Annual and seasonal changes in fine root biomass of a Quercus ilex L. forest

The biomass, production and mortality of fine roots (roots with diameter <2.5 mm) were studied in a typical Mediterranean holm oak (Quercus ilex L.) forest in NE Spain using the minirhizotron methodology. A total of 1212 roots were monitored between June of 1994 and March of 1997. Mean annual fine root biomass in the holm oak forest of Prades was 71±8 g m^–2 yr^–1. Mean annual production for the period analysed was 260+11 g m^–2 yr^–1. Mortality was similar to production, with a mean value of 253±3 g m^–2 yr^–1. Seasonal fine root biomass presented a cyclic behaviour, with higher values in autumn and winter and lower in spring and summer. Production was highest in winter, and mortality in spring. In summer, production and mortality values were the lowest for the year. Production values in autumn and spring were very similar. The vertical distribution of fine root biomass decreased with increasing depth except for the top 10–20 cm, where values were lower than immediately below. Production and mortality values were similar between 10 and 50 cm depth. In the 0–10 cm and the 50–60 cm depth intervals, both production and mortality were lower.     

Does liquid fertilization affect fine root dynamics and lifespan of mycorrhizal short roots?

We studied effects of nitrogen, other nutrients and water (liquid fertilization; LF) on fine root dynamics (production, mortality) and life span of mycorrhizal short roots in a Norway spruce stand, using minirhizotrons. Data were collected and analyzed during a two-year period at depths of 0–20 cm, 21–40 cm and 41–85 cm, six years after the start of treatment. Relative to control (C), root production was lower in LF plots at depth 0–20 cm. Root production increased significantly at depth 41–85 cm. Fine root mortality in LF plots was higher at all depths. Life span of mycorrhizal short roots in LF plots was significantly lower than C plots and at the end of the study no mycorrhizal short roots were alive. It is suggested that the water and nitrogen input lower longevity of mycorrhizal short roots and promote fine root production at deeper soil layers.     

The effect of lime and gypsum applications on a sessile oak ( Quercus petraea (M.) Liebl.) stand at La Croix-Scaille (French Ardennes) II. Fine root dynamics

Fine root distribution, quantities, dynamics and composition were studied in a sessile oak coppice stand in the French Ardennes on an acidic soil (< pH-H2O 4.5), one to five years after lime or gypsum applications. Fine root biomass and length increased and specific root length decreased after lime or gypsum treatments. The treatment responses were strongest four to five years after the applications, but the tendencies after one year were similar. The effects were pronounced in the top 15 cm but also at 30–45 cm four to five years after liming. The latter effect suggests an indirect positive feedback from the aerial parts of the trees into the deeper soil layers. Sequential sampling for two years revealed large differences in total fine root length between the years, and also indicated that fine root turnover was lower after liming or gypsum applications than in the control. This seemed to be related to a lower fine root mortality and higher longevity rather than to increased fine root production. The improved nutrient status of the fine roots corroborates this and coincides with improved foliar nutrition and tree growth. Moderate doses of lime and gypsum appeared effective in enhancing root system uptake function, resulting in increased above ground growth.     

树木根系碳分配格局及其影响因子

根系作为树木提供养分和水分的“源”和消耗C的“汇”,在陆地生态系统C平衡研究中具有重要的理论意义。尽管20多年来的研究已经认识到根系消耗净初级生产力占总净初级生产力较大的比例,但是,根系（尤其是细根）消耗C的机理以及C分配的去向一直没有研究清楚。主要原因是细根消耗光合产物的生理生态过程相当复杂,准确估计各个组分消耗的C具有很大的不确定性,常常受树种和环境空间和时间异质性、以及研究方法的限制。综述了分配到地下的C主要去向,即细根生产和周转、呼吸及养分吸收与同化、分泌有机物、土壤植食动物,及有关林木地下碳分配机理的几种假说,分析了地下碳分配估计中存在的不确定性。目的是在全球变化C循环研究中对生态系统地下部分根系消耗的C以及分配格局引起重视。     

Fine root demography in alfalfa (Medicago sativa L.)

In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N₂ fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N₂-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha^–1 and 60 kg N ha^–1, with no differences due to N₂ fixation capacity or root system architecture.     

Root dynamics in barley,lucerne and meadow fescue investigated with a mini-rhizotron technique

Root development, including depth distribution, was followed in pure barley stands (Hordeum distichum, L.) with or without nitrogen fertilization and in barley undersown with lucerne (Medicago sativa L.) or meadow fescue (Festuca pratensis, Huds.). The number of roots per 5 cm depth level down to 1 m was counted frequently during the growing season using mini-rhizotrons, i.e., transparent tubes inserted into the soil. Root biomass at different depths down to 1 m was estimated from soil cores taken one month before harvest. The results from the two methods were compared and root counts in the different treatments were compared with the above-ground growth and production. Nitrogen-fertilized barley in pure stand had the highest biomass both above and below ground. According to the mini-rhizotron observations this treatment also had a deeper and denser root system, until barley harvest, than the other treatments. After barley harvest, roots from the undersown lucerne continued to increase, whereas the number of roots in the undersown meadow fescue remained the same. The root system in barley/meadow fescue did not penetrate into the subsoil, where more than 60% of the number of roots in barley undersown with lucerne were found. In general, the mini-rhizotron results indicated a higher relative abundance of roots in the deeper layers than the root biomass estimated with the soil coring method.     

Effects of throughfall manipulation on soil nutrient status: results of 12 years of sustained wet and dry treatments

To investigate the potential effects of changing precipitation on forest ecosystems, the Throughfall Displacement Experiment (TDE) was established on Walker Branch Watershed, Tennessee, in 1993. Three different throughfall amounts were tested: ?33% (DRY); ambient (no change, AMB); and +33% (WET). Throughfall manipulations had no statistically significant effects on total C, N, exchangeable Ca²⁺, Mg²⁺, bicarbonate‐extractable P, or extractable SO₄^2? in soils after 12 years of sustained treatments. Increased K⁺ inputs in the WET treatment resulted in relative increases in exchangeable K⁺compared with the AMB and DRY treatments. Soil C, N, and extractable P declined in all treatments over the 12‐year study, and the declines in N were inexplicably large. Field observations contrasted with earlier simulations from the Nutrient Cycling Model (NuCM), which predicted greater decreases in exchangeable K⁺, Ca²⁺, Mg²⁺, and extractable P in the order WET>AMB>DRY, and no change in C, N, and extractable SO₄^2?. The failure of the NuCM model to accurately predict observed changes is attributed to the lack of mechanisms for deep rooting and the transfer of throughfall K⁺ from one plot to another in the model. Measurements of element availability using resin membranes during the final years showed higher values in wet and lower values in dry treatments compared with ambient conditions for mineral N, K, Mn, Zn, and Al, but the opposite for B, Ca, and Mg. In the cases of Ca and Mg, the patterns in resin values were similar to those at the soil exchange sites (greatest in the dry treatment) and appeared to reflect pretreatment differences. This study showed that while longer term changes in soil nutrients are likely to occur with changes in precipitation, potential changes over this 12‐year interval were buffered by ecosystem processes such as deep rooting.     

Relationships between fine root dynamics and nitrogen availability in Michigan northern hardwood forests

Minirhizotrons were used to observe fine root (Б mm) production, mortality, and longevity over 2 years in four sugar-maple-dominated northern hardwood forests located along a latitudinal temperature gradient. The sites also differed in N availability, allowing us to assess the relative influences of soil temperature and N availability in controlling fine root lifespans. Root production and mortality occurred throughout the year, with most production occurring in the early portion of the growing season (by mid-July). Mortality was distributed much more evenly throughout the year. For surface fine roots (0-10 cm deep), significant differences in root longevity existed among the sites, with median root lifespans for root cohorts produced in 1994 ranging from 405 to 540 days. Estimates of fine root turnover, based on the average of annual root production and mortality as a proportion of standing crop, ranged from 0.50 to 0.68 year^-1 for roots in the upper 30 cm of soil. The patterns across sites in root longevity and turnover did not follow the north to south temperature gradient, but rather corresponded to site differences in N availability, with longer average root lifespans and lower root turnover occurring where N availability was greater. This suggests the possibility that roots are maintained as long as the benefit (nutrients) they provide outweighs the C cost of keeping them alive. Root N concentrations and respiration rates (at a given temperature) were also higher at sites where N availability was greater. It is proposed that greater metabolic activity for roots in nitrogen-rich zones leads to greater carbohydrate allocation to those roots, and that a reduction in root C sink strength when local nutrients are depleted provides a mechanism through which root lifespan is regulated in these forests.     

高寒草甸植物群落不同根序根系特征对降雨量变化的响应

根系是草原生态系统中最重要的碳库之一,分析高寒草甸植物群落生物量和地下不同径级根系碳分配特征及根系的生长特征对降雨变化的响应,有利于了解全球变化背景下高寒草甸植物根系、土壤碳氮循环及其过程。采用微根管技术原位监测5种降雨处理下(增雨50%:1.5P、自然降雨:1.0P、减雨30%:0.7P、减雨50%:0.5P、减雨90%:0.1P)高寒草甸植物群落和根系属性(现存量、生产量、死亡量、根系寿命和周转速率)的变化特征,结果表明:(1)降雨变化对地上植物群落生物量无显著影响,但0.5P和0.1P显著增加禾本科生物量(P<0.05)。(2)总根系现存量在处理间无显著差异,但随着降雨量减少呈先增加后降低的趋势。土层间不同径级根系现存量差异显著,0-10 cm土层1.5P和0.7P1级根现存量显著增加,2级和3级根现存量显著降低;在10-20 cm土层,1.0P2级根系现存量显著高于其余处理(P<0.05)。(3)总根生产量与死亡量随降雨减少而降低,在0-10 cm土层,1.0P总根生产量和死亡量最高,0.1P显著降低了1级根生产量(P<0.05)。(4)0.1P显著增加10-20 cm土层1级根和总根寿命(P<0.05)。(5)根系周转随降雨量减少呈降低趋势,但无显著差异(P>0.05)。(6)结构方程模型进一步表明:根系现存量和生产量受土层和水分的直接影响,土层和养分对根系周转有负效应。综上所述,降雨量的变化并未显著改变地下总根系生物量,但少量降雨变化(0.7P、1.5P)会降低植物对2、3级根生物量的分配,投入更多资源以促进1级根的生长;而水分下降至轻度水分胁迫(0.1P),植物会减少地下各径级根系生物量的分配,保持低根系生物量消耗和低根系生长来维持其正常的生长状态,完成其正常的生态功能。     

Seasonal development of loblolly pine lateral roots in response to stand density and fertilization

In 1989, two levels each of stand density and fertilization treatments were factorially established in a 9-year-old loblolly pine plantation on a P-deficient Gulf Coastal Plain site in Rapides Parish, Louisiana, USA. In 1995, a second thinning was conducted on the previously thinned plots and fertilizer was re-applied to the previously fertilized plots. The morphology of new long lateral roots was evaluated at 2-week intervals in five Plexiglas rhizotrons per plot of two replications. The overall objective of this study was to evaluate the seasonal initiation of six morphological categories of long lateral roots ( 2.5 cm in length) in response to stand density and fertilization. Lateral root development exhibited a seasonal pattern with the initiation of branched lateral roots predominantly occurring in spring and summer. The initiation of non-branched lateral roots occurred throughout the year regardless of season. Stand density did not affect lateral root morphological development. However, fertilization stimulated the initiation of branched lateral roots that were greater than 1 mm in diameter.     

Root distribution,standing crop biomass and belowground productivity in a semidesert in México

In a semidesert community in México (Zapotitlán de las Salinas, Puebla) the vertical distribution of roots and root biomass was estimated at 0–100 cm depth on two sampling dates, November 1995 (wet season) and January 1998 (dry season). Root productivity at 7 to 14.5 cm depth was estimated with the in-growth core technique every two months from March 1996 to February 1998. The relationship between environmental factors and seasonal root productivity was analyzed. Finally, we tested the effect of an irrigation equivalent to 20 mm of rain on root production. Seventy four percent of the total number of roots were found at 0-40 cm depth. Very fine roots (<1 mm diameter) were found throughout the soil profile (0-100 cm). In contrast, fine roots (1-3 mm diameter) were found only from 0–90 cm depth, and coarse roots (>3 mm diameter) from 0–60 cm depth. The root biomass was 971.5 g m^–2 (S.D. = 557.39), the very fine and fine roots representing 62.9% of the total. Total root productivity, as estimated with the ingrowth core technique, was 0.031 Mg ha^–1 over the dry season and 0.315 Mg ha^–1 over the wet season. Only very fine roots were obtained at all sampling dates. Rainfall was significantly correlated with very fine root production. The difference between fine root production in non-watered (0.054 g m^–2) and watered (0.429 g m^–2) treatments was significant. The last value was the same as that predicted for a rain of 20 mm, according to the exponential model describing the relation between the production of very fine roots and rainfall at the site.     

Alley cropping with Senna siamea in South-western Nigeria: II. Dry matter,total N,and urea-derived N dynamics of the Senna and maize roots

Crop and tree roots are crucial in the nutrient recycling hypotheses related to alley cropping systems. At the same time, they are the least understood components of these systems. The biomass, total N content and urea-derived N content of the Senna and maize roots in a Senna-maize alley cropping system were followed for a period of 1.5 years (1 maize-cowpea rotation followed by 1 maize season) to a depth of 90 cm, after the application of ¹⁵N labeled urea. The highest maize root biomass was found in the 0–10 cm layer and this biomass peaked at 38 and 67 days after planting the 1994 maize (DAP) between the maize rows (112 kg ha^–1, on average) and at 38, 67 and 107 DAP under the maize plants (4101 kg ha^–1, on average). Almost no maize roots were found below 60 cm at any sampling date. Senna root biomass decreased with time in all soil layers (from 512 to 68 kg ha^–1 for the 0–10 cm layer between 0 and 480 DAP). Below 10 cm, at least 62% of the total root biomass consisted of Senna roots and this value increased to 87% between 60 and 90 cm. Although these observations support the existence of a Senna root `safety net' between the alleys which could reduce nutrient leaching losses, the depth of such a net may be limited as the root biomass of the Senna trees in the 60–90 cm layer was below 100 kg ha^–1, equivalent to a root length density of only < 0.05 cm cm^–3. The proportion of maize root N derived from the applied urea (%Ndfu) decreased significantly with time (from 21% at 21 DAP to 8% at 107 DAP), while %Ndfu of the maize roots at the second harvest (480 DAP) was only 0.6%. The %Ndfu of the Senna roots never exceeded 4% at any depth or sampling time, but decreased less rapidly compared to the %Ndfu of the maize roots. The higher %Ndfu of the maize roots indicates that maize is more efficient in retrieving urea-derived N. The differences in dynamics of the %Ndfu also indicate that the turnover of N through the maize roots is much faster than the turnover of N through the Senna roots. The recovery of applied urea-N by the maize roots was highest in the top 0–10 cm of soil and never exceeded 0.4% (at 38 DAP) between the rows and 7.1% (at 67 DAP) under the rows. Total urea N recovery by the maize roots increased from 1.8 to 3.2% during the 1994 maize season, while the Senna roots never recovered more than 0.8% of the applied urea-N at any time during the experimental period. These values are low and signify that the roots of both plants will only marginally affect the total recovery of the applied urea-N. Measurement of the dynamics of the biomass and N content of the maize and Senna roots helps to explain the observed recovery of applied urea-N in the aboveground compartments of the alley cropping system.     

帽儿山温带落叶阔叶林细根生物量、生产力和周转率

细根在森林生态系统能量流动与物质循环中占有重要地位,但其生物量、生产和周转测定尚存在很大的不确定性,而且局域尺度空间变异机制尚不清楚。本研究分析了帽儿山温带天然次生林活细根生物量和死细根生物量在0～100 cm剖面的垂直分布与0～20 cm细根的季节动态、生产力和周转率,对比了采用连续根钻法(包括决策矩阵法和极差法)和内生长袋(直径3和5 cm)估测细根生产力和细根周转率,并探讨了可能影响细根的林分因子。结果表明: 76.8%的活细根生物量和62.9%的死细根生物量均集中在0～20 cm土层,随着深度增加,二者均呈指数形式减少。活细根生物量和死细根生物量的季节变化不显著,可能与冬季几乎无降雪而夏季降雨异常多有关。2种直径内生长袋估计的细根生产力无显著差异;对数转换后决策矩阵、极差法和内生长法估计的细根生产力和细根周转率差异显著。随着土壤养分增加,活细根生物量和死细根生物量比值显著增加,死细根生物量显著减少,但活细根生物量、细根生产力和细根周转率均无显著变化;细根周转率与前一年地上木质生物量增长量呈显著正相关,但与当年地上木质生物量增长量无显著相关关系。     

Elevated CO(2) and elevated temperature have no effect on Douglas-fir fine-root dynamics in nitrogen-poor soil

Here, we investigate fine-root production, mortality and standing crop of Douglas-fir (Pseudotsuga menziesii) seedlings exposed to elevated atmospheric CO(2) and elevated air temperature. We hypothesized that these treatments would increase fine-root production, but that mortality would be greater under elevated temperature, leading to a smaller increase in standing crop. Seedlings were grown in outdoor, sun-lit controlled-environment chambers containing native soil. They were exposed in a factorial design to two levels of atmospheric CO(2) and two levels of air temperature. Minirhizotron methods were used to measure fine-root length production, mortality and standing crop every 4 wk for 36 months. Neither elevated atmospheric CO(2) nor elevated air temperature affected fine-root production, mortality, or standing crop. Fine roots appeared to root deeper in the soil profile under elevated CO(2) and elevated temperature. Low soil nitrogen (N) levels apparently limited root responses to the treatments. This suggests that forests on nutrient-poor soils may exhibit limited fine-root responses to elevated atmospheric CO(2) and elevated air temperature.     

Changes in the risk of fine-root mortality with age: a case study in peach, Prunus persica (Rosaceae)

Previous studies suggest that younger roots are more vulnerable to mortality than older roots. We analyzed minirhizotron data using a mixed-age, proportional hazards regression approach to determine whether the risk of mortality (or "hazard") was higher for younger roots than for older roots in a West Virginia peach orchard. While root age apparently had a strong effect on the hazard when considered alone, this effect was largely due to different rates of mortality among roots of different orders, diameters, and depths. Roots with dependent laterals (higher order roots) had a lower hazard than first-order roots in 1996 and 1997. Greater root diameter was also associated with a decreased hazard in both 1996 and 1997. In both years, there was a significant decrease in the hazard with depth. When considered alone, age appeared to be a strong predictor of risk: a 1-d increase in initial root age was associated with a 1.26-2.62% decrease in the hazard. However, when diameter, order, and depth were incorporated into the model, the effect of root age disappeared or was greatly reduced. Baseline hazard function plots revealed that the timing of high-risk periods was generally related to seasonal factors rather than individual root age.     

Interactive effects of soil warming and fertilization on root production, mortality, and longevity in a Norway spruce stand in Northern Sweden

The effects of soil warming and nitrogen availability on root production, longevity and mortality were studied using minirhizotrons in irrigation (C), fertilized (F), heated (H), and heated‐fertilized (HF) plots in a Norway spruce stand in northern Sweden from October 1996 to October 1997. Irrigation was included in all treatment plots. Heating cables were used to maintain the soil temperature in heated plots at 5°C above that in unheated plots during the growing season. A Kaplan–Meier approach was used to estimate the longevity of fine roots and Cox proportional hazards regression to analyze the effects of the H, F, and HF treatments on the risk of root mortality. The proportion of annual root length production contributed by winter–spring production amounted to 52% and 49% in heated plots and heated‐fertilized plots, respectively. The annual root length production in C plots was significantly higher than in other treatments, while the HF treatment gave significantly greater production compared with the F treatment. The risk of mortality (hazard ratio) relative to C plots was higher in H plots (358%) and F plots (191%). The interaction between heating and fertilizing was strongly significant. The increase in the risk of root mortality in combined fertilization and heating (103%) was lower than that in the H or F plots. The results show that nitrogen addition combined with warmer temperatures decreases the risk of root mortality, and fine root production is a function of the length of the growing season. In the future, fertilization combined with the warmer temperatures expected to follow predicted climatic change may increase root production in boreal forests at low fertility sites.     

Rapid root closure after fire limits fine root responses to elevated atmospheric CO2 in a scrub oak ecosystem in central Florida, USA

Elevated atmospheric carbon dioxide (CO₂) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long‐term CO₂ exposure. We investigated the effects of elevated CO₂ on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO₂ enrichment began immediately after a controlled burn, which simulated the natural disturbance that occurs in this system every 10–15 years. We hypothesized that (1) root abundance would increase in both treatments as the system recovered from fire; (2) elevated CO₂ would stimulate root growth; and (3) elevated CO₂ would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm^?2) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO₂ significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO₂ stimulation was initially more pronounced in the top 10 cm and 38–49 cm below the soil surface. However, these responses completely disappeared during the third year of experimental treatment: elevated CO₂ had no effect on root abundance or on the depth distribution of fine roots during years 3–7. The results suggest that, within a few years following fire, fine roots in this scrub oak ecosystem reach closure, defined here as a dynamic equilibrium between production and mortality. These results further suggest that elevated CO₂ hastens root closure but does not affect maximum root abundance. Limitation of fine root growth by belowground resources – particularly nutrients in this nutrient‐poor soil – may explain the transient response to elevated CO₂.     

Canopy and environmental control of root dynamics in a long-term study of Concord grape

Below-ground carbon allocation represents a substantial fraction of net photosynthesis in plants, yet timing of below-ground allocation and endogenous and exogenous factors controlling it are poorly understood. Minirhizotron techniques were used to examine root populations of Vitis labruscana Bailey cv. Concord under two levels of dormant-season canopy removal and irrigation. Root production, pigmentation, death and disappearance to a depth of 110 cm were determined over two wet and two dry years (1997-2000). There was continual root production and senescence, with peak root production rates occurring by midseason. Later in the season, when reproductive demands for carbon were highest and physical conditions limiting, few roots were produced, especially in dry years in nonirrigated vines. Root production under minimal canopy pruning was generally greater and occurred several weeks earlier than root production under heavy pruning, corresponding to earlier canopy development. Initial root production occurred in shallow soils, likely due to temperatures at shallow depths being warmer early in the season. Our study showed intricate relationships between internal carbon demands and environmental conditions regulating root allocation.     

Fine root length production,mortality and standing root crop dynamics in an intensively managed sweetgum (Liquidambar styraciflua L.) coppice

We used minirhizotrons and micro-video technology to study fine root production, mortality and standing root crop dynamics in an intensively managed sweetgum (Liquidambar styraciflua L.) coppice. The experiment was a split-plot design with two levels of fertilization. Low-level treatment plots received 560 kg ha-1 yr-1 fertilizer (19:9:19 NPK) via a drip irrigation system and high-level treatment plots received twice this amount. Approximately 150 cm yr-1 irrigation was applied to all treatments. There were no significant treatment differences in daily average fine root production or mortality. The phenology of fine root production and mortality, however, was characterized by strongly seasonal asynchrony. Production increased throughout the summer, peaked in September and declined sharply over the winter. Root mortality was not observed in either treatment until August, and then increased significantly throughout the winter months. There were also no significant treatment differences in standing fine root length. Standing live root length was greatest in the fall and dead root length was greatest over the winter. Roots in the upper 25 cm of the soil profile appeared to be significantly more dynamic (i.e. greater production and mortality) than deeper roots in both treatments. High levels of fertilization apparently do not alter fine root dynamics at our sites, in contrast to fertilization responses observed in other, more nutrient-poor environments.     

Experimental evaluation of an efflux-influx model of C exudation by individual apical root segments

The aim of this study was to evaluate if a model describing the efflux and the influx of C through the root surface could be fitted to experimental short-term kinetics of carbon (C) exudation by individual apical root segments in maize (Zea mays L.). The efflux of C was set constant or modelled by a power function of the distance from the apex to simulate the greater release of C around the root tip commonly reported in the literature. The influx was proportional to the C concentration in the external solution to simulate the active re-uptake of exudates by the root. Plants were exposed to full light or to shade to manipulate C allocation to roots. The model with a constant efflux gave satisfactory fits to the kinetics of exudation (average R(2)=0.66). The average gross efflux was then 2.1 mug C cm(-2) root surface h(-1). The model was improved if exudation was set more intense towards the root apex (average R(2)=0.74). The estimated gross efflux decreased then from 5.2 mug C cm(-2) h(-1) at the apex to 1.8 mug C cm(-2) h(-1) for the region located 5-25 cm from the root tip. The decrease in net exudation of individual roots due to the shading of plants was weak, which may indicate that the import of C by the primary roots studied was not reduced significantly. By describing the exudation of an apical root segment of variable length and diameter, the model is a first step in linking exudation to root system architecture models and to whole plant functioning.