Do long-term burning practices affect carbon and nitrogen flow dynamics from grassland root litter through the soil food web?

Background/Question/Methods

Soil carbon (C) and nitrogen (N) dynamics are a fundamental component of global nutrient cycling. Many human activities, including land management practices impact decomposition processes, soil communities, and soil nutrient storage and cycling. For example, annual burning of grasslands promotes higher decomposition rates (compared to unburned prairie), and alters soil community structure and C and N mineralization. Our study examines how annual burn in the tallgrass prairie affects root litter decomposition (C, N) and soil food web dynamics. This treatment (burned) versus control (unburned) has been shown to alter populations of bacteria, bacterivorous nematodes, and other invertebrates and may affect soil food web’s nutrient cycling.

Big bluestem, Andropogon gerardii, was grown from seed in a continuous labeling chamber of a ¹³C-CO₂ atmosphere, in C-free soil media fertilized weekly with a ¹⁵N- KNO₃ solution as part of an NSF project (F. Cotrufo, PI). Later, in a randomized, replicated greenhouse study, the ¹³C and ¹⁵N enriched dead roots were placed in litter bags and buried in soil collected from unburned or annually burned areas of Konza Long Term Ecological Research site. Biotic communities were extracted and analyzed for initial densities, biomass, and trophic position. Over six months, eight destructive harvests are being analyzed to trace ¹³C and ¹⁵N of roots through the soil and trophic levels (bacteria, fungi, and their nematode consumers: fungivores, bacterivores, omnivores, and predators). Microbe and nematode populations and biomass are also being analyzed. 

Results/Conclusions

Here, we present results from the first three months of our study. Root tissue had uniform highly enriched ¹³C and ¹⁵N content at an average of δ¹³C 2050.25 at 43.4% C, and δ¹⁵N 11496.96 at 1.46% N.  Results show that nematode ¹³C can reliably be determined from ~50-75 nematodes for an average natural abundance of δ¹³C -17.31. Nitrogen content of nematodes was highly variable and may require more nematodes for reliable analysis. Our data include δ¹³C and δ¹⁵N values of enriched roots, soils, bacteria, fungi, and nematode trophic levels. Additionally, with litterbag retrieval, decomposition rates are determined per treatment. Results show net C and N fluxes through soil food web and changes in soil nutrient stocks of the two soils. These results contribute to understanding of C and N flow dynamics through the soil food web, which may have differential effects on soil nutrient turnover in burned and unburned grasslands.