Interactions between Kalmia humus quality and chronic low C inputs in controlling microbial and soil nutrient dynamics

Glucose-C was repeatedly added to organic Kalmia humus from two sites of contrasting spruce productivity (i.e. rich and poor) and basal respiration, microbial biomass and metabolic quotient (qCO2) were measured over a 438 day incubation and compared to post-incubation soil mineral-N pools, anaerobic N mineralization rates, the nutritional deficiency index (NDI) of soil microbial communities and final soil weights. Repeated measures analysis of variance revealed a strong overall soil effect on basal respiration and microbial biomass and a strong overall glucose effect on microbial biomass and qCO2, whereas the analysis of within-subjects effects revealed strong interactions of the time factor with both soil and glucose on all three measurements. In the poor soil, glucose supported high microbial biomass and therefore low qCO2, until the end of the incubation, whereas in the rich soil, glucose also supported high microbial biomass and low qCO2, but these converged towards control (i.e. no glucose) values before the end of the incubation. Mineral-N pools were high in the rich control treatment only, whereas the NDI was high only in the rich + glucose treatment. Anaerobic N mineralization rates did not differ statistically among treatments. Glucose significantly decreased mass loss in the rich soil but not in the poor soil. The data support the conclusion that glucose addition to the rich soil inhibits microbial utilization of nutrient-containing soil OM for maintenance energy thus exacerbating nutritional deficiencies, whereas glucose addition to the poor soil does not affect soil N cycling. Based on results of analytical pyrolysis performed on soil subsamples prior to the incubation, we hypothesize that higher amounts of tannins measured in the poor humus may have chemically immobilized and ultimately controlled availability of N.