Soil Organic Matter Degradation in Long-Term Maize Cultivation and Insufficient Organic Fertilization

Soil organic matter carbon (C_SOM) compounds degradation was observed in long-term field experiments with silage maize monoculture. Over a period of 26 years, the content of carbon in topsoil decreased by 22% in control unfertilized plots compared to 25% and 26% in treatments fertilized annually with mineral nitrogen. With annual wheat straw application (together with mineral N), the content of C_SOM decreased by 8%. Contrary to that, the annual application of farmyard manure resulted in a C_SOM increase of 16%. The ratio of carbon produced by maize related to total topsoil C_SOM content ranged between 8.1-11.8%. In plots with mineral N fertilization, this ratio was always higher than in the unfertilized control plots. With the weaker soil extraction agent (CaCl₂), the ratio of carbon produced by maize was determined to be 17.9-20.7%. With stronger extraction agent (pyrophosphate) it was only 10.2-14.6%. This shows that maize produced mostly unstable carbon compounds. Mineral N application resulted in stronger mineralization of original and stable organic matter compared to the unfertilized control. However, the increase of maize-produced carbon content in fertilized plots did not compensate for the decrease of "old" organic matter. As a result, a tendency to decrease total C_SOM content in plots with mineral N applied was observed.