Decreased water flowing from a forest amended with calcium silicate

Responses of stomatal density and carbon isotope composition of sugar maple and yellow birch foliage to N, P and CaSiO3 fertilization

Stomatal density, stomatal length and carbon isotope composition can all provide insights into environmental controls on photosynthesis and transpiration. Stomatal measurements can be time-consuming; it is therefore wise to consider efficient sampling schemes. Knowing the variance partitioning at different measurement levels (i.e., among stands, plots, trees, leaves and within leaves) can aid in making informed decisions around where to focus sampling effort. In this study, we explored the effects of nitrogen (N), phosphorus (P) and calcium silicate (CaSiO3) addition on stomatal density, length and carbon isotope composition (δ13C) of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton). We observed a positive but small (8%) increase in stomatal density with P addition and an increase in δ13C with N and CaSiO3 addition in sugar maple, but we did not observe effects of nutrient addition on these characteristics in yellow birch. Variability was highest within leaves and among trees for stomatal density and highest among stomata for stomatal length. To reduce variability and increase chances of detecting treatment differences in stomatal density and length, future protocols should consider pretreatment and repeated measurements of trees over time or measure more trees per plot, increase the number of leaf impressions or standardize their locations, measure more stomata per image and ensure consistent light availability.

Tree variability limits the detection of nutrient treatment effects on sap flux density in a northern hardwood forest

The influence of nutrient availability on transpiration is not well understood, in spite of the importance of transpiration to forest water budgets. Soil nutrients have the potential to affect tree water use through indirect effects on leaf area or stomatal conductance. For example, following addition of calcium silicate to a watershed at Hubbard Brook, in New Hampshire, streamflow was reduced for 3 years, which was attributed to a 25% increase in evapotranspiration associated with increased foliar production. The first objective of this study was to quantify the effect of nutrient availability on sap flux density in a nitrogen, phosphorus, and calcium addition experiment in New Hampshire in which tree diameter growth, foliar chemistry, and soil nutrient availability had responded to treatments. We measured sap flux density in American beech (Fagus grandifolia, Ehr.), red maple (Acer rubrum L.), sugar maple (Acer saccharum Marsh.), white birch (Betula papyrifera Marsh.), or yellow birch (Betula alleghaniensis Britton.) trees, over five years of experiments in five stands distributed across three sites. In 2018, 3 years after a calcium silicate addition, sap flux density averaged 36% higher in trees in the treatment than the control plot, but this effect was not very significant (p = 0.07). Our second objective was to determine whether this failure to detect effects with greater statistical confidence was due to small effect sizes or high variability among trees. We found that tree-to-tree variability was high, with coefficients of variation averaging 39% within treatment plots. Depending on the species and year of the study, the minimum difference in sap flux density detectable with our observed variability ranged from 46% to 352%, for a simple ANOVA. We analyzed other studies reported in the literature that compared tree water use among species or treatments and found detectable differences ranging from 16% to 78%. Future sap flux density studies could benefit from power analyses to guide sampling intensity. Including pretreatment data, in the case of manipulative studies, would also increase statistical power.

Nano- to Global-Scale Uncertainties in Terrestrial Enhanced Weathering

Enhanced weathering (EW) is one of the most promising negative emissions technologies urgently needed to limit global warming to at least below 2 °C, a goal recently reaffirmed at the UN Global Climate Change conference (i.e., COP26). EW relies on the accelerated dissolution of crushed silicate rocks applied to soils and is considered a sustainable solution requiring limited technology. While EW has a high theoretical potential of sequestering CO₂, research is still needed to provide accurate estimates of carbon (C) sequestration when applying different silicate materials across distinct climates and major soil types in combination with a variety of plants. Here we elaborate on fundamental advances that must be addressed before EW can be extensively adopted. These include identifying the most suitable environmental conditions, improving estimates of field dissolution rates and efficacy of CO₂ removal, and identifying alternative sources of silicate materials to meet future EW demands. We conclude with considerations on the necessity of integrated modeling-experimental approaches to better coordinate future field experiments and measurements of CO₂ removal, as well as on the importance of seamlessly coordinating EW with cropland and forest management.

A review of transformative strategies for climate mitigation by grasslands

Grasslands can significantly contribute to climate mitigation. However, recent trends indicate that human activities have switched their net cooling effect to a warming effect due to management intensification and land conversion. This indicates an urgent need for strategies directed to mitigate climate warming while enhancing productivity and efficiency in the use of land and natural (nutrients, water) resources. Here, we examine the potential of four innovative strategies to slow climate change including: 1) Adaptive multi-paddock grazing that consists of mimicking how ancestral herds roamed the Earth; 2) Agrivoltaics that consists of simultaneously producing food and energy from solar panels on the same land area; 3) Agroforestry with a reverse phenology tree species, Faidherbia (Acacia) albida, that has the unique trait of being photosynthetically active when intercropped herbaceous plants are dormant; and, 4) Enhanced Weathering, a negative emission technology that removes atmospheric CO₂ from the atmosphere. Further, we speculate about potential unknown consequences of these different management strategies and identify gaps in knowledge. We find that all these strategies could promote at least some of the following benefits of grasslands: CO₂ sequestration, non-CO₂ GHG mitigation, productivity, resilience to climate change, and an efficient use of natural resources. However, there are obstacles to be overcome. Mechanistic assessment of the ecological, environmental, and socio-economic consequences of adopting these strategies at large scale are urgently needed to fully assess the potential of grasslands to provide food, energy and environmental security.

Intensified vegetation water use under acid deposition

Despite the important role vegetation plays in the global water cycle, the exact controls of vegetation water use, especially the role of soil biogeochemistry, remain elusive. In this study, we reveal a new mechanism of soil biogeochemical control of large-scale vegetation water use. Nitrate and sulfate deposition from fossil fuel burning have caused substantial soil acidification, leading to the leaching of soil base cations. Of these, calcium has a unique role in plant cells by regulating stomatal aperture, thus affecting vegetation water use. We hypothesized that the leaching of the soil calcium supply, induced by acid deposition, would increase large-scale vegetation water use. We present evidence from a long-term whole watershed acidification experiment demonstrating that the alteration of the soil calcium supply by acid deposition can significantly intensify vegetation water use (~10% increase in evapotranspiration) and deplete available soil water. These results are critical to understanding future water availability, biogeochemical cycles, and surface energy flux and to help reduce uncertainties in terrestrial biosphere models.

Give and Take: A Watershed Acid Rain Mitigation Experiment Increases Baseflow Nitrogen Retention but Increases Stormflow Nitrogen Export

In many temperate forested watersheds, hydrologic nitrogen export has declined substantially in recent decades, and many of these watersheds show enduring effects from historic acid deposition. A watershed acid remediation experiment in New Hampshire reversed many of these legacy effects of acid deposition and also increased watershed nitrogen export, suggesting that these two phenomena may be coupled. Here we examine stream nitrate dynamics in this watershed acid remediation experiment for indicators of nitrogen saturation in the terrestrial and aquatic ecosystems. Post-treatment, the (positive) slope of the relationship between nitrate concentration and discharge increased by a median of 82% ( p = 0.004). This resulted in greater flushing of nitrate during storm events, a key indicator of early stage nitrogen saturation. Hysteretic behavior of the concentration-discharge relationship indicated that the mobilization of soil nitrate pools was responsible for this increased flushing. In contrast to this evidence for nitrogen saturation in the terrestrial ecosystem, we found that nitrogen uptake increased, post-treatment, in the aquatic ecosystem, substantially attenuating growing-season nitrate flux by up to 71.1% ( p = 0.025). These results suggest that, as forests slowly recover from acid precipitation, terrestrial, and aquatic ecosystem nitrogen balance may be substantially altered.

Plant acclimation to long-term high nitrogen deposition in an N-rich tropical forest

Anthropogenic nitrogen (N) deposition has accelerated terrestrial N cycling at regional and global scales, causing nutrient imbalance in many natural and seminatural ecosystems. How added N affects ecosystems where N is already abundant, and how plants acclimate to chronic N deposition in such circumstances, remains poorly understood. Here, we conducted an experiment employing a decade of N additions to examine ecosystem responses and plant acclimation to added N in an N-rich tropical forest. We found that N additions accelerated soil acidification and reduced biologically available cations (especially Ca and Mg) in soils, but plants maintained foliar nutrient supply at least in part by increasing transpiration while decreasing soil water leaching below the rooting zone. We suggest a hypothesis that cation-deficient plants can adjust to elevated N deposition by increasing transpiration and thereby maintaining nutrient balance. This result suggests that long-term elevated N deposition can alter hydrological cycling in N-rich forest ecosystems.

Simulating carbon capture by enhanced weathering with croplands: an overview of key processes highlighting areas of future model development

Enhanced weathering (EW) aims to amplify a natural sink for CO2 by incorporating powdered silicate rock with high reactive surface area into agricultural soils. The goal is to achieve rapid dissolution of minerals and release of alkalinity with accompanying dissolution of CO2 into soils and drainage waters. EW could counteract phosphorus limitation and greenhouse gas (GHG) emissions in tropical soils, and soil acidification, a common agricultural problem studied with numerical process models over several decades. Here, we review the processes leading to soil acidification in croplands and how the soil weathering CO2 sink is represented in models. Mathematical models capturing the dominant processes and human interventions governing cropland soil chemistry and GHG emissions neglect weathering, while most weathering models neglect agricultural processes. We discuss current approaches to modelling EW and highlight several classes of model having the potential to simulate EW in croplands. Finally, we argue for further integration of process knowledge in mathematical models to capture feedbacks affecting both longer-term CO2 consumption and crop growth and yields.

Mechanisms of nitrogen deposition effects on temperate forest lichens and trees

We review the mechanisms of deleterious nitrogen (N) deposition impacts on temperate forests, with a particular focus on trees and lichens. Elevated anthropogenic N deposition to forests has varied effects on individual organisms depending on characteristics both of the N inputs (form, timing, amount) and of the organisms (ecology, physiology) involved. Improved mechanistic knowledge of these effects can aid in developing robust predictions of how organisms respond to either increases or decreases in N deposition. Rising N levels affect forests in micro- and macroscopic ways from physiological responses at the cellular, tissue, and organism levels to influencing individual species and entire communities and ecosystems. A synthesis of these processes forms the basis for the overarching themes of this paper, which focuses on N effects at different levels of biological organization in temperate forests. For lichens, the mechanisms of direct effects of N are relatively well known at cellular, organismal, and community levels, though interactions of N with other stressors merit further research. For trees, effects of N deposition are better understood for N as an acidifying agent than as a nutrient; in both cases, the impacts can reflect direct effects on short time scales and indirect effects mediated through long-term soil and belowground changes. There are many gaps on fundamental N use and cycling in ecosystems, and we highlight the most critical gaps for understanding potential deleterious effects of N deposition. For lichens, these gaps include both how N affects specific metabolic pathways and how N is metabolized. For trees, these gaps include understanding the direct effects of N deposition onto forest canopies, the sensitivity of different tree species and mycorrhizal symbionts to N, the influence of soil properties, and the reversibility of N and acidification effects on plants and soils. Continued study of how these N response mechanisms interact with one another, and with other dimensions of global change, remains essential for predicting ongoing changes in lichen and tree populations across North American temperate forests.

Nitrogen enrichment regulates calcium sources in forests

Nitrogen (N) is a key nutrient that shapes cycles of other essential elements in forests, including calcium (Ca). When N availability exceeds ecosystem demands, excess N can stimulate Ca leaching and deplete Ca from soils. Over the long term, these processes may alter the proportion of available Ca that is derived from atmospheric deposition vs. bedrock weathering, which has fundamental consequences for ecosystem properties and nutrient supply. We evaluated how landscape variation in soil N, reflecting long-term legacies of biological N fixation, influenced plant and soil Ca availability and ecosystem Ca sources across 22 temperate forests in Oregon. We also examined interactions between soil N and bedrock Ca using soil N gradients on contrasting basaltic vs. sedimentary bedrock that differed 17-fold in underlying Ca content. We found that low-N forests on Ca-rich basaltic bedrock relied strongly on Ca from weathering, but that soil N enrichment depleted readily weatherable mineral Ca and shifted forest reliance toward atmospheric Ca. Forests on Ca-poor sedimentary bedrock relied more consistently on atmospheric Ca across all levels of soil N enrichment. The broad importance of atmospheric Ca was unexpected given active regional uplift and erosion that are thought to rejuvenate weathering supply of soil minerals. Despite different Ca sources to forests on basaltic vs. sedimentary bedrock, we observed consistent declines in plant and soil Ca availability with increasing N, regardless of the Ca content of underlying bedrock. Thus, traditional measures of Ca availability in foliage and soil exchangeable pools may poorly reflect long-term Ca sources that sustain soil fertility. We conclude that long-term soil N enrichment can deplete available Ca and cause forests to rely increasingly on Ca from atmospheric deposition, which may limit ecosystem Ca supply in an increasingly N-rich world.

Fast-growing Acer rubrum differs from slow-growing Quercus alba in leaf, xylem and hydraulic trait coordination responses to simulated acid rain

We investigated the effects of historic soil chemistry changes associated with acid rain, i.e., reduced soil pH and a shift from nitrogen (N)- to phosphorus (P)-limitation, on the coordination of leaf water demand and xylem hydraulic supply traits in two co-occurring temperate tree species differing in growth rate. Using a full-factorial design (N × P × pH), we measured leaf nutrient content, water relations, leaf-level and canopy-level gas exchange, total biomass and allocation, as well as stem xylem anatomy and hydraulic function for greenhouse-grown saplings of fast-growing Acer rubrum (L.) and slow-growing Quercus alba (L.). We used principle component analysis to characterize trait coordination. We found that N-limitation, but not P-limitation, had a significant impact on plant water relations and hydraulic coordination of both species. Fast-growing A. rubrum made hydraulic adjustments in response to N-limitation, but trait coordination was variable within treatments and did not fully compensate for changing allocation across N-availability. For slow-growing Q. alba, N-limitation engendered more strict coordination of leaf and xylem traits, resulting in similar leaf water content and hydraulic function across all treatments. Finally, low pH reduced the propensity of both species to adjust leaf water relations and xylem anatomical traits in response to nutrient manipulations. Our data suggest that a shift from N- to P-limitation has had a negative impact on the water relations and hydraulic function of A. rubrum to a greater extent than for Q. alba We suggest that current expansion of A. rubrum populations could be tempered by acidic N-deposition, which may restrict it to more mesic microsites. The disruption of hydraulic acclimation and coordination at low pH is emphasized as an interesting area of future study.

Acid rain mitigation experiment shifts a forested watershed from a net sink to a net source of nitrogen

Decades of acid rain have acidified forest soils and freshwaters throughout montane forests of the northeastern United States; the resulting loss of soil base cations is hypothesized to be responsible for limiting rates of forest growth throughout the region. In 1999, an experiment was conducted that reversed the long-term trend of soil base cation depletion and tested the hypothesis that calcium limits forest growth in acidified soils. Researchers added 1,189 kg Ca(2+) ha(-1) as the pelletized mineral wollastonite (CaSiO3) to a 12-ha forested watershed within the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Significant increases in the pH and acid-neutralizing capacity of soils and streamwater resulted, and the predicted increase in forest growth occurred. An unanticipated consequence of this acidification mitigation experiment began to emerge a decade later, with marked increases in dissolved inorganic nitrogen (DIN) exports in streamwater from the treated watershed. By 2013, 30-times greater DIN was exported from this base-treated watershed than from adjacent reference watersheds, and DIN exports resulting from this experiment match or exceed earlier reports of inorganic N losses after severe ice-storm damage within the study watershed. The discovery that CaSiO3 enrichment can convert a watershed from a sink to a source of N suggests that numerous potential mechanisms drive watershed N dynamics and provides new insights into the influence of acid deposition mitigation strategies for both carbon cycling and watershed N export.

The promise and peril of intensive-site-based ecological research: insights from the Hubbard Brook ecosystem study

Ecological research is increasingly concentrated at particular locations or sites. This trend reflects a variety of advantages of intensive, site-based research, but also raises important questions about the nature of such spatially delimited research: how well does site based research represent broader areas, and does it constrain scientific discovery? We provide an overview of these issues with a particular focus on one prominent intensive research site: the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. Among the key features of intensive sites are: long-term, archived data sets that provide a context for new discoveries and the elucidation of ecological mechanisms; the capacity to constrain inputs and parameters, and to validate models of complex ecological processes; and the intellectual cross-fertilization among disciplines in ecological and environmental sciences. The feasibility of scaling up ecological observations from intensive sites depends upon both the phenomenon of interest and the characteristics of the site. An evaluation of deviation metrics for the HBEF illustrates that, in some respects, including sensitivity and recovery of streams and trees from acid deposition, this site is representative of the Northern Forest region, of which HBEF is a part. However, the mountainous terrain and lack of significant agricultural legacy make the HBEF among the least disturbed sites in the Northern Forest region. Its relatively cool, wet climate contributes to high stream flow compared to other sites. These similarities and differences between the HBEF and the region can profoundly influence ecological patterns and processes and potentially limit the generality of observations at this and other intensive sites. Indeed, the difficulty of scaling up may be greatest for ecological phenomena that are sensitive to historical disturbance and that exhibit the greatest spatiotemporal variation, such as denitrification in soils and the dynamics of bird communities. Our research shows that end member sites for some processes often provide important insights into the behavior of inherently heterogeneous ecological processes. In the current era of rapid environmental and biological change, key ecological responses at intensive sites will reflect both specific local drivers and regional trends.

Long-term trends of changes in pine and oak foliar nitrogen metabolism in response to chronic nitrogen amendments at Harvard Forest, MA

We evaluated the long-term (1995-2008) trends in foliar and sapwood metabolism, soil solution chemistry and tree mortality rates in response to chronic nitrogen (N) additions to pine and hardwood stands at the Harvard Forest Long Term Ecological Research (LTER) site. Common stress-related metabolites like polyamines (PAs), free amino acids (AAs) and inorganic elements were analyzed for control, low N (LN, 50 kg NH4NO3 ha(-1) year(-1)) and high N (HN, 150 kg NH4NO3 ha(-1) year(-1)) treatments. In the pine stands, partitioning of excess N into foliar PAs and AAs increased with both N treatments until 2002. By 2005, several of these effects on N metabolites disappeared for HN, and by 2008 they were mostly observed for LN plot. A significant decline in foliar Ca and P was observed mostly with HN for a few years until 2005. However, sapwood data actually showed an increase in Ca, Mg and Mn and no change in PAs in the HN plot for 2008, while AAs data revealed trends that were generally similar to foliage for 2008. Concomitant with these changes, mortality data revealed a large number of dead trees in HN pine plots by 2002; the mortality rate started to decline by 2005. Oak trees in the hardwood plot did not exhibit any major changes in PAs, AAs, nutrients and mortality rate with LN treatment, indicating that oak trees were able to tolerate the yearly doses of 50 kg NH4NO3 ha(-1) year(-1). However, HN trees suffered from physiological and nutritional stress along with increased mortality in 2008. In this case also, foliar data were supported by the sapwood data. Overall, both low and high N applications resulted in greater physiological stress to the pine trees than the oaks. In general, the time course of changes in metabolic data are in agreement with the published reports on changes in soil chemistry and microbial community structure, rates of soil carbon sequestration and production of woody biomass for this chronic N study. This correspondence of selected metabolites with other measures of forest functions suggests that the metabolite analyses are useful for long-term monitoring of the health of forest trees.