Carbon and nitrogen in arable soils as affected by supply of N fertilizers and organic manures

Abstract

Results from a long-term, small plot field experiment, started in 1956, comparing different mineral N fertilizers and organic amendments are presented. Continuous bare fallow was included in the plan. The treatments resulted in large differences in soil organic carbon. No steady state was established. In spite of large additions of fresh organic matter with varying nitrogen content, the C/N ratio remained as a rule around 10. The only exception was peat. This is comparatively resistant against microbial decomposition. Therefore the C/N ratio was influenced by undecomposed peat, poor in nitrogen. Nitrogen balances covering the whole experimental period are presented. Crop residues poor in nitrogen and nitrogen uptake by the crop were found to decrease nitrogen losses. Where no organic amendment was added, there was a decrease in total nitrogen in soil. Addition of organic amendments resulted in the opposite situation. Farmyard manure caused a greater increase of total nitrogen in soil than green manure, demonstrating the importance of the quality of raw material for the yield of humus material.The sizes of soil organic matter fractions were estimated. The “old” humus fraction was estimated to 23.6 t ha−1 C. The “active” and “young” humus fractions accounted for 20–61% of the soil organic carbon depending on the level of fertilization and the supply of fresh organic matter. It was stated that those fractions are of great importance for the capacity of the soil to mineralize nitrogen.