Nitrogen availability alters rhizosphere processes mediating soil organic matter mineralisation

Background and aims: 

The intrinsic nitrogen (N) supply capacity of soil is central to understanding the productivity of natural plant communities, and essential in the context of determining optimal fertilization rates for agricultural soils. However, it is largely unknown how nutrient availability affects plant mediated priming effects driving soil organic matter mineralisation and associated N-fluxes. 

Methods: 

We applied continuous, steady-state ¹³C–CO₂ labelling to Lolium perenne grown in high and low productivity grassland soils to allow quantification of SOM- and root-derived soil CO₂ efflux. Nutrient treatments (N, P and K) were applied as repeated additions to soils, and impacts on source partitioned soil CO₂ efflux were assessed relative to unamended planted and fallow soils. Plants were clipped to uniform height at weekly intervals. 

Results: 

Increasing nutrient availability in both soils resulted in a reduction in plant-mediated SOM mineralisation and clipping of plants greatly lowered root-derived respiration but increased SOM mineralisation. Nutrient addition to fallow systems had no effect on SOM mineralisation in either soil. Plant growth stimulated SOM priming, concurrent mobilisation of N from SOM and subsequent plant N uptake in the high productivity soil. Priming was not observed in the low productivity soil due to its greater inherent organic matter stability, resulting in lowered plant-mediated and basal SOM mineralisation. 

Conclusions: 

That addition of nutrients reduced SOM mineralisation in planted systems but had no effect in fallow systems is indicative of nutrient availability specifically altering plant-mediated priming of SOM mineralisation. We suggest that plant-soil interactions mediating priming effects are an important determinant of productivity and that the magnitudes of these effects are modified by nutrient availability and soil-specific controls.