Live Fine Roots Research Articles

Soil-plant hydrology of indigenous and exotic trees in an Ethiopian montane forest

Fast-growing exotic trees are widely planted in the tropics to counteract deforestation; however, their patterns of water use could be detrimental to overall ecosystem productivity through their impact on ecosystem water budget. In a comparative field study on seasonal soil-plant water dynamics of two exotic species (Cupressus lusitanica Mill. and Eucalyptus globulus Labill.) and the indigenous Podocarpus falcatus (Thunb.) Mirb. in south Ethiopia, we combined a 2.5-year record for climate and soil water availability, natural-abundance oxygen isotope ratios (delta(18)O) of soil and xylem water, destructive root sampling and transpiration measurements. Soil was generally driest under C. lusitanica with its dense canopy and shallow root system, particularly following a relatively low-rainfall wet season, with the wettest soil under E. globulus. Wet season transpiration of C. lusitanica was twice that of the other species. In the dry season, P. falcatus and C. lusitanica reduced transpiration by a factor of six and two, respectively, whereas E. globulus showed a fivefold increase. In all species, there was a shift in water uptake to deeper soil layers as the dry season progressed, accompanied by relocation of live fine root biomass (LFR) of C. lusitanica and P. falcatus to deeper layers. Under P. falcatus, variability in soil matric potential, narrow delta(18)O depth gradients and high LFR indicated fast water redistribution. Subsoil water uptake was important only for E. globulus, which had low topsoil LFR and tap roots exploiting deep water. Although P. falcatus appeared better adapted to varying soil water availability than the exotic species, both conifers decreased growth substantially during dry weather. Growth of E. globulus was largely independent of topsoil water content, giving it the potential to cause substantial dry-season groundwater depletion.

Unusual Fine Root Distributions of Two Deciduous Tree Species in Southern France: What Consequences for Modelling of Tree Root Dynamics?

The spatial distribution of fine roots of two deciduous tree species was investigated in contrasting growing conditions in southern France. Hybrid walnut trees (Juglans regia×nigra cv. NG23) and hybrid poplars (Populus euramericana cv. I214) were both cultivated with or without annual winter intercrops for 10 years on deep alluvial soils. Soil samples for measuring the fine root distribution of both trees and crops were obtained by soil coring down to 3-m depth at several distances and orientations from the tree trunk. The distribution of live fine roots from walnut and poplar trees was patchy and sometimes unexpected. In the tree-only stands, fine root profiles followed the expected pattern, as fine root density decreased with increasing depth and distance from the tree trunk. However, many fine root profiles under intercropped trees were uniform with depth, and some inverse profiles were observed. These distributions may result from a high degree of plasticity of tree root systems to sense and adapt to fluctuating and heterogeneous soil conditions. The distortion of the tree root system was more pronounced for the walnut trees that only partially explored the soil volume: in the tree-only stand, the walnut rooting pattern was very superficial, but in the intercropped stand walnut trees developed a deep and dense fine root network below the crop rooting zone. The larger poplars explored the whole available soil volume, but the intercrop significantly displaced the root density from the topsoil to layers below 1 m depth. Most tree root growth models assume a decreasing fine root density with depth and distance from the tree stem. These models would not predict correctly tree–tree and tree–understorey competition for water and nutrients in 3D heterogeneous soil conditions that prevail under low-density tree stands. To account for the integrated response of tree root systems to such transient gradients in soils, we need a dynamic model that would allow for both genotypic plasticity and transient environmental local soil conditions.

Tree-girdling to separate root and heterotrophic respiration in two Eucalyptus stands in Brazil

The release of carbon as CO2 from belowground processes accounts for about 70% of total ecosystem respiration. Insights about factors controlling soil CO2 efflux are constrained by the challenge of apportioning sources of CO2 between autotrophic tree roots (and mycorrhizal fungi) and heterotrophic microorganisms. In some temperate conifer forests, the reduction in soil CO2 efflux after girdling (phloem removal) has been used to separate these sources. Girdling stops the flow of carbohydrates to the belowground portion of the ecosystem, which should slow respiration by roots and mycorrhizae while heterotrophic respiration should remain constant or be enhanced by the decomposition of newly dead roots. Therefore, the reduction in CO2 efflux after girdling should be a conservative estimate of the belowground flux of C from trees. We tested this approach in two tropical Eucalyptus plantations. Tree canopies remained intact for more than 3 months after girdling, showing no reduction in light interception. The reduction in soil CO2 efflux averaged 16-24% for the 3-month period after girdling. The reduction in CO2 efflux was similar for plots with one half of the trees girdled and those with all of the trees girdled. Girdling did not reduce live fine root biomass for at least 5 months after treatment, indicating that large reserves of carbohydrates in the root systems of Eucalyptus trees maintained the roots and root respiration. Our results suggest that the girdling approach is unlikely to provide useful insights into the contribution of tree roots and heterotrophs to soil CO2 efflux in this type of forest ecosystem.

Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration

AbstractRadiocarbon (14C) provides a measure of the mean age of carbon (C) in roots, or the time elapsed since the C making up root tissues was fixed from the atmosphere. Radiocarbon signatures of live and dead fine (<2 mm diameter) roots in two mature Amazon tropical forests are consistent with average ages of 4–11 years (ranging from <1 to >40 years). Measurements of 14C in the structural tissues of roots known to have grown during 2002 demonstrate that new roots are constructed from recent (<2‐year‐old) photosynthetic products. High Δ14C values in live roots most likely indicate the mean lifetime of the root rather than the isotopic signature of inherited C or C taken up from the soil.Estimates of the mean residence time of C in forest fine roots (inventory divided by loss rate) are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years). By assuming positively skewed distributions for root ages, we can effectively decouple the mean age of C in live fine roots (measured using 14C) from the rate of C flow through the live root pool, and resolve these apparently disparate estimates of root C dynamics. Explaining the 14C values in soil pore space CO2, in addition, requires that a portion of the decomposing roots be cycled through soil organic matter pools with decadal turnover time.

Comparison of Fine Root Dynamics in Scots Pine and Pedunculate Oak in Sandy Soil

In this study, we investigated the relationship between the seasonality of vegetation cover and that of fine root processes in a man-made forest in northern Belgium. Due to their contrasting foliar development, we expected different seasonal patterns of fine root growth and standing biomass between Pedunculate oak (Quercus robur L.), and Scots pine (Pinus sylvestris L.). Biomass and necromass of fine and small roots were estimated by repeated core sampling in February, April, June, August and October 2003. Measurements showed that Pedunculate oaks maintained more live fine roots in winter than Scots pines. However, Scots pines produced more than twice as much fine roots in spring, such that in summer both species had similar root mass. Scots pine root production started before-, but declined during leaf unfolding. Pedunculate oak roots, in contrast, started elongating only after bud break. For both species, fine root production peaked in JuneJuly, but was more than offset by drought-induced mortality at the end of July and early August. Summer drought in 2003 was exceptionally long and intense, significantly reducing leaf area, killing most new roots, and inhibiting root decomposition, such that the obtained results cannot be typical for this forest.

Interspecific competition impacts on the morphology and distribution of fine roots in European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.)

Morphology and vertical distribution patterns of spruce and beech live fine roots (diameter ≤2 mm) were studied using a soil core method in three comparable mature stands in the Solling: (1) pure beech, (2) pure spruce and (3) mixed spruce–beech. This study was aimed at determining the effects of interspecific competition on fine root structure and spatial fine root distribution of both species. A vertical stratification of beech and spruce fine root systems was found in the mixed stand due to a shift in beech fine roots from upper to lower soil layers. Moreover, compared to pure beech, a significantly higher specific root length (SRL, P 0.10; SSA 225/212 cm2 g−1, P>0.10). Symmetric competition belowground between mixed beech and spruce was observed since live fine roots of both species were under-represented compared to pure stand. However, the higher space sequestration efficiency suggests a higher competitive ability of beech belowground.

Early impacts of forest restoration treatments on the ectomycorrhizal fungal community and fine root biomass in a mixed conifer forest

Summary The obligate symbiosis formed between ectomycorrhizal fungi (EMF) and roots of tree species in the Pinaceae influences nutrient uptake and surrounding soil structure. Understanding how EMF respond to prescribed fire and thinning will assist forest managers in selecting fuel‐reducing restoration treatments that maintain critical soil processes and site productivity. The response of EMF species richness, live fine root biomass and duff levels to various forms and combinations of thinning and burning was investigated in mixed ponderosa pine and Douglas fir stands in the Blue Mountains of Oregon. The below‐ground community composition and structure of EMF at the site was characterized using molecular methods. The EMF community consisted of a large number of infrequently detected species. The distribution of a few abundant species (Cenococcum sp., Piloderma sp., Rhizopogon salebrosus and Wilcoxina rehmii) in a majority of the treatments both before and after treatment application demonstrates that some EMF species survive or rapidly re‐establish after disturbance. EMF species richness, live root biomass and duff levels were reduced significantly by prescribed fire treatments compared to the non‐burned treatments. Synthesis and applications. These results indicate that prescribed fire results in a short‐term reduction in EMF species richness and live root biomass and may influence whether managers can achieve the desired future condition of stands with large‐tree retention and low fuel loads. EMF mortality and complete duff reduction after fire have been implicated with poor tree survival and slow stand recovery in forest ecosystems world‐wide. The results of this study, along with the recovery potential of a site, and the impending risk of stand‐replacing wildfire in stands differing in structure from historic conditions, bear consideration when reintroducing fire.

Growth and substrate quality of fine root and soil nitrogen availability in a young Betula ermanii forest of northern Japan: Effects of the removal of understory dwarf bamboo ( Sasa kurilensis)

Dwarf bamboos (mainly Sasa species) form dense undergrowth in many forests in Japan and compete with overstory vegetation for soil resources. For a better understanding of the interaction between tree and undergrowth root for soil nutrients, we compared soil N availability, root biomass and its substrate quality in two Betula ermanii stands. In one stand, the aboveground part of undergrowth ( Sasa kurilensis) had been removed for 5 years (SR) and in the other, the undergrowth was present (SP). The SR stand had significantly higher NO 3-N levels in soil (1.7–5.5 mg kg −1 at different soil depths) than the SP stand (1.2–1.7 mg kg −1), which may affect the amount and substrate quality of fine root. Although the total (live + dead) of the root and rhizome (root–rh) mass did not differ between the two stands, the total production by these categories was about half in the SR stand than the SP stand. Further, the proportion of dead root–rh mass was about two times in the SR stand than the SP stand. In the investigated depth (to 30 cm), the contribution of the C and N inputs to the total in the uppermost layer (0–10 cm deep) was greater in the SR stand (56–59%) than the SP stand (38–42%). Total root–rh of S. kurilensis in the SP stand contributed to about 38–40% of the total C and N input, and in the SR stand the contribution was still about half that of the SP stand. Further, the acid-insoluble C:N ratio in very fine root and live fine root of B. ermanii was significantly less in the SR stand. The contribution of N content in very fine root to the total root–rh was much greater in the SR (45%) stand than the SP stand (36%). These results suggest that the remaining S. kurilensis detritus and the decreased N use by this species after its removal resulted in increased N availability for B. ermanii. Natural regeneration of tree species after simultaneous death of Sasa species, which is very common in Sasa-dominant forests in Japan, may in part, be facilitated by the presence of residues of the root–rh of Sasa species.

Fine root production and litter input: Its effects on soil carbon

Carbon storage by forests has potential for contributing to ‘Kyoto Protocol’ greenhouse gas emission reduction targets, but evidence about C-storage and loss below ground is conflicting. The study addresses why soil carbon stocks are increased by land use change from forest to pasture, but are reduced by planting conifer trees, though not broadleaf trees, onto prior pasture. Can species differences in fine root production and litter input play a role? Addressing that hypothesis, a 1-year pot experiment was established as a model system in a glasshouse. Two tree species, pine (Pinus radiata) and Blue Gum (Eucalyptus globulus), and two grass species, Kangaroo Grass (Themeda triandra) and Wallaby Grass (Austrodanthonia racemosa), were sown in pots of soil taken from a native pasture with Kangaroo Grass dominant or from an adjacent pine plantation forest. After 3 months, half of the grass pots were defoliated monthly to 7 cm above ground to test for any cutting effect. Fine root production and turnover was monitored via minirhizotrons, with a destructive harvest after 1 year. Fine root oven dry mass at the end of the year varied between species: Kangaroo Grass (17 g/pot), pine (13 g/pot), Blue Gum (8 g/pot), Wallaby Grass (4 g/pot). Cutting significantly reduced fine root production in Kangaroo Grass but not in Wallaby Grass. From minirhizotron monitoring, 70% of the fine root length produced by pine during the year had disappeared, presumably by decomposition, before the final harvest. The equivalent loss for Wallaby Grass was only 26%, for Kangaroo Grass 36%, and for Blue Gum 45%. Despite the faster fine root disappearance under pines than under Kangaroo Grass, soil C declined under pines but increased under Kangaroo Grass as found in the field. Thus the experiment did not support the idea that a lower dead fine root production was the source of decline in soil C under pine. There was only a weak correlation between soil C change and the net amount of live fine root mass produced. However, the soil carbon changes in this study were positively correlated with live fine root length density in the soil. Kangaroo Grass maintained large lengths of very fine roots while pines produced a small length of thick fine roots. Accordingly, it is hypothesized that the increase of soil carbon under Kangaroo Grass compared with pine may be caused more by the activity of live fine roots than by decomposition of fine root mass to humus. This hypothesis, formulated from the model experimental system, needs to be evaluated for field sites where pines have been planted onto pastures.

Short-term effects of seasonal prescribed burning on the ectomycorrhizal fungal community and fine root biomass in ponderosa pine stands in the Blue Mountains of Oregon

The effects of seasonal prescribed fire on the belowground ectomycorrhizal community and live fine root biomass were investigated before, 1 year after, and 2 years after prescribed underburning. Ectomycorrhizas were sampled from four replications of three treatments (fall underburning, spring underburning, and a nonburned control) in a randomized complete block design. Samples were separated in two subsamples representing the upper 5 cm and lower 5 cm of a soil core. Molecular tools were used to distinguish 140 restriction fragment length polymorphism (RFLP) species of fungi directly from the ectomycorrhizas. Prior to underburning, the number of RFLP species and amount of live root biomass were similar among treatment units and between upper and lower core samples. Fall underburning largely removed live root biomass to a depth of 10 cm and significantly reduced ectomycorrhizal species richness compared with spring underburning and the nonburned control for at least 2 years. RFLP species richness and live root biomass following spring underburning were generally similar to the nonburned treatment. The successful reintroduction of fire to the ecosystem to retain high species diversity of ectomycorrhizal fungi and achieve the desired future condition of large-tree ponderosa pine retention with low fuel loads may require more than underburning in a single season.

Evaluation of soil CO2 production and transport in Duke Forest using a process‐based modeling approach

Soil surface CO2 efflux is an important component of the carbon cycle in terrestrial ecosystems. However, our understanding of mechanistic controls of soil CO2 production and transport is greatly limited. A multilayer process‐based soil CO2 efflux model (PATCIS) was used to evaluate soil CO2 production and transport in the Duke Forest. CO2 production in the soil is the sum of root respiration and soil microbial respiration, and CO2 transport in the soil mainly simulates gaseous diffusion. Simulated soil CO2 efflux in the Duke Forest ranged from 5 g CO2 m−2 d−1 in the winter to 25 g CO2 m−2 d−1 in summer. Annual soil CO2 efflux was 997 and 1211 g C m−2 yr−1 in 1997 and 1998, respectively. These simulations were consistent with the observed soil CO2 efflux. Simulated root respiration contributed 53% to total soil respiration. Soil temperature had the dominant influence on soil CO2 production and CO2 efflux while soil moisture only regulated soil CO2 efflux in the summer when soil moisture was very low. Soil CO2 efflux was sensitive to the specific fine root respiratory rate and live fine root biomass. Elevated CO2 increased annual soil CO2 efflux by 26% in 1997 and 18% in 1998, due mainly to the enhanced live fine root biomass and litterfall. On a daily to yearly basis, CO2 production is almost identical to CO2 efflux, suggesting that CO2 transport is not a critical process regulating daily and long‐term soil surface CO2 effluxes in the Duke Forest. We also developed a statistical model of soil CO2 efflux with soil temperature and moisture. Daily soil CO2 efflux estimation by the statistical model showed a similar pattern to the simulated soil CO2 efflux, but the total annual CO2 efflux was slightly lower. While the statistical model is simple, yet powerful, in simulating seasonal dynamics of soil CO2 efflux, the process‐based model has the potential to advance our mechanistic understanding of soil CO2 efflux variations in the current and future worlds.

Supply‐side controls on soil respiration among Oregon forests

AbstractTo test the hypothesis that variation in soil respiration is related to plant production across a diverse forested landscape, we compared annual soil respiration rates with net primary production and the subsequent allocation of carbon to various ecosystem pools, including leaves, fine roots, forests floor, and mineral soil for 36 independent plots arranged as three replicates of four age classes in three climatically distinct forest types.Across all plots, annual soil respiration was not correlated with aboveground net primary production (R2=0.06, P>0.1) but it was moderately correlated with belowground net primary production (R2=0.46, P<0.001). Despite the wide range in temperature and precipitation regimes experienced by these forests, all exhibited similar soil respiration per unit live fine root biomass, with about 5 g of carbon respired each year per 1 g of fine root carbon (R2=0.45, P<0.001). Annual soil respiration was only weakly correlated with dead carbon pools such as forest floor and mineral soil carbon (R2=0.14 and 0.12, respectively). Trends between soil respiration, production, and root mass among age classes within forest type were inconsistent and do not always reflect cross‐site trends.These results are consistent with a growing appreciation that soil respiration is strongly influenced by the supply of carbohydrates to roots and the rhizosphere, and that some regional patterns of soil respiration may depend more on belowground carbon allocation than the abiotic constraints imposed on subsequent metabolism.

Clean rain promotes fine root growth and soil respiration in a Norway spruce forest

AbstractSoil acidification and N saturation are considered to affect the decomposition of soil organic matter as well as growth and mortality of fine roots in many forest soils. Here we report from a field experiment where ‘clean rain’ has been applied to the soil for about 10 years under a roofed plot of a 71‐year‐old Norway spruce plantation at Solling, Central Germany. Reduced amounts of protons (−78%), sulphate (−53%), ammonium (−86%), and nitrate (−49%) were sprayed on the soil surface of the clean rain plot between 1992 and 2001. In an adjacent roofed control plot, throughfall was collected and immediately re‐sprinkled below the roof construction without any chemical manipulation. One year before the clean rain treatment started, live and dead fine root masses (≤2 mm) were determined from undisturbed soil cores down to 40 cm mineral soil depth. Total live fine root mass was significantly lower in the clean rain plot than in the control plot. After the first sampling, the soil holes were refilled with quartz sand and repeatedly sampled in June 1992, June 1996, and October 2001. There were no differences in live and dead fine root masses between the plots in 1992 and 1996. In 2001, both live and dead fine root masses of the clean rain plot were about twice as high as in the control plot, indicating that fine root growth recovered in the mineral soil following 10 years of clean rain treatment. Moreover, the clean rain treatment significantly reduced the total N concentrations of live fine roots and 1‐year‐old needles. Our results suggest that the reduced N input promoted fine root growth to compensate N deficiency. Reduced Al concentration in soil solution may have contributed to the recovery of fine root growth, however, the toxicity of Al species is largely unknown. Mean annual soil respiration rate was 24% higher in the period from 2000 to 2001, indicating that the clean rain treatment increased respiration of roots and heterotrophic microorganisms within the rhizosphere. Laboratory incubation of samples from the organic horizon and the top mineral soil revealed no differences between the plots in the decay rate of soil organic matter. Our results suggest that strong reductions in atmospheric N deposition from about 30 to 10 kg N ha−1 yr−1 and decreasing acid stress can have beneficial effects on growth of fine roots in the mineral soil within a decade. We conclude that biological recovery under reduced atmospheric loads can affect the nutrient and carbon budget of spruce soils in the long run.

Fertilization Effects on Fine Root Biomass, Production, and Turnover Rate in a Pinus rigida Plantation

We examined the effects of fertilization [control (C), 200 kg N (LNP), and 400 kg N (HNP)] on fine root (＜ 2 mm diameter) dynamics using monthly soil coring method in a 39-year-old Pinus rigida plantation of central Korea. The average fine root biomass (live + dead) (kg SE) during the first growing season for C, LNP, and HNP was 1301 54, 1084 47, and 1328 22, respectively. The fine root production (kg SE) was 2394 128 for C, 2048 101 for LNP, and 2768 150 for HNP, respectively. Over the same period, fertilization treatments had impact on N and P concentrations of live fine root. Nitrogen and P inputs (kg ) into the soil through fine root turnover for C, LNP, and HNP were 16.6 and 0.9, 17.2 and 0.9, and 24.1 and 1.6, respectively. There were no significant differences in fine root biomass and production during the first growing season after fertilization. However, fertilization increased fine root N and P concentrations, and in consequence resulted in increased N and P inputs into soil through fine root turnover.

Biomass, carbon and nitrogen dynamics of multi-species riparian buffers within an agricultural watershed in Iowa, USA

This study was conducted to determine biomass dynamics, carbon sequestration and plant nitrogen immobilization in multispecies riparian buffers, cool-season grass buffers and adjacent crop fields in central Iowa. The seven-year-old multispecies buffers were composed of poplar (Populus×euroamericana ‘Eugenei’) and switchgrass (Panicum virgatum L.). The cool-season grass buffers were dominated by non-native forage grasses (Bromus inermis Leysser., Phleum pratense L. and Poa pratensis L). Crop fields were under an annual corn-soybean rotation. Aboveground non-woody live and dead biomass were determined by direct harvests throughout two growing seasons. The dynamics of fine (0–2 mm) and small roots (2–5 mm) were assessed by sequentially collecting 35 cm deep, 5.4 cm diameter cores (125 cm deep cores in the second year) from April through November. Biomass of poplar trees was estimated using allometric equations developed by destructive sampling of trees. Poplar had the greatest aboveground live biomass, N and C pools, while switchgrass had the highest mean aboveground dead biomass, C and N pools. Over the two-year sampling period, live fine root biomass and root C and N in the riparian buffers were significantly greater than in crop fields. Growing-season mean biomass, C and N pools were greater in the multispecies buffer than in either of the crop fields or cool-season grass buffers. Rates of C accumulation in plant and litter biomass in the planted poplar and switchgrass stands averaged 2960 and 820 kg C ha−1 y−1, respectively. Nitrogen immobilization rates in the poplar stands and switchgrass sites averaged 37 and 16 kg N ha−1 y−1, respectively. Planted riparian buffers containing native perennial species therefore have the potential to sequester C from the atmosphere, and to immobilize N in biomass, therefore slowing or preventing N losses to the atmosphere and to ground and surface waters.

Impacts of plant roots on soil CO cycling and soil–atmosphere CO exchange

AbstractCarbon monoxide (CO) plays a major role in tropospheric chemical dynamics. Accordingly, global CO budgets have been reasonably well documented. Atmospheric CO consumption by soils contributes significantly to these budgets, with the magnitude of the sink generally considered to reflect a balance between microbial uptake and abiological production. However, assays of live fine roots showed that diverse intact plants produced carbon monoxide at net rates ranging from 2 to 3000 mµg gdw−−1 d−−1. CO production was greater for legumes than nonlegumes, and primarily associated with nodules. Excised roots from woody and herbaceous plants produced CO at comparable rates. CO production rates were similar for roots of intact plants and roots excised from those plants. The magnitude of net CO fluxes from roots was determined in part by the balance between simultaneous production and consumption. Surface sterilization of roots indicated that CO consumption was due, in part, rhizoplane CO‐oxidizing bacteria, but maximum CO consumption rates were typically only a small fraction of net production rates. Assays in a Brazilian agroecosystem indicated that root CO production affects soil–atmosphere CO exchange. Estimates of global CO production rates indicated that roots contribute about 170–260 Tg CO to the soil atmosphere annually, an amount comparable to current estimates of atmospheric CO uptake by soils, and much larger than estimates of net abiological soil CO production.

Fine-root response to nitrogen supply in nitrogen manipulated Norway spruce catchment areas

Investigations were carried out in order to assess the effect of decreased/increased nitrogen (N)- and sulphur (S)-deposition on the growth and vitality if fine-roots of Norway spruce ( Picea abies (L.) Karst.), in three catchment areas within the Lake Gårdsjön basin, SW Sweden. The catchment areas were subjected to decreased N- and S-deposition (G1 ROOF), increased N-deposition (G2 NITREX) and ambient levels of N- and S-deposition (F1 CONTROL). The excavated fine-roots (<1 mm in diameter) were separated into live and dead fractions and chemical analyses were carried out. The proportion of nitrogen to other macro-nutrients in the fine-roots was used as an indicator of mineral nutrient requirements of the forest trees. The most ‘vital’ fine-roots (characterised by high live to dead ratio) were found in all catchment areas in the upper parts of the organic soil horizon, which most frequently exhibited good mineral nutrient and soil water conditions. It was concluded from available climatic data on monthly precipitation and air temperature that the variation in fine-root growth was not caused by climatic changes. The live to dead ratio of the fine-roots gradually decreased with depth and was low in the mineral soil horizon. An increased percentage of living fine-roots in the ROOF catchment suggests an increased ‘vitality’ in fine-roots (increased live to dead ratios), associated with reduced N- and S-deposition. Chemical analyses of live fine-roots in the same area revealed improved mineral nutrient concentrations, e.g. higher Ca/Al ratios, increased concentrations of K, P, Ca and Mg. Fine-root vitality was improved in the NITREX catchment area, probably as a result of an increased cation release. Fine-root distributions, live to dead ratios and mineral nutrient concentrations appear to be strongly affected by N availability in the soil. Increased concentrations of K and P in proportion to N in fine-roots in the ROOF catchment indicate improved mineral nutrient conditions and lower risk of ion-imbalances. The results are in agreement with findings from other investigations suggesting those nutrient deficiencies and/or metal toxicity may strongly affect the growth of fine-roots.

Nutrient Limitation to Primary Productivity in a Secondary Savanna in Venezuela1

We examined nutrient limitation to primary productivity in a secondary savanna in the interior branch of the Coastal Range of Venezuela, which was converted from forest to savanna more than 100 years ago. We manipulated soil nutrients by adding nitrogen ( N), phosphorus and potassium ( PK), and nitrogen, phosphorus, and potassium ( NPK) to intact savanna. Eleven months after fertilization, we measured aboveground biomass and belowground biomass as live fine roots in the top 20 cm of soil, and species and functional group composition in response to nutrient additions. Aboveground biomass was highest in the NPK treatment ([mean g/m2]; control = 402, N = 718, PK = 490, NPK = 949). Aboveground production, however, appeared to be limited primarily by N. Aboveground biomass increased 78 percent when N was added alone but did not significantly respond to PK additions when compared to controls. In contrast to aboveground biomass, belowground biomass increased with PK additions but showed no significant increase with N (depth 0–20 cm; [mean g/m2]; control = 685, N = 443, PK = 827, NPK = 832). There was also a 36 percent increase in root length with PK additions when compared to controls. Whole savanna shoot:root ratios were similar for control and PK (0.6), while those for N or NPK fertilization were significantly higher (1.7 and 1.2, respectively). Total biomass response (above belowground) to nutrient additions showed a strong N and PK co-limitation ([mean g/m2]; control = 1073, N = 1111, PK = 1258, NPK = 1713). Aboveground biomass of all monocots increased with N additions, whereas dicots showed no response to nutrient additions. Trachypogon spp. (T. plumosus T. vestitus) and Axonopus canescens, the two dominant grasses, made up more than 89 percent of the total aboveground biomass in these sites. Trachypogon spp. responded to NPK, whereas A. canescens, sedges, and the remaining monocots only responded to N. Even though nutrient additions resulted in higher aboveground biomass in N and NPK fertilized plots, this had little effect on plant community composition. With the exception of sedges, which responded positively to N additions and increased from 4 to 8 percent of the plant community, no changes were observed in plant community composition after 11 months.

The age of fine-root carbon in three forests of the eastern United States measured by radiocarbon.

Using a new approach involving one-time measurements of radiocarbon (14C) in fine (<2mm diameter) root tissues we have directly measured the mean age of fine-root carbon. We find that the carbon making up the standing stock of fine roots in deciduous and coniferous forests of the eastern United States has a mean age of 3-18years for live fine roots, 10-18years for dead fine roots, and 3-18years for mixed live+dead fine roots. These 14C-derived mean ages represent the time C was stored in the plant before being allocated for root growth, plus the average lifespan (for live roots), plus the average time for the root to decompose (for dead roots and mixtures). Comparison of the 14C content of roots known to have grown within 1year with the 14C of atmospheric CO2 for the same period shows that root tissues are derived from recently fixed carbon, and the storage time prior to allocation is <2years and likely <1year. Fine-root mean ages tend to increase with depth in the soil. Live roots in the organic horizons are made of C fixed 3-8years ago compared with 11-18years in the mineral B horizons. The mean age of C in roots increases with root diameter and also is related to branching order. Our results differ dramatically from previous estimates of fine-root mean ages made using mass balance approaches and root-viewing cameras, which generally report life spans (mean ages for live roots) of a few months to 1-2years. Each method for estimating fine-root dynamics, including this new radiocarbon method, has biases. Root-viewing approaches tend to emphasize more rapidly cycling roots, while radiocarbon ages tend to reflect those components that persist longest in the soil. Our 14C-derived estimates of long mean ages can be reconciled with faster estimates only if fine-root populations have varying rates of root mortality and decomposition. Our results indicate that a standard definition of fine roots, as those with diameters of <2mm, is inadequate to determine the most dynamic portion of the root population. Recognition of the variability in fine-root dynamics is necessary to obtain better estimates of belowground C inputs.

Slash-and-burn effects on fine root biomass and productivity in a tropical dry forest ecosystem in México

Abstract We examined the effects of slash-and-burn of a tropical dry forest (TDF) and pasture establishment on fine root (≤1 mm) biomass and productivity. We also determined the seasonal changes of fine roots. The study was conducted in the coast of Jalisco, Mexico where the dominant vegetation is tropical dry forest. Two 33 m×100 m experimental plots with undisturbed TDF were slashed-and-burned by local farmers. Two adjacent plots with TDF were established as control sites. After slash-and-burning of the experimental plots, these were seeded with three pasture grasses along with two local maize varieties. Root sampling was initiated in March 1993 and roots were collected monthly until February 1994. Eight soil samples were collected randomly in each plot at each sampling date to a depth of 10 cm with a soil corer. Each soil core was divided in three depths: 0–2, 2.1–5, and 5–10 cm. Roots were separated in two size categories: fine (≤1.0 mm) and small (1.1–5 mm). Fine roots were separated into live and dead. Productivity of fine roots for each depth was estimated from the biomass data. Live fine root biomass in the 0–10 cm profile decreased due to burning but 47% of the fine root mass loss was at the 0–2 cm depth. Dead fine root biomass diminished significantly only at this depth. Total fine root productivity and mortality was 42–45% higher in TDF than in pasture in the first 5 cm of soil. About 86 and 76% of the fine root productivity in TDF and pasture, respectively, occurred in the first 5 cm of soil. Fine root turnover rates were high and similar in both ecosystems. Live fine root biomass in TDF and pasture increased in response to rainfall, particularly in the first 5 cm of soil. Mean annual live fine root biomass was significantly greater in TDF than in pasture only at the 0–2 cm depth. Mean biomass per cm of soil was greater in the first 2 cm of the profile in both TDF and pasture and it represented one-third of the fine root biomass in the 0–10 cm profile. There were no significant differences of small roots (1.1–5.0 mm in diameter) due to treatment, seasonality or dynamics. Based on root turnover and root production, our results indicate that the relative importance of belowground processes for C supply to the soil in this TDF is greater than the aboveground return.