Biological, physical, and temporal constraints on extracellular enzyme activity in prairie and corn bioenergy systems

Background/Question/Methods

Physical protection of soil organic matter is a major constraint on soil microbially-mediated decomposition.  Enmeshment of litter and root inputs into soil aggregates reduces microbial access and alters redox conditions, affecting the rate at which microbes process organic substrates.  Although it is understood soil aggregates continually disintegrate and reform in soils, it is not well known how these dynamics affect microbial activity and organic matter decomposition over time.  In this study, we investigated soil microbial activity, as measured by extracellular enzyme activity, within soil aggregates isolated from three bioenergy cropping systems:  corn, fertilized prairie, and unfertilized prairie.  Soil samples were collected monthly during the 2012 growing season at the Comparison of Biofuel Systems (COBS) experimental site in central Iowa.  Aggregate fractions were isolated by an optimal moisture procedure, and extracellular enzyme activity was determined fluorometrically.  We hypothesized that potential extracellular enzyme activity would peak in July, which corresponds to greatest plant growth, including roots, and greatest aggregate turnover due to root growth, erratic precipitation events, and increased fungal growth.  We expected to observe greatest potential enzyme activity in macroaggregates >2000 µm due to greater presence of fresh root inputs, and least in microaggregates <250 µm.

Results/Conclusions

We found changes in aggregation and aggregate turnover occurred differentially in the three cropping systems, which scaled-up to affect ecosystem-level microbial activity and organic matter decomposition. Overall, fertilized prairie systems had the greatest proportion of macroaggregates >2000 µm and corn systems the least (P<0.001).  Prairie soils were most aggregated in August, but corn systems showed little change in aggregation across the growing season.  Although potential enzyme activity within aggregate fractions did not vary significantly from each other, differential aggregation across the growing season coupled with cropping system and sampling month to drive changes in potential extracellular enzyme activity (P<0.001).  Relative to the other systems, fertilized prairie had the greatest potential enzyme activity in August and October, and unfertilized prairie had the greatest potential enzyme activity in July and September, which coincides with peak aggregate turn-over.  Corn plots always exhibited the least potential enzyme activity.  Soil microbes in corn systems are responding to both reduced soil aggregation, which alters when and how microbes interact with organic inputs, and differences in those organic inputs (e.g. corn systems have eleven times fewer root inputs than prairie systems), which scale-up to impact ecosystem level cycling of organic matter.