Microbial use of 15N-labelled maize residues affected by winter temperature scenarios

A 56-day incubation experiment was carried out to investigate decomposition and microbial use of 15N-labelled maize (Zea mays L.) residues incubated under four winter temperature scenarios. The residues were mixed to mesocosms equivalent to 1.2 mg C and 42.5 μg N g−1 dry soil, after which the samples were incubated at a constant temperature of +4 °C, a constant −3 °C, and under multiple and single freeze-thaw conditions. A constant +4 °C was most favourable for microbial substrate use, with 4- and 6-fold higher total and maize-C mineralization, respectively, in comparison with constant frost. The cumulative maize mineralization was not determined by the frequency of freeze-thaw events, but regulated by the overall time of frost and thaw conditions. The decomposition of maize straw significantly increased soil organic C mineralization (in all scenarios) and incorporation into microbial biomass (in the freeze-thaw scenarios only). The positive priming effects observed were equivalent to an additional loss of total soil organic C of between about 0.2 (continuous frost) and 0.8% (single freeze-thaw). Microbial biomass was significantly increased after maize straw amendment, with constant frost and freeze-thaw scenarios not having any negative effect on microbial biomass C compared with constant +4 °C. Highest fungal biomass was found after constant frost without fresh substrates and also after extended frost followed by a warm period when fresh plant residues were present. On average, 50% of the added maize N were recovered in the soil total N after 56 days of constant 4 °C and in the freeze-thaw scenarios, with the strongest effect after single freezing and thawing.