DYNAMICS OF THE MICROBIAL-POPULATIONS OF A RECLAIMED-POLDER SOIL UNDER A CONVENTIONAL AND A REDUCED-INPUT FARMING SYSTEM

The dynamics of microbial populations (bacteria, actinomycetes and fungi) of a field treated as a conventional farming system was compared with that of a field treated as a reduced-input system. The rhizosphere and root-free zones were investigated separately. Three approaches were used to describe the communities: by means of: (i) activity indices; (ii) a multivariate procedure (canonical correspondence analysis); and (iii) diversity indices. These methods gave complementary information concerning the behavior of the microbial populations. Canonical correspondence analysis proved to be a powerful tool to indicate differences in population characteristics between fields and sampling times. Characteristics of populations changed considerably in time. These changes were larger than differences between the two fields. In general, characteristics of populations in the rhizosphere changed more in time than those of populations in the root-free soil. Differences in population characteristics between the two fields were larger for fungi than for bacteria and actinomycetes. At the end of the growing season the fungal root populations in the two fields were significantly different. The ability of the fungi in the root zone of the conventional field to utilize polymeric substrates and aliphatic organic acids decreased strongly in this period. This was not the case in the reduced-input field. The diversity of the bacterial and fungal populations decreased in July and August, the strongest decrease occurring in the rhizosphere of the conventional field. Decrease in diversity appeared to reflect periods of drought that occurred in summer.