Root characteristics of selected field crops: Data from the Wageningen Rhizolab (1990–2002)

Since being built in 1990, the rhizotron facility in Wageningen, the Wageningen Rhizolab, has been used for experiments on crops (e.g. Alfalfa, Brussels sprouts, common velvet grass, field bean, fodder radish, leeks, lupins, maize, potato, beetroot, ryegrass, spinach, spring wheat, winter rye and winter wheat). In the experiments, horizontal glass minirhizotron tubes combined with auger sampling were used to assess rooting characteristics. For this paper we took the root data from these experiments and looked for a general relationship between thermal time/time after planting and rooting depth, the velocity of the root front and root proliferation. For certain depths (fixed by the depth at which the horizontal minirhizotrons were installed) a simple linear regression was established between the average root number per cm² minirhizotron surface area and thermal time after planting. The compartments selected for each crop were those in which there had been a control treatment and/or in which conditions for rooting were considered to be optimal. We performed regression analyses per compartment and per depth, but only for the period after planting in which a linear increase of root numbers vs. thermal time was observed. After averaging the results, the regression procedure yielded two parameters of rooting for each crop: (a) the actual or thermal time at which the first root appeared at a certain depth and (b) the root proliferation rate after the first root had appeared. In this way, inherent crop differences in rooting behaviour (rooting depth and root proliferation) became apparent. For each crop, the velocity of the root front after planting could be established (calculated in cm(°C day)^–1). This parameter differed greatly between crops. Some crops (such as leeks and common velvet grass) explored the soil profile slowly: the root front moved at a velocity of only 0.07cm(°C day)^–1. Among the crops whose roots grew down much faster (0.18–0.26cm (°C day)^–1) were cereals and fodder radish. For a day with an average temperature of 15°C these rates would have corresponded with the root front travelling approximately 1–4cm per day. In the crops studied the root front velocity did not correlate with the root proliferation rate.     

Are differences in root growth of nitrogen catch crops important for their ability to reduce soil nitrate-N content,and how can this be measured?

An experiment was made to measure root growth of nitrogen catch crops, to investigate whether differences in root growth among plant species are related to their ability to deplete the soil nitrate-N pool. Large differences were observed in root growth parameters. Monocot species had rooting depth penetration rates in the range of 1.0 to 1.2 mm d^–1 °C^–1, whereas the non-legume dicot species had rates between 1.5 and 2.3 mm d^–1 °C^–1. Substantial differences were also found in the lag time from sowing until significant root growth was observed. The estimated temperature sum needed for the crops to reach a rooting depth of 1.0 m varied from 750 d °C for fodder radish to 1375 d °C for Italian ryegrass. The depth distribution of the root system varied strongly, and at a depth of 1.0 m the non-legume dicot species generally had root intensities (number of root intersections m^–1 line on the minirhizotrons) 12 times as high as the monocot species.The amount of nitrate left in the topsoil (0–0.5 m) was only weakly correlated to a few of the measured plant and root parameters, whereas nitrate left in the subsoil (0.5–1.0 m) was clearly correlated to several root parameters. Subsoil nitrate residues were well correlated to root intensity, but showed even stronger correlations to more simple estimates of rooting depth. In the deepest soil layer measured (1.0–1.5 m), the soil water nitrate concentration was reduced from 119 g L^–1 without a catch crop to 61 g L^–1 under Italian ryegrass and to only 1.5 g L^–1 under fodder radish.The results show that to identify the important differences in root growth among catch crops, root growth must be measured in deep soil layers. In this study, none of the measurements made aboveground or in the upper soil layers were well related to subsoil nitrate depletion.     

Temporal and spatial root development of cauliflower (Brassica oleracea L. var. botrytis L.)

Row crops are often inefficient in utilizing soil resources. One reason for this appears to be inefficient rooting of the available soil volume. Five experiments were performed to study the temporal and spatial root development of cauliflower (cv. Plana). The crop was grown with 60 cm between rows, and root development was followed in minirhizotrons placed under the crop rows, 15 cm, and 30 cm from the crop rows. Soil was sampled and analyzed for nitrate content at the final harvest and once during growth. In two of the experiments N fertilizer rate was varied and in two of the other experiments two cultivars were compared (cv. Plana and Siria).The rooting depth of cauliflower was found to be linearly related to temperature sum, with a growth rate of 1.02 mm day^-1 °C^-1. Depending on duration of growth this leads to rooting depths at harvest of 85–115 cm. Soil analysis showed that the cauliflower was able to utilize soil nitrogen down to at least 100 cm.With Plana differences in root growth between row and interrow soil were only observed during early growth, but with Siria this difference was maintained until harvest. However, at harvest both cultivars had depleted row and interrow soil nitrate equally efficient. Nitrogen fertilizer did not affect overall root development significantly.The branching frequency of actively branching roots was increased in all soil layers from about 6 to 10 branches cm^-1 by increasing N fertilizer additions from 130 to 290 kg N ha^-1. Increasing N supply increased the number of actively branching roots in the topsoil and reduced it in the subsoil.The average growth rate of the roots was always highest in the newly rooted soil layers, but fell during time. At 74 days after planting very few roots were growing in the upper 60 cm of the soil whereas 70% of the root tips observed in the 80–100 cm soil layer were actively growing. Within each soil layer there was a large variation in growth rate of individual root tips.     

Soil properties and crop performance on a kaolinitic Alfisol after 15 years of fallow and continuous cultivation

A long-term field experiment was established on a kaolinitic Alfisol in Ibadan, Nigeria, in 1972. The land was cleared manually from secondary forest and used for (i) continuous no-till cropping with maize (Zea mays L.) and maize/cassava (Manihot esculenta Crantz) intercropping, (ii) planted fallow of guinea grass (Panicum maximum Jacq.), leucaena (Leucaena leucocephala de Wit), and pigeon pea (Cajanus cajan Millsp.), and (iii) natural bush regrowth in a randomized complete block design with three replications. At the end of 15 years, the fallow plots were cleared manually and cropped with maize for three years. The chemical and physical soil properties and crop performance of the newly-cleared plots were compared with those under 15 years of continuous cultivation. A total of 26 woody species were identified on the bush regrowth plots. Above-ground biomass accumulation of the bush plots was 157 Mg ha^-1 containing 1316 kg N ha^-1. Guinea grass, leucaena and natural bush regrowth plots had comparable organic C concentrations (approximately 20 g kg^-1) in the surface soil (0 to 10 cm) after 15 years. The organic C concentration in the surface soil under pigeon pea was the lowest (9.5 g kg^-1) among the four fallow treatments. Soil under 15 years of continuous no-till maize with and without residue mulch, respectively, contained approximately half (10 g kg^-1) and a quarter (5.7 g kg^-1) of the organic C under natural bush or guinea grass fallow. The levels of exchangeable Ca, K, Mg and effective cation exchange capacity (ECEC) were lower in the soils under continuous cultivation than in those under natural bush and planted fallow. Soil acidification occurred in soils under continuous cropping as depicted by the lower pH values and greater exchangeable Al and Mn concentrations compared to the fallow plots. Grain yield of maize (3 to 5 Mg ha^-1) without fertilizer application in the plots newly cleared from natural bush, guinea grass and leucaena fallow was comparable with that of continuous no-till maize with residue mulch and chemical fertilizer (N, P, K, Mg, Zn) applications. Among the four fallow treatments, maize grain and stover yields were the lowest in plots cleared from pigeon pea fallow.     

The significance of root development of spinach and kohlrabi for N fertilization

N fertilizer recommendatons are based on the N_min content in the useable soil layer. However, for spinach, information from the literature differs for both depth of useable soil layer and N fertilizer recommendations. The objectives of these experiments were to study the importance of different soil zones for N supply to spinach and to kohlrabi, and to examine the relationship between N supply in the useable soil layer and yield of spinach. Field experiments with both crops showed that about 80% of total root length was in the upper 0–15 cm soil layer and less than 5% below 30 cm. Spinach roots were present in the 15–30 cm layer only during the last 2 weeks before harvest, whereas kohlrabi roots penetrated this layer already 4 weeks before harvest. Placement of NO₃ below 30 cm depth did not influence root distribution. The top layer contributed about 80% to total N uptake for both crops. The 15–30 cm soil layer can maximally contribute 40–50 kg N ha^-1. It is concluded that N fertilizer recommendations for both crops should be based on the N_min content of the 0–30 cm soil layer. Maximum yield of spinach (300 dt f.m. ha^-1) was obtained at 150 kg N supply ha^-1. The nitrate residue was 50 kg N ha^-1 at 0–30 cm in this treatment. It is argued that the nitrate residues at harvest could be decreased by delaying the harvest for a few days, at slightly suboptimal N supply.     

Row Ratios of Intercropping Maize and Soybean Can Affect Agronomic Efficiency of the System and Subsequent Wheat

Intercropping is regarded as an important agricultural practice to improve crop production and environmental quality in the regions with intensive agricultural production, e.g., northern China. To optimize agronomic advantage of maize (Zea mays L.) and soybean (Glycine max L.) intercropping system compared to monoculture of maize, two sequential experiments were conducted. Experiment 1 was to screening the optimal cropping system in summer that had the highest yields and economic benefits, and Experiment 2 was to identify the optimum row ratio of the intercrops selected from Experiment 1. Results of Experiment 1 showed that maize intercropping with soybean (maize || soybean) was the optimal cropping system in summer. Compared to conventional monoculture of maize, maize || soybean had significant advantage in yield, economy, land utilization ratio and reducing soil nitrate nitrogen (N) accumulation, as well as better residual effect on the subsequent wheat (Triticum aestivum L.) crop. Experiment 2 showed that intercropping systems reduced use of N fertilizer per unit land area and increased relative biomass of intercropped maize, due to promoted photosynthetic efficiency of border rows and N utilization during symbiotic period. Intercropping advantage began to emerge at tasseling stage after N topdressing for maize. Among all treatments with different row ratios, alternating four maize rows with six soybean rows (4M:6S) had the largest land equivalent ratio (1.30), total N accumulation in crops (258 kg ha^-1), and economic benefit (3,408 USD ha^-1). Compared to maize monoculture, 4M:6S had significantly lower nitrate-N accumulation in soil both after harvest of maize and after harvest of the subsequent wheat, but it did not decrease yield of wheat. The most important advantage of 4M:6S was to increase biomass of intercropped maize and soybean, which further led to the increase of total N accumulation by crops as well as economic benefit. In conclusion, alternating four maize rows with six soybean rows was the optimum row ratio in maize || soybean system, though this needs to be further confirmed by pluri-annual trials.     

Effects of take-all (Gaeumannomyces graminis var. tritici) on crop N uptake and residual mineral N in soil at harvest of winter wheat

Background and aims

Take-all, caused by the fungus Gaeumannomyces graminis var. tritici, is the most damaging root disease of wheat. A severe attack often leads to premature ripening and death of the plant resulting in a reduction in grain yield and effects on grain quality (Gutteridge et al. in Pest Manag Sci 59:215–224, 2003). Premature death of the plant could also lead to inefficient use of applied nitrogen (Macdonald et al. in J Agric Sci 129(2):125–154, 1997). The aim of this study was to determine crop N uptake and the amount of residual mineral N in the soil after harvest where different severities of take-all had occurred.

Methods

Plant and soil samples were taken at anthesis and final harvest from areas showing good and poor growth (later confirmed to be caused by take-all disease) in three winter wheat crops grown on the same soil type on Rothamsted Farm in SE England in 1995, 2007 and 2008 (harvest sampling only). All crops received fertiliser N in spring at recomended rates (190–200?kg?N ha^?1). On each ocassion crops were assessed for severity of take-all infection (TAR) and crop N uptakes and soil nitrate plus ammonium (SMN) was determined. Grain yields were also measured.

Results

Grain yields (at 85% dry matter) of crops with moderate infection (good crops) ranged from 4.3 to 13.0?t ha^?1, compared with only 0.9–4.5?t ha^?1 for those with severe infection (poor crops). There were significant (P?<?0.05) negative relationships between crop N uptake and TAR at anthesis and final harvest. At harvest, good crops contained 129–245?kg?N ha^?1 in grain, straw and stubble, of which 85–200?kg?N ha^?1 was in the grain. In contrast, poor crops contained only 46–121?kg?N ha^?1, of which only 22–87?kg?N ha^?1 was in the grain. Positive relationships between SMN and TAR were found at anthesis and final harvest. The SMN in the 0–50?cm layer following harvest of poor crops was significantly (P?<?0.05) greater than that under good crops, and most (73–93%) was present as nitrate.

Conclusions

Localised patches of severe take-all infection decreased the efficiency with which hexaploid wheat plants recovered soil and fertiliser derived N, and increased the subsequent risk of nitrate leaching. The risk of gaseous N losses to the atmosphere from these areas may also have been enhanced.     

田菁内生菌定殖及其与多糖混合浸种的耐盐促生效果

【背景】土壤盐渍化已经成为日益严重的世界性问题,盐渍化不仅影响作物的产量,还会影响土壤的理化性质,抑制种子的萌发,阻碍植物正常生长,以及种子对水分和养分的吸收,进而影响作物的产量。【目的】玉米在盐渍土壤上生长受限,探究在中、高盐浓度下田菁种子内生菌与田菁胶混合浸种对玉米发芽的影响,为促进盐渍土玉米生长提供技术支持。【方法】利用LB液体培养基测定田菁种子内生菌贝莱斯芽孢杆菌ZH60的耐盐性;分别利用1%浓度田菁胶、OD600为0.8的ZH60菌悬液及两者混合液对玉米浸种3 h,自然风干后分别置于0、100和200 mmol/L NaCl的0.8%琼脂培养基上培养,测定玉米种子发芽势、发芽率、根长及芽长。将两叶一心期的玉米幼苗移至装有蛭石的花盆中培养,用荧光标记的内生菌ZH60灌根,分别于1、5、11、17、25 d取玉米根系研磨,利用平板菌落计数法测定内生菌在玉米根部的定殖量;利用激光共聚焦显微镜观察第28天ZH60在玉米根部的定殖情况。【结果】菌株ZH60耐11%的NaCl盐浓度,在中、高盐浓度下混合浸种的发芽势较对照组分别提高了28%、22%、30%;芽长提高了158%、163%、1...     

Competition in tree row agroforestry systems. 2. Distribution, dynamics and uptake of soil inorganic N

The effect of tree row species on the distribution of soil inorganic N and the biomass growth and N uptake of trees and crops was investigated beneath a Grevillea robustaA. Cunn. ex R. Br. (grevillea) tree row and Senna spectabilisDC. (senna) hedgerow grown with Zea mays L. (maize) and a sole maize crop, during one cropping season. The hypothesis was that a tree with a large nutrient uptake would have a greater competitive effect upon coexisting plants than a tree that takes up less and internally cycles nutrients. The field study was conducted on a kaolinitic Oxisol in the sub-humid highlands of western Kenya. Soil nitrate and ammonium were measured to 300 cm depth and 525 cm distance from the tree rows, before and after maize cropping. Ammonium concentrations were small and did not change significantly during the cropping season. There was > 8 mg nitrate kg^–1 in the upper 60 cm and at 90–180 cm depth at the start of the season, except within 300 cm of the senna hedgerow where concentrations were smaller. During the season, nitrate in the grevillea-maize system only decreased in the upper 60 cm, whereas nitrate decreased at almost every depth and distance from the senna hedgerow. Inorganic N (nitrate plus ammonium) decreased by 94 kg ha^–1 in the senna-maize system and 33 kg ha^–1 in the grevillea-maize system.The aboveground N content of the trees increased by 23 kg ha^–1 for grevillea and 39 kg ha^–1 for senna. Nitrogen uptake by maize was 85 kg ha^–1 when grown with grevillea and 65 kg ha^–1 with senna. Assuming a mineralisation input of 50 kg N ha^–1season^–1, the decrease in inorganic soil N approximately equalled plant N uptake in the grevillea-maize system, but exceeded that in the senna-maize system. Pruning and litter fall removed about 14 kg N ha^–1 a^–1 from grevillea, and > 75 kg N ha^–1 a^–1 from senna. The removal of pruned material from an agroforestry system may lead to nutrient mining and a decline in productivity.     

Soil organic carbon and root distribution in a temperate arable agroforestry system

Aim

To determine, for arable land in a temperate area, the effect of tree establishment and intercropping treatments, on the distribution of roots and soil organic carbon to a depth of 1.5 m.

Methods

A poplar (Populus sp.) silvoarable agroforestry experiment including arable controls was established on arable land in lowland England in 1992. The trees were intercropped with an arable rotation or bare fallow for the first 11 years, thereafter grass was allowed to establish. Coarse and fine root distributions (to depths of up to 1.5 m and up to 5 m from the trees) were measured in 1996, 2003, and 2011. The amount and type of soil carbon to 1.5 m depth was also measured in 2011.

Results

The trees, initially surrounded by arable crops rather than fallow, had a deeper coarse root distribution with less lateral expansion. In 2011, the combined length of tree and understorey vegetation roots was greater in the agroforestry treatments than the control, at depths below 0.9 m. Between 0 and 1.5 m depth, the fine root carbon in the agroforestry treatment (2.56 t ha^-1) was 79% greater than that in the control (1.43 t ha^?1). Although the soil organic carbon in the top 0.6 m under the trees (161 t C ha^?1) was greater than in the control (142 t C ha^?1), a tendency for smaller soil carbon levels beneath the trees at lower depths, meant that there was no overall tree effect when a 1.5 m soil depth was considered. From a limited sample, there was no tree effect on the proportion of recalcitrant soil organic carbon.

Conclusions

The observed decline in soil carbon beneath the trees at soil depths greater than 60 cm, if observed elsewhere, has important implication for assessments of the role of afforestation and agroforestry in sequestering carbon.     

Estimates of the residual nitrogen benefit of groundnut to maize in Northeast Thailand

Four cultivars of groundnut were grown in upland soil in Northeast Thailand to study the residual benefit of the stover to a subsequent maize crop. An N-balance estimate of the total residual N in the maize supplied by the groundnut was made. In addition three independent estimates were made of the residual benefits to maize when the groundnut stover was returned to the land and incorporated. The first estimate (Estimate 1) was an N-balance estimate. A dual labelling approach was used where ¹⁵N-labelled stover was added to unlabelled microplots (Estimate 2) or unlabelled stover was added to ¹⁵N-labelled soil microplots (Estimate 3). The nodulating groundnut cultivars fixed between 59–64% of their nitrogen (as estimated by the ¹⁵N isotope dilution method using non-nodulating groundnut as a non-fixing reference) producing between 100 and 130 kg N ha^-1 in their stover. Although the following maize crop suffered from drought stress, maize grain N and dry weights were up to 80% and 65% greater respectively in the plots where the stover was returned as compared with the plots where the stover was removed. These benefits were comparable with applications of 75 kg N ha^-1 nitrogen in the form of urea. The total residual N estimates of the contribution of the nodulated groundnut to the maize ranged from 16.4–27.5 kg N ha^-1. Estimates of the residual N supplied by the stover and fallen leaves ranged from 11.9–21.3 kg N ha^-1 using the N-balance method (Estimate 1), from 6.3–9.6 kg N ha^-1 with the labelled stover method (Estimate 2) and from 0–11.4 kg N ha^-1 with the labelled soil method. There was closest agreement between the two ¹⁵N based estimates suggesting that apparent added nitrogen interactions in these soils may not be important and that N balance estimates can overestimate the residual N in crops following legumes, even in very poor soils. This work also indicates the considerable ability of local groundnut cultivars to fix atmospheric nitrogen and the potential benefits from returning and incorporating legume residues to the soil in the upland cropping systems of Northeast Thailand. The applicability of the ¹⁵N methodology used here and possible reasons for the discrepancies between estimates 1, 2 and 3 are discussed.     

Effect of deep and shallow root systems on the dynamics of soil inorganic N during 3-year crop rotations

Unused inorganic nitrogen (N_inorg) left in agricultural soils will typically leach to deeper soil layers. If it moves below the root zone it will be lost from the system, but the depth of the root zone depends on the crop species grown. In this experiment we studied the effect of 3-year crop sequences, with different combinations of deep-rooted and shallow-rooted crops, on soil N_inorg dynamics to 2.5 m soil depth and the possibility of crop utilization of N leached to deep soil layers. We grew ten different crop sequences for 3 years. The crops and catch crops grown were selected to allow different sequences of deep-rooted and shallow-rooted crops. Very different rooting depths were obtained, from only 0.5 m (leek), to ∼1.0 m (ryegrass and barley), 1.5 m (red beet), 2.0 m (fodder radish and white cabbage) and more than 2.5 m by the chicory catch crop. The results showed a significant retention of N_inorg within the 2.5 m soil profile from one year to the next, but the retained N had leached to deeper parts of the profile during the winter season. Only little N_inorg was retained over two winter seasons. The retention in the deeper soil layers allowed N_inorg to be taken up by succeeding deep-rooted main crops or catch crops. The effects of crop rooting depth on N_inorg in the subsoil layers from 1.0 to 2.5 m were striking. White cabbage reduced N_inorg below 1.0 m with up to 113 kg N ha^-1 during its growth. Grown after catch crops, leek and red beet left on average 60 kg N ha⁻¹ less below 1.0 m than leek and red beet grown without a preceding catch crop. We conclude that it is possible to design crop rotations with improved nitrogen use efficiency by using the differences in crop rooting patterns; deep-rooted crops or catch crops can be used to recover N_inorg leached after previous crops, and catch crops can be grown before shallow-rooted crops to lift the deep N_inorg up to layers where these crops have their roots.     

Nitrate concentrations under winter wheat and in fallow soil during summer at Rothamsted

Summary Soil nitrate measurements were made on Broadbalk, under winter wheat and in adjacent fallow soil, during April-August in the years 1972 to 1975 with a nitrate ion selective electrode. The results are presented as pNO₃ values (=–log₁₀[NO₃]).For a given level of manurial N, annual mean pNO₃ values correlate with total rainfall for the period of measurement but not with percolation rate. On the N₂PKNaMg plot, a sharp decrease in nitrate concentration was consistently observed during May in fallow and cropped soil. During long dry periods, nitrate concentrations increased in the 5–25 cm zone in July and August, and on the FYM plot rose to maxima in July.Grain yield and annual mean pNO₃ values were very poorly correlated although the high pNO₃ (Nil and PKNaMg) plots gave much smaller yields than the low pNO₃ plots given N manures.The annual mean difference between (pNO₃)_fallow and (pNO₃)_crop correlated with grain yield on the N₂PKNaMg plot. In 1972, 1974 and 1975, minima in this difference were observed at Feekes' stages of development 6, 10 and 11.1 on this plot, representing depletion of soil nitrate by the crop. Quadratic curves, fitted to the effect of the period of cropping on this difference for the N₂PKNaMg and N₄PKNaMg plots during 1972–74, show maximum depletion of nitrate during June and July. Similar highly replicated measurements made on a commercial field of winter wheat (Long Hoos III at Rothamsted) in 1975 demonstrated well the depletions of nitrate in cropped soil at these stages of crop development. re]19760201     

Is low rooting density of faba beans a cause of high residual nitrate content of soil at harvest ?

It was the aim of this study was to evaluate the hypothesis that low rooting density of faba beans is the major reason for the comparable low depletion of N_min-nitrogen from the rooted soil volume during the vegetation period. Therefore a simulation study was carried out using data from a two-year field experiment with faba beans and the reference crop oats. Since the nitrate dynamics in the soil is closely coupled with the water budget, the model simulated also the water uptake by plants, movement and content in the soil applying a numerical solution of the Richard's equation. The nitrogen budget part of the model includes calculation of vertical nitrate movement in the soil, mineralisation of nitrate from organic matter and nitrate uptake by the crop. Vertical nitrate movement was simulated with the convection-dispersion equation. Mineralisation was computed from a simple first order kinetic approach using only one fraction of mineralisable organic matter. Nitrate uptake was assumed to be determined either by the nitrogen demand of the crop, which was estimated from a logistic growth equation that was fitted to measured data of N-accumulation, or by the maximum nitrate transport rate towards the root surface. The latter was computed from a steady state solution of the diffusion - mass flow equation for cylindrical co-ordinates.For oats the model calculated a maximum nitrate transport rate towards roots that was quite close to the measured N-uptake of that crop. For faba beans, however, the calculated maximum nitrate transport towards roots was much lower than total N-uptake and lower than for oats. Consequently, simulated N_min-contents below faba beans were during the growing season about 20-30 kg N ha^–1 higher than below oats. This difference matches quite close with the observed differences between the two crops. Therefore it was concluded that low nitrate uptake resulting from low rooting density is the main reason for higher residual nitrate contents below faba beans at harvest time.     

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.     

Nitrate leaching from sandy loam soils under a double-cropping forage system estimated from suction-probe measurements

Nitrate leaching from a double-cropping forage system was measured over a 2-year period (June 1994–May 1996) in the Northwest region of Portugal using ceramic cup samplers. The crops were grown for silage making and include maize (from May to September) and a winter crop (rest of the year) consisting of a mixture of cereals and Italian ryegrass. The experiment was performed on two different sites with a history of many years under the same crop and fertiliser management, but differing in the amounts of N applied as fertiliser and by regular cattle slurry applications. The annual nitrate leaching losses measured ranged from 154 to 338 kg N ha^-1. These amounts lead to annual mean concentrations between 22 and 41 mg -N L^-1 in the drained water. The coarse textured soils (sandy loams) and the climatic conditions of the region with more than 600 mm of drainage concentrated between October and March, tended to promote the leaching of all the nitrate-N left in the soil after the maize crop plus the N released by mineralization during the winter period. On these soils, the minimum amount of drainage (necessary to provide the complete leaching of all the nitrate-N in the soil profile in the end of summer), seems to be between 300 and 400 mm. The winter crops removed important quantities of N (83–116 kg N ha^-1) but, due to their late establishment in autumn they did not succeed in taking up the nitrate-N left in the soil after the maize crop. Approaches for reducing the nitrate leaching losses in this particular system are discussed.     

Intercropping enhances soil carbon and nitrogen

Intercropping, the simultaneous cultivation of multiple crop species in a single field, increases aboveground productivity due to species complementarity. We hypothesized that intercrops may have greater belowground productivity than sole crops, and sequester more soil carbon over time due to greater input of root litter. Here, we demonstrate a divergence in soil organic carbon (C) and nitrogen (N) content over 7 years in a field experiment that compared rotational strip intercrop systems and ordinary crop rotations. Soil organic C content in the top 20 cm was 4% ± 1% greater in intercrops than in sole crops, indicating a difference in C sequestration rate between intercrop and sole crop systems of 184 ± 86 kg C ha^?1 yr^?1. Soil organic N content in the top 20 cm was 11% ± 1% greater in intercrops than in sole crops, indicating a difference in N sequestration rate between intercrop and sole crop systems of 45 ± 10 kg N ha^?1 yr^?1. Total root biomass in intercrops was on average 23% greater than the average root biomass in sole crops, providing a possible mechanism for the observed divergence in soil C sequestration between sole crop and intercrop systems. A lowering of the soil δ¹⁵N signature suggested that increased biological N fixation and/or reduced gaseous N losses contributed to the increases in soil N in intercrop rotations with faba bean. Increases in soil N in wheat/maize intercrop pointed to contributions from a broader suite of mechanisms for N retention, e.g., complementary N uptake strategies of the intercropped plant species. Our results indicate that soil C sequestration potential of strip intercropping is similar in magnitude to that of currently recommended management practises to conserve organic matter in soil. Intercropping can contribute to multiple agroecosystem services by increased yield, better soil quality and soil C sequestration.     

Estimation of Nitrogen Pools in Irrigated Potato Production on Sandy Soil Using the Model SUBSTOR

Recent increases in nitrate concentrations in the Suwannee River and associated springs in northern Florida have raised concerns over the contributions of non-point sources. The Middle Suwannee River Basin (MSRB) is of special concern because of prevalent karst topography, unconfined aquifers and sandy soils which increase vulnerability of the ground water contamination from agricultural operations- a billion dollar industry in this region. Potato (Solanum tuberosum L.) production poses a challenge in the area due to the shallow root system of potato plants, and low water and nutrient holding capacity of the sandy soils. A four-year monitoring study for potato production on sandy soil was conducted on a commercial farm located in the MSRB to identify major nitrogen (N) loss pathways and determine their contribution to the total environmental N load, using a partial N budget approach and the potato model SUBSTOR. Model simulated environmental N loading rates were found to lie within one standard deviation of the observed values and identified leaching loss of N as the major sink representing 25 to 38% (or 85 to 138 kg ha^-1 N) of the total input N (310 to 349 kg ha^-1 N). The crop residues left in the field after tuber harvest represented a significant amount of N (64 to 110 kg ha^-1N) and posed potential for indirect leaching loss of N upon their mineralization and the absence of subsequent cover crops. Typically, two months of fallow period exits between harvest of tubers and planting of the fall row crop (silage corn). The fallow period is characterized by summer rains which pose a threat to N released from rapidly mineralizing potato vines. Strategies to reduce N loading into the groundwater from potato production must focus on development and adoption of best management practices aimed on reducing direct as well as indirect N leaching losses.     

Root production and root mortality of winter barley and its implication with regard to phosphate acquisition

Winter barley was grown in a long-term fertilizer experiment (14 years) using two P treatments: (i) no P fertilization over the whole time (–P) and (ii) an annual fertilization of 44 kg P ha^–1 (+P). The objective of the study was to investigate the influence of the P supply on total root production and root mortality (i.e., root turnover) and to assess the benefit of a more rapid root turnover on P acquisition. Shoot development and grain yield was reduced in the – treatment, whereas the standing root system had nearly the same size as in the +P treatment. Gross root growth was measured using the ingrowth core method. Mesh bags filled with root-free soil were buried into the rooting zone (0–30 cm) and root growth into the bags over periods of 2–3 weeks was determined. Assuming that no root mortality occured inside the bags during this short period, root length in the bags will be a measure of total root production. Total root production between April and June exceeded the size of the standing root system by a factor of 2 to 3 and was significantly higher at P deficiency. Root mortality as the difference between total root production and the size of the standing root system was also increased at P shortage. P uptake was calculated by using a mechanistic transport and uptake model. Calculations based on gross root growth and root mortality resulted in a higher uptake than calculations based on the development of the standing root system, although the length of the active roots were the same in both calculations. This was due to a better exploitation of undepleted soil areas by the growing root system. The root renewal by a continuous root growth and root mortality is discussed as a mechanism of P uptake efficiency.     

Nitrogen management in `adequate' input maize-based agriculture in the derived savanna benchmark zone of Benin Republic

Although the West-African moist savanna zone has a high potential for crop production, yields on farmers' fields are, on average, far below this potential, mainly due to the low use of external sources of nutrients. Since the mid-1990s, it has become clear that in order to upgrade crop production to levels needed to sustain the growing population without further degrading the soil resource base, inorganic fertilizers are required. Due to the physico-chemical nature of these soils and the relatively high cost of inorganic fertilizers, a general consensus exists in the research and development community that these inorganic inputs need to be complemented with organic matter. Here, we explore options to produce organic matter in-situ and evaluate the impact of combining inorganic and organic sources of N on maize yields, focusing on the densely populated derived savanna (DS) benchmark of Benin Republic. Although most of the farmers (93%) in this benchmark use inorganic fertilizer, applications rates are low (on average, 27 kg N ha^–1). A significant response to N was observed for 96% of the studied farmers' fields.Grain and herbaceous legumes were observed to produce between 383 and 8700 kg dry matter ha^–1 in the benchmark area. Inoculation with Rhizobia and inorganic P additions were shown to significantly improve biomass production on sites with low contents of Rhizobia and P. Although maize grain yield was observed to increase significantly following a legume compared with following a maize crop or natural fallow, these increases were insufficient in the case of a cowpea crop or were obtained at the cost of leaving the field `idle' for a whole year in the case of a herbaceous Mucuna fallow. Topping up a cowpea haulms equivalent of 45 kg N ha^–1 with 45 kg urea–N ha^–1 was shown to give maize yields similar to the yields obtained after applying 90 kg urea–N ha^–1 on the poorest fields. Moreover, on these fields, a positive interaction between cowpea–N and urea–N sources of 200 kg grain ha^–1 was observed. On the richest fields, the effects of applied organic matter and fertilizer were additive.Agroforestry systems are alternative cropping systems that produce organic matter in-situ. As tree roots go down below the rooting depth of food crops, sub-soil fertility was observed to influence tree biomass production. Yield increases in tree-crop intercrop systems – such as alley cropping – in the absence of inorganic inputs are often reduced by the occurrence of tree-crop competition. In cut-and-carry systems, where tree prunings are harvested from a field adjacent to the crop land, increases in maize grain yield caused by addition of those prunings were observed to be on the low side. Mixing these residues with urea, however, was shown to lead to added benefits of about 500 kg grains ha^–1, relative to the treatments with sole inputs of organic matter or urea. Although residue quality was shown to affect maize N uptake in a pot trial, its impact under field conditions was minimal for the range of considered residue qualities. In an alley cropping trial, maize yield was shown to be sustained on a non-degraded site and enhanced on a degraded site, when a minimal amount of mineral fertilizer was added with the prunings, whereas fertilizer application alone failed to do so in both cases.