How does nitrogen availability alter rhizodeposition in Lolium multiflorum Lam. during vegetative growth?

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth. Lolium multiflorum Lam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of ¹⁴C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of ¹⁴C to roots. In parallel, N fertilisation increased the root soluble ¹⁴C and the ¹⁴C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration).     

Soil types with different texture affects development of Rhizoctonia root rot of wheat seedlings

The effect of different soil textures, sandy (97.5% sand, 1.6% silt, 0.9% clay), loamy sand (77% sand, 11% silt, 12% clay) and a sandy clay loam (69% sand, 7% silt, 24% clay), on root rot of wheat caused by Rhizoctonia solani Kühn Anastomosis Group (AG) 8 was studied under glasshouse conditions. The reduction in root and shoot biomass following inoculation with AG-8 was greater in sand than in loamy sand or sandy clay loam. Dry root weight of wheat in the sand, loamy sand and sandy clay loam soils infested with AG-8 was 91%, 55% and 28% less than in control uninfested soils. There was greater moisture retention in the loamy sand and sandy clay loam soils as compared to the sand in the upper 10–20 cm. Root penetration resistance was greater in loamy sand and sandy clay loam than in sand. Root growth in the uninfested soil column was faster in the sand than in the loamy sand and sandy clay loam soils, the roots in the sandy soil being thinner than in the other two soils. Radial spread of the pathogen in these soils in seedling trays was twice as fast in the sand in comparison to the loamy sand which in turn was more than twice that in the sandy clay loam soil. There was no evidence that differences among soils in pathogenicity or soil spread of the pathogen was related to their nutrient status. This behaviour may be related to the severity of the disease in fields with sandy soils as compared to those with loam or clay soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

沙化草地恢复过程中土壤有机碳物理组分和全氮含量的变化

以宁夏中北部盐池县不同恢复阶段的沙化草地(流动沙地、半固定沙地、固定沙地和荒漠草地)土壤为研究对象,分析土壤粗砂粒有机碳、细砂粒有机碳、粘粉粒有机碳、重组有机碳、轻组有机碳和土壤全氮的变异特征,探讨沙化草地恢复过程中土壤有机碳变化机制。结果显示:(1)荒漠草地、固定沙地、半固定沙地0~30cm土层土壤,粗砂粒有机碳、细砂粒有机碳和粘粉粒有机碳含量分别比流动沙地增加了67.7%、69.8%、212.1%和48.8%、35.3%、99.9%以及33.6%、23.0%、48.9%。(2)随着沙化草地不同程度的恢复,轻组有机碳含量、分配比例和重组有机碳含量均表现为流动沙地半固定沙地固定沙地荒漠草地;重组有机碳分配比例随沙化草地恢复程度的升高呈降低趋势。(3)土壤细砂粒、粘粉粒、重组有机碳、轻组有机碳、粗砂粒有机碳、细砂粒有机碳、粘粉粒有机碳含量与土壤有机碳、土壤全氮含量均呈显著正相关关系,与粗砂粒含量均呈显著负相关关系。研究表明,轻组有机碳和粘粉粒有机碳含量对土壤有机碳的影响最大,轻组有机碳、重组有机碳、粗砂粒有机碳和粘粉粒有机碳对土壤全氮的影响最大,表明沙化草地的恢复有利于减小土壤侵蚀,改善土壤结构与质量。     

Field measurement of lupin belowground nitrogen accumulation and recovery in the subsequent cereal-soil system in a semi-arid Mediterranean-type climate

In situ ¹⁵N-labelling was used to provide a quantitative assessment of the total contribution of lupin (Lupinus angustifolius) to below-ground (BG) N accumulation during a growing season under field conditions, and to directly trace the fate of the lupin BG N in the next season, including quantifying the N benefit from lupin to a following wheat (Triticum aestivum) crop. The experiments were conducted at two sites, both experiencing a semi-arid Mediterranean-type climate in the wheat-growing region of Western Australia but with differing soil types, a deep sand (Moora) and a sand-over-clay shallow duplex soil (East Beverley, EB). Lupin shoot and root dry matter and total plant N accumulation, proportional dependence on nitrogen fixation and grain yield were greater at the deep sand site than the duplex soil site, although there was a similar proportion of shoot N to estimated total BG N at both sites. The proportion of total plant BG N decreased from the vegetative stage (42–51%) to peak biomass (25–39%) and maturity (23–34%). From 56–67% of BG N on the deep sand and 74–86% on the duplex soil was not recovered in coarse roots (>2 mm) or as soluble N, but was present in the insoluble organic N fraction. There was evidence for cycling of lupin root-derived N into soil microbial biomass and soluble organic N during lupin growth (by the late vegetative stage), but no evidence for leaching of legume derived BG N during the lupin season. Estimates of fixed N input BG were at least four times greater if based on total lupin BG N rather than on N recovered in coarse roots (>2 mm). There were no apparent losses of lupin BG N during the summer fallow period subsequent to lupin harvest at either site. Also, immediately prior to sowing of wheat there were similar proportions of lupin BG N in the inorganic (20–25%) and microbial biomass (6–9%) pools at both sites, with the majority of BG N detected in the <2 mm fraction of the soil column. However, the proportion of residual lupin BG N estimated to benefit the aboveground wheat biomass was relatively low, 10% on the deep sand and only 3% on the shallow duplex. Some (14%) residual lupin BG N was leached as nitrate to 1 m on the deep sand compared to 8% of residual lupin BG N leached to the clay layer (0.3 m) on the shallow duplex. About 27% of the residual lupin BG N on the deep sand at Moora had apparently mineralised by the end of the succeeding wheat season (i.e. recovered either in the wheat shoots, as inorganic N in the soil profile or as leached nitrate) compared to only 12% at EB. There was an unaccounted for large loss of residual lupin BG N (50%) from the duplex soil at EB during the wheat season, postulated to be chiefly via denitrification. At both sites after the wheat season a substantial proportion (32–55%) of legume derived BG N was still present as residual insoluble organic N, considered to be an important contribution to structural and nutritional long-term sustainability of these soils.     

The fate of carbon in pulse-labelled crops of barley and wheat

Wheat (cv. Gutha) and barley (cv. O'Connor) were grown as field crops on a shallow duplex soil (sand over clay) in Western Australia with their root systems contained within pvc columns. At four stages during growth, the shoots were pulse-labelled for 1.5h with¹⁴CO₂; immediately prior to labelling, the soil was isolated from the shoot atmosphere by pvc sheets. After labelling, the soil atmosphere was pumped through NaOH to trap respired CO₂ and after 2.5, 5, 7.5 and 24 h from the start of labelling, columns were destructively sampled to recover¹⁴C from the roots, soil and shoot.Both species showed similar patterns of¹⁴C distribution and changes in distribution through the growing season. During early tillering, 15–25% of the¹⁴C recovered after 24 h had been respired by the roots and rhizosphere, 17–27% was retained in the roots, 0.4–1.8% was recovered as water-soluble¹⁴C in the soil and the remainder (45–67%) was present in the shoot. These percentages changed during growth so that during grain filling only 2–3% of the¹⁴C recovered after 24 h was as respired CO₂, 2–6% was in the roots, 0.2% was in the soil and over 90% was in the shoot.The distribution of¹⁴C in components of the soil-plant system changed during the 24 h after labelling with the most rapid changes occurring generally during the first 7.5 h after labelling.Using growth measurements from adjacent plots, the amounts of C added to the soil were estimated for the whole season. Carbon input to the soil was about 48 gC m^–2 for wheat and 58 gC m^–2 for barley; the crops produced total shoot dry matter of 494 (wheat) and 735 g m^–2 (barley). Of the C input to the soil, 27.8% (wheat) and 40.3% (barley) was as respired C and only 3.3 (wheat) and 4.1% (barley) was collected as exudate (water-soluble material).     

The significance of denitrification of applied nitrogen in fallow and cropped rice soils under different flooding regimes I. Greenhouse experimènts

The role of nitrification-denitrification in the loss of nitrogen from urea applied to puddled soils planted to rice and subjected to continuous and intermittent flooding was evaluated in three greenhouse pot studies. The loss of N via denitrification was estimated indirectly using the¹⁵N balance, after either first accounting for NH₃ volatilization or by analyzing the¹⁵N balance immediately before and after the soil was dried and reflooded. When urea was broadcast and incorporated the loss of¹⁵N from the soil-plant systems depended on the soil, being about 20%–25% for the silt loams and only 10%–12% for the clay. Ammonia volatilization accounted for an average 20% of the N applied in the silt loam. Denitrification losses could not account for more than 10% of the applied N in any of the continuously flooded soil-plant systems under study and were most likely less than 5%. Intermittent flooding of soil planted to rice did not increase the loss of N. Denitrification appeared to be an important loss mechanism in continuously flooded fallow soils, accounting for the loss of approximately 40% of the applied¹⁵N. Loss of¹⁵N was not appreciably enhanced in fallow soils undergoing intermittent flooding. Apparently, nitrate formed in oxidized zones in the soil was readily denitrified in the absence of plant roots. Extensive loss (66%) of¹⁵N-labeled nitrate was obtained when 100 mg/pot of nitrate-N was applied to the surface of nonflooded soil prior to reflooding. This result suggests that rice plants may not compete effectively with denitrifiers if large quantities of nitrate were to accumulate during intermittent dry periods.     

Nitrate uptake ability by maize roots during and after drought stress

The effects of different intensities and durations of soil drought and re-watering on the nitrate uptake ability of maize roots were studied. Plants were grown in split-root containers with one part of the root system subjected to different intensities and durations of soil drought and re-watering while the other part of the root system was continuously watered to 23% (w/w) soil water content (70% water capacity). Experiments were performed in split-root containers to maintain a high growth rate, thus ensuring high nutrient demand of the shoot irrespective of the soil water regime. To avoid limitation of nitrate uptake by transport processes in the dry soil, and to ensure a uniform ¹⁴N/¹⁵N ratio at the root surface, ¹⁵N was applied to the roots by placing them into an aerated nutrient solution with 0.5 mM Ca(¹⁵NO₃)₂. Shoot elongation and biomass were only slightly affected by drought in one root compartment when the soil in the other root compartment was kept wet. Therefore, the growth-related nutrient demand of the shoot remained at a high level. At moderate levels of soil drought (10% w/w water content) the ability of the roots for N-uptake was not affected even after 10 d of drought. N-uptake ability was reduced to about 20% of the well-watered control only when the soil water content was decreased to 5%. Total soluble sugar content of the roots increased with increasing soil drought, indicating that low N-uptake ability of roots subjected to severe soil drought was not caused by low assimilate supply from the shoot. Nitrate uptake ability of roots maintained in very dry soil (5% soil water content w/w) even for a prolonged period of 8 d, recovered within 3 d following re-watering. Root growth increased one day after re-watering. A short-term experiment with excised roots formerly subjected to severe soil drought showed that nitrate uptake ability recovered in old and young root segments after 2 d of re-watering. Obviously, the increase in N-uptake ability after re-watering was caused not only by new root growth but also by recovery of the uptake ability of formerly stressed roots.     

Fibrous carrot root responses to irrigation and compaction of sandy and organic soils

Field experiments were performed in Southern Finland on fine sand and organic soil in 1990 and 1991 to study carrot roots. Fall ploughed land was loosened by rotary harrowing to a depth of 20 cm or compacted under moist conditions to a depth of 25–30 cm by three passes of adjacent wheel tracks with a tractor weighing 3 Mg, in April were contiguously applied across the plot before seed bed preparation. Sprinkler irrigation (30 mm) was applied to fine sand when moisture in the 0–15 cm range of soil depth was 50% of plant-available water capacity. For root sampling, polyvinyl chloride (PVC) cylinders (30 × 60 cm) were installed in the rows of experimental plots after sowing, and removed at harvest. Six carrot plants were grown in each of in these soil colums in situ in the field.Fine root length and width were quantified by image analysis. Root length density (RLD) per plant was 0.2–1.0 cm cm^-3 in the 0–30 cm range. The fibrous root system of one carrot had total root lengths of 130–150 m in loose fine sand and 180–200 m in compacted fine sand. More roots were observed in irrigated than non-irrigated soils. In the 0–50 cm range of organic soil, 230–250 m of root length were removed from loosened organic soils and 240–300 m from compacted soils. Specific root surface area (surface area divided by dry root weight) of a carrot fibrous root system averaged 1500–2000 cm² g^-1. Root length to weight ratios of 250–350 m g^-1 effectively compare with the ratios of other species.Fibrous root growth was stimulated by soil compaction or irrigation to a depth of 30 cm, in both the fine sand and organic soils, suggesting better soil water supply in compacted than in loosened soils. Soil compaction increased root diameters more in fine sand than it did in organic soil. Most of the root length in loosened soils (fine sand 90%, organic soil 80%) and compacted soils (fine sand 80%, organic soil 75%) was composed of roots with diameters of approximately 0.15 mm. With respect to dry weight, length, surface area and volume of the fibrous root system, all the measurements gave significant resposes to irrigation and soil compaction. Total root volumes in the 0–50 cm of soil were 4.3 cm³ and 9.8 cm³ in loosened fine sand and organic soils, respectively, and 6.7 cm³ and 13.4 cm³ in compacted sand and organic soils, respectively. In fine sand, irrigation increased the volume from 4.8 to 6.3 cm³.     

The distribution of tree and grass roots in savannas in relation to soil nitrogen and water

Here we describe the fine root distribution of trees and grasses relative to soil nitrogen and water profiles. The primary objective is to improve our understanding of edaphic processes influencing the relative abundance of trees and grasses in savanna systems. We do this at both a mesic (737 mm MAP) site on sandy-loam soils and at an arid (547 mm MAP) site on clay rich soils in the Kruger National Park in South Africa. The proportion of tree and grass fine roots at each soil depth were estimated using the δ¹³C values of fine roots and the δ¹³C end members of the fine roots of the dominant trees and grasses at our study sites. Changes in soil nitrogen concentrations with depth were indexed using total soil nitrogen concentrations and soil δ¹⁵N values. Soil water content was measured at different depths using capacitance probes. We show that most tree and grass roots are located in the upper layers of the soil and that both tree and grass roots are present at the bottom of the profile. We demonstrate that root density is positively related to the distribution of soil nitrogen and negatively related to soil moisture. We attribute the negative correlation with soil moisture to evaporation from the soil surface and uptake by roots. Our data is a snapshot of a dynamic process, here the picture it provides is potentially misleading. To understand whether roots in this system are primarily foraging for water or for nitrogen future studies need to include a dynamic component.     

雅鲁藏布江山南宽谷风沙化土地土壤养分和粒度特征

在雅鲁藏布江山南宽谷区,选择流动沙地、平缓沙砾地、半固定沙地、固定沙地和覆沙河滩地等类型样地,研究了不同深度(0—10 cm、10—20 cm和20—40 cm)土壤层的养分状况和粒度特征,探讨了风沙运动对土壤粒度组成和养分含量的影响。结果表明:1)风沙化土地土壤pH值呈中性、碱性和强碱性,土壤有机质和全氮含量均很低,但全磷和全钾均很高。土壤粒度组成表现为砂粒含量(53.83%—95.93%)>粉粒(3.3%—40.5%)>粘粒(0.77%—5.68%)。2)粘粒和粉粒含量均以覆沙河滩地(分别为4.02%和27.95%)最大、半固定沙地(分别为1.35%和5.27%)最小。粘粒含量表现为覆沙河滩地>固定沙地(2.98%)>河滩流动沙地(2.89%)>平缓沙砾地(1.69%)>河岸流动沙地(1.54%)>山坡流动沙地(1.49%)>半固定沙地。不同类型沙地粉粒含量的大小顺序与粘粒含量相似,仅在山坡流动沙地和河岸流动沙地的大小顺序有所差别。砂粒含量以半固定沙地(为93.40%)最大、覆沙河滩地最小(68.05%)。不同类型沙地的砂粒含量与粉粒含量的大小顺序正好相反。3)土壤养分含量与粘粒、粉粒、极细砂粒和细砂粒等细沙物质的相关性较强,与中砂粒、粗砂粒和极粗砂粒等粗沙物质呈负相关或相关性较弱。其中,粘粒和极细砂粒含量的增加对土壤养分的增加贡献较大。流动沙丘随风沙运动而不断往复摆动的现象和土壤细颗粒的迁移和损失,对不同类型沙地和沙丘部位的土壤养分状况及其再分配过程产生较大影响。     

On the assessment of root and soil respiration for soils of different textures: interactions with soil moisture contents and soil CO2 concentrations

Estimates of root and soil respiration are becoming increasingly important in agricultural and ecological research, but there is little understanding how soil texture and water content may affect these estimates. We examined the effects of soil texture on (i) estimated rates of root and soil respiration and (ii) soil CO₂ concentrations, during cycles of soil wetting and drying in the citrus rootstock, Volkamer lemon (Citrus volkameriana Tan. and Pasq.). Plants were grown in soil columns filled with three different soil mixtures varying in their sand, silt and clay content. Root and soil respiration rates, soil water content, plant water uptake and soil CO₂ concentrations were measured and dynamic relationships among these variables were developed for each soil texture treatment. We found that although the different soil textures differed in their plant-soil water relations characteristics, plant growth was only slightly affected. Root and soil respiration rates were similar under most soil moisture conditions for soils varying widely in percentages of sand, silt and clay. Only following irrigation did CO₂ efflux from the soil surface vary among soils. That is, efflux of CO₂ from the soil surface was much more restricted after watering (therefore rendering any respiration measurements inaccurate) in finer textured soils than in sandy soils because of reduced porosity in the finer textured soils. Accordingly, CO₂ reached and maintained the highest concentrations in finer textured soils (> 40 mmol CO₂ mol⁻¹). This study revealed that changes in soil moisture can affect interpretations of root and soil measurements based on CO₂ efflux, particularly in fine textured soils. The implications of the present findings for field soil CO₂ flux measurements are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Assimilation dynamics of soil carbon and nitrogen by wheat roots and Collembola

It has been demonstrated that plant roots can take up small amounts of low-molecular weight (LMW) compounds from the surrounding soil. Root uptake of LMW compounds have been investigated by applying isotopically labelled sugars or amino acids but not labelled organic matter. We tested whether wheat roots took up LMW compounds released from dual-labelled (¹³C and ¹⁵N) green manure by analysing for excess ¹³C in roots. To estimate the fraction of green manure C that potentially was available for root uptake, excess ¹³C and ¹⁵N in the primary decomposers was estimated by analysing soil dwelling Collembola that primarily feed on fungi or microfauna. The experimental setup consisted of soil microcosm with wheat and dual-labelled green manure additions. Plant growth, plant N and recoveries of ¹³C and ¹⁵N in soil, roots, shoots and Collembola were measured at 27, 56 and 84 days. We found a small (<1%) but significant uptake of green manure derived ¹³C in roots at the first but not the two last samplings. About 50% of green manure C was not recovered from the soil-plant system at 27 days and additional 8% was not recovered at 84 days. Up to 23% of C in collembolans derived from the green manure at 56 days (the 27 days sampling was lost). Using a linear mixing model we estimated that roots or root effluxes provided the main C source for collembolans (54−79%). We conclude that there is no solid support for claiming that roots assimilated green manure derived C due to very small or no recoveries of excess ¹³C in wheat roots. During the incubation the pool of green manure derived C available for root uptake decreased due to decomposition. However, the isotopic composition in Collembola indicated that there was a considerable fraction of green manure derived C in the decomposer system at 56 days thus supporting the premise that LMW compounds containing C from the green manure was released throughout the incubation. Responsible Editor: A. C. Borstlap.     

The effect of soil strength on the growth of pigeonpea radicles and seedlings

The effect of soil strength on the growth of pigeonpea radicles and seedlings was investigated in cores of three clay soils prepared at different water contents and bulk densities in the laboratory.Radicle elongation directly into soil cores was reduced from 50–70 mm d^-1 at strengths less than 0.5 MPa to 0 mm d^-1 at 3.5–3.7 MPa. The response to soil strength was affected by the water content of the soil, presumably as a result of reduced oxygen availability in wetter soil. This effect was apparent in soils wet to air-filled porosities less than 0.15 m³ m^-3.Radicles were more sensitive to high soil strength (>1.5 MPa) than were seedling roots which encountered the same conditions at 60 mm in the profile. Radicle growth ceased at 3.5 MPa which reduced seedling root growth by only 60%.Despite a 60% reduction in root length in the high strength zone, seedling roots compensated in zones of loose soil above and below the compacted layer, and total root length and shoot growth were unaffected. There was no evidence of a root signal response which results in reduced shoot growth in some species in response to high soil strength.The proliferation of roots in surface layers and the delayed penetration of the root system to depth in compacted soil are likely to expose seedlings to a greater risk of water-deficit in the field, particularly under dryland conditions where plants rely on stored subsoil water for growth.     

Growth and morphology of eucalypt seedling-roots, in relation to soil strength arising from compaction

Information on the response of root growth and morphology to soil strength is useful for testing suitability of existing and new tillage methods and/or for selecting plants suitable for a specific site with or without tillage. Although there is extensive published information on the root growth-soil strength relationships for annual agricultural plants, such information is scarce for woody, perennial tree species. The purpose of this study is to examine growth and morphology of the root systems of 17-day-old eucalypt seedlings with respect to variation in soil strength. Soil strength in this study was varied by compaction of a well-aggregated clay soil to bulk densities of 0.7–1.0 Mg m^-3 whilst maintaining adequate water availability and aeration for plant growth. Lengths and tip-diameters of primary and lateral roots were measured on the excavated root systems of seedlings.With increase in bulk density and also soil strength (expressed as penetrometer resistance), total length of primary and lateral roots decreased. There were 71 and 31% reduction in the lengths of primary and lateral roots respectively with an increase in penetrometer resistance from 0.4 to 4.2 MPa. This indicated primary roots to be more sensitive to high soil strength than the lateral roots. Average length of lateral roots and diameters of both primary and lateral root tips increased with an increase in soil strength as well. There was greater abundance of lateral roots (no. of lateral roots per unit length of primary root) and root hairs with increased soil strength. The observed root behaviour to variable soil strength is discussed in the context of compensatory growth of roots and overall growth of plants.     

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.     

Linking root traits and competitive success in grassland species

Background and aims

Measures of phosphorus (P) in roots recovered from soil underestimate total P accumulation below-ground by crop species since they do not account for P in unrecovered (e.g., fine) root materials. ³³P-labelling of plant root systems may allow more accurate estimation of below-ground P input by plants.

Methods

Using a stem wick-feeding technique ³³P-labelled phosphoric acid was fed in situ to canola (Brassica napus) and lupin (Lupinus angustifolius) grown in sand or loam soils in sealed pots.

Results

Recovery of ³³P was 93 % in the plant-soil system and 7 % was sorbed to the wick. Significantly more ³³P was allocated below-ground than to shoots for both species with 59–90 % of ³³P measured in recovered roots plus bulk and rhizosphere soil. ³³P in recovered roots was higher in canola than lupin regardless of soil type. The proportion of ³³P detected in soil was greater for lupin than canola grown in sand and loam (37 and 73 % lupin, 20 and 23 % canola, respectively). Estimated total below-ground P accumulation by both species was at least twice that of recovered root P and was a greater proportion of total plant P for lupin than canola.

Conclusion

Labelling roots using ³³P via stem feeding can empower quantitative estimates of total below-ground plant P and root dry matter accumulation which can improve our understanding of P distribution in soil-plant systems.

     

Effect of Soil Texture on the Distribution and Infectivity of Neoaplectana glaseri (Nematoda: Steinernematidae)

The vertical migration of infective juveniles of Neoaplectana glaseri applied to the soil surface or introduced 16 cm below the soil surface was studied in pure silica sand, coarse sandy loam, silty clay loam, and clay. The number of juveniles that migrated and infected wax moth pupae placed in the soil decreased as the proportion of clay and silt increased. The majority of nematodes moved downwards 2-6 cm from the surface, but some penetrated to a depth of 14 cm in pure silica sand and coarse sandy loam. In pure silica sand and coarse sandy loam, nematodes introduced 16 cm below the soil surface were able to infect wax moth pupae located at depths of 0-4 cm and 28-32 cm. Nematodes showed a greater tendency to disperse downwards from the point of application. Movement of the nematode was least in clay soil and limited in silty clay loam soil. The number of migrating nematodes was greatest when wax moth pupae were present.     

Contribution of Lolium perenne rhizodeposition to carbon turnover of pasture soil

Carbon rhizodeposition and root respiration during eight development stages of Lolium perenne were studied on a loamy Gleyic Cambisol by ¹⁴CO₂ pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total ¹⁴CO₂ efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first 8 days after ¹⁴C pulse labelling decreased during plant development from 14 to 6.5% of the total ¹⁴C input. Root respiration accounted for was between 1.5 and 6.5% while microbial respiration of easily available rhizodeposits and dead root remains were between 2 and 8% of the ¹⁴C input. Both respiration processes were found to decline during plant development, but only the decrease in root respiration was significant. The average contribution of root respiration to total ¹⁴CO₂ efflux from the soil was approximately 41%. Close correlation was found between cumulative ¹⁴CO₂ efflux from the soil and the time when maximum ¹⁴CO₂ efflux occurred (r=0.97). The average total of CO₂ Defflux from the soil with Lolium perenne was approximately 21 μg C-CO₂ d⁻¹ g⁻¹. It increased slightly during plant development. The contribution of plant roots to total CO₂ efflux from the soil, calculated as the remainder from respiration of bare soil, was about 51%. The total ¹⁴C content after 8 days in the soil with roots ranged from 8.2 to 27.7% of assimilated carbon. This corresponds to an underground carbon transfer by Lolium perenne of 6–10 g C m⁻² at the beginning of the growth period and 50–65 g C m⁻² towards the end of the growth period. The conventional root washing procedure was found to be inadequate for the determination of total carbon input in the soil because 90% of the young fine roots can be lost. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root water uptake of field-growing plants indicated by measurements of natural-abundance deuterium

Measurements of stable-isotope ratios of water extracted from stems and, in some studies, soils are increasingly being used to study the integrated root function of field-growing plants. This study explored if additional measurements on water extracted from roots could indicate the activity of roots in different areas of the soil profile and their influence on canopy water sources, so providing advantages over more common sampling strategies. Studies were conducted on trees and shrubs located in diverse habitats: a saline, semi-arid floodplain, a subhumid mountain-range front and a cold desert. At each site, roots, soil immediately surrounding the roots, and plant stems were sampled. Roots were taken from different depths in the soil, to approximately 2 m at one site. Overall, 80% of roots sampled had H isotope ratios different from the surrounding soil. The differences up to 37, were significant (p<0.05) at two of the sites. Thus water in most of the roots sampled did not come entirely, if at all, from the surrounding soil, illustrating movement and possible mixing of water within the root system. This condition was not simply related to the availability of water surrounding the soil, which was also measured. There were also differences in root and stem H isotope ratios (up to 17) in 67% of samples, although the difference was only significant in shallow samples from the floodplain. The general similarity in stem and root ²H values indicates that most roots sampled were involved in the main supply of water to the canopy. Patterns of root function varied between the individual sites. Trees were primarily using groundwater at the floodplain and mountain front sites, as the surface soils had mean matric potentials of-1800 kPa. At the mountain front site, the surface roots were transporting groundwater to the canopy in isolation form the surrounding soil. In contrast, surface roots at the floodplain were taking up water from the surrounding soil, although this water was not a significant source in the trees' overall water supply. This activity of surface roots would not have been evident from the ²H data without the root samples. At the cold desert the roots in moist surface soil provided the main source of water for the shrubs. There too the root data indicated different water uptake patterns than otherwise would have been assumed. The root data showed that groundwater could not have been a water source, a conclusion which had been reached in a previous study. Thus measurements of stable isotope ratios in root water may be a valuable tool in assessing water uptake patterns and root function.