Influence of Long-Term Organic Cropping Systems on Soil Microbial Population Size and Structure.

While organic farming relies on microbial activity to cycle nutrients, the efficiency and timing of nutrient release depends upon a broad set of management practices that impact the soil environment and related soil microbial populations. However, further studies are needed to determine management practices that will optimize soil microbial populations. To better understand the influence of different organic management practices on soil microbial communities, we assessed biological and chemical properties of soil from different cropping systems representative of Midwestern production systems. The study was conducted at the Wisconsin Integrated Cropping System Trial, a long-term trial (26 years) at the University of Wisconsin Arlington Agricultural Research Station in Columbia County, WI. The trial includes two organic rotations: a cash grain rotation and a dairy forage rotation, as well as a low-input pasture. Soil DNA extraction was performed followed by 16S and ITS sequencing, phospholipid fatty acid (PLFA) profiling, and soil chemical properties assessment. The 16S DNA sequencing showed that the three systems differ in their soil microbial community profiles. More specifically, while showing the most divergent bacterial population as compared to the other cropping systems characterized, the pasture system is more similar to the dairy forage than to the grain system. The PLFA extraction also demonstrated similarities in soil microbial biomass and composition between pasture and dairy forage system. The main difference between those two systems is the greater quantity of filamentous micro-organisms found in the pasture soil. As for the grain system, it has the lowest total microbial biomass as well that the lowest abundance of micro-organisms in each of the different guilds. An analysis of the interactions between soil chemical and biological properties demonstrates that within the 17 soil chemical properties assessed, pH and total nitrogen have the highest impact on soil microbial populations.