Plants and mycorrhizal fungi reduce mineral-associated organic matter nitrogen formed from decomposing litter

Plant litter decomposition is a major pathway of soil organic matter (SOM) formation and a key source of bioavailable N. Litter enters the decomposition pathway across the heterogenous soil environment, but where decomposition occurs may impact how much litter N becomes part of the mineral-associated organic matter (MAOM) pool or occluded within particulate organic matter (POM) fractions. It is suggested that soil influenced by mycorrhizal hyphae would promote MAOM formation beyond that of soil influenced by roots alone or soil without roots and hyphae (bulk soil) due to expanded, high microbial activity, but we have not yet quantified the impact of plant roots with and without mycorrhizal fungi on MAOM-N formed through the litter decomposition pathway. We conducted a greenhouse experiment using basil ( Ocimum basilicum, L.) and isotopically enriched plant residues to quantify new MAOM-N formation in the presence and absence of plant roots and arbuscular mycorrhizae (AM). The treatment with plant roots and living AM had greater new MAOM-N compared to the plant only treatment. Interestingly, the greatest new MAOM-N was found in soils without a plant or active AM. Under a limited nutrient environment, plant growth and litter N uptake decreased when AM were present. Our results show that relatively high MAOM-N formation can occur in the absence of plant roots and active AM symbionts contradicting the expectation that roots and AM fungi promote greater MAOM formation than occurs in bulk soil.