Germination and Early Growth of Slickspot Peppergrass (Lepidium papilliferum) as Affected by Desert Soil Humic Acids

Slickspot peppergrass (Lepidium papilliferum) is a biennial, or possibly perennial, endemic plant growing in the Southern Idaho high desert in visually distinct small-scale depressions in soils that collect water (so-called slickspots). Lepidium papilliferum establishes seed banks not germinating the first year but remaining dormant and viable for several years. Humic acids (HA) are universally considered to be the most important, abundant, and biologically and chemically active fractions of soil organic matter and are known to affect plant growth by various mechanisms, depending on their origin, nature, and concentration. The effects of HA in slickspot soils and how they relate to the possibility of being a factor in restoring native plants is only partially known. Thus, the objective of this study was to identify and evaluate the effects of HA isolated from three different layers within the soil profile (silt, vesicular, and clay) from inside a representative slickspot on the germination and early growth of slickspot peppergrass. Furthermore, these effects were tentatively related to the chemical, physicochemical, compositional, structural, and functional characteristics of the HA. Results of statistical analysis showed that both the type and concentration of the three HA examined exert a highly significant or significant effect on the germination and early growth of slickspot peppergrass as a function of the soil depth from which the HA originated in the slickspot. In particular, germination seemed to be enhanced, especially at higher concentrations, by the less hydrophobic HA, rich in oxygen and total sugars, present in the bottom clay soil layer, whereas root growth and shoot growth were positively influenced by the more hydrophobic and probably more polycondensed HA, rich in C, H, N, and phenolic OH present in the top layer rich in silt.