Evaluating 20 Year Soil Response to OM Manipulation: Implications for Assessment and Application.

The Long Term Soil Productivity (LTSP) network was initiated over two decades ago and has served as a flagship program for the forest soils community.  A central premise was that manipulation of organic matter (OM) inputs at time of harvesting would have effects on soil properties and site productivity with time.  Here we report on the mineral soil C and nutrient response to pulse manipulations of surface OM at some of the longest running LTSP installations located in the aspen forests of the lake states region.  Using a decadal assessment approach, we observed limited effects of OM manipulation on total soil C and N to a depth of 30 cm across 3 sites that spanned a range of soil texture and potential productivity.  Based on the soil response, single rotation effects on stand productivity would appear to be unlikely, but the long-term multi-rotation outcomes are less clear.  This uncertainty largely arises from various factors that constrain our ability to detect change in plant available nutrient pools including temporal and spatial variation of soil chemical properties and their estimation.  The base sampling design used at many LTSP installations creates additional uncertainty becasue the design generally focuses on shalllow soils and only one component of total soil nutrient pools (mineral soil fine fraction).  A reduction in stand volume following OM removal at one of the sites despite no change in soil pools demonstrates the limitations of a soil-based assessment.