Effects of long-term de-vegetation on the quantity and quality of water extractable organic matter (WEOM): biogeochemical implications

The effects of five decades of de-vegetation on the quantity and quality of water extractable organic matter (WEOM) in soils were investigated. The WEOM was sampled from the A(p)-horizon of an agricultural plot and a neighboring bare plot 5 times during the spring. The quantity of WEOM was determined by measuring its organic carbon content, and its quality was characterized by its UV absorptivity, by a humification index based on the fluorescence emission spectrum, and by its fluorescence efficiency (fluorescence divided by UV absorption). The potential substrate value of WEOC was also determined by its microbial metabolic loss over 7d. As expected, long-term de-vegetation decreased WEOC significantly (by 70%). Not expected were two results: (1) Qualitative de-vegetation effects were relatively small. In some cases they were statistically significant, but in all cases differences compared to the vegetated plots were less than 13%; and (2) Despite a major increase in vegetation (from essentially 0% to 100% plant coverage) on the agricultural plot during the spring, there was no seasonal trend to be seen in any of the measured parameters. These unexpected field based results indicate that vegetal input into the ecologically relevant dissolved organic matter pool occurs only to a minor degree directly. Most of the fresh material must be initially sequestered into the soil matrix from which it is then subsequently released. This also indicates that there is a strong "buffering" in soils of freshly introduced organic matter. These results should be considered in our attempt to understand biogeochemical cycles in soil.