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 this study, microbial parameters under different kinds of mulches and cover crops were analyzed in a field experiment with organically grown black currant (Ribes nigrum). The experiment comprised a combination of two mulches with bare soil as a control and two cover crops which were compared with bare soil, with and without an extra supply of organic fertilizer. Soil carbon (C) and nitrogen (N) as well as pH were unaffected by any of the treatments. The basal respiration rate was increased by mulching with wood chips throughout the four years of the experiment. 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 significantly after an initial supply of 200 kg N ha(-1) as solid cattle manure. The increase was significantly lower under wood chips than in bare soil, although an extra 200 kg N ha(-1) had been supplied under the 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 matter content with wood chips because of lack of N, despite a large initial input ofN and easily-available C. Possible reasons for this deficiency are either increased denitrification under the wood chips or fungal translocation of N to the wood chip layer. Results from this experiment suggests that the evaluation of a few complementary biological soil parameters can be an important tool when developing sustainable growing systems and for indicating environmental stress.