Regional Differences in Soil Carbon and Nitrogen Storage as Affected by Land Use and Soil Moisture Regime

Climate, particularly precipitation and to a lesser degree temperature fluctuations, and land use have often been shown to affect soil organic matter (SOM), carbon (C), and nitrogen (N) storage and cycling. Soil moisture regime has been suggested as a potentially important factor in determining soil organic C (SOC) content. However, few studies have specifically investigated soil moisture regime effects on SOM, SOC, or N storage or sequestration. The objective of this study was to evaluate potential regional differences in the effects of land use and soil moisture regime on changes in near-surface SOM, SOC, and total N (TN) contents and C:N ratio during a 6-year period in a silt-loam soil. Soil bulk density, SOM, and SOC, and TN changes during a 6-year period were measured on replicate samples collected from the top 10 cm in udic and aquic soil moisture regimes representing both managed agriculture and native grassland in the Ozark Highlands region of northwest Arkansas and in the Grand Prairie region of east central Arkansas. All treatment factors examined affected soil bulk density over time. Soil OM and SOC storage were affected by region, land use, and soil moisture regime, but SOM and SOC sequestration were unaffected by soil moisture regime. However, TN content increased by 0.03 kg m(-2) over time in the aquic moisture regime in the Ozark Highlands and decreased by 0.03 kg m(-2) in the Grand Prairie. Total N content also increased over time in the udic moisture regime by 0.03 kg m(-2) under native prairie and decreased by 0.03 kg m(-2) under agricultural land use. It is clear that the average degree of wetness a soil experiences, as indicated by soil taxonomy's soil moisture regime, directly affects N storage, cycling, and sequestration, and indirectly affects SOM and SOC cycling through effects on the soil C:N ratio. Soil moisture regime differences that may exist among sites and/or treatments should be at least examined and accounted for when necessary in future studies.