Factors affecting biological nitrogen fixation as assessed by the ureide technique in Glycine max. L. (Merril) and its implication on soybean production in South Africa

Abstract

Research on nitrogen fixation was limited due to the lack of a simple and accurate means of assessing nitrogen fixation activity. The development of the ureide technique, (Herridge et al., 1990; Herridge & Peoples, 1990) for assessment of nitrogen fixation on soybeans, provided us with a practical means to do research in this domain, but only if accurate sampling procedures were developed. The aim of the study was to evaluate the biological nitrogen fixation ability of commercially grown soybeans, in the main production areas of South Africa. Before this could be done the ureide technique had to be characterised for the South African conditions. The findings of this investigation were the following: The effect of time delay from sampling the soybean plant till vacuum extraction of the xylem exudate proved to be an important consideration when using the ureide technique. The delay should not be longer than 5 minutes. The time of day when the xylem exudate is extracted should also be considered. To avoid any distortion of the N2 fixation values the sampling should take place between 10:00 in the morning and 16:00 in the afternoon.

The ureide technique was implemented under field conditions by means of a nodulation trial. The cultivar LEE nodulating (NOD+) and non-nodulating (NOD-) isolines were used in conjunction with different N fertiliser application rates and by means of the ureide technique, the influence of the fertiliser rates and times of application were evaluated. We can also describe the trial from another viewpoint, namely that the nodulation trial was used to evaluate the ureide technique's ability to assess the different N2 fixation rates induced by the treatments. The influence of the N fertiliser on the N2 fixation of the NOD+ was less dramatic when it was applied during the reproductive stages of the soybean plant. N fertilisation can only be useful if the N supply to the crop is manipulated in such a way that the symbiotic N2 fixation is not inhibited. Under normal growth conditions N fertilisation hold no advantage to the farmer. The main aim of the study was achieved when the amount of N2 fixed by 32 commercial soybean crops, during the 1997 /1998 and 1998/1999 growth seasons, was assessed. Crop N yields, N2 fixation and nodulation were measured three times during crop growth and estimates of the seasonal amounts of N2 fixed were compared with amounts of seed N removed. For each crop a net N-balance was calculated to determine whether a net gain or loss of N occurred in the soil as a result of soybean cropping. Most of the crops were well nodulated. However, ureide sap samples collected indicated that the proportion of N derived form N2 fixation (Pfix) ranged between 38 and 100% at flowering and 4 and 99% at seed fill during the 1997/1998 season. The 1998/1999 seasons Pfix values ranged between 30 and 94% at flowering and 47 and 97% at seed fill. Seasonal estimates of the proportion N derived from N2 fixation (Pfix) and the amount of N2 fixed for the 1997 /1998 season ranged between 45 and 100% and 76 and 301 kg N.ha-1 respectively. For the 1998/1999 season the respective values were between 25 and 102% and 58 and 366 kg N.ha-1 . Following seed harvest these amounts of N2 fixation were used in calculating the N-balance results. The N-balance values for the 1997/1998 season ranged from -135 to 71 kg N.ha-1 and from

-157 to 47 kg N.ha-1 for the 1998/1999 season. Eight of the crops evaluated resulted in a positive net N-balance at the end of the season. The large amounts of N2 fixed by most of the crops, despite their net N-balance being negative, indicated that the potential for fixed N carry-over exists. The challenge to researchers and extension officers is to improve N2 fixation through the encouragement of correct crop and soil management practices. We were able to characterise and implement the ureide technique successfully in the South African soybean environment and were able to demonstrate its usefulness as a research tool and for nitrogen management in South African cropping systems.