Ambiental

AN IMPROVED METHOD FOR MEASURING SOIL RESPIRATION IN THE FIED
Physics Department, Rothamsted Experimental Station, Harpenden, Herts ALS 2.1Q
Sumary
(1) Soul respiration is commonly determined by measuring the rate of increase in the co2 concentration withing a chamber placed on the soil surface, with an opening to the soil.
(2) In the present method the errors associated with thetechnique have been minimized by careful design of the chamber and the sampling system, and by use of a sensitive infra-red gas analyser for the analysis of the co2 concentration.
(3) Small (5cm3) samples of the chamber air are taken at 1 min intervals over a 4 min period. From the measured co2 concentrations, the respiration rate is determined, using a graphical technique, to an accuracy of 2%and with a precisión of 10%.

INTRODUCTION

Mathematical models of crop photosynthesis (e.g. Monteith 1965) usally predict the gross carbón dioxide assimilation by the crop during daylight. To convert this gross into a net uptake, i.e. into growth, it is necessary to measure and then subtract that quantity of carbón dioxide respired by the crop. Respiration by the aerial parts of the plant maybe easily determined by measuring the co2 evolved when the plants are enclosed in a darkened chamber. Roots pose a more difficult problem; they are invariably damaged when plants are removed from the soil, and plants growing in culture solution or in pots are not representative of field plants. The method commnly used is to measure the quantities of co2 evolved from adjacent areas of cropped anduncropped soil, the difference bettween the two measurements being assumed to represent the co2 produced by root respiration (Leach 1979).
Three methods for measuring soil respiration are frequently employed:
(i) The soil is completely enclosed in a chamber (respirometer) in which co2 fluxes are monitored. Although the measurements are accurate and precise, respirometers are not porrable andthey do not reproduce the field situation (Currie 1974).
(ii) The co2 concentrations at various soil depths are measured and the fluxes calculated using estimates of the gas diffusivity of the soil. The diffusivity varies with depth and water content and has to be separately determined. Also sampling tubes have to be installed at the various depths. The method has only limited field use (deJong & Schappert 1972).
(iii) A chamber of know dimensions, and with an opening only to the soil, is placed on the soil surface and the inflow of co2 is monitored. This is the method we have used in the field. In previous applications of this method, there have been problems associated with the measurement of the quantities of co2 of evolved. Monteith. Szeicz &Yabuki (1964) measured the uptake ofco2 by an absorber placed within the chamber for a known time.
Measurement of soil respiration

Respiration was overestimated because of the enhanced diffusion gradient from the soil to the chamber. Kanemasu, Power & Sij (1974) pumped air through the chamber at a know flow-rate and measured the increase in its co2 concentration using an infra-red gas analyser . They found that the measuredrespiration rate depended on whether air was pumped into or out of the chamber as this determined the direction of mass-flow between the soil and chamber air. Lundegardh (1926) measured the co2 concentration in the chamber at the beginning and end of a known time period. However, because of the insensitivity of the method, of analysis, the concentration change had to be large, which modified the co2flux from the soil.
In the present method, whilst still making use of the increase in co2 concentration in a chamber over a known time period, the errors have been substantially decreased by using an appropriately shaped chamber, a sensitive analyser and a sequence of samples.

MATERIALS AND METHODS

The Chamber, made of Perspex, is open at the bottom, and is 10.0 cm long, 5.0 cm wide and...