WATER-EXTRACTABLE ORGANIC-MATTER IN ARABLE SOILS - EFFECTS OF DROUGHT AND LONG-TERM FERTILIZATION

The concentration of water extractable organic carbon (WEOC) was determined over 2 yr at three different depths from three agricultural fields, which have been used for fertilization productivity studies for over two decades. The overall mean value for all samples was 9.40 mug WEOC g-1. The use of mineral fertilizer alone had no significant effect while organic fertilizer resulted in an overall increase of 15 ng WEOC g-1 (soil dry weight basis) or 220 g WEOC ha-1 per 1 kg N ha-1 yr-1. This increase was usually non-linear for any given field or depth. Water flow appeared to play a role in that the deepest samples (60-90 cm) from the field with the highest leaching rate had consistently low WEOC values. The samples taken in a year (1991) with exceptionally low precipitation before sampling had significantly (P < 0.001) more (+33%) WEOC than in the other year. The magnitude of this effect was equivalent to that of the application of organic fertilizer at the rate of ca 175 kg N ha-1 yr-1. Therefore, in situ soil moisture content can have a relatively high effect on this organic carbon pool. Surprisingly the increase in WEOC due to drought was also present in the samples from the greatest depth. Again the field with the highest leaching rate was affected differently than the other two fields. No relationship was found between short term organic carbon mineralization and fertilization history. However, a significant but loose (R2 < 50%) linear relationship was found between the mineralization rate and WEOC concentration.