BEAVER DECOMPOSITION LEAVES LONG-LASTING BIOGEOCHEMICAL IMPACTS ON SURFACE AND SUBSURFACE SOILS (Invited Presentation)

Animal decomposition results in dramatic and potentially long-lived disruptions to soil biogeochemical cycling. Decomposition in surface and burial contexts likely follows different trajectories in terms of rate and timing of decay; however, we know comparatively little about these processes and their impacts on biogeochemical cycling. One important indicator of vertebrate decomposition, reflecting the sum of all nitrogen cycling, is the nitrogen stable isotopic composition of soils, reflecting: (1) the input of carcass-derived nitrogen (N); (2) the bioaccumulation of N by micro-and macro-decomposers; and (3) off-gassing of isotopically depleted N-compounds (i.e., ammonia). In response to decomposition in a surface setting in Tennessee, soils retained ¹⁵N isotopic enrichment up to at least 4 years after the decay of beavers. Within the first year, soils became ~9‰ enriched through early skeletal stage and then declined by ~1‰ within the first year. Between year 1 to 2 and years 2 to 4 after decay, ¹⁵N dropped by 2.3‰ and 1.2‰, respectively. If isotopic enrichment continues to decline by ~0.5‰ each year, it will take about 8 more years, or a total of 12 years, for the soils to be back to starting conditions. By year 4, other biogeochemical measurements, except for dissolved organic carbon (C), were indistinguishable from background conditions. In contrast, soils associated with a beaver burial in South Dakota were 4-5‰ enriched compared to controls after ~15 years. If we assume that the starting enrichment was similar to what is seen in surface decay settings or 9‰ enrichment, and a linear rate of change, soils would be expected to retain ¹⁵N enrichment for 30 years. Shifts in the elemental concentrations of soils associated with beaver burial vs. background conditions were also observed after ~15 years. P, Al, Sr, V, Pb, Zn, and Mo concentrations were higher in burial soils vs. background, while Mg, Ca, Ti, and Mn concentrations were lower. Taken together, the decomposition of animals fundamentally changes soil biogeochemical cycling, and these changes persist for years to potentially decades. Understanding the chemical trends and rates of change in soils surrounding bones can help inform similarities and differences in the early diagenetic alteration of bone in surface and burial settings.