Soil management is an important component of natural farming.  In order to avoid the use of chemical fertilizers, farmers must rely on natural fertilizers (e.g., manure) to replace nutrients removed from harvest.  Because soil microorganisms mediate the cycling of organic nutrients, natural farming is more dependent on soil microbes than conventional farming to sustain soil fertility.  The purpose of this study was to assess the relationship between soil fertility and microbial community composition and function in natural farming systems with different land use management.  Three small-scale natural farms in southeast Ohio served as the study sites with similar soil pH (~7.0). Sites A and B have received manure treatment for over two decades, but Site A had a greater application rate and was managed more intensively.  Site C is a new farm and has received a manure treatment for two years. After the last harvest, three soil cores (15 cm deep) were collected from three raised beds at each site. Soil fertility was assessed by analyzing total soil carbon (C), nitrogen (N), and soluble N. Soil microbial community composition and biomass were measured using phospholipid fatty acid (PLFA) analysis. Microbial function was determined by measuring the activity of four extracellular enzymes.

Results/Conclusions

The results of this study show that Site A had the highest levels of total C (124.7 g C kg^-1), followed by Site B (37.43 g C kg^-1) and Site C (16.25 g C kg^-1).  This pattern was similar for N, NH4, and microbial biomass, but not for CN ratio.  We observed no significant difference (P = 0.47) in total enzyme activity between sites.  Site C significantly (P = 0.04) had 13% greater total enzyme activity per microbial biomass when compared to Site B and 52% greater than Site A.  Furthermore, per unit of soil C, biomass was 75% and 42% greater in Site C when compared to Sites A and B, respectively.  Microbial community composition was significantly different (P < 0.01) between all sites based on multi-response permutation procedures.  Most notability, the fungal PLFA biomarker 18:2ω6 was absent from Sites A and B.  Results suggest that while high concentrations of organic matter results in more nutrients, soil microorganism per unit mass became less efficient to cycle nutrients via enzyme activity.  In conclusion, the results indicate that soils amended with organic fertilizer may face a saturation point in regards to the capacity of soil microorganism to cycle nutrients.