Results of field experiments using partial substratum labelling techniques are presented. These show that inorganic 15N application to soil just prior to sowing, the addition of 15N together with sucrose, incorporation of a 15N-labelled plant material into soil, as well as the 15N remaining in soil following the application of inorganic fertilizer to a previous crop provided adequate levels of 15N for field experiments to estimate N2 fixation in soybean and faba beans. These methods may be suitable for quantifying associative N2 fixation, especially if the experimental variability is low, or where N2 fixation is high. Proper site selection is therefore important. Under the conditions of the experiments reported in this paper, however, the sensitivity of detection of N2 fixation was low and could not estimate N2 fixed in plants in which the percentage of N derived from fixation was below 10–15%. The selection of the appropriate reference on standard, i.e., non-N2-fixing, crop and its judicious use in field experiments is crucial to the methodology for quantitatively measuring the amount of N2 fixed by the above methods. The most important factors in the selection of reference crops are absence of N2-fixing activity, similarity in feeding or rooting pattern of the non-N2-fixing and fixing crop, comparable growth period for reference and fixing crop, relative effect of environment on the two crops, and cropping pattern employed. In associative N2 fixation studies, an uninoculated plant in soil devoid of associative N2-fixing microorganisms would provide an ideal reference crop for an inoculated plant.
The spatial and temporal rooting pattern of Brussels sprouts and leeks