Andrew J. Burton

Soil warming, carbon–nitrogen interactions, and forest carbon budgets

Soil warming has the potential to alter both soil and plant processes that affect carbon storage in forest ecosystems. We have quantified these effects in a large, long-term (7-y) soil-warming study in a deciduous forest in New England. Soil warming has resulted in carbon losses from the soil and stimulated carbon gains in the woody tissue of trees. The warming-enhanced decay of soil organic matter also released enough additional inorganic nitrogen into the soil solution to support the observed increases in plant carbon storage. Although soil warming has resulted in a cumulative net loss of carbon from a New England forest relative to a control area over the 7-y study, the annual net losses generally decreased over time as plant carbon storage increased. In the seventh year, warming-induced soil carbon losses were almost totally compensated for by plant carbon gains in response to warming. We attribute the plant gains primarily to warming-induced increases in nitrogen availability. This study underscores the importance of incorporating carbon–nitrogen interactions in atmosphere–ocean–land earth system models to accurately simulate land feedbacks to the climate system.

Spatial Variation in Nitrogen Availability in Three Successional Plant Communities

1 Spatial variability in soil nitrogen and moisture levels was determined using a geostatistical analysis in a newly abandoned field, a mid-late successional field and a second-growth forest in south-western Michigan. 2 A greater proportion of the total variation for all variates associated with nitrogen availability was spatially dependent in the mid-successional field; the newly abandoned field and forest had similar patterns of spatial dependence in these variates. 3 The distance (range) over which there was spatial dependence was greater in the mid-successional field than the other two communities, indicating a more coarsegrained pattern of spatial heterogeneity in soil nitrogen, particularly in the surface soils (0-5 cm), than in the other two sites. 4 Examination of patterns of spatial dependence using different lag intervals generally gave similar results; though a nested pattern of N03-N availability in the midsuccessional field was detected at a finer scale of analysis, indicating spatial variation at multiple scales. 5 The results suggest that patterns of spatial variation in soil nitrogen change over time in successional plant communities, perhaps reflecting changes in the species composition or size of individual plants in these communities.

COUPLING FINE ROOT DYNAMICS WITH ECOSYSTEM CARBON CYCLING IN BLACK SPRUCE FORESTS OF INTERIOR ALASKA

Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs, and mosses were measured relative to soil CO2 fluxes. Fine root production, measured over a two-year period using minirhizotrons, varied from 0.004 ± 0.001 mm·cm–2·d–1 over winter, to 0.051 ± 0.015 mm·cm–2·d–1 during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. M...

Quantifying global soil carbon losses in response to warming

The majority of the Earth’s terrestrial carbon is stored in the soil. If anthropogenic warming stimulates the loss of this carbon to the atmosphere, it could drive further planetary warming. Despite evidence that warming enhances carbon fluxes to and from the soil, the net global balance between these responses remains uncertain. Here we present a comprehensive analysis of warming-induced changes in soil carbon stocks by assembling data from 49 field experiments located across North America, Europe and Asia. We find that the effects of warming are contingent on the size of the initial soil carbon stock, with considerable losses occurring in high-latitude areas. By extrapolating this empirical relationship to the global scale, we provide estimates of soil carbon sensitivity to warming that may help to constrain Earth system model projections. Our empirical relationship suggests that global soil carbon stocks in the upper soil horizons will fall by 30 ± 30 petagrams of carbon to 203 ± 161 petagrams of carbon under one degree of warming, depending on the rate at which the effects of warming are realized. Under the conservative assumption that the response of soil carbon to warming occurs within a year, a business-as-usual climate scenario would drive the loss of 55 ± 50 petagrams of carbon from the upper soil horizons by 2050. This value is around 12–17 per cent of the expected anthropogenic emissions over this period. Despite the considerable uncertainty in our estimates, the direction of the global soil carbon response is consistent across all scenarios. This provides strong empirical support for the idea that rising temperatures will stimulate the net loss of soil carbon to the atmosphere, driving a positive land carbon–climate feedback that could accelerate climate change.

DROUGHT REDUCES ROOT RESPIRATION IN SUGAR MAPLE FORESTS

Soil moisture deficits can reduce root respiration, but the effects have yet to be quantified at the stand level or included in models of forest carbon budgets. We studied fine-root (≤1.0 mm diameter) respiration in four sugar maple forests for three growing seasons in order to assess the combined effects of temperature, N concentration, and soil moisture on respiration rates. Fine-root respiration at the four sites was exponentially related to soil temperature and linearly related to root N concentration and soil moisture availability. Most of the variability in respiration rates was explained by temperature. Differences in soil moisture availability explained temporal variation within sites in respiration rate at a given temperature, whereas differences among sites in respiration rates resulted from site-specific differences in fine-root N concentration. Periodic moisture deficits during 1995 and 1996 were sufficient to cause declines of up to 17% in total growing-season root respiration at affected sit...

SIMULATED ATMOSPHERIC NO3− DEPOSITION INCREASES SOIL ORGANIC MATTER BY SLOWING DECOMPOSITION

Presently, there is uncertainty regarding the degree to which anthropogenic N deposition will foster C storage in the N-limited forests of the Northern Hemisphere, ecosystems which are globally important sinks for anthropogenic CO2. We constructed organic matter and N budgets for replicate northern hardwood stands (n = 4) that have received ambient (0.7-1.2 g N x m(-2) x yr(-1) and experimental NO3- deposition (ambient plus 3 g NO3(-)-N x m(-2) x yr(-1)) for a decade; we also traced the flow of a 15NO3- pulse over a six-year period. Experimental NO3- deposition had no effect on organic matter or N stored in the standing forest overstory, but it did significantly increase the N concentration (+19%) and N content (+24%) of canopy leaves. In contrast, a decade of experimental NO3- deposition significantly increased amounts of organic matter (+12%) and N (+9%) in forest floor and mineral soil, despite no increase in detritus production. A greater forest floor (Oe/a) mass under experimental NO3- deposition resulted from slower decomposition, which is consistent with previously reported declines in lignolytic activity by microbial communities exposed to experimental NO3- deposition. Tracing 15NO3- revealed that N accumulated in soil organic matter by first flowing through soil microorganisms and plants, and that the shedding of 15N-labeled leaf litter enriched soil organic matter over a six-year duration. Our results demonstrate that atmospheric NO3- deposition exerts a direct and negative effect on microbial activity in this forest ecosystem, slowing the decomposition of aboveground litter and leading to the accumulation of forest floor and soil organic matter. To the best of our knowledge, this mechanism is not represented in the majority of simulation models predicting the influence of anthropogenic N deposition on ecosystem C storage in northern forests.

Soil respiration, root biomass, and root turnover following long‐term exposure of northern forests to elevated atmospheric CO2 and tropospheric O3

The Rhinelander free-air CO(2) enrichment (FACE) experiment is designed to understand ecosystem response to elevated atmospheric carbon dioxide (+CO(2)) and elevated tropospheric ozone (+O(3)). The objectives of this study were: to understand how soil respiration responded to the experimental treatments; to determine whether fine-root biomass was correlated to rates of soil respiration; and to measure rates of fine-root turnover in aspen (Populus tremuloides) forests and determine whether root turnover might be driving patterns in soil respiration. Soil respiration was measured, root biomass was determined, and estimates of root production, mortality and biomass turnover were made. Soil respiration was greatest in the +CO(2) and +CO(2) +O(3) treatments across all three plant communities. Soil respiration was correlated with increases in fine-root biomass. In the aspen community, annual fine-root production and mortality (g m(-2)) were positively affected by +O(3). After 10 yr of exposure, +CO(2) +O(3)-induced increases in belowground carbon allocation suggest that the positive effects of elevated CO(2) on belowground net primary productivity (NPP) may not be offset by negative effects of O(3). For the aspen community, fine-root biomass is actually stimulated by +O(3), and especially +CO(2) +O(3).

Forest productivity under elevated CO2 and O3: positive feedbacks to soil N cycling sustain decade-long net primary productivity enhancement by CO2

The accumulation of anthropogenic CO₂ in the Earths atmosphere, and hence the rate of climate warming, is sensitive to stimulation of plant growth by higher concentrations of atmospheric CO₂. Here, we synthesise data from a field experiment in which three developing northern forest communities have been exposed to factorial combinations of elevated CO₂ and O₃. Enhanced net primary productivity (NPP) (c. 26% increase) under elevated CO₂ was sustained by greater root exploration of soil for growth-limiting N, as well as more rapid rates of litter decomposition and microbial N release during decay. Despite initial declines in forest productivity under elevated O₃, compensatory growth of O₃ -tolerant individuals resulted in equivalent NPP under ambient and elevated O₃. After a decade, NPP has remained enhanced under elevated CO₂ and has recovered under elevated O₃ by mechanisms that remain un-calibrated or not considered in coupled climate-biogeochemical models simulating interactions between the global C cycle and climate warming.

Anthropogenic N deposition and the fate of 15NO-3 in a northern hardwood ecosystem

Human activity has substantially increased atmospheric NO3− deposition in many regions of the Earth, which could lead to the N saturation of terrestrial ecosystems. Sugar maple (Acer saccharum Marsh.) dominated northern hardwood forests in the Upper Great Lakes region may be particularly sensitive to chronic NO3− deposition, because relatively moderate experimental increases (three times ambient) have resulted in substantial N leaching over a relatively short duration (5–7 years). Although microbial immobilization is an initial sink (i.e., within 1–2 days) for anthropogenic NO3− in this ecosystem, we have an incomplete understanding of the processes controlling the longer-term (i.e., after 1 year) retention and flow of anthropogenic N. Our objectives were to determine: (i) whether chronic NO3− additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of 15NO3− in plots receiving chronic NO3− addition. We addressed these objectives using a field experiment in which three northern hardwood plots receive ambient atmospheric N deposition (ca. 0.9 g N m−2 year−1) and three plots which receive ambient plus experimental N deposition (3.0 g NO3−-N m−2 year−1). Chronic NO3− deposition significantly increased the N concentration and content (g N/m2) of canopy leaves, which contained 72% more N than the control treatment. However, chronic NO3− deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of 15N recovered after 1 year occurred in overstory leaves and branches (10%). In contrast, we recovered virtually none of the isotope in soil organic matter (SOM), indicating that SOM was not a sink for anthropogenic NO3− over a 1 year duration. Our results indicate that anthropogenic NO3− initially assimilated by the microbial community is released into soil solution where it is subsequently taken up by overstory trees and allocated to the canopy. Anthropogenic N appears to be incorporated into SOM only after it is returned to the forest floor and soil via leaf litter fall. Short- and long-term isotope tracing studies provided very different results and illustrate the need to understand the physiological processes controlling the flow of anthropogenic N in terrestrial ecosystems and the specific time steps over which they operate.

Microbial Cycling of C and N in Northern Hardwood Forests Receiving Chronic Atmospheric NO 3 − Deposition

Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic atmospheric NO3− deposition. Experimental NO3− deposition (3 g NO3− N m−2 y−1) has significantly reduced soil respiration and increased the export of DOC/DON and NO3− across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for these ecosystem-level responses to NO3− deposition. To test this alternative, we measured microbial biomass, respiration, and N transformations in the mineral soil of four northern hardwood stands that have received 9 years of experimental NO3− deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed by NO3− deposition. We also observed no effect of NO3− deposition on annual rates of net N mineralization. However, NO3− deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO3− assimilation, the subsequent turnover of NH4+, and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient to produce the 10-fold observed increase in NO3− export, suggesting that much of the exported NO3− resulted directly from the NO3− deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed to NO3−-induced physiological changes in mineral soil microbial activity. Given the lack of response we have observed in mineral soil, our results point to the potential importance of microbial communities in forest floor, including both saprotrophs and mycorrhizae, in mediating ecosystem-level responses to chronic NO3− deposition in Lake States northern hardwood forests.