Impact of Organic Carbon On Colloid Mobilization, Aggregate Strength and Structure.

Mobilization and transport of soil colloids can result in loss of phosphorus from the root zone as well as facilitate transport of strongly sorbing contaminants to ground water and fresh water streams. The aim of this study was to investigate the impact of soil organic carbon content on soil colloid dispersibility, aggregate stability, and aggregate tensile strength.
Minimally disturbed topsoils were sampled along an organic carbon (OC) gradient, ranging from 0.015 kg kg^-1 to 0.024 kg kg^-1, at the Bad Lauchstädt long-term field experiment on fertilization strategies in Germany. Colloid dispersibility was measured on field moist soil and air-dry 1-2 mm aggregates using an end-over-end shaker followed by gravity sedimentation. Aggregate stability was measured on field moist soil using a Yoder type (i.e. short-term) wet sieving device. Both tests were carried out at increasing shaking or sieving times ranging from 15 sec. up to 24 hrs.
                      Results from the dispersibility experiment show that the low OC soil at field moist conditions more readily release colloids than the high OC soil. The opposite is the case for air-dry soil, where the high OC soil is more readily dispersible. This can be explained by the repeated wetting and drying cycles the soils over the years have gone through which has led to a higher degree of cementation of the low OC soil, due to a higher clay content to OC ratio. Cemented structural units will then easily release its clay in wet conditions but resist dispersion in dry conditions. The higher degree of clay particle cementation is also seen from the wet sieving experiment, where the low OC soil interestingly shows the lowest degree of particle loss.
                      Results from tensile strength measurements and measurements of surface and internal structure properties will also be included and discussed.