Effects of urea fertilization and liming on soil nutrient dynamics following intensive organic matter removal and vegetation control in a Douglas-fir forest

The Fall River Long-Term Soil Productivity site in coastal WA, USA, examines the long-term effects of organic matter removal and vegetation control on Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirbel) Franco) productivity. Treatments included bole-only (BO), whole-tree (WT), and whole-tree with coarse woody debris removal (WT+), with either five years of complete vegetation control (V: BOV, WTV, WT+V) or only initial control (BO). Previous studies showed that the Fall River soil initially contained high total N and low base cations, and BOV, WTV, and WT+V treatments had reduced soil NO₃, Ca, K, and Mg availability after 18 years. This study tested whether urea fertilization or liming could restore nutrient losses using soil extractions and Plant Root Simulator (PRS) probes over 2 and 5 years. Urea fertilization quickly increased NO₃ availability down to 50 cm, though WT+V plots responded the least. Urea also boosted PRS Ca, Mg, Fe, Mn, Zn, and Cu initially but subsequently reduced PRS Ca and Mg supply rates and soil pH. Liming raised exchangeable Ca and PRS Ca in the forest floor and shallow soil, but not at 20–50 cm where WT+V had the lowest Ca levels. While this study used non-operational vegetation control and organic matter removals, it underscores the role of understory vegetation and harvest residues in retaining soil nutrients and the potential for fertilizers to ameliorate nutrient losses. This study also raises concerns about the effect of urea on base cation leaching in soils with high N and low base cations, which could explain the low observed fertilizer growth response in similar coastal soils.