Differential priming of soil carbon driven by soil depth and root impacts on carbon lability

Background/Question/Methods

Root derived labile soil carbon (C) inputs regulate soil microbial activity and decomposition processes. Importantly, root-derived C inputs can stimulate decomposition of more recalcitrant soil organic carbon (SOC) by priming microbial activity, which can lead to a net loss in soil C storage. However, the mechanisms by which root-C controls the magnitude of this priming effect remain poorly understood. With this study we set out to evaluate how labile soil C availability mediates the impact of root-derived C release on priming of more recalcitrant SOC. To answer this question we conducted a controlled laboratory incubation experiment in which we supplied a synthetic root-exudate cocktail containing C compounds often found in root exudates to soils with a range of labile soil C constituents. Soils were collected from under three switchgrass (Panicum virgatum) cultivars with coarse root structures and three cultivars with fibrous root structures to a depth of 60 cm. Differences in specific root length among cultivars were expected to result in small differences in labile soil C availability, whereas differences associated with soil depth were expected to result in large differences in labile soil C availability. Soil cores were divided into 0-10 cm, 20-30 cm and 40-60 cm depth increments. After removing roots, soils across all three depths and cultivars were incubated (60 days) by adding either: (1) water (60% WHC), or (2) labile C provided as the ¹³C-labeled synthetic root exudate cocktail. We measured CO₂ respiration throughout the experiment. Addition of the strongly labeled exudate cocktail allowed us to assess impacts of labile C addition on priming in soil derived from the different cultivars and from different depths. 

Results/Conclusions

Our experiment led to three main results: (1) different cultivars of switchgrass regulate labile C availability across the soil profile differently; (2) small differences in labile C among the soils derived from different cultivars did not significantly mediate the impact of labile C additions on priming; (3) but, large differences in labile soil C contents among depths led to differences in priming effects, where greater priming effects were observed for shallow relative to deep soils across all days of the experiment. These results suggest that increased root-derived C inputs will have marginal impacts on decomposition of more stable SOC at depth and that their impact on priming will be similar in soils with small differences in available soil C.