Response time of soil microbial biomass after conversion from conventional to several different organic farming systems

Organic farming was implemented on a 660 ha area of agricultural land with a long history of conventional farm management in Trenthorst, Northern Germany. The arable land was divided in five different organic farming systems differing in crop rotations and organic fertilization in addition to grassland plots. The plots were monitored over a period of seven consecutive years from 2001 to 2008 together with four adjacent plots which remained under conventional management. To assess the impact of soil management on the microflora, soil microbial quality indicators such as total microbial biomass (C-mic), the C-mic-to-C-org ratio together with soil carbon (C-org) were analyzed annually. C-mic and C-mic -to-C-org ratio were sensitive parameters which differed in their responses dependent on the farming system and varied from no effects, to beneficial or adverse effects. Field plots under the system "DAIRY" with a high presence of legumes in the crop rotation in addition with FYM and slurry applications were the most favorable for microbial development with significant Cmic increases up to 87 % within seven years, with an annual growth rate of 12 to 53 microgramme C-mic g(-1) soil and an increase of the C-mic-to-C-org ratio of up to 60 %. Plots under the system "PIG" showed losses in C-mic of up to 25 % and a decreased C-mic-to-C-org ratio of up to 27 %. The type of crop management here could be the reason for the insufficient microbial growth. A gradual loss of Nt and P was noted in most of the organic plots over the years. As a general observation it seems necessary to examine in closer detail the nutrient composition of plots under transition to organic farming for an optimal stimulation of microbial growth. The conventionally managed plots had consistently high microbial indices, which correspond to those of plots under conversion to organic farming