Soil microbial community structure and organic matter transformation processes in organic and integrated farming systems

Summary

The DOK long-term field trial in Switzerland started in 1978 (Mäder et al., 2002). Biodynamic (BIODYN), bioorganic (BIOORG) and integrated (CONFYM) farming systems with manure fertilization, an integrated system based on mineral fertilizer only (CONMIN) and an unfertilized control (NOFERT) were investigated for changes in soil organic matter and on the structure of the soil microbial communities. Soil organic matter (SOM) in the Ap-horizon (0-20 cm) was analysed from the beginning of the DOK field trial in 1978. Roughly 20% of the initial SOM was lost when no manure was applied for 21 years as in the CONMIN and the NOFERT system, whereas SOM increased slightly by 4% in the BIODYN system that makes use of composted farmyard manure, corresponding to 1.4 livestock units per hectare (LSU). The BIOORG and the CONFYM system lost about 7%.
Soil microbial biomass analysed from 2000 and 2003 samplings revealed higher values in organically fertilised plots. Bacterial and eukaryotic biomasses followed the order: CONMIN < CONFYM ≤ BIOORG ≤ BIODYN. Phospholipid etherlipids, indicative for micro-organisms of the domain Archaea occurred in abundance compared to other organically fertilised soils and followed the order: CONMIN < BIODYN = BIOORG = CONFYM. Clear differences in PLFA based microbial community structure among the four farming systems were observed. Organic fertilization compared to no manure had the strongest effect, followed by organic farming compared to integrated, and finally the kind of organic farming management (BIODYN with composted manure vs. BIOORG with rotted manure). The incorporation of maize-derived carbon – with higher 13C content – to some of the PLFA was detected and highlights the functional role of the corresponding microbial groups in carbon transformation processes. Contrasting 13C values in the PLFA-biomarkers for Archaea in organic and conventional soils support the assumption that carbon transformation processes are affected by the farming systems.
Our results suggest that changes in SOM are indicated by changes in soil microbial biomass. The functional role of microbial groups will be further investigated. The enhanced chemical and biological soil quality found in the field plots fertilized with manure and manure compost maintains key soil functions, on which low input farming systems rely.