J. Bauhus, DOES THE ABSCISSION OF FINE ROOTS LEAD TO IMMOBILIZATION OF NITROGEN IN MICROBIAL BIOMASS DURING IN-SITU SOIL-NITROGEN MINERALIZATION MEASUREMENTS, Communications in soil science and plant analysis, 29(7-8), 1998, pp. 1007-1022
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.