Tree Encroachment and Organic Matter Dynamics in a Remnant Dry Prairie in the Lower Wisconsin River Valley, WI, USA.

A native dry prairie remnant, encroachment areas within the prairie, and an adjacent red pine plantation (Pinus resinosa) were studied to determine the influence of vegetation on C distribution, organic matter dynamics, and nutrient availability. Field transects and aerial imagery in a GIS were used to quantify woody encroachment from 1979 to 2002. Our encroachment model predicted the 6.0 ha prairie remnant to be 100% encroached within approximately 50 years of fire exclusion. Sample plots in pine, prairie, and encroachment areas on three sample dates, 1992 (from Quideau and Bockheim 1996), 2004, and 2008 were used to quantify changes in above-ground biomass and soil properties. Soil temperature was greater during the growing season (March through September) in the prairie relative to pine, but during the dormant season, pine soil temperatures were greater. Soil moisture was greater throughout the year in prairie compared to pine soils. Total ecosystem C increased 56% and the ratio of soil organic C to total ecosystem C decreased 55% as prairie was converted to forest. Particulate organic matter (POM) significantly increased relative to recalcitrant silt and clay associated soil organic matter (scSOM) with encroachment, and the POM/scSOM ratio was significantly greater in pine (0.34) and encroachment (0.25) compared to prairie (0.18) soils. Microbial biomass N was significantly greater in encroachment (53 kg N ha^-1) compared to pine (26 kg N ha^-1), with intermediate values in prairie (42 kg N ha^-1) soils. Potential N mineralization was significantly greater in pine (2.4 kg N ha^-1 d^-1) compared to prairie (0.3 kg N ha^-1 d^-1), with intermediate values for encroachment (1.4 kg N ha^-1 d^-1). Our results show labile C and nutrient availability to increase with forest encroachment and confirm others finding POM to be well correlated with landscape change, microbial biomass and activity, and nutrient availability.