Substrate quantity and quality affect microbial carbon use efficiency and priming effects of root exudates investigated with microdialysis
Takuma Koyama, Kirsten Lønne Enggrob, Jim Rasmussen, Juliana Trindade Martins, Leanne Peixoto
Abstract
Plant root exudates are an important regulator of soil organic matter (SOM) dynamics. However, we have limited knowledge regarding the microbial carbon use efficiency (CUE) of root exudates, which may be a key parameter of root exudate contribution to SOM formation. Furthermore, the magnitude of SOM decomposition induced by root exudates (priming effects) remains poorly understood. To address these knowledge gaps, we conducted soil incubation experiments using microdialysis, where 14 C-labeled substrates were continuously released into the soil over 24 hours to simulate root exudation. We assessed the substrate CUE and priming effects of sugars (glucose and sucrose), an organic acid (acetic acid) and amino acids (alanine and aspartic acids) released into the soil either individually or in mixtures with different N availabilities. The comparison of microdialysis method and traditional single-pulse addition method revealed that the substrate respiration in the two methods differed significantly from each other. Higher substrate CUE was observed for sugars (glucose and sucrose) and an organic acid (acetic acid) than amino acids (alanine and aspartic acid). Furthermore, the substrate CUE of glucose, acetic acid, and alanine decreased significantly when they were added in mixtures, which could be attributed to the increased C availability in mixture treatments. The substrate addition with low N/C ratios (0 or 1/27) significantly accelerated soil organic C decomposition, which was up to almost 100 times higher than the quantity of added substrate C. We conclude that the microdialysis method is a powerful tool to simulate root exudation. Our study demonstrated that the CUE of root exudates is substrate-specific and affected by substrate C availability, while N/C ratio of root exudates affects the magnitude of priming effects and thus the net contribution of root exudates to SOM dynamics.