Removal Of Aboveground Biomass Research Articles

Tree Harvest in an Experimental Sand Ecosystem: Plant Effects on Nutrient Dynamics and Solute Generation

The hydrochemical signatures of forested ecosystems are known to be determined by a time-variant combination of physical-hydrologic, geochemical, and biologic processes. We studied subsurface potassium (K), calcium (Ca), and nitrate (NO3) in an experimental red -pine mesocosm to determine how trees affect the behavior of these nutrients in soil water, both during growth and after a harvest disturbance. Solution chemistry was monitored for 2 years at the end of a 15-year period of tree growth, and then for 3 more years after harvest and removal of aboveground biomass. Concentrations were characterized by three distinct temporal patterns that we ascribe to changes in solute generation mechanisms. Prior to harvest, K soil-water concentrations were relatively uniform with depth, whereas Ca soil-water concentrations doubled with depth. Nitrate concentrations were below detection in soil water and discharge (drainage) water. Plant uptake and water/nutrient cycling exerted strong control during this interval. During the 1st year after harvest, K concentrations tripled in shallow soil water, relative to preharvest levels, and showed a strong seasonal peak in discharge that mimicked soil temperature. Summer soil temperatures and annual water flux also increased. Decomposition of labile litter, with complete nitrogen (N) immobilization, characterized this interval. In the third interval (years 2 and 3 after harvest), decomposition shifted from N to carbon (C) limitation, and Ca and NO3 concentrations in discharge spiked to nearly 200 and 400 μM, respectively. Relatively stable ionic strength and carbonate chemistry in discharge, throughout the study period, indicate that carbonic-acid weathering was sustained by belowground decomposition long after the harvest. This stable chemical weathering regime, along with the persistence of N limitation for a long period after disturbance, may be characteristic of early-phase primary-successional systems.

Phosphorus Translocation by Red Deer on a Subalpine Grassland in the Central European Alps

We examined the role of red deer (Cervus elaphus L.) in translocating phosphorus (P) from their preferred grazing sites (short-grass vegetation on subalpine grasslands) to their wider home range in a subalpine grassland ecosystem in the Central European Alps. Phosphorus was used because it is the limiting nutrient in these grasslands. When we compared P removal of aboveground biomass due to grazing with P input due to the deposit of feces on a grid of 268 cells (20 m × 20 m) covering the entire grassland, we detected distinct spatial patterns: the proportion of heavily grazed short-grass vegetation increased with increasing soil-P pool, suggesting that red deer preferably grazed on grid cells with a higher soil-P pool. Biomass consumption related to increased proportion of short-grass vegetation, and therefore P removal, increased with increasing soil-P pool. However, within the two vegetation types (short-grass and tall-grass), consumption was independent from soil-P pool. In addition, P input rates from defecation increased with increasing soil-P pool, resulting in a constant mean net P loss of 0.083 kg ha−1 y−1 (0.03%–0.07% of soil-P pool) independent of both soil-P pool and vegetation type. Thus, there was no P translocation between grid cells with different soil-P pools or between short-grass and tall-grass vegetation. Based on these results, it is likely that the net rate of P loss is too small to explain the observed changes in vegetation composition from tall-herb/meadow communities to short-grass and from tall-grass to short-grass on the grassland since 1917. Instead, we suggest that the grazing patterns of red deer directly induced succession from tall-herb/meadow communities to short-grass vegetation. Yet, it is also possible that long-term net soil-P losses indirectly drive plant succession from short-grass to tall-grass vegetation, because nutrient depletion could reduce grazing pressure in short-grass vegetation and enable the characteristic tall-grass species Carex sempervirens Vill. to establish.

POST-RECLAMATION LAND USE EFFECTS ON PROPERTIES AND CARBON SEQUESTRATION IN MINESOILS OF SOUTHEASTERN OHIO

Reclamation of mined soils could restore soil quality and ecosystem productivity and sequester C, but limited data exist with regard to the impact of post-reclamation land use and management on soil organic carbon (SOC) storage in reclaimed minesoils (RMS). A study was conducted to evaluate SOC and total nitrogen (TN) stocks (0-50 cm depth) and related soil physical and chemical properties for reclaimed grassland sites maintained under four distinct management practices for 24 years, namely, meadow, hay, grazing, and grazing-feeding. A nearby undisturbed grassland was used as a baseline or reference. Bulk density of the RMS (range: 1.16-1.88 mg m−3) was significantly lower for the hay and meadow than the grazed sites (P < 0.05). In addition, RMS under hay and meadow practices had greater concentration of water-stable aggregates (WSA) and larger mean weight diameter (MWD) of aggregates than the grazed sites. Soil pH and electrical conductivity (EC) were higher in the RMS than the undisturbed soils. Among the reclaimed sites, pH and EC values were generally lower (P < 0.05) in meadow than grazing and grazing-feeding practices. The SOC was generally higher in the grazing-feeding and hay sites (89 and 76 mg ha−1, respectively) than meadow (64 mg ha−1), grazing (70 mg ha−1), andundisturbed grassland (72 mg ha−1). The data suggest that removal of aboveground biomass for hay had no impact on SOC accumulation in these grasslands. Nearly 50% of the root biomass accumulated in the top 0-10 cm in the reclaimed grassland sites. The SOC content was strongly correlated with MWD (R2 = 0.70, P < 0.01), and root biomass (R2 = 0.75 P < 0.01) indicating that both roots and aggregates play a significant role in SOC accumulation in RMS.

Vegetative regeneration of biennial Oenothera species after disturbance: Field observations and experiment

It has been recently shown that some annual and biennial species of man-made habitats cope with severe disturbance by resprouting (vegetative regeneration) from their bud bank and do not only rely on regeneration from seeds. Nevertheless, information on the ecology of this phenomenon is rare. In a field study, we answered the question how frequent is resprouting from root buds in populations of the ruderal biennial herb Oenothera biennis, and how it is affected by habitat conditions. In an experiment, we tested the hypothesis that higher severity of injury and later life-cycle phase of the injured plants suppress resprouting from both axillary and root buds in O. biennis and also in its closely related congeners O. fallax and O. glazioviana. In 25 out of 29 studied ruderal populations of O. biennis severely injured individuals were found; however, only half of these populations included injured individuals that resprouted from roots. Among these populations, the number of root-sprouting individuals varied highly (from 3% to 67% of injured individuals). The largest populations and the highest percentage of root-resprouting individuals were found in urban habitats with sandy/gravelly substrate, a low vegetation cover, and a high frequency of disturbance. In the experiment with three Oenothera congeners, removal of aboveground biomass with all axillary buds largely led to the death of plants of all three species. When a portion of the basal axillary buds remained intact, individuals of O. biennis mostly failed to regenerate, whereas individuals of O. fallax and O. glazioviana survived and formed seeds. A higher severity of injury suppressed resprouting in Oenothera congeners in this experiment. However, the relationship between life-cycle phase and the ability to resprout remains unclear in Oenothera species. This study showed that resprouting after severe injury is an important feature of Oenothera individuals occurring in man-made habitats and may represent an alternative strategy to regeneration from the seed bank under disturbance conditions.

Differential responses of Bolivian timber species to prescribed fire and other gap treatments

We followed the establishment and growth response of 13 commercial tree species to canopy opening, above-ground biomass removal, and experimental burns of low and high intensities in a lowland dry forest in Bolivia. Three patterns of response to treatments were observed among the most abundant commercial tree species. (1) Shade-intolerant species regenerated mostly from seed and had the highest survival and growth rates following high-intensity burns. (2) Shade-tolerant species were abundant in gap control and plant removal treatments. Treatments had little effect on the height growth of these species. (3) Individuals of root sprouting species were most abundant following plant removal and low-intensity burn treatments. Treatments had little effect on the height growth of these species. The wide variation in species’ responses to gap treatments found in this study not only reinforces the concept that species are distributed along a continuum of shade-tolerance levels, but that other aspects of species’ biology, such as seed dispersal type or sprouting behavior, further differentiate regeneration strategies. The variety of regeneration strategies found among the species at this forest site will require a flexible management scheme that mixes more intensive silvicultural treatments such as prescribed burning with less intensive treatments.

Aboveground Biomass Removal by Burning and Raking Increases Diversity in a Reconstructed Prairie

Abstract Prescribed spring burning often contributes to a predominance of C4 grasses and low forb abundance and is impractical at many sites, especially near development. We tested raking after mowing as an alternative to prescribed burning in a reconstructed Minnesota prairie. We also tested mowing without raking as a possible means of maintaining prairie communities. Frequency, flowering stem abundance, and cover were measured for all plant species and native functional groups (C4 grasses, C3 graminoids, forbs, legumes, and annual or biennial forbs). Mowing alone did not differ from the control in its effect on any functional groups of plants. Round‐headed bush clover (Lespedeza capitata), a legume, and Black‐eyed Susan (Rudbeckia hirta), a biennial, increased in frequency with treatments that removed biomass (i.e., fire or raking), but they did not have significantly more flowering stems. Thus, new plants established well from seed, whereas the vitality of mature plants did not change. Raking had similar effects to burning on most functional groups, although flowering stems of C4 grasses were significantly more abundant after fire than after raking. Burning reduced some C3 forbs and grasses and favored the dominance of C4 grasses. Therefore, raking after mowing in the spring provides an alternative to prescribed burning that has many of the same positive aspects as fire but does not promote aggressive C4 grasses to the same extent.

Potential of agroforestry for carbon sequestration and mitigation of greenhouse gas emissions from soils in the tropics

Losses of carbon (C) stocks in terrestrial ecosystems and increasing concentrations of greenhouse gases in the atmosphere are challenges that scientists and policy makers have been facing in the recent past. Intensified agricultural practices lead to a reduction in ecosystem carbon stocks, mainly due to removal of aboveground biomass as harvest and loss of carbon as CO2 through burning and/or decomposition. Evidence is emerging that agroforestry systems are promising management practices to increase aboveground and soil C stocks and reduce soil degradation, as well as to mitigate greenhouse gas emissions. In the humid tropics, the potential of agroforestry (tree-based) systems to sequester C in vegetation can be over 70 Mg C ha−1, and up to 25 Mg ha−1 in the top 20 cm of soil. In degraded soils of the sub-humid tropics, improved fallow agroforestry practices have been found to increase top soil C stocks up to 1.6 Mg C ha−1 y−1 above continuous maize cropping. Soil C accretion is linked to the structural development of the soil, in particular to increasing C in water stable aggregates (WSA). A review of agroforestry practices in the humid tropics showed that these systems were able to mitigate N2O and CO2 emissions from soils and increase the CH4 sink strength compared to cropping systems. The increase in N2O and CO2 emissions after addition of legume residues in improved fallow systems in the sub-humid tropics indicates the importance of using lower quality organic inputs and increasing nutrient use efficiency to derive more direct and indirect benefits from the system. In summary, these examples provide evidence of several pathways by which agroforestry systems can increase C sequestration and reduce greenhouse gas emissions.

Review of estimation of plant rhizodeposition and their contribution to soil organic matter formation

The methods used for estimating rhizodeposition of plants (carbon (C) deposition of living roots), and the results obtained for different plant species are reviewed. Three tracer techniques using C isotopes to quantify rhizodeposition are discussed: pulse labelling, continuous labelling, and natural 13C abundance. Only the tracer methods provided adequate results for the whole rhizodeposition. The differences in the below-ground C translocation pattern between cereals and grasses are discussed. Cereals (wheat and barley) transfer 20 – 30% of total assimilated C into the soil. Half of this amount is subsequently found in the roots and about one-third in CO2 evolved from the soil by root respiration and microbial utilization of root-borne organic substances. The remaining part of below-ground translocated C is incorporated into the soil microorganisms and soil organic matter (SOM). The portion of assimilated C allocated below the ground by cereals decreases during growth and by increasing N fertilization. Pasture plants translocated about 30 – 50% of assimilates below-ground, and their translocation patterns were similar to those of crop plants. On average, the total C amounts translocated into the soil by cereals and pasture plants are approximately the same (1.5 Mg C ha−1; calculated for the productivity of about 6 Mg grain yield), when the same growth period is considered. However, during one vegetation period the cereals and grasses allocated beneath the ground about 1.5 and 2.2 Mg C ha−1, respectively. Finally, a simple approach is suggested for a rough calculation of C input into the soil and for root-derived CO2 efflux from the soil. Contribution of C4 carbon (from maize and some C4 grasses) to turnover of SOM (C3 soils) estimated by natural 13C abundance is reviewed. In average for Ap horizons, the portion of maize derived carbon increases of about 0.98% of SOM content per year. Factors influencing the contribution of maize-derived carbon to soil organic carbon are discussed. The contribution of maize derived carbon decreases with soil depth, without fertilization, after removal of above ground biomass, and with soil tillage.

Macroinvertebrates in North American tallgrass prairie soils: effects of fire, mowing, and fertilization on density and biomass

The responses of tallgrass prairie plant communities and ecosystem processes to fire and grazing are well characterized. However, responses of invertebrate consumer groups, and particularly soil-dwelling organisms, to these disturbances are not well known. At Konza Prairie Biological Station, we sampled soil macroinvertebrates in 1994 and 1999 as part of a long-term experiment designed to examine the effects and interactions of annual fire, mowing, and fertilization (N and P) on prairie soil communities and processes. For nearly all taxa, in both years, responses were characterized by significant treatment interactions, but some general patterns were evident. Introduced European earthworms ( Aporrectodea spp. and Octolasion spp.) were most abundant in plots where fire was excluded, and the proportion of the total earthworm community consisting of introduced earthworms was greater in unburned, unmowed, and fertilized plots. Nymphs of two Cicada genera were collected ( Cicadetta spp. and Tibicen spp.). Cicadetta nymphs were more abundant in burned plots, but mowing reduced their abundance. Tibicen nymphs were collected almost exclusively from unburned plots. Treatment effects on herbivorous beetle larvae (Scarabaeidae, Elateridae, and Curculionidae) were variable, but nutrient additions (N or P) usually resulted in greater densities, whereas mowing usually resulted in lower densities. Our results suggest that departures from historical disturbance regimes (i.e. frequent fire and grazing) may render soils more susceptible to increased numbers of European earthworms, and that interactions between fire, aboveground biomass removal, and vegetation responses affect the structure and composition of invertebrate communities in tallgrass prairie soils.

Impacts of grazing on the alkalinized–salinized meadow steppe ecosystem in the Songnen Plain, China – A simulation study

A process-based model was built to describe the ecological processes of an alkalinized–salinized meadow steppe ecosystem, including the hydrological and alkalization–salinization processes in the soil, as well as the succession and growth dynamics of the grassland communities. A numerical integration model and a water and salt balance model were integrated into a physically-based model, describing the dynamics of soil moisture, salt concentration, exchangeable sodium percentage (ESP) and pH. Meteorological variables and soil characteristics were the main environmental factors used to estimate the growth dynamics of three herbaceous communities that were dominated by Aneurolepidium chinense, Chloris virgata, and Suaeda glauca, respectively. Model validation showed good agreement between the simulated results and the observed data. Simulation studies were conducted to evaluate the potential changes in hydrological and alkalization–salinization processes, succession and growth dynamics from 1991 to 1998, under five grazing intensities, namely 0%, 25%, 50%, 75% and 90% above-ground biomass removal (AGBR). The simulations show that soil moisture decreased markedly under the 50%, 75% and 90% AGBR, but increased slightly under the 25% AGBR. The de-alkalization and de-salinization processes would be predominant under the 0% AGBR, and the processes became a little slower under the 25% AGBR. In contrast, the 50%, 75% and 90% AGBR accelerated the degradation of soil properties. The grassland was dominated by A. chinense under the 0% AGBR, and by A. chinense and C. virgata under the 25% AGBR. C. virgata could grow on slightly alkalinized–salinized soil and became a dominant species after three years of 50% AGBR. The soil degraded quickly and only S. glauca could grow on the severe alkalinized–salinized soil if the grassland received 75% or 90% AGBR. The grassland grew well under the 0% AGBR, and the biomass stayed at moderate level under 25% AGBR. The 50%, 75% and 90% AGBR decreased the grassland growth greatly. After accumulating the grazed biomass for each year, the 25% AGBR would provide the highest production, and the grassland production would decrease sharply with the increasing of grazing intensities. The simulation results indicate that 25% AGBR is significant for preserving the soil from degradation, and maintaining high grassland production.

Fast regeneration of the tussock grass Ampelodesmos mauritanica after clearing

Disturbance by clearing is common in Mediterranean shrublands of NE Spain as a fire prevention technique to reduce juel loads. We compared the regeneration of the vegetation in grass-dominated communities in cleared stands with adjacent non-cleared stands. We especially focussed on the response of the expansive perennial grass Ampelodesmos mauritanica in a 6 month (recently cleared), two year old (once cleared) stands and in a one-year-old stand cleared in two consecutive years (twice cleared). Cleared plots recovered quickly from clearing due to the dominance in the community of species that resprout from belowground organs after disturbance. The number of species was not significantly different between cleared and adjacent non-cleared plots. Total plant cover was lower in recently cleared plots than in adjacent non-cleared plots, but not significantly different in once and twice cleared plots. Clipped Ampelodesmos mauritanica plants had high survival and vigorous resprouting after clearing. Although in cleared plots Ampelodesmos mauritanica plants were smaller than in adjacent non-cleared plots, cover was not significantly different. Seedling recruitment was higher in recently cleared plots than in adjacent non-cleared plots. Our results suggest that clearing is not a very effective management tool for reducing Am pelodesmos mauritanica abundance and as a fire-prevention technique because this dominant grass shows no mortality after clearing and accumulates fuel loads very fast after aboveground biomass removal.

THE INFLUENCE OF A RIPARIAN SHRUB ON NITROGEN CYCLING IN A SONORAN DESERT STREAM

Riparian zones often act as nutrient filters, removing NO3 from water flowing through riparian soils. The role of vegetation in NO3 retention remains unclear and may be direct (uptake) or indirect (stimulation of microbial activity). We studied the riparian shrub Baccharis salicifolia (seepwillow) in Sycamore Creek (Arizona, USA), to determine (1) if sites colonized by seepwillow were sinks for NO3, and (2) the mechanism by which seepwillow causes NO3 retention. Subsurface water was sampled along flowpaths from an uncolonized gravel bar through seepwillow sites at several depths and on several gravel bars. NO3 concentration was significantly lower in seepwillow sites than in uncolonized sites, at least to 20 cm below the water table. Predictions of three hypotheses were tested to explain NO3 losses: (Hi) by plant uptake, (H2) by stimulation of denitrification by seepwillow, and (H3) a prior condition unrelated to seepwillow. Six experiments were used to test these hypotheses. Transplant experiments, plant size relationships, and root distri- bution experiments all demonstrated the importance of seepwillow (rejection of H3). Other tests involving removal of aboveground biomass, denitrification measures, and mass balance calculations showed a predominance of denitrification over uptake (rejection of Hi). We conclude that the main effect of seepwillow is to produce organic matter creating conditions favorable to denitrification and a loss of NO3 from subsurface water. Since denitrification is a permanent loss of N to the atmosphere, and uptake only temporarily retains N, the interaction between plants and microbes has important implications for the maintenance of water quality in streams and downstream reservoirs.

The Influence of a Riparian Shrub on Nitrogen Cycling in a Sonoran Desert Stream

Riparian zones often act as nutrient filters, removing NO3 from water flowing through riparian soils. The role of vegetation in NO3 retention remains unclear and may be direct (uptake) or indirect (stimulation of microbial activity). We studied the riparian shrubBaccharis salicifolia (seepwillow) in Sycamore Creek (Arizona, USA), to determine (1) if sites colonized by seepwillow were sinks for NO3, and (2) the mechanism by which seepwillow causes NO3 retention. Subsurface water was sampled along flowpaths from an uncolonized gravel bar through seepwillow sites at several depths and on several gravel bars. NO3 concentration was significantly lower in seepwillow sites than in uncolonized sites, at least to 20 cm below the water table. Predictions of three hypotheses were tested to explain NO3 losses: (H1) by plant uptake, (H2) by stimulation of denitrification by seepwillow, and (H3) a prior condition unrelated to seepwillow. Six experiments were used to test these hypotheses. Transplant experiments, plant size relationships, and root distribution experiments all demonstrated the importance of seepwillow (rejection of H3). Other tests involving removal of aboveground biomass, denitrification measures, and mass balance calculations showed a predominance of denitrification over uptake (rejection of H1). We conclude that the main effect of seepwillow is to produce organic matter creating conditions favorable to denitrification and a loss of NO3 from subsurface water. Since denitrification is a permanent loss of N to the atmosphere, and uptake only temporarily retains N, the interaction between plants and microbes has important implications for the maintenance of water quality in streams and downstream reservoirs.

Harvesting intensity affects forest structure and composition in an upland Amazonian forest

Forest structure and floristic composition were studied in a series of 0.5 ha natural forest plots at four sites near Porto Trombetas in Pará State, Brazil, 11–12 years after being subjected to differing levels of above-ground biomass harvest and removal. In addition to undisturbed control plots, experimental treatments included: removal of most trees ≥45 cm DBH (low intensity harvest); removal of trees <20 and ≥60 cm DBH (moderate intensity harvest); clear-cutting (100% above-ground biomass removal). Post-harvest basal area growth generally increased with harvest intensity, and total basal areas for trees ≥5 cm DBH were, at the time of our study, 60% (in the clear-cut) to about 80% of those in the control plots. Biomass harvests stimulated recruitment and growth of residual trees, particularly in the smaller diameter classes, but had little effect on species richness for small trees, seedlings, vine, herbs, and grasses. Species richness for trees ≥15 cm DBH was greater in the control and low-intensity (74–75 species) than in the moderate intensity (47 species) and clear-cut (26 species) treatment plots. While the tree flora within all harvest treatments was broadly similar to the undisturbed (control) plots and included similar numbers of species of the major plant families typical of the surrounding forests, the more intensive harvest treatments, especially the clear-cut, were dominated by a higher proportion of short-lived, early successional tree species.

Dissolved nitrogen dynamics in groundwater under a coastal Massachusetts forest.

The benthic algal biomass is distributed very patchily throughout each site. As a result, even though six core samples were taken on each sampling day, more samples might be needed to truly estimate the actual biomass of each quadrat. The tremendous variability-not only within each area but also within each quadrat-confounds the algal chlorophyll measurements. It is possible that significant algal growth occurred, but the algae were grazed heavily and thus did not show an actual increase in biomass. It is also possible that our removal of aboveground plant biomass resulted in desiccation of the marsh surface. Desiccation could limit algal growth directly and by preventing the remineralization of nutrients necessary for future algal growth. Regrowth of marsh plants in the hayed site occurred particularly rapidly (to more than 70% of the biomass of an S. patens reference) over the 30 days of the experiment. Thus any stimulation of algal growth by increased light due to haying is likely to be short

Changes induced by elk browsing in the aboveground biomass production and distribution of willow (Salix monticola Bebb): their relationships with plant water, carbon, and nitrogen dynamics.

Willows are dominant woody plants of many high-elevation riparian areas of the western USA, and constitute an important food resource for various ungulates, which tend to concentrate in riparian areas. The response of willow to browsing was analyzed in the elk winter range of Rocky Mountain National Park, by considering the effect of elk browsing on Salix monticola Bebb, one of the most common willow species in this area. Unbrowsed and browsed treatments were established during the 1997 growing season (May to October), using eight long-term exclosures built in the fall of 1994. Plants in the browsed treatment were in the areas open to browsing, but they were protected from browsing by small exclosures during the experimental period. Winter browsing by elk induced the following measured responses in plant morphology and development: (1) higher shoot biomass production but similar leaf biomass and leaf area per plant, (2) a lower number of and bigger shoots, (3) a lower number of and bigger leaves, and (4) flower inhibition. In addition, we infer that browsing induces (5) lower belowground allocation and (6) a more negative N balance but a higher soil N uptake. We conclude that elk browsing negatively affects willow even though willow compensate for aboveground biomass removal. Continuous browsing produces long-term changes in willow morphology which constrain plant growth and development. High browsing utilization, as occurred in this experiment, could therefore reduce the competitive ability and survivorship of willow, in particular under drier environmental conditions.

Impacts of agricultural management practices on C sequestration in forest-derived soils of the eastern Corn Belt

Soil organic matter has recently been implicated as an important sink for atmospheric carbon dioxide (CO 2). However, the relative impacts of various agricultural management practices on soil organic matter dynamics and, therefore, C sequestration at spatial scales larger than a single plot or times longer than the typical three year experiment have rarely been reported. Results of maintaining agricultural management practices in the forest-derived soils of the eastern Corn ( Zea mays L.) Belt states of Kentucky, Michigan, Ohio and Pennsylvania (USA) were studied. We found annual organic C input and tillage intensity were the most important factors in affecting C sequestration. The impact of rotation on C sequestration was primarily related to the way it altered annual total C inputs. The removal of above-ground plant biomass and use of cover crops were of lesser importance. The most rapid changes in soil organic matter content occurred during the first five years after a management practice was imposed with slower changes occurring thereafter. Certain management practices, e.g. no-tillage (NT), increased the soil's ability to sequester atmospheric CO 2. The impact of this sequestration will be significant only when these practices are used extensively on a large percentage of cropland and when the C-building practices are maintained. Any soil C sequestered will be rapidly mineralized to CO 2 if the soil organic matter building practices are not maintained.

Wheat Yields, Nitrogen Uptake, and Soil Moisture Following Winter Legume Cover Crop vs. Fallow

AbstractLong‐term use of fallow in dryland wheat (Triticum aestivum L.) systems can increase erosion and decrease soil fertility. One possible solution is to grow a green manure crop during the fallow period. To evaluate the short‐term effects of substituting a winter legume green manure for fallow in a 2‐yr wheat rotation, a series of experiments were conducted in the Sacramento Valley of California from 1994 to 1996. Soil water content, inorganic soil N, and yield of fertilized wheat after fallow were compared with those of unfertilized wheat following a woollypod vetch‐field pea [Vicia villosa Roth subsp. varia (Host) Corb.; syn. V. dasycarpa Ten.‐Pisum sativum] green manure crop. Supplemental experiments (1995–1996) compared unfertilized wheat following (i) fallow with incorporation of legume biomass imported from another plot, (ii) growth and incorporation of green manure plus supplemental legume biomass from another plot, or (iii) growth of a legume crop with removal of aboveground biomass. Soil water content after a green manure crop was 6.6 cm less than after fallow in 1994 (to the 90‐cm depth), but only 1.5 cm less in 1995 (to 150 cm). During the wet winter of 1995–1996, fertilized wheat plots after fallow had higher inorganic soil‐N levels than unfertilized plots following a green manure crop, but wheat yields were similar. Where a green manure crop was grown and incorporated, there was no wheat yield response to additional legume biomass. Similarly, without a green manure crop, there was no yield difference between fertilized plots receiving 112 vs. 28 kg N ha−1. Results could be different in drier years or in soils with lower N fertility, but the data indicate that possible long‐term yield benefits of green manuring are not necessarily preceded by lower short‐term yields.

Predominance of ecophysiological controls on soil CO2 flux in a Minnesota grassland

Ecosystem studies often study soil CO2 flux as a function of environmental factors, such as temperature, that affect respiration rates by changing the rate of utilization of carbon substrates. These studies tend not to include factors, such as photosynthesis, that affect the supply of carbon substrates to roots and root-associated processes. We examined the role of decreased carbohydrate source on soil CO2 flux and root respiration in an annually-burned grassland through manipulations of light intensity and removal of above ground biomass. We also quantified the contribution of root respiration to soil CO2 flux by measuring the respiration rates of excised roots. Two days of shading caused a 40% reduction in soil CO2 flux, while clipping was associated with a 19% reduction in soil CO2 flux. Both reductions were independent of soil and air temperature at the time of measurement. The relative decrease in soil CO2 flux observed in the clipping experiment was similar in magnitude to an observed decrease in root respiration per gram of root, linking decreased root activity and soil CO2 flux. From these experiments, we conclude that variation in factors that affect carbon availability to roots can be important determinants of soil CO2 flux and should be included explicitly in studies that measure or model soil CO2 flux.

Soil carbon dynamics in the humid tropical forest zone

Conversion of natural forests to agriculture in the humid tropics leads to a reduction in ecosystem carbon storage due to the immediate removal of aboveground biomass and a gradual subsequent reduction in soil organic carbon. A considerable part of soil carbon is protected from microbial attack by a range of physical and chemical mechanisms and is not sensitive to landuse change. We analyzed the soils data base for Sumatra (Indonesia) developed by the Center for Soil and Agroclimate Research (CSAR) to estimate effects of landuse on soil C content. Sumatra has a considerable diversity of soils ranging from those of recent origin in the highlands, to older sedimentary and heavily leached soils in the pedimont peneplain and large areas of wetland soils along the coast. Peat soils (Histosols) and other wetland soils (Aquic and Fluvic suborders) contain the greatest soil C reserves, followed by young volcanic soils (Andisols). Agricultural use of these soils can have a disproportionately large effect on C release to the atmosphere. On the major part of the upland soils the difference in (top) soil C content between natural forest and agricultural land is in the range 0.5–1.0% C, equivalent to a change in total C stock of 10–20 Mg ha −1. These results agree with data collected in S. Sumatra in the 1930s. C org of forest soils is related to soil pH, and is lowest in the pH range 5.0–6.0. Wetland conditions, lower pH, higher altititude (lower temperature) and higher clay and silt content all contributed to higher soil C contents in a multiple regression analysis of the whole data set. Existing models and data sets are insufficient to predict changes in soil C contents under various landuse practices. Carbon isotope studies, and especially the δ 13C method may be used to study the effects of landuse change, especially when the vegetation was changed from one dominated by C3 plants (most forest species) to one dominated by grasses and crops with a C4 photosynthetic pathway. Results from Brazil documented a gradual decline of organic matter originating from the forest system and its partial replacement by organic matter derived from inputs of sugarcane during the first fifty years of cultivation. Forest conversion to well managed grasslands may lead to an increased soil C storage, after an initial decline. The consequences of erosion on losses of soil C depend on the scale at which these losses are considered, because of sedimentation processes. When net erosion losses are not expressed per unit area, but per length scale to the power l.6, erosion losses appear to be largely independent of scale. The `fractal dimension of erosion’ (on average around 1.6) probably is a landscape characteristic and estimates of its value are needed for extrapolation. Better understanding of soil C deposition sites is needed to evaluate overall erosion effects and test whether or not erosion can contribute to net C sequestration.