adding of organic materials, mainly turves taken from podzolic top soils and peats from nearby localities.     

Extractable lipid contents and colour in particle-size separates and bulk arable soils

Chemical alteration of plant biomass to soil organic matter is often accompanied by characteristic trends, e.g. with decreasing particle size and increasing depth organic carbon and nitrogen concentrations and stable carbon isotope values (δ¹³C) often increase. In agricultural soils, systematic studies of soil organic carbon (SOC) distribution in bulk soils and particle‐size separates of depth profiles are scarce. In this study, three soil profiles from one site with different monoculture crops were analysed for organic carbon and nitrogen concentrations, stable carbon isotopes, bulk extractable lipids, and soil colour. In contrast to most previous observations, stable carbon isotope values were constant over soil depth and within particle‐size separates, probably as a result of little biomass input due to the harvesting techniques applied and the presence of fossil carbon. Bulk extractable lipids contributed 1–10% to the total SOC. Significantly more lipids could be extracted from rye‐ than from maize‐derived SOC. Lipid yields normalized to soil mass increased with decreasing particle size and decreased with depth. When normalized to organic carbon concentration, sand‐size fractions had the largest lipid yields. Soil colour, expressed as Munsell values, was lightest in sand‐ and silt‐size separates. A cross‐plot of Munsell values and their SOC concentrations revealed characteristic, non‐overlapping areas for each particle‐size class and the bulk soils. Clay‐size separates and bulk soils were almost identical in Munsell values, although for clay‐size separates SOC concentrations were much larger than for bulk soils. Thus, the SOC‐rich clay‐size separates exerted the dominant influence on the colour of the bulk soils. Determination of colour and extractable lipid contents could be useful additional parameters for soil characterization.     

稳定13C同位素示踪技术在农田土壤碳循环和团聚体 固碳研究中的应用进展

农田土壤有机碳库是全球碳循环的重要组成部分,其积累和分解直接影响陆地生态系统碳贮藏与全球碳平衡。土壤团聚体是土壤结构的物质基础和土壤肥力的重要载体,也是土壤有机碳的固定场所。稳定~(13)C同位素示踪技术是研究土壤碳动态变化的有效手段,能够揭示新输入碳在土壤及团聚体中赋存状态、周转过程以及微生物的调节机制。本文主要归纳与阐述了稳定~(13)C同位素示踪技术在农田土壤有机碳循环及土壤团聚体固碳机理方面的研究进展,提出~(13)C同位素示踪技术在未来土壤碳循环和固碳机制方面的主要研究方向。     

Repeated annual drought has minor long‐term influence on δ13C and alkane composition of plant and soil in model grassland and heathland ecosystems

As a result of global climate change the incidence of drought conditions in Europe is predicted to increase in the future, which also influences plant resistance. Lipids are important plant constituents that protect plants against drought stress and contribute to the intermediate stable carbon (C) pool in soil. However, the extent to which drought influences lipid cycling in the plant–soil system is unknown and, therefore, it remains questionable how the ecosystem recovers after drought. We focused on plant and soil samples from two different plant communities (temperate grassland and heathland) that had been exposed to 5 years of 4.5–6.0 weeks repeated annual drought. They were sampled one year after the last drought to check the recovery of the plant–soil system. Samples were analyzed for their bulk C, stable C and nitrogen (N) isotope (δ¹³C, δ¹⁵N) and lipid composition. Contrary to our expectation, no strong influence of five years of repeated annual drought was observed for above‐ground biomass, roots and soils in the model ecosystems with respect to elemental (C and N concentrations, C : N ratio) bulk isotope (δ¹³C, δ¹⁵N) composition and the total extractable lipid concentration. Thus, plants did not sustain a significant change in their C and lipid concentration as well as their composition after five years of repeated annual drought. This might be related to the comparatively short drought period related to the overall growth season and provides evidence for recovery of the C and lipid dynamics in temperate grassland and heathland model ecosystems exposed to annual drought.     

EA-IRMS法测定不同类型土壤有机碳稳定性同位素组成

利用元素分析仪-稳定同位素比例质谱仪（EA-IRMS）分析系统,以国际标准物质Urea为基准,标定了高纯钢瓶CO2参考气,其δ13CvsPDB值为（-29.523±0.181）‰。通过试验对比检验了EA-IRMS分析系统的稳定性、线性和测定δ13C值的样品分析精度,建立了测定土壤中有机碳稳定性同位素的分析方法。其离子强度在1.0～7.0V之间具有良好的线性,在1.5～5.0V内的线性优于总体线性;样品分析精度优于0.15‰;样品含碳量大于5μg即可满足测定δ13C的分析要求。应用此方法实测了18份不同类型土壤的有机碳稳定同位素组成,获得其13C天然丰度平均值为1.082%;不同类型的土壤有机碳稳定同位素组成存在较大的差异。     

Effects of drought stress on photosynthesis,rhizosphere respiration,and fine‐root characteristics of beech saplings: A rhizotron field study

Soil drought influences the C turnover as well as the fine‐root system of tree saplings. Particularly during the period of establishment, the susceptibility to drought stress of saplings is increased because of incompletely developed root systems and reduced access to soil water. Here, we subjected beech saplings (Fagus sylvatica L.) to different levels of drought stress. Beech saplings were planted in rhizotrons, which were installed in the soil of a Norway spruce forest before bud burst. Soil moisture was manipulated in the following year during May to September. We measured photosynthetic net CO₂ uptake, volume production of fine roots, and rhizosphere respiration during the growing season. Biometric parameters of the fine‐root system, biomass, and nonstructural carbohydrates were analyzed upon harvest in October. Photosynthesis and rhizosphere respiration decreased with increasing drought‐stress dose (cumulated soil water potential), and cumulative rhizosphere respiration was significantly negatively correlated with drought‐stress dose. Fine‐root length and volume production were highest at moderate soil drought, but decreased at severe soil drought. The proportion of fine‐roots diameter < 0.2 mm and the root‐to‐shoot ratio increased whereas the live‐to‐dead ratio of fine roots decreased with increasing drought‐stress dose. We conclude that the belowground C allocation as well as the relative water‐uptake efficiency of beech saplings is increased under drought.     

近30年川中丘陵区不同土地利用方式土壤碳氮磷生态化学计量特征变化

基于1981年第二次土壤普查数据和2012年实地采样数据,分析了紫色丘陵区1981—2012年不同土地利用方式下表层土壤碳氮磷含量及生态化学计量特征变化。结果表明:研究区近30年来土壤碳、氮含量增幅分别为109.98%、27.27%,磷含量基本稳定。1981年土壤C︰N︰P比为7.28︰1︰1,2012年C︰N︰P比为16.41︰1.37︰1。各土地利用方式土壤碳、氮含量均有不同程度提升,尤以林草地、园地提升最为明显;水田、旱地、园地土壤磷含量基本稳定而林草地下降明显;土壤C/N、C/P、N/P均有提升。30年来表层土壤碳储量明显增加,氮储量基本稳定而磷储量有所下降。     

Effects of Thinning and Induced Drought on Microbiological Soil Properties and Plant Species Diversity at Dry and Semiarid Locations

Soil properties and plant species diversity are key elements of forest ecosystem functioning and are affected directly by climate change. The aim of this work was to study plant species diversity, physicochemical and soil microbiological properties and enzymatic activities after induced drought conditions and thinning at semiarid (Calasparra) and dry (Yeste) Pinus halepensis Mill. forest stands. Different plots affected by a wildfire event 17 years ago, with or without thinning 5 years after the fire event, were selected. A 15% rainfall reduction over 3 years was also carried out. Physicochemical soil properties (soil texture, pH, carbonates, total organic carbon, electrical conductivity and total N and P), soil enzymes (urease, phosphatase, β‐glucosidase and dehydrogenase activities), carbon mineralisation and soil microbial biomass carbon were analysed in the selected study areas. Shannon and Simpson indices were calculated, and total plant cover and plant species richness were evaluated. The results showed no differences in microbiological soil properties and soil enzyme activities when comparing thinned and unthinned plots; conversely, plant species diversity indices were affected by thinning. Induced drought affected only total cover and species richness, which were lower at Yeste. Significant site variation was also observed in soil properties, species richness and total plant cover, as opposed to the plant species diversity indices. We conclude that soil properties recover in the midterm after thinning activities, and they resist against a 15% rainfall reduction under potential climate change conditions remaining steady after induced drought. The plant community presents different responses to drought depending on the experimental site. Copyright © 2014 John Wiley & Sons, Ltd.     

宽谷丘陵区农牧开发模式监测评价体系研究

宽谷丘陵区是以黄土高原丘陵沟壑区第Ⅴ副区为主体,涉及第Ⅱ、第Ⅲ、第Ⅳ副区的部分区域。该区域沟壑纵横,常年降水稀少,气候干旱,水土流失严重,人民生活水平低。水土流失、干旱、低产、贫困是该区突出的生态、社会和经济问题。选择黄土丘陵沟壑区第Ⅲ副区的中山沟流域东山村作为示范点,在治理水土流失的同时,配套集雨节灌措施,培育和发展葡萄等特色产业,大力推广节水农业,通过减少化肥、农药施用量,以减缓面源污染。并在综合确定监测指标的同时,通过定点监测、调查访问等监测方法,采用层次分析法建立了示范效益监测与评价指标体系,从而科学合理地评价示范效应及生态经济系统。     

Relationships between C and N availability, substrate age, and natural abundance C and N signatures of soil microbial biomass in a semiarid climate

Soil microbial organisms are central to carbon (C) and nitrogen (N) transformations in soils, yet not much is known about the stable isotope composition of these essential regulators of element cycles. We investigated the relationship between C and N availability and stable C and N isotope composition of soil microbial biomass across a three million year old semiarid substrate age gradient in northern Arizona. The δ¹⁵N of soil microbial biomass was on average 7.2‰ higher than that of soil total N for all substrate ages and 1.6‰ higher than that of extractable N, but not significantly different for the youngest and oldest sites. Microbial ¹⁵N enrichment relative to soil extractable and total N was low at the youngest site, increased to a maximum after 55,000 years, and then decreased slightly with age. The degree of ¹⁵N enrichment of microbial biomass correlated negatively with the C:N mass ratio of the soil extractable pool. The δ¹³C signature of soil microbial biomass was 1.4‰ and 4.6‰ enriched relative to that of soil total and extractable pools respectively and showed significant differences between sites. However, microbial ¹³C enrichment was unrelated to measures of C and N availability. Our results confirm that ¹⁵N, but not ¹³C enrichment of soil microbial biomass reflects changes in C and N availability and N processing during long-term ecosystem development.