Plant-derived compounds stimulate the decomposition of organic matter in arctic permafrost soils

Abstract

Arctic ecosystems are warming rapidly, which is expected to promote soil organic matter (SOM) decomposition. In addition to the direct warming effect, decomposition can also be indirectly stimulated via increased plant productivity and plant-soil C allocation, and this so called "priming effect" might significantly alter the ecosystem C balance. In this study, we provide first mechanistic insights into the susceptibility of SOM decomposition in arctic permafrost soils to priming. By comparing 119 soils from four locations across the Siberian Arctic that cover all horizons of active layer and upper permafrost, we found that an increased availability of plant-derived organic C particularly stimulated decomposition in subsoil horizons where most of the arctic soil carbon is located. Considering the 1,035 Pg of arctic soil carbon, such an additional stimulation of decomposition beyond the direct temperature effect can accelerate net ecosystem C losses, and amplify the positive feedback to global warming.

Figure 1. Map of sampling sites across the Siberian Arctic.

The dotted line indicates the polar circle. The map was created in R using the packages sp and rworldmap.

Figure 2. Losses of native SOC from different horizons of arctic permafrost soils after 25 weeks of incubation (dark grey bars).

Losses induced by the addition of cellulose or protein in comparison to control samples are indicated in light grey. Bars represent means with standard errors, different letters indicate significant differences between horizons at p < 0.05. See Supplementary Fig. S1 for the development of SOC- and substrate-derived respiration over time.

Figure 3. Response of cumulative SOC mineralization in different horizons of arctic permafrost soils to addition of cellulose or protein.

Response ratios were calculated as ratios of samples amended with cellulose or protein over control samples. Bars represent means with standard errors, significant differences in SOC mineralization between amended and control samples are indicated (Welch’s paired t-tests; ***p < 0.001; **p < 0.01; *p < 0.05). For response ratios at individual sampling sites see Supplementary Table S5.

Figure 4. Response of the microbial biomass in different horizons of arctic permafrost soils to addition of cellulose or protein.

Response ratios were calculated as ratios of samples amended with cellulose or protein over control samples. Bars represent means with standard errors, significant differences in microbial biomass between amended and control samples are indicated (Welch’s paired t-tests; **p < 0.01; *p < 0.05). For microbial biomass in control samples see Supplementary Table S3, and for response ratios at individual sampling sites see Supplementary Table S5.

Figure 5. Microbial substrate use efficiency of cellulose- or protein-derived C in different horizons of arctic permafrost soils.

Substrate use efficiency was calculated as the ratio of substrate-derived C in microbial biomass over substrate-derived C in biomass and cumulative respiration after 25 weeks of incubation. Bars represent means with standard errors. Significant differences between cellulose and protein treatments are asterisked (***p < 0.001; **p < 0.01; *p < 0.05), significant differences between horizons for cellulose or protein are indicated by different letters (p < 0.05). For data on individual sampling sites see Supplementary Table S6.