Leaching of soil organic carbon and nitrogen in sandy soils after cultivating grass-clover swards

Several studies have focused on the formation and losses of dissolved organic matter in forest systems, whereas a limited number have dealt with this aspect in agricultural soils. The purpose of this study was to estimate the leaching of dissolved organic carbon (DOC) and nitrogen (DON), with focus on the period after cultivating grass-clover swards. Grass-clovers were ploughed in the spring prior to sowing cereals followed by either catch crops or bare soil. The concentrations of DOC and DON decreased with soil depth and ranged at 90-cm soil depth between 7 and 21 mg C L-1 and between 1 and 3 mg N L-1, respectively, in a sandy loam soil, and between 16 and 63 mg C L-1 and between 1 and 10 mg N L-1, respectively, in a coarse sandy soil. The resulting DOC/DON ratios were in the range between 2 and 42, with higher values in the coarse sandy soil than in the sandy loam soil. The total percolation was 218 mm in the sandy loam soil and 596-645 mm in the coarse sandy soil, which resulted in an annual leaching of 22-40 kg DOC ha(-1) year(-1)supercript stop and 3-4 kg DON ha(-1) year(-1) in the sandy loam soil, and 174-310 kg DOC ha(-1) year(-1) and 10-31 kg DON ha(-1) year(-1) in the coarse sandy soil. It was shown that higher amounts of DOC were lost by leaching under the catch crops than from bare soil, that losses of DON were higher from bare soil than from soils with catch crops and that DON contributed significantly to the total N loss. Thus, DON needs to be taken into account in N-balance calculations.