Influence of various forms of green manure amendment on soil microbial community composition, enzyme activity and nutrient levels in leek

The influence of different forms of green manure amendment on selected soil microbial characteristics and crop productivity was investigated in a leek cropping system. Mulching with fresh red clover, and incorporation of red clover-derived biogas slurry and compost were tested as alternatives to direct incorporation of a red clover crop. The specific aim of this study was to investigate how these different green manure forms, fresh and processed, influenced soil microbial biomass, microbial community composition and soil enzyme activity. An initial increase in abundance of bacteria and fungi was observed after direct incorporation and amendment with red clover-derived slurry and compost, but amendment with fresh red clover sustained a higher bacterial and fungal biomass until the end of the cropping season. Mulching stimulated arbuscular mycorrhizal (AM) fungi at the end of the cropping season. The treatments with fresh red clover, direct incorporation and mulch, tended to differ in their microbial community composition from the treatments with processed red clover. The protease, acid phosphatase and ary1sulphatase activities were highest in the direct incorporation treatment, whereas enzyme activity in treatments with processed red clover was never higher than in the control treatments. There were no differences in leek harvest yield, but the nitrogen (N), phosphorus (P) and sulphur (S) concentrations in the leek crop at harvest increased in response to higher amounts of slurry and compost amendment. Arylsulphatase activity was negatively correlated with leek S concentration at harvest, indicating that the activity was regulated by the nutrient status of the cropping system. We concluded that direct incorporation of a red clover crop was most effective for enhancing and sustaining a high microbial biomass and high rates of enzyme activity in soil. (c) 2006 Elsevier B.V. All rights reserved.