Estimating the Pace of Nitrogen Transformations in Old Growth Forest Plots Undergoing Long-Term Soil Organic Matter Manipulations.

The summer of 2007 marked the 10^th anniversary of the Detritus Inputs and Removals Treatments (DIRT) study at the H.J. Andrews Experimental Forest.  This long-term manipulation experiment examines how soil organic matter (SOM) quality and quantity affects SOM chemistry, nutrient availability and turnover. Isotope dilution methods were utilized to estimate gross N mineralization and nitrification rates across six treatments that controlled the quality and rate of SOM inputs (control, double wood, double litter, no roots, no litter and no inputs). Ammonium production and consumption decreased, compared to the control, in the treatment where annual aboveground woody debris is doubled.  Doubling litter or removing belowground inputs did not impact NH₄⁺ production and consumption. Gross nitrification was measured in only the double wood treatment. This suggests that NH₄⁺ consumption in all other treatments occurred by immobilization. To estimate gross organic N turnover, double labeled (¹³C and ¹⁵N) glutamic acid was used in the control, no roots and no inputs (no roots, litter or woody debris) treatments. At 6 hours, respiration measurements revealed similar levels of percent C derived from the labeled substrate across all treatments. At 24 hours, the glutamic acid accounted for a smaller percentage of respired C in the control plots than the other treatments. Organic N turnover estimates show that substrate consumption or soluble organic N production or both, occur at a faster rate in the control plots. Immobilization and fast turnover of organic N suggests that the microbial communities in these plots are likely N limited, even after 10 years of manipulating C inputs. A surprising result was the increased NO₃^- production in response to woody debris additions, which might be expected to exacerbate N limitation.