Interactive priming effect of labile carbon and crop residues on SOM depends on residue decomposition stage: Three-source partitioning to evaluate mechanisms

Inputs of crop residues and labile C (e.g. root exudates) can affect the decomposition rate of soil organic matter (SOM) through the priming effect (PE). Most previous priming studies describe the addition of single labile or residue C, ignoring the interactions of labile C and fresh or decaying crop residues commonly present in field conditions. Using a dual 13C/14C labelling approach in a 62-day incubation, we investigated the effects of adding labile C (40 μg glucose-C g−1 soil) together with wheat shoot or root residues (3.1 mg C g−1 soil) on SOM priming at three residue decomposition stages: intensive (day-1), reduced (day-9) and stabilised (day-24). To estimate the PE, total soil CO2 efflux and microbial biomass were partitioned for three sources: labile C (14C-glucose), plant residues (13C-labelled) and SOM (unlabelled). Without glucose, roots were decomposed less than shoots but induced 1.4-fold stronger cumulative SOM priming (365 μg C g−1 soil) than shoots. Addition of glucose increased SOM priming, with a stronger effect in the presence of shoot than root residues. Glucose addition at the intensive stage of shoots decomposition slightly increased SOM priming. However, compared with residues alone PE, the addition of glucose during reduced residue decomposition stage, increased SOM priming by 60% (roots) to 104% (shoots). Remarkably, this SOM priming after glucose addition was followed by a decline in residue decomposition and by an increase (up to 50%) in SOM-derived C in microbial biomass. Hence, following glucose addition, microorganisms utilised more SOM rather than feeding on decaying residues during reduced decomposition stage. During stabilised residue decomposition stage, the impact of glucose on SOM priming declined again, while the residue decomposition rate remained unaffected. Furthermore, a large proportion of added glucose (up to 10%) was retained in microbial biomass and its mineralisation rate declined strongly (compared with intensive and reduced decomposition stage). Therefore, the glucose amount was not sufficient to influence microbial activities determining SOM or stabilised residue decomposition rates. Overall, SOM decomposition increased by 1- to 4-fold more than the amount of added glucose C, which resulted in a negative net soil C balance compared with residues alone. Thus, we demonstrated for the first time that 1) the interactive effects of glucose (trace amount) and residues on SOM priming depend on plant residue type (higher under shoots than roots) and 2) stage of residues decomposition (higher SOM priming when labile C was added after the end of intensive decomposition stage of plant residues).