Mineral associated organic matter in practice

Soil organic matter (SOM) plays a vital role in most soil functions related to agriculture. It is a building block of soil structure, it buffers pH and nutrient availability, and it supports the soil food web. Up to now in agricultural practices, including agriculture labs, SOM has only been characterized as one pool. Recently, more attention has been on the fractionation into particulate organic matter (POM) and mineral associated matter (MAOM) in relation to SOM dynamics. MAOM will be the most stable pool of SOM and mineralization is probably dominated by rhizosphere processing, and therefore controlled by plant nutrient demand. Based on the idea that microbial biology plays a key role in both the formation and degradation of MAOM, we propose that adjusting agricultural management to optimize the build-up of MAOM is the way forward in minimizing nutrient losses to surface waters.

In this study we measured POM and MAOM, based on size fractionation, in pairs of agricultural plots with contrasting soil management. Furthermore we followed mineralization rate via continuous measurements of EC, moisture content and soil temperature, and based on ion-binding resin bags placed at 10, 30 and 60 cm depth.

Overall, texture is a strong predictor of the amount of MAOM, but on top the application of compost appears to have a positive effect, both on grass- and cropland. But we have indications that in some cases our MAOM fractions are dominated by fine POM, probably caused by the practice of incorporation of organic manure into the soil. Initial results show that nitrogen leaching is more associated with POM than with MAOM.