Effects of level and quality of organic matter input on carbon storage and biological activity in soil: Synthesis of a long‐term experiment

The aim of this paper was to synthesize a series of investigations from a long‐term field experiment addressing the turnover of organic matter in soil. This paper is based on four organic matter inputs (soil treatments): none (fallow), small amounts of roots+stubble cropped without N fertilizer (no‐N), moderate additions of animal manure, and moderate additions of peat. After 42 years, soil carbon stocks declined in the fallow and no‐N treated soil but increased in the animal manure and peat‐amended soil. Gentle fractionation of soil particles and aggregates revealed that the silt‐sized fraction contained most of the soil C and was most responsive to changes in input of organic matter. The clay‐sized fraction (<2 μm) acted as a sink for C, and amounts of clay‐sized C increased in all treatments, including fallow. The contribution of size fractions to C storage decreased in the following order: silt > clay > fine sand > coarse sand. The highest natural abundance of 13 C and 15 N was found in the clay‐sized fraction, the fallow being most enriched in 13 C and the animal manure‐treated soil in 15 N, indicating that the organic matter of the clay‐sized fraction had been turned over most intensively. The 13 C inventory showed that the transfer from silt‐ to clay‐sized carbon was most intensive in the soil treated with animal manure and least intensive in the peat‐treated soil. Bacterial diversity increased from sand‐ to clay‐sized fractions revealed by 16S rRNA genes. Fungal activity was highest in coarse‐sized fractions as indicated by enzyme measurements. The quality and amount of organic matter input had no significant effect on the community structure of soil bacteria.