Quantity and spectroscopic properties of soil dissolved organic matter (DOM) as a function of soil sample treatments: air-drying and pre-incubation

The dissolved organic matter (DOM) in soils is essentially defined by the way in which it is obtained. Therefore, we need to understand as to how pre-treatment of a soil will affect the characteristics of DOM, since this fraction may be strongly influenced by a soil's water content. The effect of two different pre-treatments on DOM from the A-horizons of a large variety of ecosystems and regions were compared. In both cases the soils were allowed to air-dry. In one case the air-dried soil was directly extracted (AD), while in the other case it was preincubated for 1 week at 50% of its water holding capacity (INCU). AD is simpler, but INCU brings the soil, and especially its microbial population, back to a standardised state, which is more representative of the usual state in the field. Both methods are used whenever an adjustment of the soil water content is essential to compare different regions or to eliminate short term weather effects. A significant regression indicated that the dissolved organic carbon (DOC) extracted from INCU samples was only 20% of AD DOC. Both the absorptivity (UV absorption divided by DOC) of 86% of the samples, and a fluorescence emission spectrum based Humification Index in all cases increased as a result of preincubation. This would indicate that labile compounds released during drying were metabolised during the incubation. However, the magnitude of this increase varied, and no correlation with soil organic and microbial carbon, pH, or texture could be detected. The results show that DOM extracted from AD and INCU soils is not comparable and that the differences are mainly due to the impact of air-drying on the microbial activity.