Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH₄NO₃ (0, 5 and 15 g N m^–2 yr^–1) have been made in two forest stands:Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons.Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased.The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m^–2 yr^–1), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO₃-N> NH₄-N> DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.     

Foliar free polyamine and inorganic ion content in relation to soil and soil solution chemistry in two fertilized forest stands at the Harvard Forest,Massachusetts

Polyamines (putrescine, spermidine, and spermine) are low molecular weight, open-chained, organic polycations which are found in all organisms and have been linked with stress responses in plants. The objectives of our study were to investigate the effects of chronic N additions to pine and hardwood stands at Harvard Forest, Petersham, MA on foliar polyamine and inorganic ion contents as well as soil and soil solution chemistry. Four treatment plots were established within each stand in 1988: control, low N (50 kg N ha^-1 yr^-1 as NH₄NO₃), low N + sulfur (74 kg S ha^-1 yr^-1 as Na₂SO₄), and high N (150 kg N ha^-1 yr^-1 as NH₄NO₃). All samples were analyzed for inorganic elements; foliage samples were also analyzed for polyamines and total N. In the pine stand putrescine and total N levels in the foliage were significantly higher for all N treatments as compared to the control plot. Total N content was positively correlated with polyamines in the needles (P 0.05). Both putrescine and N contents were also negatively correlated with most exchangeable cations and total elements in organic soil horizons and positively correlated with Ca and Mg in the soil solution (P 0.05). In the hardwood stand, putrescine and total N levels in the foliage were significantly higher for the high N treatment only as compared to the control plot. Here also, total foliar N content was positively correlated with polyamines (P 0.05). Unlike the case with the pine stand, in the hardwood stand foliar polyamines and N were significantly and negatively correlated with foliar total Ca, Mg, and Mn (P 0.05). Additional significant (P 0.05) relationships in hardwoods included: negative correlations between foliar polyamines and N content to exchangeable K and P and total P in the organic soil horizon; and positive correlations between foliar polyamines and N content to Mg in soil solution. With few exceptions, low N + S treatment had effects similar to the ones observed with low N alone for both stands. The changes observed in the pine stand for polyamine metabolism, N uptake, and element leaching from the soil into the soil solution in all treatment plots provide additional evidence that the pine stand is more nitrogen saturated than the hardwood stand. These results also indicate that the long-term addition of N to these stands has species specific and/or site specific effects that may in part be explained by the different land use histories of the two stands.     

Long-Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests

This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH₄NO₃) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September) at four rates: control (no fertilizer addition), low N (5 g N m^-2 y^-1), high N (15 g N m^-2 y^-1), and low N + sulfur (5 g N m^-2 y^-1 plus 7.4 g S m^-2 y^-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of ¹⁵N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996. Received 2 April 1999; Accepted 29 October 1999.     

Relationship between decomposition of cellulose in the soil and tree stand characteristics in natural boreal forests

The relationship between the decomposition of cellulose placed on and buried in the forest floor and various tree stand characteristics was studied at sites with minimal anthropogenic influence. The 22 study sites, including both forested upland and peatland plots, were clustered in 4 catchments between 61°–69° N in Finland. The stands were 60 to 320 years old and composed of varying proportions of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies Karst.) and deciduous species (mainly t Betula spp.). Cellulose strips (softwood pulp) were placed on the forest floor surface and buried at four depths down to 5 cm for two 1-year periods and the weight loss measured. Decomposition did not significantly correlate with stand age, but was strongly and positively correlated with stand basal area, mean tree height and stem volume. This was valid at all depths, and even after differences due to climate between catchments had been taken into account. The stem volume of Scots pine dominated plots had the highest correlation. Our results showed that decomposition of organic matter on and in the forest floor is related to the stand characteristics. This relationship should be considered when comparing soil processes different stands, even when comparing stands of the same tree species composition.     

Effects of European Earthworm Invasion on Soil Characteristics in Northern Hardwood Forests of Minnesota, USA

European earthworms are colonizing worm-free hardwood forests across North America. Leading edges of earthworm invasion in forests of northern Minnesota provide a rare opportunity to document changes in soil characteristics as earthworm invasions are occurring. Across leading edges of earthworm invasion in four northern hardwood stands, increasing total earthworm biomass was associated with rapid disappearance of the O horizon. Concurrently, the thickness, bulk density and total soil organic matter content of the A horizon increased, and it’s percent organic matter and fine root density decreased. Different earthworm species assemblages influenced the magnitude and type of change in these soil parameters. Soil N and P availability were lower in plots with high earthworm biomass compared to plots with low worm biomass. Decreases in soil nitrogen availability associated with high earthworm biomass were reflected in decreased foliar nitrogen content for Carex pensylvanica, Acer saccharum and Asarum canadense but increased foliar N for Athyrium felix-femina. Overall, high earthworm biomass resulted in increased foliar carbon to nitrogen ratios. The effects of earthworm species assemblages on forest soil properties are related to their feeding and burrowing habits in addition to effects related to total biomass. The potential for large ecosystem consequences following exotic earthworm invasion has only recently been recognized by forest ecologists. In the face of rapid change and multiple pressures on native forest ecosystems, the impacts of earthworm invasion on forest soil structure and function must be considered.     

Variation of soil carbon pools in Pinus sylvestris plantations of different ages in north China

Plantations play an important role in absorbing atmospheric CO₂ and plantation soil can serve as an important carbon (C) sink. However, the stocks and dynamics of soil C in differently aged plantation forests in north China remain uncertain. In this study, we measured soil inorganic carbon (SIC), soil organic carbon (SOC) and total nitrogen content (STN), the light (LF) and heavy fractions (HF) of soil organic matter (SOM) to a depth of 1 m in 3 different ages (10-, 30-, 40-year-old) of Pinus sylvestris var. mongolica (Mongolia pine) plantations in 2011 and 2012. Soil pH, texture and moisture were also measured to explore the causes of SOC dynamics for different stand ages. Our results showed that no significant difference in SIC content was observed at different soil depths. As forest age increases, SIC content as well as the C and N content in SOM, LF and HF initially rose and then decreased, while the LF in SOC initially decreased and then increased. Although the C:N ratio of SOC and HF did not significantly change, the C:N ratio of LF increased with depth. SOC dynamics at different stand ages were significantly correlated with soil moisture and clay content. Soil pH and moisture explained 58.63% of the overall variation of SOC at different depths. Moreover, the SOC increased during the early stage of afforestation, mostly because of the increase in recalcitrant C; however, the decrease of SOC with increasing stand age was also mainly affected by C loss in the recalcitrant C pool.     

Ectomycorrhizal root growth in scots pine stands in response to manipulation of litter and humus layers

 The effect on ectomycorrhizal root growth in a nitrogen-enriched planted stand of Scots pine (Pinus sylvestris L.) on podzolic sandy soil to manipulation of litter and humus layers (removal, doubling and control treatments) was examined, and compared to ectomycorrhizal root growth in an untreated naturally established Scots pine stand on nutrient-poor non-podzolic sandy soil. Half a year after manipulation of litter and humus layers in the planted stand, ingrowth-cores to a depth of 60 cm were installed in both stands. Scots pine roots were sampled four times during two growing seasons. Ectomycorrhizal roots were found at all sampled soil depths to 60 cm in all plots. Root growth and ectomycorrhizal development were greater in the naturally established stand than in all plots in the planted stand. Numbers of ectomycorrhizal root tips in the litter and humus removal plots were generally higher than in the control plots in the planted stand until May 1992. Doubling litter and humus did not significantly affect root length or the numbers of ectomycorrhizal root tips. The N _dissolved , NH₄ ⁺ and NO₃ ^– concentrations and the organic matter content in the upper 5 cm of the mineral soil in the planted stand on podzolic sandy soil were generally higher and the pH significantly lower than in the naturally established stand on non-podzolic sandy soil. Root growth and ectomycorrhizal development in the secondary stand may have been negatively affected by the chemical composition of the podzolic sandy soil. Accepted: 19 March 1997     

氮磷添加及林分密度对大叶相思林土壤化学性质的影响

大气氮(N)沉降随着人类的活动而日趋严重, 加上中国热带亚热带红壤普遍缺磷(P), 许多森林生态系统由于广泛使用磷肥而产生P富集, 直接影响了森林土壤化学特性。林分密度改变林地的光照、温度、湿度和凋落物持水量, 从而影响土壤特性。为了解外源性N和P添加与林分密度对大叶相思(Acacia auriculiformis)林地土壤化学性质的影响, 为大叶相思人工林的种植密度和土壤养分管理提供科学依据, 该研究于2013到2015年, 以4种不同密度(1 667、2 500、4 444和10 000 trees·hm ^-2)的10年生大叶相思人工林为研究对象, 分别进行添加N、P和N+P处理, 在试验结束时采集0-10 cm土壤, 对其pH、有机质含量、N含量、P含量和钾(K)含量进行了测定分析。结果表明: 施N和N+P均显著降低了土壤的pH和速效K含量, 显著提高了林地土壤的碱解N含量。施N还显著提高了林分土壤的全N含量, 施P显著提高了土壤pH, 降低了林分土壤的全N含量。施P和N+P显著提高了土壤有机质、全P和有效P含量。随着林分密度的增加, 各处理的土壤有机质、全N、碱解N、全P、有效P和速效K含量显著提高。N、P添加处理和密度处理对大叶相思林的土壤pH、有机质和N、P、K含量有显著的交互作用。总体来看, N添加、P添加、林分密度及其交互作用对大叶相思的土壤化学性质有显著影响。     

Chronic nitrogen additions suppress decomposition and sequester soil carbon in temperate forests

The terrestrial biosphere sequesters up to a third of annual anthropogenic carbon dioxide emissions, offsetting a substantial portion of greenhouse gas forcing of the climate system. Although a number of factors are responsible for this terrestrial carbon sink, atmospheric nitrogen deposition contributes by enhancing tree productivity and promoting carbon storage in tree biomass. Forest soils also represent an important, but understudied carbon sink. Here, we examine the contribution of trees versus soil to total ecosystem carbon storage in a temperate forest and investigate the mechanisms by which soils accumulate carbon in response to two decades of elevated nitrogen inputs. We find that nitrogen-induced soil carbon accumulation is of equal or greater magnitude to carbon stored in trees, with the degree of response being dependent on stand type (hardwood versus pine) and level of N addition. Nitrogen enrichment resulted in a shift in organic matter chemistry and the microbial community such that unfertilized soils had a higher relative abundance of fungi and lipid, phenolic, and N-bearing compounds; whereas, N-amended plots were associated with reduced fungal biomass and activity and higher rates of lignin accumulation. We conclude that soil carbon accumulation in response to N enrichment was largely due to a suppression of organic matter decomposition rather than enhanced carbon inputs to soil via litter fall and root production.     

Soil CO2 efflux in a mixed pine-oak forest in Valsaín (central Spain)

Soil-surface CO2 efflux and its spatial and temporal variation were investigated in a southern Mediterranean, mixed pine-oak forest ecosystem on the northern slopes of the Sierra de Guadarrama in Spain from February 2006 to July 2006. Measurements of soil CO2 efflux, soil temperatures, and moisture were conducted in nine 1963-m2 sampling plots distributed in a gradient around the ecotone between Pinus sylvestris L. and Quercus pyrenaica Lam. forest stands. Total soil organic matter, Walkey-Black C, particulate organic matter, organic matter fraction below 53 microm, total soil nitrogen content, total soil organic carbon content, and pH were also measured under three representative mature oak, pine, and mixed pine-oak forest stands. Soil respiration showed a typical seasonal pattern with minimums in winter and summer, and maximums in spring, more pronounced in oak and oak-pine stands. Soil respiration values were highest in pine stands during winter and in oak stands during spring and summer. Soil respiration was highly correlated with soil temperatures in oak and pine-oak stands when soil moisture was above a drought threshold of 15%. Below this threshold value, soil moisture was a good predictor of soil respiration in pine stands. Greater soil organic matter, particulate organic matter, Walkey-Black C, total organic C, and total N content in pine compared to oak sites potentially contributed to the greater total soil CO2 efflux in these stands during the winter. Furthermore, opposing trends in the organic matter fraction below 53 microm and soil respiration between plots suggest that in oak stands, the C forms are less affected by possible changes in use. The effects of soil properties on soil respiration were masked by differences in soil temperature and moisture during the rest of the year. Understanding the spatial and temporal variation even within small geographic areas is essential to assess C budgets at ecosystem level accurately. Thus, this study bears important implications for the study of large-scale ecosystem dynamics, particularly in response to climatic change.     

帽儿山5种林型土壤碳氮磷化学计量关系的垂直变化

采用土壤分层取样法测定了帽儿山地区相同气候条件、不同立地条件下林龄相近的2种人工针叶林(红松林和兴安落叶松林)和3种天然落叶阔叶林(蒙古栎林、杨桦林、硬阔叶林)的土壤碳(C)、氮(N)、磷(P)元素含量,以及土壤容重、土壤厚度等,研究土壤C、N、P含量和密度及化学计量关系的垂直变化特征.结果表明: 不同林型间土壤C、N、P含量和密度差异显著,其中硬阔叶林土壤O层和A层的C、N含量和密度均显著高于其他林型.所有林型土壤的C、N含量均随土层加深而下降;落叶阔叶林土壤P含量随土层加深而显著下降,但针叶林各土层间P含量差异不显著.不同林型间A层土壤C/N、O层土壤N/P,以及A和B层土壤C/P均存在显著差异.不同林型间土壤C-N存在显著的线性关系,且其斜率和截距在林型间差异均不显著,但土壤N-P、C-P关系只在阔叶林中存在显著相关关系.这表明不同林型间土壤C-N耦联关系有趋同现象,而土壤N-P和C-P关系随林型而变.     

Jack pine foliar δ15N indicates shifts in plant nitrogen acquisition after severe wildfire and through forest stand development

Background and aims

Natural abundance of the stable nitrogen (N) isotope ¹⁵N can elucidate shifts in plant N acquisition and ecosystem N cycling following disturbance events. This study examined the potential relationship between foliar δ¹⁵N and depth of plant N acquisition (surface organic vs. mineral soil) and nitrification as conifer stands develop following stand-replacing wildfire.

Methods

We measured foliar δ¹⁵N along an 18-site chronosequence of jack pine (Pinus banksiana) stands, 1 to 72 years in age post-wildfire. Foliar δ¹⁵N was compared to total δ¹⁵N of the organic (Oe + Oa) and mineral (0–15 cm) soil horizons, and organic horizon N mineralization and nitrification as functions of total mineralization.

Results

Foliar δ¹⁵N declined with stand age, yet wildfire effects were heterogeneous. Jack pine seedlings on burned, mineral soil patches in the youngest stand were significantly more enriched than those on unburned, organic patches (P?=?0.007). High foliar values in the youngest stands relative to mineral-horizon δ¹⁵N indicate that nitrification also likely contributed to seedling enrichment.

Conclusions

Our results suggest jack pine seedlings on burned patches obtain N from the mineral soil with potentially high nitrification rates, whereas seedlings on unburned patches and increasingly N-limited, mature jack pine acquire relatively more N from organic horizons.     

Microfungal community structure from forest soils in Northern Thrace Region, Turkey

The soil fungi in the pure stand of oak (Quercus petraea), beech (Fagus orientalis), and pine (Pinus nigra) were investigated by the dilution plate method at Yildiz Mountain in Thrace region. The mycobiota, as well as the number of isolates per plate, was determined at various soil depths. Principal component analysis of the soil profiles indicated that there was variation in mycobiota composition and the variation was attributed to differences among the ecosystems. When comparing conifer and hardwood soils, using Sorenson’s similarity index, fungal community composition corresponded more closely between the hardwood stands than with the conifer stand. Fungal community composition appears to be influenced by the organic compounds entering soil from plant litter.     

凉水自然保护区阔叶红松林林分空间结构特征及其与影响因子关系简

根据凉水自然保护区28块典型阔叶红松林样地的5个林分空间结构参数和18个影响因子数据,采用典范对应分析（CCA）方法,对凉水自然保护区阔叶红松林林分空间结构与影响因子间关系进行分析。研究结果表明：（1）研究区域阔叶红松林整体具有较好的林分空间结构,其水平分布格局主要表现为随机分布,树木生长整体处于中庸状态,林木的整体混交程度较高;（2）林分空间结构的CCA排序较好的揭示了该区林分空间结构与影响因子的关系;CCA第一排序轴反映了林龄、坡度、阔叶比和坡向的变化,第二排序轴反映了坡向、土壤有机质和平均胸径的变化,上述6因子的组合是决定林分空间结构特征的主要影响因子;（3）影响林分空间结构的变量中,地形、土壤和林分因子共解释了林分空间结构变化的59.20%,其中纯地形因子占30.68%,纯林分因子占19.01%,纯土壤因子占8.21%,未能解释部分为40.80%。     

帽儿山温带森林演替初期土壤碳、氮、磷计量特征的变化

2004年在帽儿山森林生态站设置土壤置换试验,将0～30 cm农田土置换成邻近天然次生林淋溶层土(A处理)、淀积层土(B处理)和母质层土(风化砂,C处理),分别模拟森林皆伐次生演替、无种子库次生演替和原生演替,2014年研究温带落叶阔叶林不同演替类型在自然演替初期土壤碳、氮、磷计量特征的变化.结果表明: 演替10年,A处理土壤碳、氮、磷含量无显著变化,B处理土壤碳和氮含量分别降低34.7%和38.6%,而C处理土壤碳和氮含量分别增加63.4%和198.4%.植被演替后,氮-碳异速生长关系斜率显著降低,磷-氮异速生长速率显著升高.10年演替后,仅C处理土壤C∶N减小44.5%,N∶P增加283.6%,其他处理变化不显著.土壤碳、氮、磷含量与活细根现存量、死细根现存量均存在显著相关关系,植被演替可能主要通过改变有机质输入驱动土壤碳、氮、磷含量及其计量关系.     

Soil Organic Carbon and Water Retention after Conversion of Grasslands to Pine Plantations in the Ecuadorian Andes

Tree plantations in the high elevations of the tropics constitute a growing land use, but their effect on ecosystem processes and services is not well known. We examined changes in soil organic carbon (C) and water retention in a chronosequence of Pinus radiata stands planted in páramo grasslands in Cotopaxi province, Ecuador. Water retention at 10, 33, and 1,500 kPa declined with stand age, with soils in the oldest pine stands retaining 39%, 55%, and 63% less water than grassland soils at the three pressures tested. Soil organic C in the 0–10-cm depth also declined with stand age, from 5.0 kg m^–2 in grasslands to 3.5 kg m^–2 in 20–25-year-old pine stands (P < 0.001); at greater depth in the A horizon, C contents decreased from 2.8 to 1.2 kg m^–2 (P = 0.047). There were no significant differences among age classes in the AC and C horizons (P = 0.15 and P = 0.34, respectively), where little or no weathering of the primary material has occurred. Inputs of C may be affected by the significantly higher carbon–nitrogen (C:N) ratio of the litter under older pine stands (P = 0.005), whereas outputs are influenced by substrate quality as well as soil environmental factors. Soil ratios at the 0–10 cm depth were significantly higher in grasslands and young pine stands (P < 0.001), whereas carbon–phosphorous (C:P) ratios at 0–10-cm depth followed a similar but not significant trend. However, there was no significant difference in short-term decomposition rates (P = 0.60) when the soils were incubated under uniform temperature and moisture conditions. In páramo ecosystems, where high soil moisture plays an important role in retarding decomposition and driving high C storage, the loss of water retention after afforestation may be the dominant factor in C loss. These results suggest that soil C buildup and water retention respond rapidly to changes in biota and need to be assessed with regard to implications for C sequestration and watershed management.     

The fate of15N enriched throughfall in two coniferous forest stands at different nitrogen deposition levels

The stable isotope¹⁵N was added as (¹⁵NH₄)₂SO₄ to throughfall water for one year, to study the fate of the deposited nitrogen at different levels of N deposition in two N saturated coniferous forests ecosystems in the Netherlands. The fate of the¹⁵N was followed at high-N (44–55 kg N ha^–1 yr^–1) 1) and low-N (4–6 kg N ha^–1 yr^–1) deposition in plots established under transparent roofs build under the canopy in a Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and Scots pine (Pinus sylvestris L.) forest.The applied¹⁵N was detectable in needles and twigs, the soil and soil water leaching below the rooting zone (90 cm depth). Total¹⁵N recovery in major ecosystem compartments was 71–100% during two successive growing seasons after the start of a year-round¹⁵N application to throughfall-N. Nine months after the year-round¹⁵N application, the¹⁵N assimilated into tree biomass was 29–33% of the¹⁵N added in the Douglas fir stand and less than 17% in the Scots pine stand. At the same time total¹⁵N retention in the soil (down to 70 cm) of the high-N plots was about 37% of the deposited¹⁵NH₄-N, whereas 46% and 65% of the¹⁵N was found in the soil of the low-N deposition plots at the Douglas fir and Scots pine stand, respectively. The organic layers accounted for 60% of the¹⁵N retained in the soil. The total N deposition exceeded the demand of the vegetation and microbial immobilization. Total¹⁵N leaching losses within a year (below 90 cm) were 10–20% in the high-N deposition plots in comparison to 2–6% in the lowered nitrogen input plots. Relative retention in the soil and vegetation increased at lower N-input levels.Species differences in uptake and tree health seem to contribute to lower¹⁵N recoveries in the Scots pine trees compared to the Douglas fir trees. The excessive N deposition and resulting N saturation lead to conditions were the health and functioning of biota were negatively influenced. At decreased N deposition, lower leaching losses together with increased soil and plant retention indicated a change in the fate of the¹⁵N deposited. This may have resulted from changes in ecosystem processes, and thus a shift along the continuum of N saturation to N limitation.     

The effect of nitrogen addition on soil organic matter dynamics: a model analysis of the Harvard Forest Chronic Nitrogen Amendment Study and soil carbon response to anthropogenic N deposition

Recent observations indicate that long-term N additions can slow decomposition, leading to C accumulation in soils, but this process has received limited consideration by models. To address this, we developed a model of soil organic matter (SOM) dynamics to be used with the PnET model and applied it to simulate N addition effects on soil organic carbon (SOC) stocks. We developed the model’s SOC turnover times and responses to experimental N additions using measurements from the Harvard Forest, Massachusetts. We compared model outcomes to SOC stocks measured during the 20th year of the Harvard Forest Chronic Nitrogen Amendment Study, which includes control, low (5 g N m^?2 yr^?1) and high (15 g N m^?2 yr^?1) N addition to hardwood and red pine stands. For unfertilized stands, simulated SOC stocks were within 10 % of measurements. Simulations that used measured changes in decomposition rates in response to N accurately captured SOC stocks in the hardwood low N and pine high N treatment, but greatly underestimated SOC stocks in the hardwood high N and the pine low N treatments. Simulated total SOC response to experimental N addition resulted in accumulation of 5.3–7.9 kg C per kg N following N addition at 5 g N m^?2 yr^?1 and 4.1–5.3 kg C per kg N following N addition at 15 g N m^?2 yr^?1. Model simulations suggested that ambient atmospheric N deposition at the Harvard Forest (currently 0.8 g N m^?2 yr^?1) has led to an increase in cumulative O, A, and B horizons C stocks of 211 g C m^?2 (3.9 kg C per kg N) and 114 g C m^?2 (2.1 kg C per kg N) for hardwood and pine stands, respectively. Simulated SOC accumulation is primarily driven by the modeled decrease in SOM decomposition in the Oa horizon.     

Soil carbon fluxes and stocks in a Great Lakes forest chronosequence

We measured soil respiration and soil carbon stocks, as well as micrometeorological variables in a chronosequence of deciduous forests in Wisconsin and Michigan. The chronosequence consisted of (1) four recently disturbed stands, including a clearcut and repeatedly burned stand (burn), a blowdown and partial salvage stand (blowdown), a clearcut with sparse residual overstory (residual), and a regenerated stand from a complete clearcut (regenerated); (2) four young aspen ( Populus tremuloides ) stands in average age of 10 years; (3) four intermediate aspen stands in average age of 26 years; (4) four mature northern hardwood stands in average age of 73 years; and (5) an old-growth stand approximately 350-years old. We fitted site-based models and used continuous measurements of soil temperature to estimate cumulative soil respiration for the growing season of 2005 (days 133–295). Cumulative soil respiration in the growing season was estimated to be 513, 680, 747, 747, 794, 802, 690, and 571 g C m⁻² in the burn, blowdown, residual, regenerated, young, intermediate, mature, and old-growth stands, respectively. The measured apparent temperature sensitivity of soil respiration was the highest in the regenerated stand, and declined from the young stands to the old-growth. Both, cumulative soil respiration and basal soil respiration at 10 °C, increased during stand establishment, peaked at intermediate age, and then decreased with age. Total soil carbon at 0–60 cm initially decreased after harvest, and increased after stands established. The old-growth stand accumulated carbon in deep layers of soils, but not in the surface soils. Our study suggests a complexity of long-term soil carbon dynamics, both in vertical depth and temporal scale.     

Molecular characterization of methanotrophic communities in forest soils that consume atmospheric methane

Methanotroph abundance was analyzed in control and long-term nitrogen-amended pine and hardwood soils using rRNA-targeted quantitative hybridization. Family-specific 16S rRNA and pmoA/amoA genes were analyzed via PCR-directed assays to elucidate methanotrophic bacteria inhabiting soils undergoing atmospheric methane consumption. Quantitative hybridizations suggested methanotrophs related to the family Methylocystaceae were one order of magnitude more abundant than Methyloccocaceae and more sensitive to nitrogen-addition in pine soils. 16S rRNA gene phylotypes related to known Methylocystaceae and acidophilic methanotrophs and pmoA/amoA gene sequences, including three related to the upland soil cluster Alphaproteobacteria (USCalpha) group, were detected across different treatments and soil depths. Our results suggest that methanotrophic members of the Methylocystaceae and Beijerinckiaceae may be the candidates for soil atmospheric methane consumption.