Abstract. In a context of global change, soil has been identified as a potential carbon (C) sink, depending on land-use strategies. To detect the trends of carbon stocks after the implementation of new agricultural practices, early indicators, which can highlight changes in short timescales are required. This study proposes the combined use of stable isotope probing and chemometrics applied to solid-state 13C NMR spectra to unveil the dynamics of storage and mineralization of soil C pools. We focused light organic matter fractions isolated by density fractionation of soil water stable aggregates because they respond faster to changes in land-use than the total soil organic matter. Samples were collected from an agricultural field experiment with grassland, continuous cropping, and ley grassland under temperate climate conditions. Our results indicated contrasting aggregate dynamics depending on land-use systems with grassland returning to soil larger amount of C as belowground inputs than cropping systems. Those fresh inputs are preferentially incorporated at the level of microaggregates, which are enriched in C in comparison with those of cropped soils. Land-use changes with the introduction of ley grassland provoked a decoupling of the storage/degradation processes after the grassland phase. The newly-derived maize inputs were barely degraded during the first three years of maize cropping, whereas grassland-derived material was depleted. As a whole, results suggest large microbial proliferation as showed by 13C NMR under permanent grassland, then reduced within the first years after the land-use conversion, and finally restored. The study highlighted a fractal structure of the soil determining a scattered spatial distribution of the cycles of storage and degradation of soil organic matter related to detritusphere dynamics. In consequence, vegetal inputs from a new land-use are creating new detritusphere microenvironments rather than sustaining the previous dynamics, resulting in a legacy effect of the previous crop. Increasing the knowledge on the soil C dynamics at fine scale will be helpful to refine the prediction models and land-use policies.
The effect of crop species and N fertilization on soil organic matter