Soil Micro-Organisms As Influenced By Farming Systems and Management Practices.

Soil microorganisms guarantee numerous soil functions, such as nutrient cycling, soil structure formation and the regulation of soil pathogens. Restoring soil fertility and increasing stability of soil is an urgent call to farmers. Soils also serve as a habitat for an almost uncountable diversity of soil microbes, which often is closely linked to the above ground one. Active, well-structured soils guarantee sustainable yields and safeguard natural resources simultaneously.

Farming systems as a whole, but also individual practices such as tillage, the use of organic manures and crop rotations have a strong influence on soil microbial activities and biodiversity. Long-term trials are ideal bases to identify effects of agricultural management on the soil microbial communities. In this talk we will focus on results gained in a long-term system comparison trial in Therwil (Basel-Land), Switzerland, called DOK experiment. In this experiment, bio-dynamic (D), bio-organic (O), and conventional (K, konventionell in German) farming systems are compared since 1978 in a ley rotation. We found that soil microbial biomass and soil microbial activities (soil enzymes) were increased in organic as compared to conventional systems. Microbial communities, as assessed by phospholipid fatty acids and molecular finger prints (terminal restriction fragment length polymorphism, TRFLP) were clearly affected by farming systems and crops in the rotation. Diverse microbial populations more efficiently used soil organic carbon sources, as indicated by a lower metabolic quotient (qCO₂). In incubation studies, microbial biomass (chloroform fumigation extraction method) was identified as driver of decomposition of freshly added ¹⁴C labeled straw. Biomass was also correlated with soil structure, and to wheat total above nitrogen yield of wheat, showing its bench mark character for above ground biomass production.

Mycorrhizal fungi, associated with most arable crops, play key roles in resilient, sustainable cropping systems. A higher mycorrhizal root colonization, and a higher diversity of mycorrhiza, determined by spore morpho-typing and by micro-satellite technique was detected in the biological systems, whereas Gigaspora and Scutellosporaspecies, known as sensitive indicators for farm management, occurred more frequently in soils of biological  systems. A steady increase of mycorrhizal species was identified along with land use intensity from monoculture, to crop rotation, to extensive meadows.

In a long-term tillage experiment under organic management in Frick (Canton Aargau, Switzerland), tillage increased soil organic carbon in the upper soil layer, and also augmented soil microbial biomass and activity (dehydrogenase). In the reduced tillage plots, also soil structure and water holding capacity were greater than in ploughed plots. Here intraspecific diversity of Glomus intraradices (Rhizophagus irregularis) was higher in reduced tillage plots than in ploughed ones.

It is suggested that the higher nutrient use efficiency and energy efficiency found in organic systems is related to the more active and diverse micro-flora in the soil. The findings will be discussed in the frame of a series of long-term experiments in Europe (TILMAN-ORG project).