Predicting Mobilization and Macropore Transport of Colloids and Phosphorus in Agricultural Soils.

Around 50 % of the Danish arable land is tile-drained. Loss of phosphorus, P, to surface waters from tile-drain outflow can be considerable and very variable among fields. In order to reduce P loss from tile-drained areas it is essential to know the cause of the variability among fields. Leaching of P to tile-drains will mainly occur through macropores where P can be transported quickly to drains either dissolved or adsorbed to mobile colloids. It is our hypothesis that there is a geological variation in the probability for macropore flow and colloid-facilitated transport of P, and that it is possible to relate this probability to readily available pedological and soil physical data. Two important processes for the transport of P to tile-drains are macropore transport and the in-situ mobilization of colloids. The macropore structures of interest in the upper meter are mainly earthworm channels and structural inter-aggregate pores. We establish relationships between simple soil variables and the number, distribution, size and orientation of pores and relate this to how they react hydraulically. Dependent on e.g., texture, porosity and pH, the structural elements can have a different shape, size, and grade of development. These morphological relationships are related to soil hydraulic functioning using hydraulic data and model simulations. Much of the P leached to tile-drain is sorbed to colloids and there is a linear relationship between particle matter and particulate P in drain water. A key parameter in colloid mobilization is the amount of water-dispersible colloids a soil can release. A relationship between soil characteristics and the release of water-dispersible colloids is presented and linked to facilitated transport of P. We try to link the number of earthworm burrows, hydraulic data, and release of water-dispersible colloids to simple soil variables and leaching of P.