Wageningen Rhizolab Research Articles

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 0.07 cm (°C day)−1. Among the crops whose roots grew down much faster (0.18–0.26 cm (°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 1–4 cm per day. In the crops studied the root front velocity did not correlate with the root proliferation rate.

Modelling belowground processes to explain field-scale emissions of nitrous oxide

Globally, soils are the most important source of nitrous oxide (N 2O). In several recent studies, N 2O fluxes from soils have been monitored. Regression relationships between N 2O fluxes and variables like average soil moisture content, nitrogen content and temperature did in general not explain more than 50% of the flux variance. The quantitative insight into the relation between measured fluxes on the one hand, and the underlying basic processes and their determining factors on the other, is still limited. This study aims to contribute to this insight by modelling underground processes to explain N 2O profiles in the soil. On sandy grassland soils in the Wageningen Rhizolab, we monitored aboveground N 2O fluxes as well as underground profiles of several variables. A process-based model was adapted to simulate the relevant processes. In the model we assumed that the sandy soil profile consisted of homogeneous soil layers. The modelled processes included biological and physical processes. Nitrous oxide is produced during denitrification and is transported via diffusion. The simulated O 2 and CO 2 profiles were satisfactory, indicating that the (bulk) respiration rates used in the model were realistic. The N 2O profiles, however, were less well simulated. With respect to N 2O production, the assumption of homogeneity within the soil layers was probably not correct for the sandy soil. We discuss modelling approaches to explain observed N 2O profiles by describing partial anaerobicity in soil layers. Of these, the so-called ‘randomly distributed-pore model’ [Eur. J. Soil Sci. 46 (1995) 507] and an adapted version for regularly distributed pores appear good candidates for incorporation in field-scale models for a layered sandy soil. Such models would describe processes like bulk respiration and the transport and plant uptake of water and nutrients. The randomly distributed-pore model is a relatively simple model for a steady state situation. For sandy soil layers, the model predicts that a substantial part of the volume can be anoxic under a range of conditions including conditions that often occurred in our experiment: a ratio (volumetric moisture content)/(saturated volumetric moisture content)≥0.7 and oxygen percentages in the air-filled pores between 5 and 21%. The regularly distributed-pore model also predicts the occurrence of anoxic fractions under common experimental conditions, but these fractions are significantly smaller than those according to the randomly distributed-pore model.

LINGRA‐CC: a sink–source model to simulate the impact of climate change and management on grassland productivity

A simulation model for the prediction of grassland (Lolium perenne) productivity under conditions of climate change is described and validated for grass growing in the Wageningen Rhizolab, Wageningen, The Netherlands. In this work the model was used to study the impact of different management strategies on the productivity of grassland under present and increased atmospheric CO2 concentrations. In LINGRA‐CC simulated key processes are light utilization, leaf formation, leaf elongation, tillering and carbon partitioning. The daily growth rate is determined by the minimum of a sink and a source term. As in a previous model (LINGRA), the potential growth of the sink depends on the mean daily temperature, and can be modified by the effects of the availability of assimilates on tillering. The growth of roots is calculated from the amount of carbohydrates the shoot is unable to utilize when the number or activity of the sinks is small (overflow hypothesis). The main difference between LINGRA and LINGRA‐CC is the way the source of assimilates for growth is calculated. Assimilate production depends on intercepted radiation, and a photosynthetic light‐use efficiency (LUE) calculated as a function of CO2, temperature, light intensity and the Rubisco concentration of upper leaves. Other differences are that in LINGRA‐CC, the specific shoot area for new growth depends on the level of reserves. Data from two independent experiments with L. perenne swards, grown in enclosures at two levels of CO2 during 1994 and 1995, were used to calibrate and validate the model, respectively. The model predicted well the observed amounts of harvested biomass, and the dynamics of the leaf area index, tiller number and specific shoot area. LINGRA‐CC was used to study the effects of different combinations of cutting interval and cutting height on biomass production, at ambient (350 μmol mol−1 CO2) and double (700 μmol mol−1 CO2 ) CO2 conditions. Under both ambient and doubled CO2, maximum biomass was produced with cuttings of leaf area index >1, and at cutting intervals of 20 and 17 d for ambient and increased CO2 environments, respectively. Under high CO2 conditions the cutting interval for maximum yield was 15% shorter than at ambient CO2. However, the gain in harvested biomass obtained by reducing the cutting interval by 3 d under high CO2 conditions was negligible.

Effects of potato-cyst nematodes (Globodera pallida) and soil pH on root growth, nutrient uptake and crop growth of potato

Potato-cyst nematodes (Globodera pallida) cause severe yield losses in potato. Plants infected with potato-cyst nematodes generally have reduced concentrations of nitrogen, phosphorus and potassium in the foliage. This study investigated whether reduced growth of nematode-infected potato is caused by nutrient limitation. Experiments in the field and in containers showed that phosphorus concentration correlated best with total crop biomass at early stages of growth. The role of phosphorus in nematode damage was further investigated in the field and in the Wageningen Rhizolab. The experimental field was infested with potato-cyst nematodes and two levels of nematode density were established by fumigation with a nematicide. Prior applications of calcium carbonate resulted in pHKCl levels of 4.8 and 6.1. Two levels of phosphorus fertiliser were applied: either 0 or 225 kg P ha−1. In the Wageningen Rhizolab, soil of both pH levels from the field was used after treatment with 1 MRad gamma irradiation to kill the nematodes. Subsequently, half of the soil was inoculated with cysts to give a nematode density of 30 viable juveniles per gram of soil. In the field, nine weeks after planting, the total crop biomass ranged from 107 g m−2 for the treatment with nematodes at pHKCl 6.1 without phosphorus fertiliser to 289 g m−2 for the fumigated treatment at pHKCl 4.8 with phosphorus fertiliser. The differences in total biomass for the various treatments were explained by differences in foliar phosphorus concentration. Nematodes induced or aggravated P deficiency and reduced total biomass. This was not the only damage mechanism as at high, non-limiting levels of foliar phosphorus concentration, nematodes still reduced total biomass. In the Wageningen Rhizolab, directly after planting, the number of roots visible against minirhizotrons was reduced by nematodes. However, the increase of root number in the nematode treatment continued longer than in the control, until root number was higher than that of the control. The compensary root growth of the nematode treatment was restricted to the top 30 cm and nematodes reduced rooting depth. High soil pH reduced growth, mainly by reducing the availability of phosphate. Both nematodes and high soil pH reduced nutrient uptake per unit root length. Our results lead us to suggest an interaction between nematodes and soil pH, with nematode damage being higher at pHKCl 6.1 than at pHKCl 4.8.

Root, soil water and nitrogen dynamics in a catch crop - soil system in the Wageningen Rhizolab

Catch crops (winter rye [Secale cereale] and fodder radish [Raphanus sativus]) were grown on lysimeters with rhizotron facilities in Wageningen, Netherlands, from September-March (1993-94) and August-March (1994-95) in order to study root growth and water and nitrogen dynamics under different regimes of irrigation and N supply. Catch crops took up 20-30 g N/msuperscript 2, of which 37-48% was present in dead leaves in March. Rooting depth increased by 2.6 cm/day for both species at the start of the growing season. Catch cropping reduced the NO-3-N concentration in the soil considerably, initially in the top layers and then further down the soil profile. The reduction in total leached N was similar to the total crop N uptake. Nitrate-N concentrations in leached water were reduced by 49-85 mg/litre (62-99%), depending upon N availability and irrigation. Due to catch cropping the NO-3-N concentration in the percolate decreased with increasing irrigation (or precipitation), whereas the amount of N leached increased with irrigation. Evapotranspiration from a catch cropped soil was close to the potential evapotranspiration under optimal growth conditions.

The influence of potato cyst nematodes ( Globodera pallida) and drought on rooting dynamics of potato ( Solanum tuberosum L.)

Minirhizotron root video observations of two experiments (in 1991 and 1992) in the Wageningen Rhizolab were used to investigate the extent to which potato cyst nematodes ( Globodera pallida) without (1991) or with accompanying drought (1992) influenced the rooting dynamics of potatoes. The main effect of potato cyst nematodes in 1991 was a retarded root length. At a depth of 10 cm and without nematodes, the maximum total root length was produced 60 days after planting. At a depth of 45 cm, this point was reached after 80 days. With nematodes (2.5, 10 and 40 living larvae per gram of soil), however, the date at which maximum root length was produced was retarded with at least 20 days, depending on the infestation level of the soil. At a depth of 45 cm, nematodes at the highest infestation level also eventually resulted in a shorter root length. At a depth of 10 cm, nematodes resulted in a somewhat greater root length. The effects of nematodes on the longevity of potato roots were analysed by following the fate of individual root segments. It was concluded that in the first experiment (1991), the rate at which roots decayed was higher without nematodes than with nematodes. Combining the data for root length production with root length decay, it was found that without nematodes, the standing living root length was only higher in the first part of the season. Also, from the second experiment (in 1992), in which the effect of nematodes was studied in combination with drought, potato roots did not show a reduced longevity with nematodes. The yield of potatoes in the presence of nematodes is discussed.

Carbon balance and water use efficiency of frequently cut Lolium perenne L. swards at elevated carbon dioxide

The impact of doubled atmospheric [CO2] on the carbon balance of regularly cut Lolium perenne L. swards was studied for two years under semi‐field conditions in the Wageningen Rhizolab. CO2 and H2O vapour exchange rates of the swards were measured continuously for two years in transparent enclosures. The light utilization efficiencies of the swards ranged between 1.5 g CO2 MJ–1 global radiation (high light, ambient [CO2]) and 2.8 g CO2 MJ–1 (low light, doubled [CO2]). The above‐ground net primary productivity (NPP) in the enclosures was greater by 29% in 1994 and 43% in 1995 in the doubled [CO2] treatments, but only 20% and 25% more carbon was recovered in the periodical cuts. Thus, NPP increased significantly more than did the harvested above‐ground biomass. The positive [CO2] effect on net carbon assimilation is therefore associated with a preferential allocation of extra carbon to the roots and soil.In addition to higher canopy photosynthesis and leaf elongation rates, a small part of the positive [CO2] effects on NPP could be attributed to a decrease of the specific respiration of the shoots. On a canopy basis however, respiration was equal or slightly higher at doubled [CO2] due to the higher amount of standing biomass.Comparison of NPP and carbon recovered in different harvests showed that allocation to roots and soil was highest in spring, it was low in early summer and increased again in late summer and autumn.The total gross amount of carbon partitioned to the roots and soil during the two year period was 57% more at doubled [CO2]. The total amount of carbon that was sequestered in the soil after subtraction of the respiratory losses was 458 g m–2 and 779 g m–2 in the ambient and doubled [CO2] treatments, respectively.The average water use efficiency (WUE) of the swards was increased by a factor 1.5 at doubled [CO2]. Both WUE and its positive interaction with [CO2] varied between years and were positively correlated with global irradiance. At doubled [CO2], the higher WUE was fully compensated for by a higher leaf area index. Therefore, total transpiration on a canopy basis was equal for the ambient and the doubled [CO2] concentrations in both years.

Root growth dynamics of Brussels sprouts (Brassica olearacea var.gemmifera) and leeks (Allium porrum L.) as reflected by root length, root colour and UV fluorescence

Minirhizotron observations of roots of leeks and Brussels sprouts grown in the Wageningen Rhizolab were used to study the dynamics of root length. Day of appearance and the time of decay were assessed for individual root segments visible on the minirhizotron surface.

Soil mineral nitrogen availability to young maize plants as related to root length density distribution and fertilizer application method

Minirhizotron observations from 4 experiments in 1992 and 1993 with maize on a sandy soil in the Wageningen Rhizolab, Netherlands, showed strong vertical and lateral root density gradients during the first 9 weeks after emergence. Root length density (Lrv), as determined in core samples 9 weeks after emergence, was positively related (P

The spatial and temporal rooting pattern of Brussels sprouts and leeks

Rooting intensity and root distribution of Brussels sprouts and leeks were assessed in field experiments and in the Wageningen Rhizolab, a rhizotron facility built in 1990, in relation to their contrasting N uptake behaviour (Brussels sprouts usually take up N rapidly after planting, resulting in soil depletion, while leeks generally use only half of the available N). In field experiments rooting was quantified in the various layers of the profile as the volumetric root length density (RLD; cm/cmsuperscript 3) and in the Rhizolab with horizontal glass minirhizotrons as the number of roots per cmsuperscript 2 (NR). Regression of RLD and NR on thermal time (accumulated average daily temperature above ground) after planting revealed that the rooting depth of Brussels sprouts increased faster with thermal time than that of leeks (0.13 vs. 0.08 cm/degree-day, respectively in one of the Rhizolab experiments). Furthermore, leeks showed an unusual distribution of roots in the profile with maximal rooting intensity at depths of 10-20 cm. In all experiments the proliferation of roots (calculated as the increase in NR or RLD with thermal time) was slower in leeks than in Brussels sprouts, especially in the deeper layers of the soil profile. Therefore leeks can be considered as a shallow rooting crop. The field experiments generally showed the same difference between the crops, Brussels sprouts having greater root density, deeper rooting and faster root proliferation than leeks. For both crops, however, root proliferation at the deeper layers was much slower in the field than in the Rhizolab, probably due to unfavourable conditions in the subsoil caused by higher bulk densities or temporary high water tables. The consequences of different rooting patterns for N utilization are discussed.

Effects of climate change on productivity of cereals and legumes; model evaluation of observed year‐to‐year variability of the CO2 response

AbstractThe effect of elevated [CO2] on the productivity of spring wheat, winter wheat and faba bean was studied in experiments in climatized crop enclosures in the Wageningen Rhizolab in 1991–93. Simulation models for crop growth were used to explore possible causes for the observed differences in the CO2 response. Measurements of the canopy gas exchange (CO2 and water vapour) were made continuously from emergence until harvest. At an external [CO2] of 700 μmol mol−1 Maximum Canopy CO2 Exchange Rate (CCERmax) at canopy closure was stimulated by 51% for spring wheat and by 71% for faba bean. At the end of the growing season, above ground biomass increase at 700 μmol mol−1 was 58% (faba bean), 35% (spring wheat) and 19% (winter wheat) and the harvest index did not change. For model exploration, weather data sets for the period 1975‐88 and 1991–93 were used, assuming adequate water supply and [CO2] at 350 and 700 μmol mol−1. For spring wheat the simulated responses (35–50%) were at the upper end of the experimental results. In agreement with experiments, simulations showed smaller responses for winter wheat and larger responses for faba bean. Further model explorations showed that this differential effect in the CO2 response may not be primarily due to fundamental physiological differences between the crops, but may be at least partly due to differences in the daily air temperatures during comparable stages of growth of these crops. Simulations also showed that variations between years in CO2 response can be largely explained by differences in weather conditions (especially temperature) between growing seasons.

Rooting Characteristics and Nitrogen Utilisation of Brussels Sprouts and Leeks

ABSTRACT Under field conditions a crop of Brussels sprouts (Brassica oleracea, var. gemmifera) usually utilises the plant-available nitrogen better than leeks (Alliumporrum L.). To investigate the role of the root system in nitrogen utilisation, the two crops were grown in the Wageningen Rhizolab, a new rhizotron facility. Large differences in rooting pattern were observed. Leeks had a relatively shallow rooting system with maximum root growth at a depth of 15 to 20 cm and a low rooting intensity. The rooting pattern of Brussels sprouts could explore the soil profile to a depth of one metre in a very short period. Large differences in nitrogen uptake pattern also showed up between the crops. Brussels sprouts depleted the entire rooted profile of nitrogen in a relatively short period (June to mid-August) whereas in leeks the nitrogen uptake was greatest in the second part of the season. The implications of these rooting and nitrogen uptake patterns for fertilisation management strategies to minimise N loss...

The influence of cyst nematodes and drought on potato growth

In two experiments in the Wageningen Rhizolab with potato cv. Mentor planted in soils with or without potato cyst nematodes (Globodera pallida, W) the number of roots per cm2 was observed two weekly by video camera, in horizontally placed minirhizotrons at depths varying from 5 to 100 cm. In both experiments initial root growth was more rapid in the top soil of the uninfected soil. In the first experiment under optimal water supply, root formation continued longer in the top 30 cm of infested soil leading to twice as many roots at the end of the growing season as in uninfested soil. In the subsoil from 30 to 100 cm, however, root formation was strongly reduced by cyst nematodes leading to an uneven distribution of roots throughout the profile. In the second experiment potato cyst nematodes only increased rooting in the top soil with reduced irrigation. Potato cyst nematodes did not affect the water use efficiency of the crop whereas reduced irrigation increased water use efficiency by about 22%. Without potato cyst nematodes the soil profile was depleted of mineral nitrogen until a depth of 1 m whereas with high initial population densities no nitrogen was taken up in the subsoil between 30 and 100 cm. The spational heterogenity of roots and nitrogen in the soil is an important mechanism of damage. This finding may lead to improved cultural practices and breeding for tolerance.

The Wageningen Rhizolab — a facility to study soil-root-shoot-atmosphere interactions in crops

A research facility is described for the integrated study of soil-root-shoot-atmosphere relationships in crops. The Wageningen Rhizolab has been in use since 1990, and consists of two rows, each with eight below-ground compartments; aligned along a corridor. A rain shelter automatically covers the experimental area at the start of rainfall. Compartments are 125 cm×125 cm and 200 cm deep. Each compartment has a separate drip irrigation system. Crop canopy photosynthesis, respiration, and transpiration can be measured simultaneously and continuously on four out of eight compartments at a time. Each compartment can be filled with a selected soil material (repacked soil) and is accessible from the corridor over its full depth [...]

The Wageningen Rhizolab ? a facility to study soil-root-shoot-atmosphere interactions in crops

Roots in the Wageningen Rhizolab are observed using two methods: (i) non-destructively, using horizontal, glass minirhizotrons at intervals of 14 days between observations; (ii) with destructive sampling using augers on three dates in the season. This paper reports changes with depth and time in root numbers per unit interface area of the minirhizotron tube (number of intersections) of four crop species (wheat, Brussels sprouts, leek and potato). The number of root intersections of Brussels sprouts, wheat and potato declined with depth at any time, whereas leek showed a different pattern because maximum root growth was observed at a depth of 10–20 cm. Root density generally decreased in the following order: Brussels sprouts, wheat, potato and leek. Plots of root length densities, Lrv(cm. cm-3), obtained by auger sampling, versus the number of intersections showed considerable variation in slope with species, time in the season and year, implying that a single, universal equation to convert minirhizotron observations into volumetric root densities does not exist. Causes of variation in the slopes are discussed. It is concluded that limited auger sampling combined with minirhizotron observations yield adequate quantitative estimates of relevant root properties.