Development of aggregates after application of maize residues in the presence of mycorrhizal and non-mycorrhizal pea plants

• Small and large macroaggregates were increased by saprotrophic microorganisms.
• They increased the free, but had no effect on the occluded light fraction.
• The effects of myc+ Frisson on all treatments were generally small.

It has been demonstrated before that growing pea (Pisum sativum L.) plants reduce the decomposition of 15N labeled maize residues. This reduction was especially strong in pots planted with the mycorrhizal wild-type Frisson (myc+) in comparison with the non-mycorrhizal mutant P2 (myc−). The aim of the present experiment was to assess the contribution of AMF and saprotrophic microorganisms to the formation of aggregates and accumulation of C and N as occluded light fraction organic matter in the soil of this pot experiment. In the myc+ treatment of the size class < 53 μm, the dry weight yield and the δ13C value of soil organic C were significantly higher than in the myc− treatment. After application of maize leaf residues, the percentage of dry weight yield generally increased in the two macroaggregate size classes at the expense of the small and large microaggregates. This demonstrates the important role of saprotrophic microorganisms in macroaggregate formation. Consistently, the content of maize-derived C declined significantly in the occluded light fraction of the myc+ treatment, showing that less plant residues were accumulated in aggregates. Overall, the presence of saprotrophic microorganisms induced aggregate formation and accumulation of C within aggregates. The presence of mycorrhiza did not affect aggregation or soil C partitioning, despite a higher root biomass. It should be further investigated whether the positive effects of the maize-decomposing microorganisms on aggregation persist for longer periods.