LONG-TERM EFFECTS OF FERTILIZERS, MANURE, STRAW AND CROP-ROTATION ON TOTAL-N AND TOTAL-C IN SOIL

In long-term experiments it was found that different crop rotations, applications of farm manure and fertilizer-N, and annually ploughed-in cereal straw have an effect on the total-N and total-C contents of the topsoil. C:N ratios remained unaffected by different crop rotations in a 31-year experiment on loam soil, but a long-term increase in N in relation to C seems to have occurred. The residual carbon in the soil amounted to approximately 7% of the C-additions in straw applied annually for 21, 23 and 31 years in experiments on loam and clay loam. Increase in total-N in the soil was roughly 30-40% of the nitrogen added in straw in two of the three field experiments. In an 8-year field lysimeter experiment the mineralization rate of the soil nitrogen corresponded to a half-life breakdown of 55 and 43 years for unfertilized cereal and fallowed plots, respectively. In five experiments with 8-50 years of N-additions, a half-life period of 15 years of nitrogen not utilized or lost in the year of application is calculated from the residual N found in the soil. The nitrogen source was mostly calcium nitrate. Somewhat higher residual N percentages in the soil with N added in farm manure in the same experiments, may largely be due to the much lower crop utilization percentages for farm manure nitrogen in the first year.