Roots,soil water and crop yield: tree crop interactions in a semi-arid agroforestry system in Kenya

Variations in soil water, crop yield and fine roots of 3–4 year-old Grevillea robusta Cunn. and Gliricidia sepium (Jacq.) Walp. growing in association with maize (Zea mays L.) were examined in semiarid Kenya during the long rains of 1996 and 1997. Even although tree roots penetrated more deeply than maize roots, maximum root length densities for both tree species and maize occurred in the top 200 mm of the soil profile where soil moisture was frequently recharged by rains. Populations of roots in plots containing trees were dominated by tree roots at the beginning of the growing season but because tree roots died and maize root length increased during the cropping season, amounts of tree and maize roots were similar at the end of the season. Thus, there was evidence of temporal separation of root activity between species, but there was no spatial separation of the rooting zones of the trees and crops within that part of the soil profile occupied by crop roots. Tree root length density declined with increasing distances from rows of trees and with depth in the soil profile. Although Grevillea trees were largest, plots containing G. sepium trees always contained more tree roots than plots containing G. robusta trees and Gliricidia was more competitive with maize than Grevillea. Overall, Gliricidia reduced crop yield by about 50% and Grevillea by about 40% relative to crop yield in control plots lacking trees and reductions of crop yield were greatest close to trees. There was less soil moisture in plots containing trees than in control plots. Such difference between control plots and plots containing trees were maximal at the end of the dry season and there was always less soil moisture close to trees than elsewhere in the plots. Plots containing Gliricidia trees contained less soil water than plots containing Grevillea trees.     

Competition in tree row agroforestry systems. 1. Distribution and dynamics of fine root length and biomass

Complementarity in the distribution of tree and crop root systems is important to minimise competition for resources whilst maximising resource use in agroforestry systems. A field study was conducted on a kaolinitic Oxisol in the sub-humid highlands of western Kenya to compare the distribution and dynamics of root length and biomass of a 3-year-old Grevillea robusta A. Cunn. ex R. Br. (grevillea) tree row and a 3-year-old Senna spectabilis DC. (senna) hedgerow grown with Zea mays L. (maize). Tree roots were sampled to a 300 cm depth and 525 cm distance from the tree rows, both before and after maize cropping. Maize roots were sampled at two distances from the tree rows (75–150 cm and 450–525 cm) to a maximum depth of 180 cm, at three developmental stages. The mean root length density (L_rv) of the trees in the upper 15 cm was 0.55 cm cm⁻³ for grevillea and 1.44 cm cm⁻³ for senna, at the start of the cropping season. The L_rv of senna decreased at every depth during the cropping season, whereas the L_rv of grevillea only decreased in the crop rooting zone. The fine root length of the trees decreased by about 35% for grevillea and 65% for senna, because of maize competition, manual weeding, seasonal senescence or pruning regime (senna). At anthesis, the L_rv of maize in the upper 15 cm was between 0.8 and 1.5 cm cm⁻³. Maize root length decreased with greater proximity to the tree rows, potentially reducing its ability to compete for soil resources. However, the specific root length (m g⁻¹) of maize was about twice that of the trees, so may have had a competitive uptake advantage even when tree root length was greater. Differences in maize fine root length and biomass suggest that competition for soil resources and hence fine root length may have been more important for maize grown with senna than grevillea. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of tree species and crown pruning on root length and soil water content in semi-arid agroforestry

Soil cores were taken to estimate root length prior to transplanting and after 60 days growth of a dry season sorghum crop in an agroforestry experiment in a semi-arid region of north-east Nigeria. The experiment compared sorghum grown alone and with two tree species (Acacia nilotica subsp adstringens and Prosopis juliflora) and one management treatment (pruning of tree crowns). Data on soil water content were collected from 6 days before and 20, 60 and 110 days after sorghum transplanting. The main findings were: (i) Per unit root length, A. nilotica had a more negative effect on sorghum above and below ground than P. juliflora. This appeared to be correlated with greater rates of water extraction from layers of soil shared with crop roots; (ii) Crown pruning substantially reduced the competitive effect of P. juliflora on crop yield but did not affect the impact of A. nilotica on intercropped sorghum. Since the impact of pruning on tree-crop competition varies with species, tree species selection and management will be a key factor in determining the feasibility of dryland agroforestry systems.     

Root length dynamics in agroforestry with Gliricidia sepium as compared to sole cropping in the semi-deciduous rainforest zone of West Africa

Tree root systems may improve soil fertility through carbon inputs, uptake of leachable nutrients and maintenance of soil biomass, but can at the same time reduce crop yields by competition for water and nutrients. Quantitative information about the positive and negative effects of tree roots and their changes in space and time are necessary for the optimization of agroforestry associations. An alley cropping experiment was layed out as a randomized complete block design on a Plinthic Lixisol/Ferralic Cambisol with Gliricidia sepium hedgerows at 5 m distance, including a sole cropping control. The development of root systems was monitored by sequential soil coring (eight samplings) during one year, with maize and groundnut as crops. Additional information is presented from a single sampling for rice during the foregoing year. Pronounced fluctuations of live root length density indicated an important variability in the nutrient and water uptake capacity of the vegetation. At low total root length density, the hedgerows affected the root development in the agroforestry plots directly by the presence of their root systems. At high root length density, they affected root development mainly by improving crop root growth and influencing the composition of the spontaneous vegetation. The root length density of the hedgerows was too low to compete with the crops for soil resources. The hedgerows tended to increase root length densities in the subsoil when few roots were present, thus possibly reducing the risk of nutrient leaching. However, the length density of the perennial root systems decreased during the cropping season, presumably as an effect of repeated pruning, and attained minimum values almost at the same time as the crops. Trees with denser root systems which are less frequently pruned may be more efficient in achieving closer nutrient cycles, though at the cost of higher root competition with crops.     

Fine root turnover of irrigated hedgerow intercropping in Northern Kenya

Fine root turnover (<2 mm) was determined from repeated measurements of root distribution up to 120 cm soil depth by core sampling in four month intervals. Sole cropped Sorghum bicolor and Acacia saligna were compared with the agroforestry combination in an alley cropping system in semiarid Northern Kenya. Three methods for the calculation of root production were used: the max-min, balancing-transfer and compartment-flow method. The highest root biomass was found in the topsoil for all cropping systems, though trees had a deeper root system. Trees and crops had a similar amount of below-ground biomass during the vegetation period (0.3 and 0.4 Mg DM ha^-1 120 cm^-1), but in the agroforestry combination root biomass was more than the sum of the sole cropped systems (1.1 Mg DM ha^-1 120 cm^-1). The tree system showed a very static root development with little fluctuation between seasons, whereas root biomasses were very dynamic in the crop and tree + crop systems. Root production was highest in the tree + crop combination with 2.1 Mg DM ha^-1 a^-1, with about 50% less in sole cropped trees and crops. Root N input to soil decreased in the order tree + crop>tree>crop system with 13.5, 11.0 and 3.2 kg N ha^-1 a^-1, and cannot be estimated from total below-ground biomass or carbon turnover, as N is accumulated in senescing roots. Such low N input to soil stresses the need for investigating other processes of nutrient input from roots to soil. Areas of highest N input were identified in the topsoil under the tree row in the tree system. Resource utilisation and C and N input to soil were highest with a combination of annual and perennial crops.     

Root distributions partially explain 15N uptake patterns in Gliricidia and Peltophorum hedgerow intercropping systems

The relative distributions of tree and crop roots in agroforestry associations may affect the degree of complementarity which can be achieved in their capture of below ground resources. Trees which root more deeply than crops may intercept leaching nitrogen and thus improve nitrogen use efficiency. This hypothesis was tested by injection of small doses of (¹⁵NH₄)₂SO₄ at 21.8 atom% ¹⁵N at different soil depths within established hedgerow intercropping systems on an Ultisol in Lampung, Indonesia. In the top 10 cm of soil in intercrops of maize and trees, root length density (L_rv) of maize was greater than that of Gliricidia sepium trees, which had greater L_rv in this topsoil layer than Peltophorum dasyrrachis trees. Peltophorum trees had a greater proportion of their roots in deeper soil layers than Gliricidia or maize. These vertical root distributions were related to the pattern of recovery of ¹⁵N placed at different soil depths; more ¹⁵N was recovered by maize and Gliricidia from placements at 5 cm depth than from placements at 45 or 65 cm depth. Peltophorum recovered similar amounts of ¹⁵N from placements at each of these depths, and hence had a deeper N uptake distribution than Gliricidiaor maize. Differences in tree L_rv across the cropping alley were comparatively small, and there was no significant difference (P<0.05) in the uptake of ¹⁵N placed in topsoil at different distances from hedgerows. A greater proportion of the ¹⁵N recovered by maize was found in grain following ¹⁵N placement at 45 cm or 65 cm depth than following placement at 5 cm depth, reflecting the later arrival of maize roots in these deeper soil layers. Thus trees have an important role in preventing N leaching from subsoil during early crop establishment, although they themselves showed a lag phase in ¹⁵N uptake after pruning. Residual ¹⁵N enrichment in soil was strongly related to application depth even 406 days after ¹⁵N placement, demonstrating the validity of this approach to mapping root activity distributions.     

Interspecific differences in root architecture among maize and triticale genotypes grown under drought,waterlogging and soil compaction

Environmental stresses (soil compaction, drought, waterlogging) cause changes in plants’ root system structure, also affecting the growth of above-ground parts. The aim of this study was to estimate phenotypic variation among maize and triticale genotypes in root penetration ability through petrolatum-wax-layer (RPA). Also, the effect of shortage or excess of soil water on dry matter of shoots and roots and morphological changes in root system structure in sensitive and resistant maize and triticale genotypes grown in low or high soil compaction level was evaluated. To estimate RPA index, the petrolatum-wax-layer method (PWL) was used. The strength of three petrolatum-wax concentrations 60, 50 and 40 % was 0.52, 1.07 and 1.58 MPa, respectively. High coefficients of variation (CV) were observed in 0.52 and 1.07 MPa and for maize were 19.2 and 21.7 %, and for triticale, 12.5 and 18.3 %, respectively. The data indicate that the use of PWL technique is an effective screening method, and makes it possible to divide the genotypes into resistant and sensitive groups. The second part of this study investigated a multistress effect of soil compaction combined with drought or waterlogging on root and shoot growth and morphological changes in root system structure of maize and triticale genotypes differing in susceptibility to environmental stresses. Seedlings were grown for 4 weeks in root-boxes under conditions of low (LSC 1.1 g cm^?3) or severe (SSC 1.6 g cm^?3) soil compaction. Drought or waterlogging stresses were applied for 2 weeks from 14th to 28th day. In comparison to LSC treatment, in SSC treatment the decrease in dry matter of shoots and roots was greater for sensitive genotypes of maize and triticale (Ancora, CHD-147). Soil drought or waterlogging caused greater decrease of dry matter of shoots and roots in seedlings grown in SSC in comparison to LSC. The root penetration index (RPI) was estimated as a ratio of root dry matter in 15–40 cm root-box layer to total root dry matter. On the basis of RPI it was possible to group the genotypes according to their ability to distribute roots in soil profile. In comparison to LSC, SSC exerted a strong influence on the length of seminal and seminal adventitious roots, as well as the number and length of L- and S-type lateral roots developed on seminal and nodal roots. In both species the restriction effect of soil compaction on number and length of roots was more severe in sensitive (Ankora, CHD-147) than in resistant (Tina, CHD-247) genotypes. The restriction in roots propagation was greater in triticale than in maize. Exposure to drought or waterlogging in the case of genotypes grown in LSC and SSC treatments caused a decrease in number and length of particular components of root system structure. In both species the decrease of root number and length in plants grown under waterlogging was greater than under drought. The observed changes in root system were greater in sensitive (Ankora, CHD147) than in resistant (Tina, CHD-247) genotypes. Statistically significant correlations were found between RPA and RPI and also between these indexes and soil compaction, drought and waterlogging susceptibility indexes. This indicates that genotypes resistant to soil compaction were resistant to drought or waterlogging and also that genotypes resistant to drought were resistant to waterlogging.     

Interspecific water use strategies of a Juglans regia and Isatis tinctoria/Senna tora agroforestry

Aims Understanding the interspecific water relations is important for designing agroforestry systems. The objective of this study was to determine the water use strategies of component species in a walnut (Juglans regia)-woad (Isatis tinctoria)/sicklepod (Senna tora) agroforestry system.Methods Water sources of component species in a walnut-woad/sicklepod agroforestry system were investigated with the technique of stable deuterium isotope tracing at a site of hilly area in Northern China during 2012-2013.Important findings Results showed that the soil water content in the agroforestry system was 26.74% and 7.93% greater than in the pure woad field in the first half year, and 17.39% and 13.65% greater than in the pure sicklepod field in the second half year (sicklepod growth period), in 2012 and 2013, respectively. The lowest water content was found in the middle of tree rows, and the highest water content was found in the northern side of tree rows or under the trees. In the soil layers measured, the pure woad and pure sicklepod systems had greater hydrogen stable isotope ratios (δ D value) of soil water than in the agroforestry system. During the period of woad growth, more than half of the water absorbed by walnut was from the deeper soil layer (30-80 cm). In contrast, the walnut trees mainly utilized shallow layer (0-30 cm) soil water during the period of sicklepod growth. These findings suggest that walnut has a two-state root system: during the period of woad growth, shallow roots of walnut are not active when soil is dry whereas the sicklepod growth occur in rainy season, and the shallow roots of walnut are active and utilize more shallow soil water supplemented by rainwater. More than 85% of water used by both the woad and the sicklepod were from the shallow layer soil. At the seedling stage, the roots of woad, cannot grow into the deeper soil layer, and the absorbed water is completely from the shallow layer in the pure woad system. However, 5.7% of the water absorbed by the intercropped woad was from the deeper soil layer in 2012, and the proportion increased further (9.7%) in the following year when there was less precipitation. The results confirmed that hydraulic lift effect of walnut occurred on shallow layer crop in dry season, and this effect become greater under drier conditions. Therefore, deeper roots of walnut improved water condition in the walnut- woad/sicklepod agroforestry systems compared to pure crop systems. The walnut mainly utilized water from the deeper layer to avoid water competition with the shallow layer. In the dry season, crops benefited from the water provided by walnut roots through hydraulic lift. Walnut and intercropped plants exhibited water facilitation in the agroforestry systems, suggesting that this configuration is a suitable practice in this area.     

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.     

华北低丘山区果药复合系统种间水分利用策略

了解林农复合系统的种间水分关系至关重要.该文通过稳定氘同位素研究了华北低丘山区核桃(Juglans regia)-菘蓝(Isatis tinctoria)/决明(Senna tora)复合系统各组分的水分来源, 试图明确该果药复合系统的种间水分利用策略, 为该区林农配置模式的选择提供理论依据.研究结果表明: 果药复合系统的土壤含水量明显高于单作菘蓝和单作决明地块, 在2012年,2013年上半年比单作菘蓝高26.74%和7.93%, 在下半年比单作决明高17.39%和13.65%.在果药复合系统内部, 土壤含水量以核桃树行中间位置的最低,树行北侧和树下最高.在各个土层深度, 单作系统的土壤水氢稳定同位素比率(δD值)均比复合系统的高.在菘蓝生长时期的春旱期, 复合系统中核桃的大部分水分来源于30-80 cm深层土壤水, 表明此时期核桃表层根系活性不高; 而决明生长时期正值雨季, 此时核桃优先利用雨水补充的0-30 cm浅层土壤水,表层根系活性增强.在任何生长时期, 菘蓝和决明85%以上的水分都来自浅层土壤水.在菘蓝苗期, 其根系尚未扎入深层土壤中, 单作菘蓝的水分完全来源于浅层土壤, 而在2012年间作菘蓝却有5.7%的水分来自于深层土壤, 在更为干旱的2013年该比例上升到9.7%, 该结果证实了核桃在旱季存在"水力提升"作用, 供浅根系作物吸收利用, 并且越干旱, 该水力提升作用越强.在华北低丘山区核桃-菘蓝/决明复合系统中, 深根性核桃改善了复合系统的土壤水分状况, 在旱季主要利用深层土壤水以避开与浅层作物的水分竞争,并能将深层土壤水提升至浅层土壤供菘蓝吸收利用, 核桃与两种药材表现为水分互利关系, 因而该模式适合在该地区发展.     

Cultivar variation in the effect of chlorsulfuron in depressing the uptake of copper in wheat

The application of herbicides has induced symptoms of nutrient deficiencies under some circumstances. This glasshouse study examined the effect of chlorsulfuron on the uptake and utilization of copper (Cu) in four cultivars of wheat plants (Triticum aestivum L. cvs. Kulin, Cranbrook, Gamenya and Bodallin) on a Cu-responsive soil. Application of chlorsulfuron depressed the concentration of Cu in wheat plants receiving either inadequate or adequate Cu. In plants with inadequate Cu supply, chlorsulfuron increased the severity of Cu deficiency. Shoot weight was markedly decreased by chlorsulfuron at all levels of Cu, through decreasing the number of tillers and the elongation of leaves. This decreased growth of shoots occurred prior to the effect on Cu concentration in tissues. The retranslocation of Cu in old tissues over time was unaffected by chlorsulfuron. In all wheat cultivars, the decreased growth of shoots were correlated with the concentration of Cu in the youngest fully emerged leaf blade with critical levels of 1.6−1.7 at day 25 and 0.9−1.0 μg g⁻¹ d. wt. at day 60. The application of chlorsulfuron tended to increase the critical level at day 25 but not at day 60. In addition, Kulin seems to be most, and Cranbrook least, sensitive to chlorsulfuron. This sensitivity was associated with the sensitivity of the cultivars to Cu deficiency. It is suggested that chlorsulfuron application induces Cu deficiency in wheat plants mainly due to effects on the uptake of Cu. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root pruning reduces root competition and increases crop growth in a living mulch cropping system

Abstract

In two field experiments, growth of white cabbage in pure stands was compared with that of cabbage grown in living mulch systems to reduce pest attacks. The roots of the living mulch were pruned early in the season, with the aim of reducing competition and increasing growth of the white cabbage. Root pruning was shown to increase the above-ground biomass of white cabbage, with two prunings giving higher cabbage yields than one, but there were clear differences between the living mulch species tested (red clover, birdsfoot trefoil, salad burnet, winter rye). Below-ground growth and competition were examined by measuring root distribution in minirhizotrons and uptake of ¹⁵N placed at different soil depths. These studies showed that the ability of mulch species to compete for resources at depth was restricted by pruning, and that this was crucial for the development of the white cabbage crop.     

核桃-小麦复合系统中细根生长动态及竞争策略

以核桃(Juglans regia)-小麦(Triticum aestivum)间作复合系统为研究对象,用微根窗和根钻相结合的方法采样,研究复合系统中核桃和小麦细根年内年际的生长动态和竞争适应策略,为农林复合系统的经营管理和竞争模型的建立提供理论依据和技术支持。结果表明,间作核桃和小麦根系均在上半年有一个大的生长高峰(5月和4月),在下半年有一个小的生长高峰(9月和11月),二者的竞争主要发生在上半年的大生长高峰期。在各年份各土层,间作核桃的根长密度均低于单作核桃,且在从第7年开始存在显著差异。在0—20 cm土层间作小麦根长密度在第3—7年间获得迅速提高,从第7年开始显著高于单作小麦,但在20 cm以下土层则相反。间作使核桃和小麦细根生态位实现了分离,11年的观察期内间作核桃比单作核桃细根的垂直分布中心下移了6.59 cm,间作小麦比单作小麦的上移了8.59 cm。在根系竞争策略方面,小麦根系是通过短期内的快速生长,迅速占据土壤空间获得竞争优势;而核桃根系是通过根系的逐年积累,逐步占据土壤空间从而获得竞争优势。可以干扰核桃根系积累过程的"竞争-干扰-再平衡"农林复合经营管理策略可以让复合系统中核桃和小麦保持各自竞争优势的情况下实现共存。在根系形态方面,自身细根直径较小者小麦在剧烈竞争区域以增加细根直径减小比根长来适应竞争,而自身细根直径较大者核桃则相反。     

Vertical and horizontal development of the root system of carrots following green manure

Cover crops grown as green manure or for other purposes will affect nitrogen (N) distribution in the soil, and may thereby alter root growth of a succeeding crop. During two years, experiments were performed to study effects of nitrogen supply by green manure on root development of carrots (Daucus carota L). Total root intensity (roots cm⁻² on minirhizotrons) was significantly affected by the green manures, and was highest in the control plots where no green manure had been grown. Spread of the root system into the interrow soil was also affected by green manure treatments, as the spread was reduced where spring topsoil N_min was high. Although N supply and distribution in the soil profile differed strongly among the treatments, no effect was observed on the rooting depth of the carrot crops. Across all treatments the rooting front penetrated at a rate of 0.82 and 0.68 mm day⁻¹ °C⁻¹ beneath the crop rows and in the interrow soil, respectively. The minirhizotrons only allowed measurements down to 1 m, and the roots reached this depth before harvest. Extrapolating the linear relationship between temperature sum and rooting depth until harvest would lead to rooting depths of 1.59 and 1.18 m under the crop rows and in the interrow soil respectively. Soil analysis showed that the carrot crop was able to reduce N_min to very low levels even in the 0.75 to 1.0 m soil layer, which is in accordance with the root measurements. Still, where well supplied, the carrots left up 90 kg N ha⁻¹ in the soil at harvest. This seemed to be related to a limited N uptake capacity of the carrots rather than to insufficient root growth in the top metre of the soil. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interactions of maize and Italian ryegrass in a living mulch system: (1) Shoot growth and rooting patterns

It has been demonstrated that the use of living mulches solves some of the environmental problems associated with the conventional cropping of maize (Zea mays L.). However, plant growth and yield are often reduced in such a cropping system. Since shoot competition between the main crop and the cover crop can be avoided by regular cutting of the cover crop, it was hypothesized that decreases in maize growth and yield in a living Italian ryegrass (Lolium multiflorum Lam.) mulch must be related to below ground interactions between the two species and that these may be traced back to the characteristics of their root systems. Two cropping systems, maize grown alone in bare soil (conventional cropping, BS) or together with a living Italian ryegrass mulch (LM), were studied in lysimeters (1.0 m² surface area and 1.1 m depth) placed outdoors, near Zurich Switzerland, for a duration of three years. In the LM treatment a strip, 0.3 m wide, in the center of the plot around the maize row was free of grass. For comparison, an Italian ryegrass (RG) treatment, managed as the LM treatment but without maize plants, was also included in the study. Minirhizotrons (54 mm inner diameter) were horizontally installed at ten soil depths between 0.0 and 1.0 m, perpendicular to the orientation of the maize rows. The development of the maize shoot and the rooting patterns were observed non-destructively. LM strongly modified the maize crop by decreasing growth and duration of the leaf area, and thus biomass and grain yield at harvest by as much as 78 and 72%, respectively. Maximum root densities in the three treatments were observed around the time of maize anthesis. However, BS maize was unable to build up root densities similar to those observed in Italian ryegrass plots at the time of maize sowing. The root densities of the LM and the RG treatments were usually similar. The inability of the maize plants to establish a competitive root system in the LM limits the supply of nutrients and water and therefore reduces growth and yield. Improving the productivity of maize in living mulches will depend on the ability to achieve a better separation of the rooting volumes of the two species, so that specific steps to facilitate the main crop and control the living mulch can be taken.     

Hydraulic conductivities of competing root systems of Grevillea robustaand maize in agroforestry

Models of water uptake in mixed stands of vegetation commonly assume that water is partitioned among competing root systems in proportion to relative root length densities. Such an approach assumes implicitly that roots of different species have equivalent hydraulic properties. This was tested for root systems of Grevillea robustaA. Cunn. and maize (Zea maysL.) at a semi-arid site in Kenya. The hydraulic conductances for roots of both species were measured in situat the scale of the whole root or root system using a high pressure flow meter (HPFM). Hydraulic conductivities (_r) were expressed per unit root length. Root lengths were estimated for maize plants by soil coring and for G. robustausing a fractal branching model calibrated against soil coring. Mean _r was 1.88×10^–7 ±0.28×10^–7kg s^–1 MPa^–1 m^–1 for G. robustaand 1.25×10^–7 ±0.13×10^–7kg s^–1 MPa^–1 m^–1 for maize. Values of _r were not significantly different (P<0.05), suggesting that the assumption of hydraulic equivalence for root systems of the two species may be valid, at least when hydrostatic gradients are the major driving force for water uptake. Differences in conductivities between these species could arise, however, because of variation in the hydraulic properties of roots not accounted for here, for example because of root age, phenology or responses to the soil environment.     

Drought resistance and root morphology in sorghum

Two 2 m³ plots of soil were prepared to different water contents and each isolated from surrounding soil by impermeable plastic material. Nine sorghum varieties were germinated in the plots and allowed to grow without further watering. Time-to-wilt after emergence was measured, and several parameters relating to water flow of the seedling and nodal roots were determined. There was a good positive correlation between both seminal root and nodal root relative conductvity and time-to-wilt. In a second experiment, plants were germinated and grown in pots, and after two weeks of growth without further watering were inspected for survival in the unwilted state. The per cent survival was calculated. There was a negative correlation of seminal root relative conductivity with per cent survival, and a high negative correlation of the number of seminal roots with per cent survival. It is concluded that high relative conductivity indicates drought resistance if the plants are growing with less restricted roots as in open soil, while if the plants are grown in pots the reverse is the case. Experiments linking root conductivity with survival conducted in pots are poor predictors of performance in less restricted rooting conditions.     

SEASONAL VARIATIONS IN THE PROPORTIONS OF SUBERIZED AND UNSUBERIZED ROOTS OF TREES IN RELATION TO THE ABSORPTION OF WATER

Growing root tips usually constituted less than 1 per cent and mycorrhizal roots less than 6 per cent of the total root surface under a 34-year-old pine stand. Growing root tips usually constituted less than 1 per cent of the total root surface under a yellow poplar stand, although one sample taken in May contained 9 per cent of unsuberized roots. The water permeability of various types of roots was measured under a pressure gradient of 31 cm of mercury. It differed widely among individual roots, ranging from an average of 6.6. mm³/cm²/hr for suberized pine roots 1.33 mm in diameter, to 36.6 mm³ for suberized pine roots 3 mm in diameter, and 178 mm³/ cm²/hr for unsuberized roots grown in water culture. Water intake through a group of unsuberized roots grown in soil averaged 37.4 mm³/cm²/hr. The permeability of yellow poplar roots varied even more, ranging from essentially zero to 30,000 mm³/cm²/hr. It is concluded that the major part of water absorption in pine occurs through suberized roots, some through mycorrhizal roots, and relatively little through growing root tips. Likewise, in yellow-poplar most of the water probably enters through suberized roots. Further study is needed of the role of suberized roots in water and salt absorption.     

Subsoil root activity in tree-based cropping systems

An increasing number of studies indicate that (i) nutrient and water resources can be abundant in the subsoil and (ii) trees have deep root systems that can possibly reach these resources. It is less clear whether subsoil resources are actually improving water and nutrient status of tree-based cropping systems and whether they are significantly increasing crop production and yield. To answer such a question, the distribution of nutrient and water uptake by trees needs to be quantified. So-called `root activity distributions' give valuable information about actual subsoil use by trees whereas studies on root length or mass distributions do not often correlate with uptake distributions. Despite the usually lower relative root activity in the subsoil compared to the topsoil per unit soil, the large volume of subsoil in comparison to mostly shallow topsoil is an important resource for crop nutrient and water uptake. The present study compares published root activity distributions using the model Activity=A _max(1–k ^depth). The obtained regression constants k of 0.91–0.99 determined in this publication reflect the values computed by an earlier published survey for root biomass ranging from tundra to those of temperate forest biomes. Thus, tree crops can have shallow root activity and 75% of their total root activity in the first 0.1 m of soil, or very deep root activity with more than 90% below 0.1 m. Neither environmental factors (i.e., climate and soil properties available from these publications) nor plant species explain differences of root activity distributions with depth. The deepest root activity is found for fruit trees such as citrus, guava and mango. Shaded crops such as coffee and cacao tend to have shallower root activity than fruit trees. Monocots including oil palm, coconut or banana have root activity that can be both deep and shallow. Regional and temporal variations of subsoil root activity for the same tree species are significant and generally larger than differences between species. Root activity patterns of tree crops appear to be sufficiently flexible to allow for subsoil resource use. Consequently, management such as pruning, fertilization, liming and irrigation are shown to significantly affect subsoil root activity.     

玉米/大豆间作条件下的作物根系生长及水分吸收

通过田间试验研究了玉米/大豆条带间作群体的根系分布及土壤水分吸收规律.结果表明：水分充足条件下,土壤剖面内玉米和大豆根系的分布模式近似于三角形;玉米根系水平分布范围较大,侧向伸展长度约为58 cm,16～22 cm土层的玉米根系侧向伸展最远,玉米根系不仅分布于间作条带行间,而且生长到大豆条带的行间;大豆根系水平分布于相对有限的区域内,侧向伸展长度约为26 cm.作物根质量密度随着距作物行（玉米或大豆）距离的增加而减少,玉米行和边行大豆根质量密度的90%分布于0～30 cm土层.距玉米行10 cm处玉米的根质量密度高于大豆,距玉米行20 cm处大豆的根质量密度大于玉米,两种作物根质量密度的85%都分布于0～30 cm土层内.间作条带内水分变化主要集中在0～30 cm土层,水分变化量依次为：玉米区域>大豆区域>条带行间.表明在水分充足条件下,间作作物优先在自己的区域吸水,根系混合区吸水滞后发生.