Short-term effects of manipulated increase in acid deposition on soil, soil solution chemistry and fine roots in Scots pine (Pinus sylvestris) stand on a podzol

A manipulated increase in acid deposition (15 kg S ha⁻¹), carried out for three months in a mature Scots pine (Pinus sylvestris) stand on a podzol, acidified the soil and raised dissolved Al at concentrations above the critical level of 5 mg l⁻¹ previously determined in a controlled experiment with Scots pine seedlings. The induced soil acidification reduced tree fine root density and biomass significantly in the top 15 cm of soil in the field. The results suggested that the reduction in fine root growth was a response not simply to high Al in solution but to the depletion of exchangeable Ca and Mg in the organic layer, K deficiency, the increase in NH₄:NO₃ ratio in solution and the high proton input to the soil by the acid manipulation. The results from this study could not justify the hypothesis of Al-induced root damage under field conditions, at least not in the short term. However, the study suggests that a short exposure to soil acidity may affect the fine root growth of mature Scots pine.     

Assessing fine-root biomass and production in a Scots pine stand – comparison of soil core and root ingrowth core methods

Soil core and root ingrowth core methods for assessing fine-root (< 2 mm) biomass and production were compared in a 38-year-old Scots pine (Pinus sylvestris L) stand in eastern Finland. 140 soil cores and 114 ingrowth cores were taken from two mineral soil layers (0–10 cm and 10–30 cm) during 1985–1988. Seasonal changes in root biomass (including both Scots pine and understorey roots) and necromass were used for calculating fine-root production. The Scots pine fine-root biomass averaged annually 143 g/m² and 217 g/m² in the upper mineral soil layer, and 118 g/m² and 66 g/m² in the lower layer of soil cores and ingrowth cores, respectively. The fine-root necromass averaged annually 601 g/m² and 311 g/m² in the upper mineral soil layer, and 196 g/m² and 159 g/m² in the lower layer of soil cores and ingrowth cores, respectively. The annual fine-root production in a Scots pine stand in the 30 cm thick mineral soil layer, varied between 370–1630 g/m² in soil cores and between 210 – 490 g/m² in ingrowth cores during three years. The annual production calculated for Scots pine fine roots, varied between 330–950 g/m² in soil cores and between 110 – 610 g/m² in ingrowth cores. The horizontal and vertical variation in fine-root biomass was smaller in soil cores than in ingrowth cores. Roots in soil cores were in the natural dynamic state, while the roots in the ingrowth cores were still expanding both horizontally and vertically. The annual production of fine-root biomass in the Scots pine stand was less in root ingrowth cores than in soil cores. During the third year, the fine-root biomass production of Scots pine, when calculated by the ingrowth core method, was similar to that calculated by the soil core method. Both techniques have sources of error. In this research the sampling interval in the soil core method was 6–8 weeks, and thus root growth and death between sampling dates could not be accurately estimated. In the ingrowth core method, fine roots were still growing into the mesh bags. In Finnish conditions, after more than three growing seasons, roots in the ingrowth cores can be compared with those in the surrounding soil. The soil core method can be used for studying both the annual and seasonal biomass variations. For estimation of production, sampling should be done at short intervals. The ingrowth core method is more suitable for estimating the potential of annual fine-root production between different site types.     

Short-term increases in relative nitrification rates due to trenching in forest floor and mineral soil horizons of different forest types

The representation of NO₃ ^– dynamics within forest growth simulation models could improve forest management. An extensive literature review revealed an 88% probability of measuring a higher relative nitrification index (i.e. RNI = [NO₃ ^–] ÷ [NO₃ ^– + NH₄ ⁺]) in mineral soil horizons than in forest floors, across a wide range of conifer and hardwood ecosystems. We then hypothesised that humus form and fine root density could be used as two crude variables to predict changes in in situ, potential and relative nitrification rates. Twenty-seven trench plots were established in 1999, across nine contrasting hardwood and coniferous stands in the Eastern Townships of Québec. Forest floor and mineral soil samples were collected from each plot, and from a 1 m radius surrounding each plot, on three dates during summer 2000. In situRNI values increased significantly in trench plots as the season progressed. Potential nitrification rates (i.e. NO₃ ^– concentrations following incubation) were two orders of magnitude higher in forest floor than in mineral soil samples. RNI was significantly higher in mineral soil than in forest floor samples after incubations, but the relative increase in RNI due to trenching was higher in forest floor samples. Indices of available C were significantly higher in forest floor than mineral soil samples, and decreased only in forest floor samples during incubations. Likewise, trenching significantly reduced available C in forest floor samples only. Seasonal changes in soil temperature and fine root growth were the most plausible explanations for seasonal changes in NO₃ ^– dynamics, whereas other factors such soil acidity and moisture appeared less important in determining NO₃ ^– dynamics in this study. We conclude that crude assessments of humus form and fine root density have the potential to be used as calibration parameters for the simulation of NO₃ ^– dynamics in forest growth and yield models.     

TRAP: a modelling approach to below-ground carbon allocation in temperate forests

Tree root systems, which play a major role in below-ground carbon (C) dynamics, are one of the key research areas for estimating long-term C cycling in forest ecosystems. In addition to regulating major C fluxes in the present conditions, tree root systems potentially hold numerous controls over forest responses to a changing environment. The predominant contribution of tree root systems to below-ground C dynamics has been given little emphasis in forest models. We developed the TRAP model, i.e. Tree Root Allocation of Photosynthates, to predict the partitioning of photosynthates between the fine and coarse root systems of trees among series of soil layers. TRAP simulates root system responses to soil stress factors affecting root growth. Validation data were obtained from two Belgian experimental forests, one mostly composed of beech (Fagus sylvatica L.) and the other of Scots pine (Pinus sylvestris L.). TRAP accurately predicted (R = 0.88) night-time CO₂ fluxes from the beech forest for a 3-year period. Total fine root biomass of beech was predicted within 6% of measured values, and simulation of fine root distribution among soil layers was accurate. Our simulations suggest that increased soil resistance to root penetration due to reduced soil water content during summer droughts is the major mechanism affecting the distribution of root growth among soil layers of temperate Belgian forests. The simulated annual rate of C input to soil litter due to the fine root turnover of the Scots pine was 207 g C m^–2 yr^–1. The TRAP model predicts that fine root turnover is the single most important source of C to the temperate forest soils of Belgium.     

In situ measurement of fine root water absorption in three temperate tree species—Temporal variability and control by soil and atmospheric factors

Miniature heat balance-sap flow gauges were used to measure water flows in small-diameter roots (3–4 mm) in the undisturbed soil of a mature beech–oak–spruce mixed stand. By relating sap flow to the surface area of all branch fine roots distal to the gauge, we were able to calculate real time water uptake rates per root surface area (J_s) for individual fine root systems of 0.5–1.0 m in length. Study aims were (i) to quantify root water uptake of mature trees under field conditions with respect to average rates, and diurnal and seasonal changes of J_s, and (ii) to investigate the relationship between uptake and soil moisture θ, atmospheric saturation deficit D, and radiation I. On most days, water uptake followed the diurnal course of D with a mid-day peak and low night flow. Neighbouring roots of the same species differed up to 10-fold in their daily totals of J_s (<100–2000 g m⁻² d⁻¹) indicating a large spatial heterogeneity in uptake. Beech, oak and spruce roots revealed different seasonal patterns of water uptake although they were extracting water from the same soil volume. Multiple regression analyses on the influence of D, I and θ on root water uptake showed that D was the single most influential environmental factor in beech and oak (variable selection in 77% and 79% of the investigated roots), whereas D was less important in spruce roots (50% variable selection). A comparison of root water uptake with synchronous leaf transpiration (porometer data) indicated that average water fluxes per surface area in the beech and oak trees were about 2.5 and 5.5 times smaller on the uptake side (roots) than on the loss side (leaves) given that all branch roots <2 mm were equally participating in uptake. Beech fine roots showed maximal uptake rates on mid-summer days in the range of 48–205 g m⁻² h⁻¹ (i.e. 0.7–3.2 mmol m⁻² s⁻¹), oak of 12–160 g m⁻² h⁻¹ (0.2–2.5 mmol m⁻² s⁻¹). Maximal transpiration rates ranged from 3 to 5 and from 5 to 6 mmol m⁻² s⁻¹ for sun canopy leaves of beech and oak, respectively. We conclude that instantaneous rates of root water uptake in beech, oak and spruce trees are above all controlled by atmospheric factors. The effects of different root conductivities, soil moisture, and soil hydraulic properties become increasingly important if time spans longer than a week are considered.     

The significance of Cognettia sphagnetorum(Enchytraeidae) on nitrogen availability and plant growth in wood ash-treated humus soil

A laboratory microcosm experiment was established to study whether the role of Cognettia sphagnetorum (Enchytraeidae) in affecting Scots pine (Pinus sylvestris) seedling growth is influenced by wood ash-amendment, i.e., neutralisation of the raw humus soil. Coniferous forest soil, inoculated with soil microbes and nematodes, was either treated with wood ash or left as ash-free control. Wood ash (corresponding to an amount of 5000 kg ha^–1) was either spread on the soil surface or mixed into the soil. Enchytraeid and pine seedling biomass, abundance of nematodes, and water leachable NH₄ ⁺-N and NO₃ ^–-N were measured 26 and 51 weeks after initiation of the experiment and root length and N concentration of needles were measured 51 weeks after initiation of the experiment. Wood ash when mixed into the soil, reduced the biomass of C. sphagnetorum. Nematodes were unaffected by the treatments. In the ash-free soils C. sphagnetorum had little influence on pine growth, but it did decrease root length and root to shoot ratio. In the absence of enchytraeids wood ash decreased pine biomass production and root length. However, the presence of enchytraeids in the ash-treated soil compensated the ash-induced negative effects on the pine performance. Enchytraeids increased and wood ash decreased water leachable NH₄ ⁺-N in the presence but not in the absence of enchytraeids, while water leachable NO₃ ^–-N was not affected by the treatments. It was concluded that C. sphagnetorum can be important in ensuring nutrient cycling and plant growth in situations when an ecosystem encounters disturbances.     

Effects of soil phosphorus availability,temperature and moisture on soil respiration in Eucalyptus pauciflora forest

Rates of soil respiration (CO₂ efflux) were measured for a year in a mature Eucalyptus pauciflora forest in unfertilized and phosphorus-fertilized plots. Soil CO₂ efflux showed a distinct seasonal trend, and average daily rates ranged from 124 to 574 mg CO₂ m^–2 hr^–1. Temperature and moisture are the main variables that cause variation in soil CO₂ efflux; hence their effects were investigated over a year so as to then differentiate the treatment effect of phosphorus (P) nutrition.Soil temperature had the greatest effect on CO₂ efflux and exhibited a highly significant logarithmic relationship (r² = 0.81). Periods of low soil and litter moisture occurred during summer when temperatures were greater than 10 °C, and this resulted in depression of soil CO₂ efflux. During winter, when temperatures were less than 10 °C, soil and litter moisture were consistently high and thus their variation had little effect on soil CO₂ efflux. A multiple regression model including soil temperature, and soil and litter moisture accounted for 97% of the variance in rates of CO₂ efflux, and thus can be used to predict soil CO₂ efflux at this site with high accuracy. Total annual efflux of carbon from soil was estimated to be 7.11 t C ha^–1 yr^–1. The model was used to predict changes in this annual flux if temperature and moisture conditions were altered. The extent to which coefficients of the model differ among sites and forest types requires testing.Increased soil P availability resulted in a large increase in stem growth of trees but a reduction in the rate of soil CO₂ efflux by approximately 8%. This reduction is suggested to be due to lower root activity resulting from reduced allocation of assimilate belowground. Root activity changed when P was added to microsites within plots, and via the whole tree root system at the plot level. These relationships of belowground carbon fluxes with temperature, moisture and nutrient availability provide essential information for understanding and predicting potential changes in forest ecosystems in response to land use management or climate change.     

Seasonal dynamics of fine root biomass, root length density, specific root length, and soil resource availability in a Larix gmelinii plantation

Fine root turnover is a major pathway for carbon and nutrient cycling in terrestrial ecosystems and is most likely sensitive to many global change factors. Despite the importance of fine root turnover in plant C allocation and nutrient cycling dynamics and the tremendous research efforts in the past, our understanding of it remains limited. This is because the dynamics processes associated with soil resources availability are still poorly understood. Soil moisture, temperature, and available nitrogen are the most important soil characteristics that impact fine root growth and mortality at both the individual root branch and at the ecosystem level. In temperate forest ecosystems, seasonal changes of soil resource availability will alter the pattern of carbon allocation to belowground. Therefore, fine root biomass, root length density (RLD) and specific root length (SRL) vary during the growing season. Studying seasonal changes of fine root biomass, RLD, and SRL associated with soil resource availability will help us understand the mechanistic controls of carbon to fine root longevity and turnover. The objective of this study was to understand whether seasonal variations of fine root biomass, RLD and SRL were associated with soil resource availability, such as moisture, temperature, and nitrogen, and to understand how these soil components impact fine root dynamics in Larix gmelinii plantation. We used a soil coring method to obtain fine root samples (⩽2 mm in diameter) every month from May to October in 2002 from a 17-year-old L. gmelinii plantation in Maoershan Experiment Station, Northeast Forestry University, China. Seventy-two soil cores (inside diameter 60 mm; depth intervals: 0–10 cm, 10–20 cm, 20–30 cm) were sampled randomly from three replicates 25 m × 30 m plots to estimate fine root biomass (live and dead), and calculate RLD and SRL. Soil moisture, temperature, and nitrogen (ammonia and nitrates) at three depth intervals were also analyzed in these plots. Results showed that the average standing fine root biomass (live and dead) was 189.1 g·m⁻²·a⁻¹, 50% (95.4 g·m⁻²·a⁻¹) in the surface soil layer (0–10 cm), 33% (61.5 g·m⁻²·a⁻¹), 17% (32.2 g·m⁻²·a⁻¹) in the middle (10–20 cm) and deep layer (20–30cm), respectively. Live and dead fine root biomass was the highest from May to July and in September, but lower in August and October. The live fine root biomass decreased and dead biomass increased during the growing season. Mean RLD (7,411.56 m·m⁻³·a⁻¹) and SRL (10.83 m·g⁻¹·a⁻¹) in the surface layer were higher than RLD (1 474.68 m·m⁻³·a⁻¹) and SRL (8.56 m·g⁻¹·a⁻¹) in the deep soil layer. RLD and SRL in May were the highest (10 621.45 m·m⁻³ and 14.83m·g⁻¹) compared with those in the other months, and RLD was the lowest in September (2 198.20 m·m⁻³) and SRL in October (3.77 m·g⁻¹). Seasonal dynamics of fine root biomass, RLD, and SRL showed a close relationship with changes in soil moisture, temperature, and nitrogen availability. To a lesser extent, the temperature could be determined by regression analysis. Fine roots in the upper soil layer have a function of absorbing moisture and nutrients, while the main function of deeper soil may be moisture uptake rather than nutrient acquisition. Therefore, carbon allocation to roots in the upper soil layer and deeper soil layer was different. Multiple regression analysis showed that variation in soil resource availability could explain 71–73% of the seasonal variation of RLD and SRL and 58% of the variation in fine root biomass. These results suggested a greater metabolic activity of fine roots living in soil with higher resource availability, which resulted in an increased allocation of carbohydrate to these roots, but a lower allocation of carbohydrate to those in soil with lower resource availability. __________ Translated from Acta Phytoecologica Sinica, 2005, 29(3): 403–410 [译自: 植物生态学报, 2005, 29(3): 403–410]     

Root biomass changes after drainage and fertilization of a low-shrub pine bog

The stump and root systems of Scots pine (Pinus sylvestris) and field-layer vegetation were sampled before (1984) and three growing seasons after drainage and fertilization (1987) of a low-shrub pine bog. Average below-ground biomass of the field layer was 548 gDW m^–2 in 1984, with no significant treatment effects during experimentation. The stump-plus-root biomass of the pine stands was 1464 gDW m^–2 in the virgin state, and had increased to 1854 gDW m^–2 three years after the NPK-fertilizer treatment. The distribution over fractions also changed with this treatment. The fraction of fine roots ( < 1 mm) in stump-root biomass increased from 4% (56 gDW m^–2) to 11% (196 gDW m^–2), while the other compartments changed less. Total pine root length was 729 mm^–2 in 1984. Root length increased by 94% to 1380 mm^–2 on NPK-fertilized plots. Most of the fine pine roots were in the surface layer (0–10 cm), 79% in 1984 and 88% in 1987, and few pine roots were deeper than 20 cm. Maximum root length of fine pine roots ( < 1 mm) was estimated to be 2710 mm^–2 at about 800 gDW m^–2 (NPK treatment), and the corresponding maximum for small pine roots (=1–10 mm) was 227 mm^–2 at 809 gDW m^–2. Drainage stimulated net growth of fine roots, but this treatment also caused higher mortality rates of small roots. The fine roots responded to fertilization with higher net growth rate, and secondary growth of the large roots ( > 10 mm) was improved. The observed changes in root biomass and structure are explained as strategic adaptations to altered hydrological and nutritional circumstances in the root zone after drainage and fertilization.     

Stand fine root biomass and fine root morphology in old-growth beech forests as a function of precipitation and soil fertility

Only very limited information exists on the plasticity in size and structure of fine root systems, and fine root morphology of mature trees as a function of environmental variation. Six northwest German old-growth beech forests (Fagus sylvatica L.) differing in precipitation (520 – 1030 mm year^–1) and soil acidity/fertility (acidic infertile to basic fertile) were studied by soil coring for stand totals of fine root biomass (0–40 cm plus organic horizons), vertical and horizontal root distribution patterns, the fine root necromass/biomass ratio, and fine root morphology (root specific surface area, root tip frequency, and degree of mycorrhizal infection). Stand total of fine root biomass, and vertical and horizontal fine root distribution patterns were similar in beech stands on acidic infertile and basic fertile soils. In five of six stands, stand fine root biomass ranged between 320 and 470 g m^–2; fine root density showed an exponential decrease with soil depth in all profiles irrespective of soil type. An exceptionally small stand fine root biomass (<150 g m^–2) was found in the driest stand with 520 mm year^–1 of rainfall. In all stands, fine root morphological parameters changed markedly from the topsoil to the lower profile; differences in fine root morphology among the six stands, however, were remarkably small. Two parameters, the necromass/biomass ratio and fine root tip density (tips per soil volume), however, were both much higher in acidic than basic soils. We conclude that variation in soil acidity and fertility only weakly influences fine root system size and morphology of F. sylvatica, but affects root system structure and, probably, fine root mortality. It is hypothesized that high root tip densities in acidic infertile soils compensate for low nutrient supply rates, and large necromasses are a consequence of adverse soil chemical conditions. Data from a literature survey support the view that rainfall is another major environmental factor that influences the stand fine root biomass of F. sylvatica.     

Wood-ash recycling affects forest soil and tree fine-root chemistry and reverses soil acidification

Wood ash was applied to a forest ecosystem with the aim to recycle nutrients taken from the forest and to mitigate the negative effects of intensive harvesting. After two years, the application of 8,000 kg ha⁻¹ of wood ash increased soil exchangeable Ca and Mg. Similarly, an increase in Ca and Mg in the Norway spruce fine roots was recorded, leading to significant linear correlations between soil and root Ca and soil and root Mg. In contrast to these macronutrients, the micronutrients Fe and Zn and the toxic element Al decreased in the soil exchangeable fraction with the addition of wood ash, but not in the fine roots. Only Mn decreased in soil and in fine roots leading to a significant linear correlation between soil and root Mn. In soil, as well as in fine roots, strong positive correlations were found between the elements Ca and Mg and between Fe and Al. This indicates that the uptake of Mg resembles that of Ca and that of Al that of Fe. With the wood ash application, the pH increased from 3.2 to 4.8, the base saturation from 30% to 86%, the molar basic cations/Al ratio (BC/Al) of the soil solution from 1.5 to 5.5, and the molar Ca/Al ratio of the fine roots from 1.3 to 3.7. Overall, all below-ground indicators of soil acidification responded positively to the wood ash application within two years. Nitrate concentrations increased only slightly in the soil solution at a soil depth of 75–80 cm, and no signs of increased heavy metal concentrations in the soils or in the fine roots were apparent. This suggests that the recycling of wood ash could be an integral part of sustainable forest management because it closes the nutrient cycle and reverses soil acidification.     

Radiation and soil temperature interactions on the growth and physiology of eastern white pine ( Pinus strobus L.) seedlings

A greenhouse experiment was set up during one growing season to test the hypothesis that soil temperature controls a significant part of the light response of eastern white pine (Pinus strobus L.) seedlings that is observed in the field. The experimental design was a three by three factorial split-plot design, with three levels of light availability: 10%, 40% and 80% of full light; and three levels of soil temperature: 16 °C, 21 °C, and 26 °C in the soil at midday. The results show significant interactions between light and soil temperature factors on several variables (gas exchange, root growth, leaf-mass ratio and leaf–mass per unit area), but not on shoot dry mass. These interactions indicate that, in the field, a significant proportion of the light response of young eastern white pine could result from changes in soil temperature, especially under conditions of limiting water availability. Our results suggest that soil temperature must be taken explicitly into account as a driving variable when relating the growth of young eastern white pine to photosynthetic radiation.     

The phenology of fine root growth in a maple-dominated ecosystem: relationships with some soil properties

A two-year study was undertaken in a maple-dominated watershed of southern Québec, Canada, to examine relationships between trends in fine root growth, stem diameter growth, soil moisture, soil temperature, mineralized-N and extractable-P. Until September, soil temperature was consistently higher in 1995 than in 1994. Apart from the first sampling in mid-May, soil moisture was higher in 1994 than in 1995. In 1994, most fine roots were produced before leaf expansion, whereas in 1995, fine root production peaked in July. Annual fine root production was estimated to be 2.7 times higher in 1994 than in 1995. Stem growth was strongly associated with the seasonal and annual variation in soil temperature. Root and diameter growth were asynchronous in 1994 but not in 1995. Fine root production was associated with two groups of variables: a soil fertility (mineralized-N and extractable-P) group and a physical soil environment (moisture and temperature) group. Our results are consistent with the negative effect of high soil-N fertility on fine root production but are inconclusive as to the positive effect of high soil-P fertility. Soil conditions that are detrimental to root growth such as high N availability and anaerobiosis could modify the normal dynamics of fine root growth.     

Effects of water and nutrient applications in a Scots pine stand to tree growth and nutrient cycling

In a Scots pine forest stand, demineralized water and a complete set of nutrients with water were applied to the soil by means of frequent irrigation for four years in order to eliminate water and nutrient shortage of the trees. Apart from this optimization, dissolved (NH₄)₂SO₄ was irrigated at a rate of 120 kg N ha^-1 y^-1 to create a situation of N excess. Effect of treatments on tree growth and chemical composition of soil water and vegetation were monitored. From the first treatment year onwards basal area growth increased by ca. 35% as a result of the increased water supply. Nutrient applications increased K and P concentrations in pine needles immediately, but growth was enhanced only in the fourth treatment year and coincided with an improved K supply. Most of the applied P and K was retained in the soil, and only 6% was recovered in the vegetation. Tree nutrient status did not respond on Ca and Mg applications, whereas Ca and Mg seepage losses were increased with ca. 5 kg ha^-1 y^-1. The applied NH₄ was mostly retained in the 0–20 cm surface soil and caused a drastic increase of Al in soil solution. Tree growth was stimulated initially by extra NH₄, but was hampered after three years obviously because of a decreased P nutrition. The applied base cations were absorped to the soil and the accompanying anions were leached, thus temporarily increasing the acidification of the soil solution.     

Tree species mediated soil chemical changes in a Siberian artificial afforestation experiment

Natural and human-induced changes in the composition of boreal forests will likely alter soil properties, but predicting these effects requires a better understanding of how individual forest species alter soils. We show that 30 years of experimental afforestation in Siberia caused species-specific changes in soil chemical properties, including pH, DOC, DON, Na⁺, NH₄ ⁺, total C, C/N, Mn²⁺, and SO₄ ^2-. Some of these properties –- pH, total C, C/N, DOC, DON, Na⁺ –- also differed by soil depth, but we found no strong evidence for species-dependent effects on vertical differentiation of soil properties (i.e., no species × depth interaction). A number of soil properties –- NO₃ ^–, N, Al³⁺, Ca²⁺, Fe³⁺, K⁺, Mg²⁺ and Cl^– –- responded to neither species nor depth. The six studied species may be clustered into three groups based on their effects on the soil properties. Scots pine and spruce had the lowest pH, highest C/N ratio and intermediate C content in soil. The other two coniferous species, Arolla pine and larch, had the highest soil C contents, highest pH values, and intermediate C/N ratios. Finally, the two deciduous hardwood species, aspen and birch, had the lowest C/N ratio, intermediate pH values, and lowest C content. These tree-mediated soil chemical changes are important for their likely effects on soil microbiological activities, including C and N mineralization and the production and consumption of greenhouse gases.     

Effect of soil moisture at different temperatures on Rhizoctonia root rot of wheat seedlings

The effect of soil moisture at different temperatures on root rot of wheat seedlings caused by Rhizoctonia solani AG-8 was studied in temperature controlled water tanks under glasshouse conditions. Four moisture levels (15, 30, 50 and 75% of soil water holding capacity at saturation which were equal to –10, –7, –5 and –3 kPa, respectively) were tested in tanks maintained 10, 15, 20 or 25 °C. The role of microbial activity in the effect of soil moisture and temperature on disease severity was also studied by including treatments of steam treated soil. Results showed that at soil moisture levels optimum for plant growth (50 and 75% WHC) disease was more severe at a lower temperature (10 °C), but under relatively dry conditions (15% WHC) disease levels were similar at all temperatures tested. In warm soils (20 and 25 °C) at high soil moisture levels (50 and 75% WHC), disease was more severe in steam treated soil than in non-steam treated soil, indicating that the suppression of disease in natural soil under these conditions was associated with high soil microbial activity.     

Transpiration/biomass ratio for carrots as affected by salinity,nutrient supply and soil aeration

The influence of salinity, nutrient level and soil aeration on the transpiration coefficient, defined as amount of water transpired/unit biomass produced (transpiration/biomass ratio) of carrots was investigated under non-limiting conditions with respect to water supply.Under optimum conditions and favorable nutrient supply, the transpiration coefficient amounted to 280–310 g H₂O g^–1 storage root dry weight (RDW). The transpiration coefficient did not change significantly up to salt concentration of 16 mS cm^–1 in the soil solution under otherwise optimum conditions. Higher salt concentrations or low nutrient levels increased the transpiration coefficient to values of 390–540 g H₂O g^–1 RDW. It is suggested that the transpiration coefficient is not affected by salinity as long as toxic effects and nutrient imbalances do not occur. The transpiration coefficient was not increased by impeded soil aeration. Biomass production was more negatively influenced by adverse soil conditions (salinity, low nutrient level, impeded soil aeration) than was the transpiration coefficient.     

Effect of soil moisture and phosphate level on root hair growth of corn roots

Summary Root hairs have been shown to enhance P uptake by plants growing in low P soil. Little is known of the factors controlling root hair growth. The objective of this study was to investigate the influence of soil moisture and P level on root hair growth of corn (Zea mays L.). The effect of volumetric soil moistures of 22% (M₀), 27% (M₁), and 32% (M₂) and soil (Raub silt loam, Aquic Argiudoll) P levels of, 0.81 (P₀), 12.1 (P₁), 21.6 (P₂), 48.7 (P₃), and 203.3 (P₄) mol P L^–1 initially in the soil solution, on shoot and root growth, P uptake, and root hair growth of corn was studied in a series of pot experiments in a controlled climate chamber. Root hair growth was affected more by soil moisture than soil P. The percentage of total root length with root hairs and the density and length of root hairs on the root sections having root hairs all increased as soil moisture was reduced from M₂ to M₀. No relationship was found between root hair length and soil P. Density of root hairs, however, was found to decrease with an increase in soil P. No correlation was found between root hair growth parameters and plant P content, further suggesting P plays a secondary role to moisture in regulating root hair growth in soils. The increase in root hair growth appears to be a response by the plant to stress as yield and P uptake by corn grown at M₀ were only 0.47 to 0.82, and 0.34 to 0.74, respectively, of that measured at M₁ across the five soil P levels. The increase in root hair growth at M₀, which represents an increase of 2.76 to 4.03 in root surface area, could offset, in part, the reduced rate of root growth, which was the primary reason for reduced P uptake under limited soil moisture conditions.Journal Paper No. 10,066 Purdue Univ. Agric. Exp. Stn., W. Lafayette, IN 47907. Contribution from the Dep. of Agron. This paper was supported in part by a grant from the Tennessee Valley Authority.     

Moisture retention properties of a mycorrhizal soil

The water relations of arbuscular mycorrhizal plants have been compared often, but virtually nothing is known about the comparative water relations of mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis typically affects soil structure, and soil structure affects water retention properties; therefore, it seems likely that mycorrhizal symbiosis may affect soil water relations. We examined the water retention properties of a Sequatchie fine sandy loam subjected to three treatments: seven months of root growth by (1) nonmycorrhizal Vigna unguiculata given low phosphorus fertilization, (2) nonmycorrhizal Vigna unguiculata given high phosphorus fertilization, (3) Vigna unguiculata colonized by Glomus intraradices and given low phosphorus fertilization. Mycorrhization of soil had a slight but significant effect on the soil moisture characteristic curve. Once soil matric potential (_m) began to decline, changes in _m per unit change in soil water content were smaller in mycorrhizal than in the two nonmycorrhizal soils. Within the range of about –1 to –5 MPa, the mycorrhizal soil had to dry more than the nonmycorrhizal soils to reach the same _m. Soil characteristic curves of nonmycorrhizal soils were similar, whether they contained roots of plants fed high or low phosphorus. The mycorrhizal soil had significantly more water stable aggregates and substantially higher extraradical hyphal densities than the nonmycorrhizal soils. Importantly, we were able to factor out the possibly confounding influence of differential root growth among mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis affected the soil moisture characteristic and soil structure, even though root mass, root length, root surface area and root volume densities were similar in mycorrhizal and nonmycorrhizal soils.     

A cyclical but asynchronous pattern of fine root and woody biomass production in a hardwood forest of southern Quebec and its relationships with annual variation of temperature and nutrient availability

In contrast to the well-documented seasonal variation in growth of below- and above-ground components of trees, the annual variation in below- and aboveground production is not well understood. In this study, we report on the monitoring of an unmanaged hardwood forest ecosystem in a small watershed of southern Quebec between 1993 and 1999. Below- and above-ground biomass production, leaf and soil solution chemistry, and air temperature were measured at different regular intervals and are reported on an annual basis. The objective of the study was to describe the annual dynamics of carbon partitioning between below- and above-ground tree components and to gain a better understanding of the soil and climatic factors that govern it. Fine root production peaked one year earlier than woody biomass production and years with high production of fine roots had low woody biomass production. All models that included May temperature in the calculation of the predicting/independent variables were significant predictors of total tree biomass production (r > 0.87). Fine root production was associated negatively with the previous year average growing season temperature (r < -0.84). Soil solution NO₃ ^–, NH₄ ⁺ and NO₃ ^– + NH₄ ⁺ concentrations were positively correlated with fine root production (r > 0.72) and negatively correlated with woody biomass production (r < -0.84). Leaf N and P concentrations were negatively correlated (r = -0.99 and r = -0.98, respectively) with fine root production for the period of 1994–1998. Our results suggest that a cool growing season, and in particular a cool month of October, is likely to result in low fine root production and nutrient uptake the following year and, therefore, to increase soil N availability and decrease leaf N. This initial response is thought to be the first step of a feedback loop involving plant N nutrition, soil N availability, fine root growth and aboveground biomass production that led to a cyclical (3–4 years) but asynchronous production of fine roots and aboveground biomass production.