Residual effects of different N fertilizer treatments on growth, N uptake and yield of oilseed rape, wheat and barley

In the long term, changes of N input (e.g. amount of N fertilization) or N output (e.g. N offtake in the grain) can affect soil N content and, in consequence, potential soil N mineralization, which may promote crop growth and support yield formation. The aim of this study was to quantify the residual effects of a former N fertilization on N mineralization, crop growth, N uptake and grain yield. From 1990/1991 to 1998/1999, different N treatments including slurry (none, 80 kg N ha-1 as pig slurry in autumn, in spring, in autumn + spring) and mineral N fertilization (0-240 kg N ha-1) were tested in a winter oilseed rape (OSR) -- winter wheat -- winter barley rotation on a pseudogleyic sandy loam (Luvisol) at Hohenschulen Experimental Station near Kiel in NW Germany. Each year, treatments occurred in all three crops of the rotation and were located on the same plots. Accumulated over the experimental period, the N balance (N supply - N offtake - N leaching) of the different N treatments varied between -740 and +1300 kg N ha-1. In 1999/2000-2001/2002, all plots remained unfertilized in order to avoid interactions with the actual N fertilization. The former N treatments enhanced dry matter and total N uptake of wheat in 2000-2002 compared with the former unfertilized control, however, differences were not significant (P > 0.05) at most sampling dates. Also soil mineral N (SMN) in 0-30 cm remained unaffected during spring growth. Lately, on average of all treatments, grain yield of wheat decreased from 692 g m-2 in 2000 to 687 g m-2 in 2001 and 357 g m-2 in 2002. Barley yielded 481, 462 and 207 g m-2, and OSR 375, 350 and 176 g m-2, respectively. Total N uptake in the above-ground biomass of wheat at harvest was reduced from 109 kg N ha-1 in 2000 to 76 kg N ha-1 in 2001 and 59 kg N ha-1 in 2002, respectively. Barley accumulated 69, 57 and 33 kg N ha-1 in the above-ground biomass, and OSR 120, 111 and 59 kg N ha-1, respectively. In all crops, residual N effects were largest in the first year (2000) and decreased with the years. The residual effects of slurry and mineral N fertilizer were similar. Considering the differences of 2000 kg N ha-1 in the N balance, the residual effect on N uptake remained at a very low level. The crops used on average 3% of the N surplus within the first three years without N fertilization, i.e. about 1% per year.