Nutrients In Terrestrial Ecosystems Research Articles

Effects of temperature-related changes on charred bone in soil: From P release to microbial community

Phosphorus (P) is one of the most common limited nutrients in terrestrial ecosystems. Animal bones, with abundant bioapatite, are considerable P sources in terrestrial ecosystems. Heating significantly promotes P release from bone bioapatite, which may alleviate P limitation in soil. This study aimed to explore P release from charred bone (CB) under heating at various temperatures (based on common natural heating). It showed that heating at ∼300 °C significantly increased the P release (up to ∼30 mg/kg) from CB compared with other heating temperatures. Then, the subsequent changes of available P and pH induced evident alternation of soil microbial community composition. For instance, CB heated at ∼300 °C caused elevation of phosphate-solubilizing fungi (PSF) abundance. This further stimulated P mobility in the soil. Meanwhile, the fungal community assembly process was shifted from stochastic to deterministic, whereas the bacterial community was relatively stable. This indicated that the bacterial community showed fewer sensitive responses to the CB addition. This study hence elucidated the significant contribution of heated bone materials on P supply. Moreover, functional fungi might assist CB treated by natural heating (e.g., fire) to construct P “Hot Spots”.

Chronic anthropogenic disturbance alters litter decomposition and nutrient concentrations and stocks across a Caatinga dry forest chronosequence

Litter decomposition plays a key role in the cycling and storage of nutrients in terrestrial ecosystems. However, very little is known about decomposition processes and their underlying drivers in tropical dry forests, particularly in human-modified landscapes. Here, we evaluate decomposition rates and nutrient concentrations and stocks in the litter of a Caatinga dry forest chronosequence, in northeast Brazil. Litter decomposition was monitored for a year across forest stands of varying ages and old-growth forests. The litter decomposition rate was highly variable across forest stands, although on average decomposition occurred slowly, with approximately 50% of the initial litter mass still present after one year. The decomposition rate is responsive to forest type and negatively affected by chronic disturbance. Litter nutrient concentration decreased along the decomposition process, exhibiting a nutrient-specific mineralization rate. Response to drivers was also nutrient specific, with precipitation and chronic disturbances playing prominent roles. Nutrient stocks in the remaining litter were higher in old-growth forests, and final stocks of certain nutrients also responded to multiple drivers. Our results suggest that the Caatinga dry forest experiences a slow rate of litter decomposition and mineralization, although rates exhibit considerable variability across forest stands and environmental conditions. Thus, chronic disturbance plays a role by either facilitating or reducing litter decomposition, mineralization rate, and the stocks transferred to the soil. Consequently, these factors have implications for nutrient availability, including goat consumption and subsequent loss of nutrients.

Effects of Heavy Metals on Nitrogen in Soils of Different Ecosystems in the Karst Desertification of South China

Nitrogen, as a crucial limiting nutrient in terrestrial ecosystems, plays a vital role in determining land quality. Heavy metals, as drivers of soil substance transformation, are important indicators for assessing ecosystem function. Currently, the relationship between soil nitrogen and heavy metals in karst desertification areas remains unclear. Therefore, this study focuses on the soil of grassland, forest, and agroforestry ecosystems in a karst desertification area to investigate the relationship between heavy metals and nitrogen distribution using ecological stoichiometry. The findings revealed the following: (i) Total nitrogen (TN) and available nitrogen (AN) exhibited the trend of agroforestry * > forest > grassland, while soil microbial biomass nitrogen (SMBN) showed the trend of forest * > grassland * >> agroforestry; (ii) Chromium (Cr), Ferrum (Fe), Niccolum (Ni), and Plumbum (Pb) showed the trend of agroforestry * > grassland > forest, while Cuprum (Cu) demonstrated the trend of agroforestry > grassland > forest, and Zincum (Zn) exhibited the trend of grassland > forest * >> agroforestry. The Nemerow comprehensive pollution index were 0.77 for grassland, 0.69 for forest, and 0.94 for agroforestry; (iii) The sensitivity of soil nitrogen and heavy metals ranked as grassland > agroforestry > forest. The research findings aim to provide a scientific reference for karst desertification control, ecological protection and restoration, and enhancement of ecosystem function.

Standardized Data to Improve Understanding and Modeling of Soil Nitrogen at Continental Scale

AbstractNitrogen (N) is a key limiting nutrient in terrestrial ecosystems, but there remain critical gaps in our ability to predict and model controls on soil N cycling. This may be in part due to lack of standardized sampling across broad spatial–temporal scales. Here, we introduce a continentally distributed, publicly available data set collected by the National Ecological Observatory Network (NEON) that can help fill these gaps. First, we detail the sampling design and methods used to collect and analyze soil inorganic N pool and net flux rate data from 47 terrestrial sites. We address methodological challenges in generating a standardized data set, even for a network using uniform protocols. Then, we evaluate sources of variation within the sampling design and compare measured net N mineralization to simulated fluxes from the Community Earth System Model 2 (CESM2). We observed wide spatiotemporal variation in inorganic N pool sizes and net transformation rates. Site explained the most variation in NEON’s stratified sampling design, followed by plots within sites. Organic horizons had larger pools and net N transformation rates than mineral horizons on a sample weight basis. The majority of sites showed some degree of seasonality in N dynamics, but overall these temporal patterns were not matched by CESM2, leading to poor correspondence between observed and modeled data. Looking forward, these data can reveal new insights into controls on soil N cycling, especially in the context of other environmental data sets provided by NEON, and should be leveraged to improve predictive modeling of the soil N cycle.

Dynamic of boron in forest ecosystems traced by its isotopes: A modeling approach

Understanding the factors that control the cycling of nutrients in terrestrial ecosystems and in the Critical Zone is of fundamental importance given its role for example in nutrient availability to sustain forest productivity, and ultimately in soil carbon storage.In this paper, we developed a model to assess the dynamic of boron in forest ecosystems and to appraise how the impacts on boron cycling by internal or external factors should be reflected in the changes of its isotopic compositions across an ecosystem. Despite the scarcity of data, we tested this model on two case studies and were able to reproduce the distribution of boron isotopes between the different pools of these two contrasted ecosystems. The model shows a time dependency of the boron isotopic composition of the different biotic and abiotic compartments of the ecosystem. When the forest grows, a transient enrichment in the heavy isotope up to 20‰ relative to the values at steady-state is observed in the biomass and the soil solutions. The magnitude of this enrichment, and the return time to steady state, are sensitive to B supply and plant demand for boron. We also tested the responses of B dynamic to natural or anthropogenic disturbances and we show that they are well reflected in the variations of the B isotopic compositions of the different pools. As a whole, we show that B isotopes are a good potential tracer of nutrient cycling and by extension, a promising proxy for tracing the global functioning of terrestrial biosphere at present and in the past.

Earthworms negate the adverse effect of arbuscular mycorrhizae on living bacterial biomass and bacterial necromass accumulation in a subtropical soil

Earthworms and arbuscular mycorrhizal fungi (AMF) have substantial individual effects on the supply and absorption of soil nutrients in terrestrial ecosystems. How their interactions regulate the soil living microbial community and microbial necromass accumulation remains unknown. We hypothesized that the effect of their interaction on the microbial community and necromass accumulation may relate to their controls on status of soil N and P. We conducted a greenhouse experiment manipulating the AMF Rhizophagus intraradices and the earthworm Pontoscolex corethrurus in microcosms planted with a fern Dicranopteris dichotoma. Phospholipid fatty acids and amino sugars were used to indicate the living microbial community and necromass accumulation, respectively. The AMF consistently reduced the biomass of bacterial groups and the concentration of total amino sugars, glucosamine and muramic acid, in the absence of earthworms, but had no effect in their presence. The earthworms increased the concentration of soil total dissolved N, water-soluble N and water-soluble P, and the activity of β–1,4–N-acetylglucosaminidase and phosphatase, and the amount of P in total plant biomass. Redundancy analysis showed that the bacterial biomass was positively correlated with most soil labile N indices and negatively correlated with the amount of N in total plant biomass. Pearson correlation revealed that the bacterial nutritional stress indicator Sat/Mono was negatively correlated with soil NH4+ concentration and positively correlated with the amount of N in total plant biomass. The results suggested that earthworms might negate the suppression effect of AMF on bacterial abundance and necromass accumulation by alleviating AMF-induced N deficiency.

Earthworm gut: An overlooked niche for anaerobic ammonium oxidation in agricultural soil

Soil fauna takes an active part in accelerating turnover of nutrients in terrestrial ecosystems. Anaerobic ammonium oxidation (anammox) has been widely characterized, however, whether anammox is active in earthworm gut and the effect of earthworm on anammox in soil remain unknown. In this study, the activity, abundance and community of anammox bacteria in earthworm guts and soils from microcosms were determined using a 15N-tracing technique, quantitative PCR, and anammox bacterial 16S rRNA gene amplicon sequencing. Results showed that anammox rates in guts ranged between 5.81 and 14.19 nmol N g−1 dw gut content h−1, which were significantly (P < 0.01) higher than that in their surrounding soils during 30 day incubation. On the contrary, abundances of hzsB genes encoding subunit B hydrazine synthase in guts were significantly (P < 0.05) lower than those in their surrounding soils. Anammox rates, denitrification N2 production rates and hzsB genes in soils with earthworms were significantly (P < 0.05) lower than those in control soils. Anammox bacterial compositions differed significantly (P < 0.05) between gut and soil, and earthworm altered anammox bacterial communities in soils. Brocadia, Kuenenia and abundant unclassified anammox bacteria were detected in collected soils and gut contents, in which Brocadia was only detected in guts. These results suggested that microbes in earthworm gut increase, but present of earthworm reduces anammox and denitrification associated N loss by altering the anammox bacterial community compositions in soils.

Mechanisms of Carbon Sequestration in Highly Organic Ecosystems - Importance of Chemical Ecology.

Organic matter decomposition plays a major role in the cycling of carbon (C) and nutrients in terrestrial ecosystems across the globe. Climate change accelerates the decomposition rate to potentially increase the release of greenhouse gases and further enhance global warming in the future. However, fractions of organic matter vary in turnover times and parts are stabilized in soils for longer time periods (C sequestration). Overall, a better understanding of the mechanisms underlying C sequestration is needed for the development of effective mitigation policies to reduce land‐based production of greenhouse gases. Known mechanisms of C sequestration include the recalcitrance of C input, interactions with soil minerals, aggregate formation, as well as its regulation via abiotic factors. In this Minireview, we discuss the mechanisms behind C sequestration including the recently emerging significance of biochemical interactions between organic matter inputs that lead to C stabilization.

Responses of Foliar Nutrient Status and Stoichiometry to Nitrogen Addition in Different Ecosystems: A Meta‐analysis

AbstractAtmospheric nitrogen (N) deposition alters the cycling of nutrients in terrestrial ecosystems. However, there is limited knowledge regarding how N addition affects foliar nutrients beyond N and phosphorus (P) and their stoichiometry. We conducted a meta‐analysis, including 2,004 observations from 134 fertilization studies, to synthesize the effects of N addition on multiple foliar nutrients and stoichiometry in terrestrial ecosystems and to examine their potential controls. Overall, we found that N addition significantly decreased foliar P, potassium (K), calcium (Ca), magnesium (Mg), and calcium:aluminium (Ca:Al) by 3.22%, 7.58%, 9.55%, 5.65%, and 15.17%, respectively, but significantly increased foliar N, Al, N:K, N:Ca, and N:Mg by 19.02%, 6.99%, 21.06%, 27.65%, and 11.33%, respectively. Among the forest ecosystems, foliar N and P exhibited greater changes in temperate or boreal forests, and foliar K, Mg, and Mn showed greater decreases or increases in tropical forests after N addition. Nitrogen addition significantly affected foliar K (−7.55%), Mg (−6.46%), and Al (+6.81%) in forests but not in grasslands. Similarly, foliar K, Mg, and Al showed significant changes in woody plants but not in herbaceous plants. In addition, we found that environmental factors (e.g., ambient N deposition, mean annual precipitation, and soil pH) affected foliar nutrient responses of N, K, and Mg to N addition. In summary, our findings indicate that N addition affects foliar nutrient contents, with the extent of these effects differing among ecosystem type. This is important for understanding and predicting plant growth and nutrient dynamics under N deposition scenarios.

Temperature sensitivity of microbial Fe(III) reduction kinetics in subalpine wetland soils

Microbially-mediated iron (Fe) cycling controls the fate of organic matter, contaminants, and nutrients in terrestrial ecosystems including wetland soils. However, the effects of temperature variations due to seasonal differences on Fe(III) reduction rates and kinetics in such ecosystems remains poorly understood. To evaluate the potential temperature impact on dissimilatory microbial Fe(III) reduction it is crucial to determine environmentally-relevant reaction rates and kinetic parameters. Here, we investigate the relationship between soil temperature and microbial Fe(III) reduction kinetics in mineral soils from two subalpine wetlands with distinct hydrologic and edaphic conditions. We conducted flow-through experiments (FTR) at three temperatures (6, 12, and 18 °C) using intact soil cores collected from 30 cm [(higher organic carbon (Corg) and total nitrogen (TN)] and 70 cm (lower Corg and TN) soil depths in order to determine the apparent Fe(III) affinity constant (Km), apparent maximum Fe(III) reduction rates (Vmax) and temperature sensitivity (Q10 and Ea) of Fe(III) reduction. We used Fe(III)-NTA, a model compound for aqueous labile and complexed Fe present in natural organic matter. Our results show that changes in apparent Vmax and Km are driven primarily by temperature. Significant differences in apparent Vmax at 18 °C relative to 6 and 12 °C (P < 0.05) suggest that dissimilatory microbial Fe(III) reduction in wetland soils accelerates during warmer summer days. However, temperature alone fails to explain the large variability of the apparent parameters Q10 (1.5–8.9) and Ea (26–148 kJ mol−1) for the two wetland types and depths (30 and 70 cm). Strong relationship between both parameters of temperature sensitivity (Q10 and Ea) and reactive soil Fe content at 30 cm and Corg/TN at 70 cm depth demonstrate the notable impact of soil properties on the temperature sensitivity for mirobial Fe(III) reduction in these wetland soils. Our results emphasize the importance of soil temperature on Fe(III) reduction kinetics and must be considered when predicting dissimilatory Fe(III) reduction under different seasonal temperatures or in wetlands located at different temperature regimes.

Responses of Dodonaea viscosa growth and soil biological properties to nitrogen and phosphorus additions in Yuanmou dry-hot valley

Nitrogen (N) and phosphorus (P) are limited nutrients in terrestrial ecosystems, and their limitation patterns are being changed by the increase in N deposition. However, little information concerns the plant growth and the soil biological responses to N and P additions among different soils simultaneously, and these responses may contribute to understand plant-soil interaction and predict plant performance under global change. Thus, this study aimed to explore how N and P limitation changes in different soil types, and reveal the relationship between plant and soil biological responses to nutrient additions. We planted Dodonaea viscosa, a globally distributed species in three soil types (Lixisols, Regosols and Luvisols) in Yuanmou dry-hot valley in Southwest China and fertilized them factorially with N and P. The growth and biomass characters of D. viscosa, soil organic matter, available N, P contents and soil carbon (C), N, P-related enzyme activities were quantified. N addition promoted the growth and leaf N concentration of D. viscosa in Lixisols; N limitation in Lixisols was demonstrated by lower soil available N with higher urease activity. P addition promoted the growth and leaf P concentration of D. viscosa in Luvisols; severe P limitation in Luvisols was demonstrated by a higher soil available N: P ratio with higher phosphatase activity. Urease activity was negatively correlated with soil available N in Nlimited Lixisols, and phosphatase activity was negatively correlated with soil available P in P-limited Luvisols. Besides, the aboveground biomass and leaf N concentration of D. viscosa were positively correlated with soil available N in Lixisols, but the aboveground biomass was negatively correlated with soil available P. Our results show similar nutrient limitation patterns between plant and soil microorganism in the condition of enough C, and the nutrient limitations differ across soil types. With the continued N deposition, N limitation of the Lixisols in dry hot valleys is expected to be alleviated, while P limitation of the Luvisols in the mountaintop may be worse in the future, which should be considered when restoring vegetation.

Phosphorus fertilising potential of fly ash and effects on soil microbiota and crop

The production of fast pyrolysis bio oil (FPBO) constitutes one of the newest technologies for gaining a liquid biofuel from woody biomass. During this process biomass fly ashes (FAs), rich in minerals and salts, are produced. However, FAs are often disposed in landfills and their fertilising potential has been underestimated. A greenhouse trial was set up to test the impact of FA on soil physico-chemical and microbiological properties with a special focus on phosphorus, one of the main limiting nutrients in terrestrial ecosystems. FA were added into an acidic grassland soil at a rate of 2% with wheat (Triticum aestivum subsp. spelta) used as test plant. Soil and plants were collected after an incubation period of 60 and 100 days. Ash application increased soil pH and electrical conductivity, and improved soil nutritional status by increasing soil total, inorganic, and plant available phosphorus over time. Accordingly, higher plant yields were observed in ash-treated soils. The effect of FA on microbial biomass, assessed as double stranded DNA content, was time dependent and increased significantly with plant presence. Acid phosphomonoesterase activity significantly decreased following ash addition. However, neither alkaline phosphomonoesterase (ALP) activity nor the abundance and composition of the ALP gene (phoD) harboured by bacteria were affected by FA application. On the whole, FA from FPBO production seems to improve soil nutrient status and plant growth without inheriting detrimental effects on soil microbial communities in the mid-term.

Climate and Spring Phenology Effects on Autumn Phenology in the Greater Khingan Mountains, Northeastern China

Vegetation phenology plays a key role in terrestrial ecosystem nutrient and carbon cycles and is sensitive to global climate change. Compared with spring phenology, which has been well studied, autumn phenology is still poorly understood. In this study, we estimated the date of the end of the growing season (EOS) across the Greater Khingan Mountains, China, from 1982 to 2015 based on the Global Inventory Modeling and Mapping Studies (GIMMS) normalized difference vegetation index third-generation (NDVI3g) dataset. The temporal correlations between EOS and climatic factors (e.g., preseason temperature, preseason precipitation), as well as the correlation between autumn and spring phenology, were investigated using partial correlation analysis. Results showed that more than 94% of the pixels in the Greater Khingan Mountains exhibited a delayed EOS trend, with an average rate of 0.23 days/y. Increased preseason temperature resulted in earlier EOS in most of our study area, except for the semi-arid grassland region in the south, where preseason warming generally delayed EOS. Similarly, EOS in most of the mountain deciduous coniferous forest, forest grassland, and mountain grassland forest regions was earlier associated with increased preseason precipitation, but for the semi-arid grassland region, increased precipitation during the preseason mainly led to delayed EOS. However, the effect of preseason precipitation on EOS in most of the Greater Khingan Mountains was stronger than that of preseason temperature. In addition to the climatic effects on EOS, we also found an influence of spring phenology on EOS. An earlier SOS led to a delayed EOS in most of the study area, while in the southern of mountain deciduous coniferous forest region and northern of semi-arid grassland region, an earlier SOS was often followed by an earlier EOS. These findings suggest that both climatic factors and spring phenology should be incorporated into autumn phenology models in order to improve prediction accuracy under present and future climate change scenarios.

ПРОСТРАНСТВЕННОЕ ВАРЬИРОВАНИЕ СОДЕРЖАНИЯ ВТОРИЧНЫХ МЕТАБОЛИТОВ, УГЛЕРОДА И АЗОТА В ПОДСТИЛКАХ СЕВЕРОТАЕЖНЫХ ЕЛЬНИКОВ*, "Лесоведение"

RUSSIAN JOURNAL OF FOREST SCIENCE, 2018, No. 1, P. 37-47, DOI: 10.7868/S0024114818010035 SPATIAL VARIABILITY OF SCEONDARY METABOLITES, CARBON AND NITROGEN IN LITTERS OF SPRUCE FORESTS IN NORTHERN TAIGA N. A. Artemkina 1 , N. V. Lukina 2 , M. A. Orlova 2 1 Institute of Industrial Ecology Problems of the North, Kola Science Centre, Russian Academy of Sciences Fersmana st. 14a, Apatity, Murmansk Oblast, 184200, Russia E-mail: artemkina@inep.ksc.ru 2 Center for Forest Ecology and Productivity of the Russian Academy of Sciences Profsoyuznaya st. 84/32 bldg. 14, Moscow, 117997, Russia Received 30 September 2016 Plant residues and litter composition play important role in the organic matter decomposition and soil formation. We studied spatial variations of secondary metabolites (lignin, high- and low-molecular phenols, tannins), total carbon and nitrogen in litters across the profiles and laterally within forest ecosystems and between them in geochemically conjugated landscapes. The study took place in spruce forests of northern taiga (Murmansk Oblast). Clear decreasing trends of high- and low-molecular weight phenols and tannins were found between L and H layers across elementary biogeoareas (EBGA) at various levels in landscape. The maximal concentrations of phenolic compounds were found in litters of autonomous and transit spruce EBGAs. Across accumulative sites high-molecular phenols concentrated in litters of spruce EBGAs, tannins concentrated in ledum EBFAs, and low-molecular phenolic compounds in sphagnum EBGAs. The latter had the lowest concentration of lignin and distinct lignin/N ratio. Significant differences in concentrations of secondary metabolites, total carbon and nitrogen were found in litters of forests growing at different levels of landscape. Forest litters of transit and accumulative levels had significantly higher concentrations of tannins and phenols than autonomous. 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Nutrient limitation of soil microbial processes in tropical forests

AbstractSoil fungi and bacteria are the key players in the transformation and processing of carbon and nutrients in terrestrial ecosystems, yet controls on their abundance and activity are not well understood. Based on stoichiometric principles, soil microbial processes are expected to be limited by mineral nutrients, which are particularly scarce in often highly weathered tropical forest soils. Such limitation is directly relevant for the fate of soil carbon and global element cycles, but its extent and nature have never been assessed systematically across the tropical biome. Here, we address the relative importance of nitrogen, phosphorus, and other nutrients in limiting soil microbial biomass and process rates in tropical forests. We conducted an in‐depth literature review and a meta‐analysis of the available nutrient addition experiments in tropical forests worldwide. Our synthesis showed predominant and general phosphorus limitation of a variety of microbial processes across tropical forests, and additional nitrogen limitation in tropical montane forests. The apparent widespread microbial phosphorus limitation needs to be accounted for in the understanding and prediction of biogeochemical cycles in tropical forests and their future functioning. Other mineral nutrients or carbon may modify the importance of phosphorus, but more experimental studies are urgently needed.

Phosphorus uptake and availability and short-term seedling growth in three Ontario soils amended with ash and biochar

Phosphorus (P) can be a limiting nutrient in terrestrial ecosystems and adding biochar or wood ash can increase plant-available P. We added wood ash and biochar to microcosms containing three acidic Ontario soils planted with red pine or sugar maple seedlings and observed seedling growth responses, as well as amendment-induced changes in soil P pools, microbial P, and enzyme activity. Neither ash nor biochar consistently increased seedling growth; instead sugar maple and red pine seedlings often had opposing responses to the same amendment–soil combination. Overall, these results indicate that it is important to carefully consider both the chemical and physical characteristics of the soil and the ash or biochar, as well as the nutrient requirements of the target tree species, to effectively use these amendments to reduce P limitation.

Geospatial Distribution of Soil Organic Carbon in the Bhima River Watershed Region of Solapur District, Maharashtra, India

Objectives: To determine the Soil Organic Carbon Density (SOCD) and to observe distribution of Soil Organic Carbon (SOC) in the Bhima River sub-watershed of Solapur. Methods/Analysis: The spatial distribution of Soil Organic Carbon and it density was shown by Inverse Distance Weighted (IDW) interpolation technique using ArcGIS and the subsequent map generated in the process account for the fertility across the watershed and its longitudinal section. Total 36 sampling points were used for soil sampling and are processed for the output map. The land cover type of barren land and agriculture field were selected for soil sampling. Walkley and Black method was used for analysis of soil organic carbon and organic matter was calculated as a derived parameter. Findings: It is found that average soil bulk density was 1.15 gm cm-3 and the difference in SOCD from the minimum to maximum was 2.85 kg/m2 with average of 1.87 kg/m2. The lower values carbon was at high elevation and higher values were recorded nearer to mouth of river. There seems to be insignificant correlation between bulk density and SOC. Applications/Improvements: The SOC comparison between different land cover, clearly shows that agricultural terrains were richer as compared to barren lands. The outcome of the research could be of importance for the policy makers and management of agriculture, as SOC is the major nutrient in terrestrial ecosystems.

87Sr/86Sr, Ca/Sr, and Ge/Si ratios as tracers of solute sources and biogeochemical cycling at a temperate forested shale catchment, central Pennsylvania, USA

Plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems. Hydrologic and other losses are offset by inputs from atmospheric deposition and weathering. This multi-tracer study investigates these effects using 87Sr/86Sr, Ca/Sr, and Ge/Si ratios from solid (soil profiles and bedrock), biological (leaves and sap waters), and water (pore-, ground-, stream-waters) samples to study Ca, Sr, Ge, and Si sources and cycling in a forest catchment underlying gray shale in the temperate climate of central Pennsylvania. Leaves and sap waters were found to have similar Ge/Si ratios <1μmol/mol which is consistent with biological fractionation occurring at the root–soil water interface. Ge/Si ratios in soil porewaters were higher near the surface and increased over the growing season suggesting plant preferential uptake of Si over Ge. Ca/Sr ratios in oak leaves (1360±40mol/mol) were significantly higher than those in maple leaves (650±20mol/mol). Ca/Sr ratios were also generally higher in leaves than in sap waters which are consistent with preferential Sr adsorption or Sr uptake during transpiration. 87Sr/86Sr ratios in both leaves and sap waters were similar for a given site implying that trees access similar pools of Sr and Ca, although there are site-to-site differences. Additionally, 87Sr/86Sr ratios in plant tissues were most similar to shallow pore waters (0–50cm) suggesting that mineral nutrients are obtained by trees from the near sub-surface. 87Sr/86Sr ratios in soil solutions from ridgetop and swale sites are best explained by mixing Sr derived from shale and atmospheric deposition. Valley bottom soil reservoirs and stream and groundwater samples include Sr and Ca derived from dissolution of two isotopically distinct generations of carbonates. A preliminary estimate of the Sr and Ca stream fluxes and isotopic mass balances imply propagation of a carbonate weathering front of ca. 300m Myr−1, a value larger than previously reported for regolith weathering advance rates calculated based on cosmogenic nuclides and U-series isotopes. The data for Ca, Sr, Si, and Ge in soil, soil solutions, and stream waters reflect the interaction of slower weathering processes with rapid, biologically driven cycling between soils and biomass.

Decomposition of beech (Fagus sylvatica) and pine (Pinus nigra) litter along an Alpine elevation gradient: Decay and nutrient release

Litter decomposition is an important process for cycling of nutrients in terrestrial ecosystems. The objective of this study was to evaluate direct and indirect effects of climate on litter decomposition along an altitudinal gradient in a temperate Alpine region. Foliar litter of European beech (Fagus sylvatica) and Black pine (Pinus nigra) was incubated in litterbags during two years in the Hochschwab massif of the Northern Limestone Alps of Austria. Eight incubation sites were selected following an altitudinal/climatic transect from 1900 to 900masl. The average remaining mass after two years of decomposition amounted to 54% (beech) and 50% (pine). Net release of N, P, Na, Al, Fe and Mn was higher in pine than in beech litter due to high immobilization (retention) rates of beech litter. However, pine litter retained more Ca than beech litter. Altitude retarded decay (mass loss and associated C release) in beech litter during the first year only but had a longer lasting effect on decaying pine litter. Altitude comprises a suite of highly auto-correlated characteristics (climate, vegetation, litter, soil chemistry, soil microbiology, snow cover) that influence litter decomposition. Hence, decay and nutrient release of incubated litter is difficult to predict by altitude, except during the early stage of decomposition, which seemed to be controlled by climate. Reciprocal litter transplant along the elevation gradient yielded even relatively higher decay of pine litter on beech forest sites after a two-year adaptation period of the microbial community.

Ecosystem Succession and Nutrient Retention: Vitousek and Reiners' Hypothesis

What mechanisms mediate the flow of energy and cycling of nutrients during ecological succession? Are they linked in time and across space? In 1969, Eugene P. Odum proposed a series of hypotheses that caused the ecological community to think critically about patterns and processes (sensu Watt 1947) during ecological succession and how they might be linked in a causal manner. Odum's perspective on the process of succession, or “the strategy of ecosystem development” as he termed it, was undoubtedly shaped by the intellectual influence of his mentor, Victor Shelford, as well as Shelford's contemporaries such as Fredrick Clements. They shared the idea that ecological communities possessed emergent properties, wherein the whole is greater than the sum of its parts (sensu Phillips 1934). Inasmuch, the 24 hypotheses Odum (1969) articulated were an amalgam of holism and reductionism that created his rationale for the way energy flows and nutrients cycle within ecosystems. It is within this context that Peter Vitousek and Bill Reiners derived ideas that are fundamental to our understanding of energy flow and nutrient cycling in terrestrial ecosystems, ideas that have profoundly shaped my thinking as an ecologist. In his classic paper, Odum (1969) eloquently drew parallels between successional patterns of biomass accumulation in laboratory microcosms and forests, and, moreover, explained how biomass accumulation ceased late in succession because respiration consumed the products of photosynthesis; patterns and processes that he proposed were shared among all ecosystems. He argued further that the “conservation” of growth-limiting plant nutrients should be low in simple, species-poor, earlysuccessional ecosystems, whereas nutrient conservation should be greatest in late-successional ecosystems due to the complexity of biotic interactions among a diverse set of organisms. In retrospect, Odum's rationale regarding the emergence of nutrient conservation from the complexity of late successional ecosystems was probably shaped by a holistic perspective likely derived from his intellectual upbringing, whereas his ideas regarding biomass accumulation were based on physiological observations (e.g., microcosms) and insightful extrapolation (e.g., forests). Odum's attempt to derive pattern from process, or vice versa, fueled the critical thinking of many ecologists, and over the following decades, set in motion events that have generated great insight into ecosystem-level processes. I believe one of the most important was the Nutrient Retention Hypothesis proposed by Peter Vitousek and Bill Reiners. Vitousek and Reiners (1975) revealed a critical disconnect in Odum's rationale for patterns of biomass accumulation and nutrient conservation during secondary succession; Odum failed to realize they were linked, wherein one drove the other. Vitousek and Reiners (1975) argued that, if biomass accumulation in an ecosystem ceased late in secondary succession, then, in contrast to Odum's prediction, nutrient conservation should be low, because the net incorporation of growth-limiting nutrients into living plant biomass also must cease (Fig. 1). This led to the prediction that nutrient retention, or “conservation” as Odum had termed it, should be greatest early in succession when ecosystems are accumulating biomass (i.e., net ecosystem productivity is greater than zero; Fig. 1). Whereas nutrient retention should be low in late-successional ecosystems, because the pace of biomass accumulation has ceased, (i.e., net ecosystem productivity = 0), thereby creating a low demand for growth-limiting plant nutrients; the input of nutrients should therefore be equivalent to their loss (Fig. 1). This simple, elegant, and mechanistic logic simultaneously united the flow of energy and cycling of nutrients within ecosystems, albeit Lindemann (1942) had made remarkable headway on this matter several decades earlier. A conceptual diagram depicting Vitousek and Reiners' (1975) nutrient retention hypothesis, which links the flow of energy and the cycling of nutrients in terrestrial ecosystems. Panel a illustrates the rate at which biomass accumulates ( i.e., net ecosystem productivity) during primary and secondary succession, which is the balance between rates of net primary productivity and microbial decay. Early in succession, biomass accumulates because net primary productivity exceeds the rate of decay ( i.e., net ecosystem productivity > 0). In contrast, rates of net primary productivity and decomposition are equivalent late in succession; here, biomass ceases to accumulate and net ecosystem productivity approaches 0. Panel b illustrates how the loss of growth-limiting, non-limiting, and nonessential nutrients correspond to patterns of ecosystem biomass accumulation in Panel a. An assumption is that rates of nutrient input remain constant over successional time, as illustrated in Panel b (see nutrient input rate). When net ecosystem productivity is positive during succession (i.e., net primary productivity > microbial decay), the loss of growth-limiting plant nutrients approaches zero, because growth-limiting nutrients are needed to build accumulating biomass. In contrast, the loss of non-limiting nutrients is less affected by patterns of net ecosystem productivity, because there is a lower biological need for them; nonessential nutrients are not affected by ecosystem productivity. An important prediction encapsulated in the nutrient retention hypothesis is that the loss of nutrients from terrestrial ecosystem is greatest following disturbance. In this situation, rates of decay dramatically exceed net primary productivity; therefore net ecosystem productivity is < 0. This causes the mineralization of nutrients from decaying organic matter to exceed biological demand, and nutrient loss attains a maximum. Human activity has intentionally and unintentionally manipulated net ecosystem productivity on a global basis, and has therefore also altered the rates at which nutrients are retained and lost from terrestrial ecosystems. What set Vitousek and Reiners' work apart from others was devising a true ecosystem-level experiment that provided a statistical test of their hypothesis. If their Nutrient Retention Hypothesis was correct, then the export of growth-limiting plant nutrients should be greatest in late-successional ecosystems and dramatically lower in early-successional ecosystems in which biomass was accumulating. But, how does one test this at the scale of an entire ecosystem? Vitousek and Reiners cleverly used a paired-watershed approach in which the size, topography, and geologic characteristics of two adjacent watersheds were as identical as possible and only differed from one another in the successional stage of the dominant vegetation; in this case, high elevation spruce forest in the northeastern United States. Using these “natural” experiments, they identified paired watersheds that were sealed by underlying bedrock and drained by a single stream, making stream flow the only means for nutrients to exit these watersheds. Monitoring the concentration of growth-limiting (NO3−), essential (Ca), and nonessential (Na) plant nutrients in stream flow over time enabled them to test their hypothesis, and the competing hypothesis proposed by Odum (1969). Their work revealed that the concentration of NO3−, a growth-limiting nutrient, in stream flow exiting late-successional sprucedominated watersheds was significantly greater than that exiting watersheds blanketed by earlysuccessional forest. Moreover, they observed no difference in the concentration of a nonessential plant nutrient ( i.e., Na) in the stream water draining from early- and late-successional watersheds, providing evidence that the rate of biomass accumulation caused patterns of nutrient retention and loss. With a mechanistic rationale and an ecosystem-scale “natural” experiment, Vitousek and Reiners (1975) demonstrated that the flow of energy and cycling of nutrients were indeed linked in time. It is the juxtaposition of these competing hypotheses that has impressed on me how one's intellectual upbringing can shape their ecological “world view” and the importance of questioning the underlying assumptions of one's own thinking. Further, it exemplified how ecological knowledge is iteratively built from a foundation set in place by others, as well as the importance of devising a complete connection between an ecologically important hypothesis and a clever experiment that provides the appropriate statistical test. Vitousek and Reiners' work exemplifies these essential scientific characteristics, strengthening ecology as a science and increasing our understanding of how ecosystems function. For me, it served as a scientific model to emulate and helped me understand how and why biogeochemical cycles are linked in time and space. Most importantly, the ecological dynamics depicted in Vitousek and Reiners' Nutrient Retention Hypothesis lie at the heart of understanding a myriad of human influences on biogeochemical cycles, including coastal eutrophication, nitrogen saturation of terrestrial ecosystems (Aber et al. 1989, 1998), as well as the accumulation of anthropogenic carbon dioxide in the Earth's atmosphere. As we further embark through a time period in which humans have dramatically altered the flow of energy and cycling of nutrients at a global scale, it is our foundational knowledge of ecosystem processes, like that generated by Vitousek and Reiners (1975), which enables us to understand, anticipate, and hopefully limit our collective impact on the planet.