EFFECTS OF MULCHING AND COVER CROPPING ON SOIL MICROBIAL PARAMETERS IN THE ORGANIC GROWING OF BLACK-CURRANT

Cover cropping and mulching to sustain and improve soil fertility and for weed control are common practices in organic growing systems. In t his study, microbial parameters under different kinds of mulches and c over crops were analyzed in a field experiment with organically grown black currant (Ribes nigrum). The experiment comprised a combination o f two mulches with bare soil as a control and two cover crops which we re compared with bare soil, with and without an extra supply of organi c fertilizer. Soil carbon (C) and nitrogen (N) as well as pH were unaf fected by any of the treatments. The basal respiration rate was increa sed by mulching with wood chips throughout the four years of the exper iment. During the last two years of the experiment, substrate induced respiration was also measured but was not found to be affected by any of the mulches. The potential ammonium (NH4+) oxidation increased sign ificantly after an initial supply of 200 kg N ha(-1) as solid cattle m anure. The increase was significantly lower under wood chips than in b are soil, although an extra 200 kg N ha(-1) had been supplied under th e wood chips. Furthermore, the black currant bushes suffered from a N deficiency in the wood chip treatment. The results: showed that there was no substantial lasting build-up of microbial biomass or organic ma tter content with wood chips because of lack of N, despite a large ini tial input ofN and easily-available C. Possible reasons for this defic iency are either increased denitrification under the wood chips or fun gal translocation of N to the wood chip layer. Results from this exper iment suggests that the evaluation of a few complementary biological s oil parameters can be an important tool when developing sustainable gr owing systems and for indicating environmental stress.