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 Lauchstädt und 12 other European sites, the carbon (C) and nitrogen (N) dynamics in soils, the determination of decomposable soil organic matter (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 an inert and a decomposable fraction. The inert C is strongly correlated with clay content, while most changes in both C and N occur in the readily 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 for the calculation of the decomposable C and thus for the N release from 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 exceeding 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.