Residual effects of previous nitrogen application in two Hungarian long-term field trials

Uniform long-term field trials were set up on two sites in the autumn of 1965 in a wheat-wheat-corn-corn rotation to investigate the direct effects of N at rates of up to 348 kg N ha(-1). Site 1 (Keszthely) was a sandy loam Mollic Cambisol. Site 2 (Szentgyorgyvolgy) was a clay loam Stagnic Luvisol. The field trials were unchanged for 29 yr, until autumn 1994, when the initial "old" N application rates were discontinued, thus allowing the residual effects of the previous annual N rates to be evaluated. A new, or "recent" N application regime (0, 100, 150, and 200 kg N ha(-1) year(-1)) was applied at each "old" N rate. From 1995 to 1997, the 3-year residual effect on the weight and N concentration of young plant samples, on grain yields, and on the nitrate contents of the 0-1 m soil profile were measured in early spring. In summer 1998, the nitrate contents of the 0-3 m soil profile were also measured. The effects of recent N applications on the above parameters were also determined for the old 87 and 348 kg N ha(-1) rates. At Site 1, there was no NO3-N accumulation in the 0-1 or 0-3 m layers, as the residual effect of previous N applications. The recent application of 100-200 kg N ha(-1) resulted in an accumulation peak at a depth of 2.5 m. However, there was a NO3-N accumulation peak at a depth of 1.5 m of the clay loam-textured soil profile at Site 2, as the result of old N applications, whereas recently applied 100-200 kg N ha(-1) rates resulted in a slightly higher accumulation at the same depth. At neither site was there a significant increase in young plant weight or %N as the result of either residual, or recent N. However, the residual effect of the 29 yr of N application resulted in a 0.2-1.3 t ha(-1) increase in the wheat grain yield in the first year at Site 1, and a 0.4-1.3 and 0.8-2.0 t ha(-1) increase in corn yields in the second and third years, respectively. Effects of recently applied N were more pronounced. There were also substantial effects of recent and residual N on grain yields at Site 2. The residual effect of 29 yr of N application gave grain increases of 0.2-2.8 t ha(-1) in wheat in the first year and 1.1-2.8 and 0.0-1.1 t ha(-1) in corn in the second and third years, respectively. The effects of recently applied N were again more pronounced. Although there was a significant residual effect of previous N applications even in the third year, the advantage of recently applied N over residual N increased with time.