G. Brouwer

Auger Sampling, Ingrowth Cores and Pinboard Methods

This chapter outlines those methods for assessing root systems structure and function in the field which are based on washing roots free from the soil in which they grew. Some of these methods are included in previous reviews (Kolesnikov 1971; Bohm 1979). The methods are either disruptive or totally destructive to the root system being studied and to the immediate environment (Taylor et al. 1991).

Review of Quantitative Root Length Data in Agriculture

ABSTRACT Quantitative data on specific root length (length/dry weight ratio) and root length density in different layers of the soil are summarized from literature for various crops.

Production and decay of structural root material of winter wheat and sugar beet in conventional and integrated cropping systems

Abstract Production of structural root material of sugar beet and winter wheat was quantified by analysis of root growth and decay in a time series of minirhizotron images, combined with a single auger sampling. Cumulative root production of winter wheat was about 1700 kg ha−1 for conventional crp management and 1960 kg ha−1 for integrated (less pesticides and mineral fertilizer, less intensive soil tillage and more organic manure) crop management; in 1990 the difference between the two management systems was statistically significant. At harvest time 85% and 68% (in 1986 and 1990, respectively) of this structural root production remained as intact roots in the soil in both management systems. For sugar beet total fine root production was estimated at 1150 kg ha−1 in 1987 and 1989, with a significantly lower amount on the field on which minimum tillage was introduced in 1986; on average 47% of total root production remained as intact roots at harvest. Winter wheat root decay was studied with litter pots after crop harvest and in the following growing season. Initially, the N concentration in remaining roots increased while dry weight decreased. No net immobilisation or mineralisation of N and P during autumn was evident. During the next growing season net mineralisation was proportional to loss of root weight in an exponential decay with a half-life of 600 degree days (daily temperature sum). This N release pattern during the next growing period thus contributes to the synchrony between N demand and supply, but no difference between the two management systems was found.

Trench Profile Techniques and Core Break Methods

This chapter describes methods for root observations based on mapping or counting root intersections with planes of observation in the soil. Normally these planes of observation are either vertical or horizontal. Compared with the methods based on washed root samples discussed in Chapter 6, these “profile wall” methods have advantages as well as disadvantages. A major disadvantage of the profile wall methods is that only a small part of a root is visible on such an intersection and it is not easy to distinguish between roots of different species, or between live or dead roots. Even the question of whether a whitish thread-like object sticking out of a plane is a root and not an enchytraeid (pot worm) or other soil organism may take some experience to answer (potworms move when touched). Creating access to planes of observation via trenches can be a rather destructive activity which is not welcome on small experimental plots, especially those intended for long-term experiments. On the positive side, however, profile wall methods can give a quick estimate of overall root distribution and can give detailed information on spatial patterns of roots in their interaction with physical, chemical and biological characteristics of the soil profile. If maps are made of root occurrence as well as any other readily observable feature, the toolbox of geographical information systems and quantitative map analysis can be used to analyze patterns, be it in only two dimensions.

An inflatable minirhizotron system for root observations with improved soil/tube contact

Commonly used minirhizotrons consisting of a transparent tube inserted into the soil seldom attain good contact between the tube and the soil, which leads to root growth occurring in a gap rather than in the soil. A new system is described involving an inflatable flexible rubber wall, made from a modified motorcycle tube. Pressure ensures a proper tube/soil contact so that the environmental circumstances for root growth along the tube more closely correspond to those in the undisturbed soil. Before the endoscope slide is introduced into the minirhizotron for taking pictures, the inflatable tube is removed, so that there is no-often opaque-wall between the endoscope and the roots. This improves the picture quality and facilitates the analysis of root images.

Quantification of air-filled root porosity: A comparison of two methods

Air-filled porosity of the root cortex is important for aeration of roots in situations where the external oxygen is insufficient. A quantitative theory predicting the depth a vertically growing root can penetrate into the soil is now available for simultaneous internal and external oxygen transport as a function of the air-filled porosities of soil and root (De Willigen and Van Noordwijk, 1987). Maximum depth of root penetration in the soil depends on root diameter, respiration rate, conductance of root epidermis-plus-exodermis for oxygen and air-filled porosity of both soil and root. Reliable methods for quantification of the air-filled porosity of roots or root segments are needed for practical applications of this theory. Two measurement techniques will be discussed here, direct measurements on microscopic sections and the pycnometer method as described by Jensen et al. (1969). To obtain root material with the significant variation in porosity, maize plants were grown with and without aeration, including some factors stimulating or reducing the normal formation of air spaces via ethylene (Konings, 1983).

Gas-filled root porosity in response to temporary low oxygen supply in different growth stages

The development of gas-filled root porosity in response to temporary low oxygen supply was tested for a range of edible and ornamental crops: rice, maize, wheat, sugar beet, tomato, cucumber, sweet pepper, carnation, gerbera and rose. In a first experiment, the roots of tomato, maize and gerbera had a higher gas-filled root porosity, Ep (% v/v), when grown permanently in a non-aerated instead of aerated solution. The Ep of roots increased during two weeks when half the root system of a young plant was transferred to a non-aerated solution; in older plants this response was not seen. Carnation had a negligible gas-filled porosity in all treatments. In a second experiment, a comparison was made between high (20 kPa) and low (about 2 kPa) O2 partial pressure in a recirculating nutrient solution. Half of the root system was transferred to low O2 at various growth stages. In most species older plants did not increase Ep on exposure to low O2. For tomato, sweet pepper and rose, Ep was normally in the range 3–8% (v/v). Young plants of cucumber, wheat and sugar beet also had an Ep in that range, but in older plants values ranged from 1 to 3%. Transverse root sections examined by light microscopy showed, on average, 60% more intercellular spaces in the root cortex than the measurements of gas-filled porosity, probably because some gaps and spaces in the cortex were not gas-filled. This effect was most pronounced in tomato. A negative pressure in the cortex may be needed for gaps to be gas-filled. An exodermis may increase the effectiveness of gas spaces in the cortex by closing the gas channels and, by offering some resistance to water uptake, allowing a negative pressure head in the cortex which keeps gaps gas-filled. A redox dye method was developed to study the length of root which is effectively supplied with oxygen, as a function of Ep. Results indicated that for every percent Ep the root can remain aerated over at least 1 cm in a non-aerated medium under the conditions of the test.

Heavy-metal uptake by crops from polluted river sediments covered by non-polluted top soil. 11. Cd uptake by maize in relation to root development

Cadmium uptake by maize from polluted river sediments covered with a clean top layer of variable thickness is discussed in relation to root distribution. Two pathways for uptake are distinguished: roots penetrating the contaminated layer or contaminants moving into the root zone. Relative Cd uptake proved to be roughly proportional to the fraction of total root length found in the contaminated layer. A deeper water table induced a deeper root development and more Cd uptake for a given thickness of clean topsoil. A model based on exponential decrease of root length density with depth is acceptable as first approximation only. Little or no evidence was found for contaminants moving into the root zone during the ten years of the experiment.Cadmium uptake by maize from polluted river sediments covered with a clean top layer of variable thickness is discussed in relation to root distribution. Two pathways for uptake are distinguished: roots penetrating the contaminated layer or contaminants moving into the root zone. Relative Cd uptake proved to be roughly proportional to the fraction of total root length found in the contaminated layer. A deeper water table induced a deeper root development and more Cd uptake for a given thickness of clean topsoil. A model based on exponential decrease of root length density with depth is acceptable as first approximation only. Little or no evidence was found for contaminants moving into the root zone during the ten years of the experiment.