Measuring water extractable organic matter in North American grassland soils using standarized methods for continental scale comparisons

Background/Question/Methods

Terrestrial inputs of organic matter are a major subsidy of nutrients and energy to many aquatic systems. Inland waters act as “active pipes” that connect terrestrial systems to the ocean and not only transport organic materials but also transform them along the way. Despite this integrative nature, a majority of research has focused on the study of terrestrial and marine systems while neglecting the linkages between them.  Our research focused on the organic matter export from land into freshwater aquatic systems. Few studies have quantified the organic matter export potential of soils and the studies that have used varying and contradicting methods making it hard to compare across study.  Here we used soil samples from participants in the Nutrient Network to perform a standardized extraction method to quantify the water extractable organic matter (WEOM) from grasslands across the United States. This allowed us to make more general conclusions about the controls of organic matter export potential in grassland ecosystems on a continental scale.

Results/Conclusions

After analysis of over 15 sites across the United States, we have found that grassland soils differ significantly in their export of organic carbon and nitrogen. In our sites, soils with a high percentage of sandy material exported the greatest percentage of the carbon and nitrogen stored in them. Additionally, mean carbon and nitrogen export within site were tightly coupled suggesting a comment mechanism of export. Organic matter export was not significantly related to mean annual precipitation at the site but was significantly related to the organic content of the soil. We conclude that easily measurable soils characteristics, such as particle size and organic content, can be used to set the upper bound on potential organic matter leaching from grassland soils.