Following carbon through the soil food web: Do long-term burning practices affect carbon trophic dynamics in grasslands?

Background/Question/Methods

Soil carbon (C) dynamics are a central component of global C cycling. Human activities, such as land management practices, impact decomposition processes, soil communities, and soil C storage. For example, annual burn of grasslands promotes higher decomposition rates (compared to unburned) and impacts soil C mineralization. In addition, annual burning alters populations of bacteria, bacterivore nematodes, and other invertebrates and thus, may affect soil food web’s C cycling. Our greenhouse study, through a novel ¹³C tracer approach, examines how tallgrass prairie’s annual burn affects root litter decomposition, soil food web structure, and C dynamics.

Andropogon gerardii was grown from rhizomes in a continuous labeling chamber of ¹³C-CO₂ atmosphere. In a randomized, replicated greenhouse study, the ¹³C-labeled dead roots were buried in soil collected from 2 management systems: a) annually burned (AB) or b) unburned (UB) areas at the Konza Prairie Biological Station. Decomposition was assessed over 6 months at 8 destructive harvests. Microbes were extracted by phospholipid fatty acid (PLFA) procedure and their nematode consumers (fungivores, bacterivores, omnivores, predators) were water-extracted (Baermann Funnel technique). These biota were analyzed for biomass, microbial community composition, faunal trophic position, and C isotope signature of different communities to trace root-C through soil trophic levels. 

Results/Conclusions

We will present the results of our 6-month study. Root tissue had uniform highly enriched ¹³C content. With litterbag retrieval, total decomposition showed no differences between litter mass loss in the two soils. However, there were differences in microbial communities and nematode populations in both soils. PLFA data analyses suggest significantly lower microbial abundance in AB. The microbial community structure changed over time of decomposition, with significant increases in fungi to bacteria ratios in UB and gram-positive to gram-negative bacteria ratios in both soils. Nematode populations increased after litter addition in UB and decreased in AB, and shifted over time of decomposition in both soils.

Litter-derived C was rapidly incorporated into PLFA-C and nematode-C, but relative incorporation into different communities varied temporally. By 3 days, litter-C was incorporated into fungi, bacteria, and bacterivorous nematodes in both soils, and also in fungivorous nematodes in AB. By 14 days, litter-C was also incorporated into omnivorous and predaceous nematodes in UB. By 6 months, the amount of litter-derived C became more concentrated in higher trophic levels in both soils. These results contribute to the understanding of soil food web’s C dynamics, which may differentially affect soil C turnover in burned and unburned grasslands.