Nitrogen fertilisation reduces the contribution of root-derived carbon to mineral-associated organic matter formation at low and high defoliation frequencies in a grassland soil

Background and aims: Rhizodeposition is organic matter released by living plant roots that can be transformed by microbes into particulate organic matter (POM), but that can also become more stable through the adsorption of organic matter onto soil minerals (mineral-associated organic matter, MAOM), thereby playing an important role in mitigating climate change. We examined how root-derived carbon (C) as a proxy for rhizodeposition contributed to POM and MAOM formation in a grassland affected by nitrogen (N) fertilisation and defoliation frequency, and to what degree rhizodeposition was incorporated into microbial biomass. 

Methods: We applied N fertiliser (0 vs. 40 kg N ha⁻¹ yr⁻¹) and defoliation frequencies (3–4 vs. 6–8 clipping events year⁻¹, simulating low and high grazing intensity) for three years, then used a ¹³CO₂ pulse labelling technique to examine the incorporation of rhizodeposition into microbial biomass, POM and MAOM fractions. 

Results: With N fertilisation, rhizodeposition contributed less to the formation of MAOM compared to the formation of POM, while defoliation frequency decreased the contribution of rhizodeposition into both POM and MAOM, particularly with N fertilisation. Although the MAOM fraction was relatively rich in N (C: N ratio of 10.5 vs. 13.5 for POM), our results suggest that adding inorganic N promoted the formation of POM more than of MAOM from rhizodeposition.

Conclusion: A large proportion of rhizodeposition was taken up by microbes that eventually could contribute to POM and MAOM formation. Our results provide insightful information regarding the stabilisation of rhizodeposition into different soil organic matter pools.