DOES THE ABSCISSION OF FINE ROOTS LEAD TO IMMOBILIZATION OF NITROGEN IN MICROBIAL BIOMASS DURING IN-SITU SOIL-NITROGEN MINERALIZATION MEASUREMENTS

A possible artefact of in situ measurements of nitrogen (N) net minera lization may result from the inclusion of abscised fine roots in incub ated soil. Fine roots usually have a relatively wide carbon (C)/N rati o and if fine root carbon is available to micro-organisms they will im mobilize N. In this study nitrogen and carbon in microbial biomass wer e compared between incubated soil and surrounding soil at the end of f ield incubation periods. It was hypothesized that immobilization of N would result in higher microbial N in incubated soil when compared to the surrounding soil. Incubations were carried out in steel tubes at s ites of different fine root density. No net immobilization of N occurr ed during incubations. There were no significant increases of microbia l N in incubated soil suggesting that immobilization of N in microbial biomass did not occur. A comparison of K2SO4-extractable organic N co ncentrations between incubated and surrounding soil also indicated tha t immobilization of N into extractable metabolites was unlikely. Howev er, under herbaceous vegetation significantly lower microbial N concen trations were measured in incubated soil compared to the surrounding s oils. This may suggest a decrease in microbial biomass during incubati ons due to cessation of C input from fine roots. If the reduction in m icrobial biomass leads to the mineralization of microbial tissue, net N mineralization using the in situ incubation technique may be overest imated.