Soluble organic nitrogen in agricultural soils

The existence of soluble organic forms of N in rain and drainage waters has been known for many years, but these have not been generally regarded as s ignificant pools of N in agricultural soils. We review the size and functio n of both soluble organic N extracted from soils (SON) and dissolved organi c N present in soil solution and drainage waters (DON) in arable agricultur al soils, SON is of the same order of magnitude as mineral N and of equal s ize in many cases; 20-30 kg SON-N ha(-1) is present in a wide range of arab le agricultural soils from England. Its dynamics are affected by mineralisa tion, immobilisation, leaching and plant uptake in the same way as those of mineral N, but its pool size is more constant than that of mineral N. DON can be sampled from soil solution using suction cups and collected in drain age waters. Significant amounts of DON are leached, but this comprises only about one-tenth of the SON extracted from the same soil. Leached DON may t ake with it nutrients, chelated or complexed metals and pesticides. SON/DON is clearly an important pool in N transformations and plant uptake, but th ere are still many gaps in our understanding.