Stratification of soil chemical and microbial properties under no-till after liming

Soils under continuous no-till (NT) often produce stratified soil acidification due to fertilizer placement and subsequent lack of mixing. Because aluminum (Al) becomes more bioavailable with lower soil pH, Al toxicity is an emerging problem in many soils throughout the Palouse region in the dryland cropping areas of the inland Pacific Northwest of the United States. When various lime materials are surface applied to NT soils, the depth to which these materials may impact soil pH, available Al, and microbial community response has not been explored in the fine-silty soils of this arid region. The objectives of the current study were to: (1) determine if soil sampling at small, discrete depth increments captures stratification of chemical and microbial properties in NT soils and (2) evaluate the impacts of two types of surface-broadcast lime on Al availability and microbial community composition at each depth interval. In April 2014 and 2015 (6 mo and 18 mo after liming), soil samples were collected at 2-cm depth increments and analyzed for soil pH and metals bioavailability, as well as microbial community composition using terminal restriction fragment length polymorphism analysis. In 2015, surface soil (0-2 cm) pH increased from 4.6 in untreated control to 6.6 in limed plots, and KCl-extractable Al decreased from approximately 50 mg kg−1 to below 10 mg kg−1 in the 2-4 cm depth of soil. The surface soil microbial community composition changed significantly in response to lime application, and was highly correlated with soil pH and Al concentrations. No significant effects of treatment were detected when data were pooled across the 10 cm depth. The 2-cm sampling increments provided statistically significant resolution in soil chemical stratification and related shifts in microbial community composition, revealing different effects at different depths. We are able to conclude from this study that (1) A finely resolved, spatially explicit sampling design is necessary to capture soil chemical stratification complexity within a variety of variables, and microbial community response over time; and (2) Surface application of lime in Palouse NT systems will take >18 mo to be effective at depths below 6 cm.