TURNOVER OF SOIL ORGANIC-MATTER (SOM) AND LONG-TERM BALANCES - TOOLS FOR EVALUATING SUSTAINABLE PRODUCTIVITY OF SOILS

Using data from long-term experiments at the Loess-Chernozem site, Bad Lauchstadt und 12 other European sites, the carbon (C) and nitrogen ( N) dynamics in soils, the determination of decomposable soil organic m atter (SOM), the effect on yield of SOM as well as carbon and nitrogen balances are discussed. Both C and N in SOM have to be divided into a n inert and a decomposable fraction. the inert C is strongly correlate d with clay content, while most changes in both C and N occur in the r eadily decomposable fraction. In the experiments considered the latter ranges between 0.2 to 0.6% C and 0.02 to 0.06% N. The annual changes of the C-org content amount only to about 0.01% C-org corresponding to 500 kg/ha, even under extreme changes of the fertilizing system. Hot water extractable C(C-hwe) has proved to be an appropriate criterion f or the calculation of the decomposable C and thus for the N release fr om soil. different methods to maintain a SOM balance are compared and first guideline values for an agronomically and ecologically justified SOM content of arable soils are recommended. In arable soils the exce eding of an upper C-org value influences neither crop yield nor the C and N balance in a positive way. In terms of ecology and environment, set-aside-programmes or fallows in a crop rotation affect the balances negatively. Atmospheric N deposition can amount to about 50 kg/ha.yr.