Mixing plant residues of different quality reduces priming effect and contributes to soil carbon retention

Cellulose-rich rice straw (RS) decomposes readily in soils, with much of C lost as CO2 and only a small fraction stabilized as soil organic carbon (SOC). Additionally, the addition of high-energy containing residues may stimulate native SOC decomposition (priming effect, PE) and the high C/N ratio of RS may lead to soil N-mining. We hypothesized that mixing high-C/N ratio RS with low-C/N ratio groundnut stover (GN) would improve the chemical composition of the input, thereby stimulating microbial growth and hence increasing the retention of residue-derived C in the soil as well as decreasing PE. To prove this, we designed a 112-day incubation experiment with unamended 13C-enriched C4 soil, soil amended with single 13C-depleted residues of C3 plants, RS and GN, or their mixture. The partitioning of applied C and native soil C between CO2 and microbial biomass was performed using their differences in 13C abundance, while the uncertainties in our estimates of soil C fluxes were quantified by a sensitivity analysis of the two-end member isotopic model. From 42 days after incorporation, the mixture reduced CO2-C loss by 9-18% compared to either residue applied alone. Compared to RS, the mixture respired less CO2, inducing a 29% reduction in PE, and increased C retention as microbial biomass by 22% during the first 56 days due to the preferential utilization of residue C. This resulted in reduced native soil C and N mining during the later decomposition stages. According to the isotopic sensitivity analysis, our conclusions were valid despite variations in isotopic composition of the chemical components of the residues. However, this might not be the case if the delta 13C values of preferentially used readily available components (e.g., cellulose) and/or of whole residues are very different.