The use of carbon isotopes as tracers is essential for measuring carbon flows in an intact whole plant-soil system. Here, we describe an Experimental Soil Plant Atmosphere System (ESPAS) to perform pulse-labelling and steadystate labelling experiments with 13CO2 and 14CO2. The ESPAS facility is an environmental research tool that is used to measure the carbon fluxes from the atmosphere to the roots and into the soil and the microbial biomass and to study decomposition of plant residues and soil organic matter. The influence of environmental conditions in the atmosphere or in soil on the carbon allocation and turnover in the plant-soil ecosystem can be quantified. The design and the technical description of the phytotrons is presented and evidence is provided that the phytotrons are equivalent. For this purpose, Triticum aestivum plants were cultivated in the phytotrons for 39 days and shoot growth, root growth and water use were compared. No significant differences were observed for plant growth and water use. As an example of the practical application of the equipment, an experiment with elevated atmospheric CO2 is presented. Data are given on the uptake of 14C under ambient (350 #L L -l) and elevated (700 #L L-ICO2 in Lolium perenne and Festuca arundinacea and the distribution of 14C among different plant-soil compartments i.e. shoot, root, root-soil respiration, and soil. We conclude that these phytotrons yield detailed information on gross carbon flows in a whole plant-soil system that can not be obtained without sensitive carbon tracers. Such data are important for proper calibration of simulation models on soil organic matter.

During the autumn of 1989 a feed contamination induced a widespread lead intoxication of cattle in the northern provinces of The Netherlands (Groningen and Friesland). Over 300 farms were involved, affecting about 15,500 animals (mostly dairy cattle). For a period of one to four weeks these animals took up a thousand kg of lead. This resulted in lead levels in milk, livers, and kidneys above the regulatory safety limits. Due to the chelating therapy, which was rapidly applied by the local veterinarians, only about 30 animals died of an acute lead intoxication. A joint action of the governmental and private authorities prevented exposure of consumers to lead-contaminated animal products. Based on observations, measurements and literature data, predictions were made of the lead levels to be expected in animal products and the time needed for depletion of these levels. The appropriate animals were ear-tagged to ensure their identification, and the decline in time of the lead levels in milk and offals was conscientiously monitored. In the second week of 1990 the lead concentrations were decreased to levels well below the regulatory limits, and hence the tags were removed. The present paper reports our observations and conclusions, especially regarding treatment, predictions and outcome of this incident.

SUMMARYIn this study the impact of an elevated CO, level on allocation of assimilates and water use efficiency of Douglas fir [Pseudotsiiga menziesii (Mirb.) Franco] was investigated. Juvenile Douglas firs were exposed to a long-term treatment at 350 and 700^1 T' CO^ for 14 months and subsequently crosswise transferred to phytotrons for a short-term treatment with 350 and 700 fi\ 1"^ COj for 4 wk in an atmosphere continuously labelled with "CO^. No interactive efTects on total net uptake of ^^COj between long-term treatment and short-term treatment were observed. The short-term treatment with 700 n\ 1"' COj increased the total net uptake of "CO^ by 22 %, compared with the 350 //I 1"^ CO^ treatment. The long-term pretreatment did not affect the total net uptake, suggesting that photosynthetic acclimation had not occurred. However, expressed per unit of needle mass a 14% reduction was observed in the trees pretreated at 700 /tl 1"^ COj. This was not because of a reduced sink strength of the root system. This reduced uptake per unit of needle mass after long-term treatment may have implications for carbon storage in forest ecosystems. The results showed that an initial growth stimulation can eventually be annulled by developing physiological or morphological adaptions. "CO^ in the root/soil respiration increased in the short-term treatment with 700 //I 1"^ CO^, indicating a stimulated use of current carbon compounds either by roots or microorganisms. The water use efficiency during the short-term treatment with 700 /tl T' COg increased by 32 %, but was not affected by the long-term pretreatment. Water use per unit needle mass during the short-term treatment was decreased both by the short-term treatment and by the long-term pretreatment by about 15%. Some of the observed effects appeared to be persistent, such as decreased water use per unit needle mass, whereas others, stimulation of total net "CO2 uptake and water use efficiency, were transient.

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