Linkages between soil organic matter and magnetic mineral formation in agricultural fields in Northfield, MN

Abstract:

There are many physical, chemical, and biological indicators commonly used to quantify soil health; one of the most common is soil organic matter (SOM). SOM is quantified using a variety of methods, most often loss-on-ignition, Walkley-Black, or other field-estimates. Here, we describe an approach to constraining soil health based on a correlation between SOM and pedogenic iron oxide minerals, which can be quantified quickly and at low cost using magnetic properties. We present data from two agricultural fields in Northfield, Minnesota. In each field, we collected 100 topsoil samples along a 40 m by 20 m grid to determine spatial variability in SOM and soil magnetic properties; we also collected 2 soil cores to determine variability with depth. SOM was measured in each sample, as was a suite of magnetic properties: the frequency dependence of magnetic susceptibility, isothermal remanent magnetization (IRM), anhysteretic remanent magnetization (ARM), and hysteresis properties. Strong positive correlations were observed between SOM and three magnetic properties: the frequency dependence of susceptibility, ARM, and the ARM/IRM ratio (all three proxies for the concentration of soil-formed magnetite of different grain sizes). We suggest a mechanistic basis for this association whereSOM appears to limit magnetic mineral production through microbially-driven iron reduction processes, where SOM acts as an electron donor during wet-dry cycles. Continued work to demonstrate both empirical and theoretical validation of this correlation will improve confidence in using soil magnetization as a cost-effective and efficient tool for evaluating SOM in large agricultural fields.