WHAT SOIL PROPERTIES REGULATE RESPIRATION RATE AS AN INDICATOR OF SOIL HEALTH?

Soil health refers to a holistic perspective on how a soil functions in an agricultural ecosystem, with particular emphasis on biological processes. Since 2008, Iowa State University has conducted a long-term, large-scale, randomized complete block experiment to compare four biofuel cropping systems: continuous corn (CC), continuous corn with rye cover crop (CCW), unfertilized reconstructed prairie (P), and a nitrogen-fertilized reconstructed prairie (PF). In this presentation, we focus especially on the 24-hour soil respiration rate (measured after 8 growing seasons in 2016) as an indicator of soil health where biofuel crops are grown.

The research site is set on a representative suite of till-derived soils. The soil in the treatment plots had pH values of 6.1– 7.6, soil organic C concentrations of 25 – 30 g kg^-1, and clay concentrations of 244 – 282 g kg^-1.

We found that the 24-h CO₂ respiration rates of samples from the cropping systems treatments were variable, but overall, the mean values of the replicate plots were greater in the perennial cropping systems (P and PF) than in the annual cropping systems (CC and CCW). There was no consistent pattern to the respiration rates related to when the samples were collected – either before planting in the spring or after crop harvest in the fall.

We explored the impacts of a variety of “static” soil characteristics on the “dynamic” measurement of CO₂ respiration to see if the biological activity measured in the respiration test could be predicted by other indicators. The following parameters were not correlated with respiration rate: soil organic C, soil total N, water-extractable organic C, water-extractable total N, water-extractable organic N. On the other hand, for soil samples collected in the spring of 2016, the CO₂ respiration rate was positively correlated with water-extractable mineral N (R² = 0.71) in samples from the annual cropping systems (CC and CCW).

We are currently seeking other potential regulators of 24-h CO₂ respiration rates in these high-organic matter soils, including the possibility that particulate organic matter (i.e., decomposing crop root residues) are the main drivers of the soil respiration rate. Since respiration is a key indicator of soil health, we hope that this effort will lead to a better understanding of soil health in general.