Impacts of residue quality and N input on aggregate turnover using the combined 13C natural abundance and rare earth oxides as tracers

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The effects of residue quality and nitrogen (N) input on soil aggregate turnover are not clarified. In this study, aggregation pathways and C cycling were tracked by using rare earth oxides (REOs) and 13C natural abundance as tracers, respectively. Two residues (maize straw, C/N = 32; miscanthus straw, C/N = 220) coupled with or without N application (200 kg N ha−1) were incorporated into the REO labelled soil (δ13C = −23.2‰). Soil respiration, aggregate stability (MWD), REO concentrations and 13C were measured after 0, 7, 14 and 28 days of incubation. The maize straw incorporation resulted in a greater soil respiration and MWD as compared with the poor residue quality miscanthus straw. The N addition improved the miscanthus straw decomposition and MWD, whereas this effect was negligible for the maize straw treatments. The change in the MWD was related to the soil respiration (P < 0.001). As a result, the aggregate turnover of the soil incorporated with maize straw was faster than that of the soil incorporated with miscanthus straw. The N input increased the aggregate turnover of the soil incorporated with miscanthus straw but this effect was minimal for the maize straw amended soil. The new C accumulated more, but decomposed faster in macroaggregates than in microaggregates. A significant positive relationship was observed between the 13C concentration and the aggregate turnover rate (P < 0.01) in sole straw amended treatments, whereas this relationship became weak after N input. Our results demonstrate that aggregate turnover depends on the residue quality and N input when the residue decomposition is under N-limited conditions.