Phenolic inputs by invasive species could impart seasonal variations in nitrogen pools in the introduced soils: A case study with Polygonum cuspidatum

One cascading feature of the spread of introduced species of plants is their capacity to alter levels of resource availabilities for other species. However, the effects of introduced species on temporal patterns of soil nutrient availability remain unexplored, which could be critical for invasion where the growing seasons are short. To test the hypothesis that the invasive species could alter seasonal patterns of resource availability via suppression of soil processes through secondary compounds in autumnal litter-fall, we measured nutrient dynamics in soils inside and outside stands of invasive Polygonum cuspidatum at four sites in northeastern US over the growing season. In the uppermost soil (0-5 cm depth), both concentrations of inorganic N and rates of N mineralization were 60% lower inside than outside stands of knotweed in spring but became as high or higher inside than outside by fall. Carbon and nitrogen mineralizing soil microbial enzymes also followed a similar seasonal trend with lower activity inside the knotweed stands than outside during spring and these differences disappearing by summer. Concentrations of dissolved organic N (DON) and phenolic compounds showed the opposite seasonal pattern and a strong correlational trend; they were nearly 3 times higher inside than outside stands in spring and similar inside and outside in fall. The estimated flux of tannins from knotweed litter to soil was calculated as 189.5 +/- 39.2 g m(-2) year(-1). Further, exogenous application of knotweed tannin reduced the soil N-mineralization and paralleled the field observed inhibition of mineralization. At 5-15 cm depth, concentrations of phenolics were uniformly low and differences between soils inside and outside stands were mostly non-significant. Our results suggests that invasion by polyphenol rich, yet fast growing exotic species would make the soil N pools more dynamic, which could facilitate invasion by creating stints of apparent deficiency and sufficiency of resources, and due to the localization of the invader effect the amelioration practices could be focused on the surface 0-5 cm layer of the mineral soil. (C) 2012 Elsevier Ltd. All rights reserved.