Earthworm dynamics in tallgrass prairies in the Ozark Highlands region of northwest Arkansas

Tallgrass prairie was a historic and is presently a unique and endangered natural ecosystem throughout the Ozark Highlands region in the mid-southern United States. Tallgrass prairies have been disturbed for agricultural production and/or urban expansion; thus, as a result, efforts have been undertaken to reclaim areas previously used for agricultural production to mitigate the loss of tallgrass prairies. Limited research exists on earthworm dynamics in mounded tallgrass prairies or prairie restorations, particularly in the Ozark Highlands region of northwest Arkansas. The objectives of this study were to i) evaluate the effects of soil moisture regime (SMR; aquic and udic), landscape position (LP, mound and inter-mound), and sample date (October 2018 and May 2019) on earthworm densities in two mounded, native prairies, ii) to evaluate the effect of time (i.e., May 2007 and May 2019), ecosystem (i.e., native prairie and prairie restoration), and their interaction on earthworm density in a native prairie remnant and a chronosequence of three prairie restorations (i.e., 1979, 2000, and 2002), and iii) to identify relationships between earthworm density (i.e., juvenile, adult, and total) and soil physical and chemical properties in the upper 20-cm of soil across all ecosystem combinations and restoration sites sampled in the Ozark Highlands region of northwest Arkansas. Juvenile and total earthworm densities among two mounded prairie remnants were five times greater (P < 0.05) in the udicand aquic-inter-mound combinations than in any other SMR-LP combination in October 2018 and May 2019, respectively. Adult earthworm densities were two times greater (P < 0.05) in the aquic-inter-mound at Searles Prairie than in any other SMR-LP combination in either mounded prairie. Juvenile earthworm densities were three times greater (P < 0.05) in May 2019 than in May 2018 at Chesney Prairie, but adult and total earthworm densities did not change over the same period of time. Among a native prairie remnant and a chronosequence of three prairie restorations, juvenile earthworm density decreased (P = 0.03) 25 and 35% in the 2000 and 2002 restorations from May 2007 to May 2019, while total earthworm density decreased (P = 0.03) by 33 and 42% in the 2000 in the 2002 restorations over the same 12-year period of time, but neither juvenile or total earthworm density changed over time in the oldest restoration or native prairie. Adult earthworm density did not change over time (P > 0.05) among the native prairie and chronosequence of prairie restorations. Combined across all data for Searles and Chesney Prairies, juvenile and total earthworm densities were positively correlated (P < 0.05) with VWC (r = 0.33 and 0.34, respectively), calcium (r = 0.20 and 0.21, respectively), and iron (r = 0.53 and 0.52, respectively), but negatively correlated (P < 0.05) with manganese (r = -0.34 and -0.32, respectively) among two mounded prairies. Combined across the native prairie and three prairie restorations, adult earthworm density was positively correlated (P < 0.05) with sand content (r = 0.49) and pH (r = 0.49) and negatively correlated (P < 0.05) with silt (r = -0.56) and sulfur content (r = -0.47). This study extended overall understanding of earthworm populations in mounded prairie systems and provided an important target for prairie restoration activity. To promote earthworm densities similar to native prairies, consideration should be given to numerous soil properties prior to earthworm inoculation in prairie restorations. Table of