Tree Encroachment Impacts Carbon Dynamics in a Sand Prairie in Wisconsin

A sandy prairie remnant in the Lower Wisconsin River Valley, encroachment areas within the prairie, and an adjacent red pine (Pinus resinosaAiton) plantation were studied to determine the influence of woody cover on C dynamics. Field transects, aerial imagery, and a geographic information system were used to quantify encroachment from 1979 to 2002. A linear encroachment model predicted 100% encroachment of the 6.0‐ha prairie in 50 yr. Four field plots in each of pine, prairie, and encroachment areas were sampled and soils collected (0–18, 18–38, and 38–75 cm) in 2004 and 2008. Total ecosystem C was greater in pine (126.6 Mg C ha−1) and encroachment areas (71.8 Mg C ha−1) than prairie (48.3 Mg C ha−1). In the 0 to 38 cm, coarse particulate organic matter (POM) (4.1, 6.3, and 7.5 Mg C ha−1) and the POM C/N ratio (15.1, 16.2, and 20.2) increased with woody encroachment (prairie, encroachment areas, and pine, respectively). Changes in POM suggest more organic inputs and slower decomposition, but due to minimal protection of C within aggregates, increased total soil C was not observed with woody advancement (46.4–47.2 Mg C ha−1). Microbial biomass (0–38 cm) was greatest in encroachment areas, followed by prairie, and then pine (108, 84, and 51 kg N ha−1, respectively), probably a result of more favorable microclimate and substrate at the ecotone boundary. Potential N mineralization (0.6, 2.8, and 4.8 kg N ha−1d−1), extractable NH4+(28, 33, and 57 kg ha−1), and Bray‐1 P (380, 402, and 541 kg ha−1) (0–38 cm) increased with woody cover, and increased nutrient availability could lead to a greater aboveground C sink through increased tree growth.