Crop residues can affect N leaching over at least two winters

Abstract

Overwinter nitrate leaching and periodic measurements of soil mineral N (SMN) to 90 cm depth, were taken for 20 months after components of various arable crop residues had been ploughed into a free-draining sandy loam textured soil in central UK. Treatments were arranged to incorporate either, below ground residues; above ground residues with their own below ground residues; or, the above ground residues separately with the below ground residue of a preceding under-fertilised barley crop. The combination of these treatments allowed separate quantification of the SMN and N leached from the above and below ground residues. The residues were incorporated (12/9/1995) and a crop of barley fertilised with 90 kg ha −1 N sown (14/9/1995). Immediately after the residues incorporation SMN was greatest where cabbage, potato and over-fertilised wheat had been grown. It was least following under-fertilised wheat and sugar beet. The N leached in the following winter (1995/1996) was strongly related to this preceding SMN (6/11/1995). There was considerable variation in N leached in 1995/1996 after the incorporation of below ground residues (18–145 kg ha −1 N). The greatest losses were after incorporating the over-fertilised wheat, potatoes and cabbage (58–145 kg ha −1 N), the smallest after sugar beet (18 kg ha −1 N). The net effect of above ground residues on N leached in 1995/1996 also varied, but their additional effect was far less (−19 to 15 kg ha −1 N). The abover-ground residues of sugar beet and cabbage increased N leaching by 5–15 kg ha −1 in 1995/1996, while the above-ground residues of cereals tended to reduce leaching (−19 to 2 kg ha −1). By the harvest of the subsequent under-fertilised barley crop (30/7/1996), and all subsequent sampling dates, there was no effect of previous crop residue treatments on SMN. N leached in 1996/1997, however, varied significantly. Leaching was again greatest from the below-ground residues of potato (69–83 kg ha −1 N) and cabbage (112–115 kg ha −1 N). Leaching from these residues of sugar beet remained small (15–24 kg ha −1) in 1996/1997. N leached in this second winter from the below-ground, with and without above-ground residues, was therefore, very strongly related to N leached the previous winter ( r 2=0.93). Relative to the below ground residues, the above ground residue again had far less effect on N leached in 1996/1997. The above ground residue of sugar beet again increased N leaching by the greatest margin (9–10 kg ha −1 in 1996–1997), cereals again tended to reduce N leaching (−16 to 4 kg ha −1). N leached in this second winter from the above ground residues was, therefore, very strongly related to N leached the previous winter in 1995/1996 ( r 2=0.69).