Quantitative estimates of root densities at minirhizotrons differ from those in the bulk soil

Aims A key issue related to the usefulness of the minirhizotron technique is whether root presence and behaviour in the soil zone at the minirhizotron interface are consistent with those in the bulk soil. We wanted to test the null hypotheses that there were no differences in root densities or specific root length (SRL) between those positions. The effects of different nutrient and water regimes, and different tree species (grey alder and basket willow) on the root variables were also examined. Methods We quantified root mass and length densities, and calculated SRL, of three diameter classes from cores sampled around minirhizotrons and in the bulk soil, i.e. quartz sand. Fine root dynamics were monitored in the minirhizotrons during three consecutive years prior to sampling. The study was conducted on individual root systems within buried and covered lysimeters, placed in a stand structure, and with different water and nutrient regimes. Results Significant discrepancies in root densities were found between positions. Plants subjected to limited water or nutrient supply had up to three times higher densities at the minirhizotrons. However, effects of species and treatments showed a similar pattern for the diameter class < 1 mm between the two sampling positions and minirhizotron observations. The pulses of fine root growth and decay were coordinated in time for both species and treatments. The SRL was not affected by the tube in any diameter class. Conclusions We concluded that the minirhizotron technique alone was of limited use for estimating root densities in the bulk soil. But the results showed that minirhizotrons could be useful, for example, combined with soil coring or in comparative studies. The patterns of fine root growth and decay were similar for species and treatments, and SRL was unaffected, suggesting that minirhizotrons can be used in studies of root dynamics and morphology. There is, however, a need for further studies concerning the influence on root survival and decay.