Phosphorus fertiliser form determined the allocation of root-derived organic carbon to soil organic matter fractions

The efficiency of soil organic carbon (SOC) sequestration as a suitable negative emission technology depends mainly on plant-derived organic carbon input and its allocation to stabilised SOC pools. These processes may be affected by fertiliser use and soil type. The purpose of this study was to investigate the effect of the organic fertiliser poultry manure compost, the mineral fertiliser rock phosphate and their mixture on organic carbon (OC) transfer from plant to soil. We studied also its allocation to protected SOC pools of a Luvisol and a Neoluvisol. We carried out a growth chamber experiment with 13 C-enriched atmosphere, where ryegrass plants were grown for 7 weeks on the two different soil types in the presence of the three different fertilisers. We quantified root-derived OC input in three SOC density fractions and soil microbial biomass at the end of the experiment. Our results indicated that addition of poultry manure compost and its mixture with rock phosphate led to more root biomass and more root-derived OC transfer to active pools compared to rock phosphate alone. The amendments with poultry manure compost also led to higher microbial biomass contents compared to mineral fertilisation due to higher available organic phosphorus. We also noticed variations in the dynamics of the stabilised OC pools amongst soils, which could be attributed to the different impacts of phosphorus fertiliser types on SOC stabilisation processes. We concluded that organic and mineral phosphorus fertilisers may have a contrasting impact on OC flow from plant to soil and in particular on the allocation of root-derived OC to labile or stable SOC fractions. The magnitude of these effects was more influenced by soil type than their pattern, which was similar for the two soil types. • Poultry manure enhanced microbial biomass carbon by about 28–125%. • Soil microbial biomass increased up to 2-fold by manure and rock phosphate mixture. • Phosphate rock decreased the carbon derived from the plant and microbial biomass. • Manure increased plant-derived carbon by 1.9-fold in organic matter fractions. • Organic phosphorus plus microbial biomass explained plant-derived carbon by 66–96%.