Influence of Soil Amendment History on Decomposition of Recently Applied Organic Amendments

Long-term use of organic soil amendments, compared with unamended or mineral fertilized soils, can change soil organic matter content, microbial biomass content, the microbial community structure, and the activity of enzymes involved in organic matter decomposition. It is not clear, however, whether long-term use of organic amendments, by means of these changes, leads to modified decomposition rates of newly added organic amendments. Therefore, this study was used to test the hypothesis that amendment history has an influence only on the decomposition of recalcitrant organic amendments and not on less recalcitrant organic amendments. Soils used for experimentation were taken from a field experiment where contrasting organic amendment regimes of farmyard manure, pine (Pinus sylvestris L.) bark, vegetable crop residues, and an unamended control had been applied for 35 yr. In a full factorial, laboratory-based incubation experiment, each soil was treated with each of these amendments and net C and N mineralization and microbial biomass C contents were monitored during a 147-d period. Collected data were then used to estimate gross turnover rates of newly added amendments with a modeling approach based on the soil organic matter module of the Daisy model. The modeling results suggested that the turnover of farmyard manure and pine bark, not however of crop residues, should be simulated in consideration of an amendment history effect. In contrast, the results of the ANOVA indicated that amendment history had an insignificant effect on net C and N mineralization from recently applied amendment. We concluded that the effects of amendment history on gross turnover rates of recently added organic amendments may depend on the type of amendment but that these effects on net C and N mineralization are minor in magnitude and hence irrelevant to N fertilization practice.