THE RESPONSE OF SOIL ORGANIC-MATTER TO MANURE AMENDMENTS IN A LONG-TERM EXPERIMENT AT ULTUNA, SWEDEN

In a long-term field experiment started in 1956 on a clay loam soil at Uppsala, Sweden, changes of organic carbon in the topsoils receiving various organic amendments at the rate of 2000 kg C ha(-1) year(-1) we re studied to determine soil organic matter characteristics, variation s of delta(13)C in the soil and to estimate a carbon balance. Fallow a nd mineral fertilizer without N led to a significant decrease of soil organic matter (SOM) in the soil, green manure maintained the SOM cont ent, and animal manure and peat increased the SOM content significantl y. The stable portion of the added organic materials after 37 years of continuous input was 12.8, 27.3, and 56.7%, for green manure, animal manure and peat, respectively. This was reflected by half-lives of org anic carbon originating from the amendments between 3.0 (green manure) and 14.6 years (peat). The isotopic composition of SOM changed both d ue to mineralization (continuous fallow) and the addition of amendment s isotopically different from soil humus (green manure, animal manure) . The isotopic effect was used to calculate the percentage of carbon d erived from animal manure present for the year 1993. This value (55.4% ) was larger than that derived from the carbon balance, which indicate d a priming effect of the animal manure on the initial soil humus. Min eralization of microbially available organic substances led to an incr ease in the degree of humification on plots not receiving organic amen dments. Adding peat and animal manure resulted in a decrease of the hu mification index due to the continuous input of poorly humified materi al. The extinction ratio (E-4/E-6) and ratio of fulvic acid to humic a cid changed considerably in the peat treated plots. Fourier transform infrared (FTIR)-measurements of the extracts showed that peat characte ristics can be detected in peat treated soils. The other amendments di d not alter the characteristics of the extractable humic substances.