Richard D. Boone

Roots exert a strong influence on the temperature sensitivity of soil respiration

The temperature sensitivity of soil respiration will largely determine the effects of a warmer world on net carbon flux from soils to the atmosphere. CO2 flux from soils to the atmosphere is estimated to be 50–70 petagrams of carbon per year and makes up 20–38% of annual inputs of carbon (in the form of CO2) to the atmosphere from terrestrial and marine sources,. Here we show that, for a mixed temperate forest, respiration by roots plus oxidation of rhizosphere carbon, which together produce a large portion of total effluxed soil CO2, is more temperature-sensitive than the respiration of bulk soil. We determine that the Q10 value (the coefficient for the exponential relationship between soil respiration and temperature, multiplied by ten) is 4.6 for autotrophic root respiration plus rhizosphere decomposition, 2.5 for respiration by soil lacking roots and 3.5 for respiration by bulk soil. If plants in a higher-CO2 atmosphere increase their allocation of photosynthate to roots these findings suggest that soil respiration should be more sensitive to elevated temperatures, thus limiting carbon sequestration by soils.

LONG-TERM IMPACTS OF AGRICULTURE ON SOIL CARBON AND NITROGEN IN NEW ENGLAND FORESTS

Abandonment and reforestation of agricultural lands has been a major influence on the landscape of eastern North America. Cultivation and soil amendments can dramatically alter soil nutrient pools and cycling, yet few studies have examined the long-term (>50 yr) influence of pasturing and cultivation on soil processes in the forests that develop after abandonment. Twelve forested sites at Harvard Forest in central New England were compared 90–120 yr after abandonment from agricultural use. We measured soil carbon (C), nitrogen (N), and phosphorus (P); light fraction C, N, and δ15N; microbial chloroform-N; net N mineralization and nitrification; nitrification potential; and culturable nitrifiers on sites with differing land-use history and vegetation. The sites had similar soil series and topography but were arrayed along a soil disturbance gradient from permanent woodlots (selective logging but no mineral soil disturbance) to formerly pastured sites (clearcut and grazed but no deep [>10 cm] soil disturbance) to formerly cultivated sites (cleared-with-plow horizon 15–20 cm thick). Mineral soil C (0–15 cm soil depth) was very similar among all sites, but the forest floor C was lower in the cultivated sites than in the woodlots of both stand types. Mineral soil in cultivated sites contained 800 kg N/ha and 300 kg P/ha more than woodlots, a relative increase of 39% for N and 52% for P. The cultivated soils had lower C:N and C:P ratios, largely driven by higher soil N and P. The light fraction appeared to be more sensitive to prior land use than the bulk soil organic matter. The C content and C:N ratio of light fraction were lower in cultivated soils, which suggests that input and/or turnover of organic matter pools of relatively recent origin remain altered for a century after abandonment. Similar δ15N for the light and heavy fraction organic matter pools in cultivated soils suggests that cultivation accelerates the mineralization of humus N, increasing the proportion of N available for plant uptake, resulting in a convergence of the light and heavy fractions. The N pool in the woodlot soils may have been subject to preferential losses of small amounts of 14N over a longer time period, resulting in a more pronounced divergence between the light fraction (reflecting recent plant inputs) and the mineral-associated heavy fraction (more recalcitrant). Nitrification was strongly influenced by land-use history, with highest rates in formerly cultivated sites. In contrast, soil net N mineralization and chloroform-N were more strongly influenced by present vegetation. Nitrifying bacteria were relatively abundant in all pastured and cultivated sites; however, only the cultivated hardwood sites, with lowest C:N ratios (16–18), had substantial net nitrification. Historical manure inputs may explain the more rapid soil net nitrification rates, by decreasing soil C:N ratios and thus reducing nitrate immobilization in the mineral soil of cultivated sites. Regionally, 65% of the land area was pastured, and a proportion of the nutrients obtained from grazing was transferred to the cultivated croplands, which comprise ≤15% of the land area. Pastures generally had intermediate nutrient ratios and N transformations but were often more similar to woodlots, which suggests that plowing and amendments, rather than forest clearance, have the greatest impact on soil organic matter and nutrients. The influence of land-use history on soil N and P and nitrification rates was more dramatic in hardwood sites, which indicates that characteristics of the recovering vegetation and/or changes in plant community composition associated with prior land use are important factors in the rate of recovery. Our findings lead to the surprising conclusion that 19th century agricultural practices decreased forest floor nutrient content and ratios, and increased nitrifier populations and net nitrate production for approximately a century after abandonment. Consideration of site history clearly deserves more attention in the design of field experiments, and in our understanding of patterns of element distributions and transformations in dynamic forested landscapes.

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.

Controls on soil respiration: Implications for climate change

The disruption of the global C cycle by human activity in both developed and developing countries is one of the key environmental issues facing human populations as we move into the 21st century.

Plant and Soil Responses to Chronic Nitrogen Additions at the Harvard Forest, Massachusetts

Data are presented on changes in plant and soil processes in two forest types (red pine plantation and oak-maple forest) at the Harvard Forest, Petersham, Massachusetts, in response to 3 yr of chronic N fertilization. The hardwood stand exhibited greater N limitation on biological function than the pine stand prior to fertilization as evidenced by a lower net N mineralization rate, nearly undetectable rates of net nitrification, and very low foliar N content. N additions were made in six equal applications throughout the growing season, and consisted of 5 and 15 g°m-2 °yr-1 of N as ammonium nitrate. The pine stand showed larger changes than the hardwood stand for extractable N, foliar N, nitrification, and N leaching loss. Retention of added N was essentially 100% for all but the high application pine plot from which significant N leaching occurred in the 3rd yr of application. From 75 to 92% of N added to fertilized plots was retained in the soil, with larger fractions retained in the hardwood stand than the pine stand for all treatments. As hypothesized, the stands are exhibiting highly nonlinear patterns of nitrogen output in response to continuous nitrogen inputs. The implications of this nonlinearity for regional eutrophication of surface waters and atmospheric deposition control policy are discussed.

FOREST BIOGEOCHEMISTRY AND PRIMARY PRODUCTION ALTERED BY NITROGEN SATURATION

Results from four intensive site-level manipulations and one extensive field survey in northern temperate and boreal forests show a consistent set of responses to chronic N additions. These include 1) initial and often large increase in net N mineralization followed by decreases, 2) increases in net nitrification. 3) increases in N concentration in foliage, and 4) decreased Mg∶N and Ca∶Al ratios, and declining tree growth and vigor in all evergreen stands. These results are synthesized into a set of proposed summary relationships that define the temporal pattern of responses of N-limited systems to N additions.

Controlling Site to Evaluate History: Vegetation Patterns of a New England Sand Plain

The widespread and long-lasting impact of human activity on natural eco- systems indicates that land-use history must be treated as an integral aspect of ecological study and a critical component of conservation planning. The New England landscape has undergone a complete transformation as forests were converted to agriculture in the 18th and 19th centuries followed by succession to woodland as a result of widespread agricultural abandonment. Despite the prevalence of human impacts, the effect and lon- gevity of land-use practices on modern forest conditions are poorly understood. In the present study of pitch pine-scrub oak vegetation on a sand plain in the Connecticut Valley of Massachusetts, we address the following questions: (1) what is the relative importance of human and natural disturbance and environmental factors in controlling vegetation composition, structure, and landscape patterns; (2) what are the mechanisms underlying human impacts on vegetation, and what is the duration of these impacts; and (3) what are the implications of land-use history for the interpretation and conservation of these communities? Sand plain vegetation was selected for investigation because the homo- geneity of site conditions facilitates the interpretation of land-use and natural disturbance impacts, and because the uncommon vegetation and constituent species are priorities for conservation efforts. Paleoecological data suggest that pre-European fires were common on the study area, perhaps ignited by a large regional Indian population. The area was noted historically as an extensive pine plain and was used for wood products from the 18th to the mid-19th century. Eighty-two percent of the area was subsequently plowed for agriculture before being abandoned in the early 20th century. Soil analyses confirm the homogeneity of site conditions and suggest that land uses (plowing, woodlot/pasture) were determined according to ownership pattern rather than site factors. Previously cultivated parcels have distinct Ap (plow horizons) 15-33 cm deep, whereas uncultivated parcels have A horizons 3-10 cm in depth. Soil physical and chemical characteristics are similar among land uses and modern vegetation types. Aerial photographs document a dramatic transformation in plant cover over the last 50 yr. In 1939, the vegetation was grassland or shrub-heath (49%), open-canopy forest (29%), and scrub-oak shrublands (15%). In 1985, 73% of the study area was forested with pitch pine (40%), hardwood (12%), or mixed stands (21%), 9% was in open-canopy stands, and 3% was covered by grass or shrubs. Vegetation/land-use relations are striking. Pitch pine occurs almost exclusively (97%) on former plowed sites, whereas scrub oak stands occur preferentially (89%) on sites that have not been plowed. Land use explains the greatest variation in modern vegetation as well as the distribution and abundance of many taxa. Fire has been common across the study area but has influenced vegetation largely within patterns resulting from prior land use. Land-use patterns and factors controlling vegetation composition and structure are broadly paralleled at similar sites elsewhere in the Connecticut Valley. The study indicates that conservation biologists interested in preserving species, com- munities, and landscape patterns on sand plains in the northeastern United States need to incorporate a dynamic perspective of biological systems that includes the overriding impact of prior land use. In order to appreciate, study, and display these land-use and vegetation patterns it is essential to conserve the mosaic of assemblages and historical uses within a landscape setting.

Observations of Thermokarst and Its Impact on Boreal Forests in Alaska, U.S.A.

A hinged gate and flexible loading seal for an inclined conveyor, more particularly for an inclined apron conveyor with pushers for carrying loose, bulk material. The pushers, longitudinally spaced either in staggered arrangement across the conveyor surface or each fully traversing the conveyor width, reduce gross downward movement of the material during conveyor ascent. A segmental gate is hinged above the conveyor in the vicinity of transfer from an unloading conveyor, allows upward passage of the pushers and interrupts downward movement of conveyed material. A flexible, resilient seal fixed above the conveyor and positioned downstream of the hinged gate prevents further downward retreat of escaping conveyed material and redeposits same upstream on advancing pushers.

Soil carbon and nitrogen in a pine-oak sand plain in central Massachusetts: Role of vegetation and land-use history

Abstract Over the last 150 years much of the landscape of eastern North America has been transformed from predominantly agricultural lands to forest. Although cultivation strongly affects important ecosystem processes such as biomass accumulation, soil organic matter dynamics, and nitrogen cycling, recovery of these processes after abandonment is insufficiently understood. We examined soil carbon and nitrogen pools and nitrogen dynamics for 16 plots on a central Massachusetts sand plain, over 80% of which had been cultivated and subsequently abandoned at least 40 years ago. The two youngest old-field forests, located on sites abandoned 40–60 years prior to our sampling, had the lowest mineral soil carbon content (0–15 cm), 31% less than the average of unplowed soils. Soil carbon concentration and loss-on-ignition were significantly higher in unplowed soils than in all plowed soils, but these differences were offset by the higher bulk density in formerly plowed soils, leading to no significant differences in C content between plowed and unplowed soil. Soil C:N ratios were lower in formerly plowed soils (26.2) than in unplowed soils (28.0). While soil N content was not affected by land-use history or vegetation type, net N mineralization showed much greater variation. In situ August net nitrogen mineralization varied nearly 40-fold between stand types: lowest in pitch pine and white pine stands (−0.13 and 0.10 kg N ha−1 28 day−1), intermediate in scrub oak stands (0.48 kg N ha−1 28 day−1) and highest in aspen and mixed oak stands (1.34–3.11 kg N ha−1 28 day−1). Mineralization was more strongly related to present vegetation than to land-use history or soil N content. Appreciable net nitrification was observed only in the most recently abandoned aspen plot (0.82 kg N ha−1 28 day−1), suggesting that recent disturbance and residual agricultural lime stimulated nitrification. Carbon:nitrogen ratios increased and pH declined with stand age. Higher bulk density, lower loss-on-ignition and C:N ratios, and slightly lower C concentrations in the surface mineral soil are the persistent legacies of agriculture on soil properties. Short-term agricultural use and the low initial C and N concentrations in these sandy soils appear to have resulted in less persistent impacts of agriculture on soil C and N content and N cycling.

Light-fraction soil organic matter: origin and contribution to net nitrogen mineralization

Abstract Two investigations were conducted to examine the nature of light-fraction ( −3 ) soil organic matter (OM): (1) determination of the relative contributions of above- and belowground litter to the light fraction (LF) pool in two oak stands (Noe Woods and Wingra Woods) that are part of a long-term (>30yr) litter manipulation study in southern Wisconsin, U.S.A.; and (2) evaluation of the monthly variation in LF mass and LF N mineralization potential, plus the contribution of LF to total net N mineralization, for a cornfield, pine stand, and maple stand in western Massachusetts, U.S.A. The long-term treatments at the Wisconsin plots include no-litter, 2 × litter, and one-time removal of the A horizon. In the first study, LF mass under the no-litter treatment was 64 and 32% lower for the Noe and Wingra sites, respectively, relative to the controls (unaltered litter inputs); LF mass under the 2 × litter treatment was 85% higher at Noe, but 15% lower at the Wingra site. The LF represented only 6.7 and 4.3% of the total mineral-soil C losses under no-litter and 7.0% (Noe alone) of the total mineral-soil C gain under 2 × litter. The results suggest that aboveground foliar litter contributed one-third (Wingra) to two-thirds (Noe) of the LF pool. Additionally, the heavy fraction (HF) declined quickly in response to relatively short-term changes in litter input and functioned as the primary long-term C sink. In the second study, LF represented 11% (corn), 13% (pine), and 2% (maple) of the N mineralization potential (anaerobic incubation) for the whole mineral soil; contribution of LF to net N mineralization could not be determined for the cornfield, but was calculated to be 3 kg N ha −1 yr −1 for both the pine and maple stands. N mineralization potential for the pine and maple sites was consistently lower for LF OM than for HF OM. Both LF mass (corn and pine) and LF N mineralization potential (all sites) showed differences by sampling month. LF constituted 5% (maple), 13% (corn), and 14% (pine) of mineral soil OM. Findings suggest that HF is the primary N source in coarse-textured mineral soils, and also that LF is a relatively minor N source in forest stands with mor-type soil.