Altered food web structure and C-flux pathways associated with mineralisation of organic amendments to agricultural soil

Abstract

Application of organic materials to agricultural soils offers the potential to divert these from conventional waste streams and to reduce the use of mineral fertilisers for crop production. In addition to direct impacts of organic materials on soil physico-chemical conditions, such amendments may also affect the structure and functioning of soil food webs and the coupling of plant- and soil biological activity. In this study we investigated the impact of cattle slurry and municipal green compost applications on soil microbial and nematode communities. Specifically, we applied steady-state 13C-labelling to quantify the impacts of the amendments on transfer of plant-derived C through components of the soil food web and to determine the activity of the soil biota in mineralising amendment- and soil organic matter derived-C. Both the slurry and compost amendments significantly promoted barley N-acquisition and growth. The municipal compost amendment, but not slurry application, strongly affected soil microbial community structure and increased nematode abundance, relative to unamended soil. This was associated with increased flux of plant-derived C through bacterial (particularly Gram negative) populations in compost-amended soil and was associated with increased mineralisation of amendment- and soil organic matter-derived substrates. For slurry-amended soils, the amendment did not alter soil community structure and mineralisation of soil C was not coupled to the flux of plant-derived C through below-ground pools. The results are indicative of N-supply to barley in compost-amended soil being dependent on the functioning of the soil food web (including C-flux to it from living roots), whereas N-supply from slurry was independent of this coupling between plant and soil processes. We suggest that plant/food web interactions are important targets for development of effective management strategies for organic waste applications to agricultural soils in the context of optimising the balance between nutrient supply to crops and the susceptibility of nutrients to losses from soil.