Soil Variability along a Nitrogen Mineralization and Nitrification Gradient in a Nitrogen-Saturated Hardwood Forest

Abstract

Some N-saturated watersheds of the Fernow Experimental Forest (FEF), West Virginia, exhibit a high degree of spatial heterogeneity in soil N processing. We used soils from four sites at FEF representing a gradient in net N mineralization and nitrification to consider the causes and consequences of such spatial heterogeneity. We collected soils with extremely high vs. low rates of N processing within each of two watersheds: WS3, treated for 15 yr with (NH 4 ) 2 SO 4 , and WS4, untreated reference (control). Mineral soil was analyzed for extractable NH 4 , NO 3 , Ca, and Al before and during 28-d incubations at 10, 20, and 30°C after 7, 14, 21, and 28 d. To address the fourth question, we decreased C:N ratios in the soil exhibiting lowest field rates of net N mineralization and no net nitrification (control-low N) by adding glycine and increased C:N ratios in the soil exhibiting highest field rates of net N mineralization and nitrification by adding sucrose. Incubations under controlled conditions supported the N-processing gradient found in the field under in situ conditions in the following order from highest to lowest rates of N mineralization and nitrification: control-high N > N-treated-high N > N-treated-low N > control-low N, the latter exhibiting no net nitrification in the field. Net Ca mineralization increased with net nitrification along the gradient from zero to highest rates. Soil Al appeared to inhibit net nitrification, being lowest in the soil with highest net nitrification and highest in the soil exhibiting no net nitrification. Glycine additions to this latter soil greatly stimulated net N mineralization but failed to initiate net nitrification. Sucrose additions resulted in net immobilization of NH 4 and NO 3 in soil with highest net N mineralization and nitrification. Results demonstrate that increased nitrification may enhance Ca mobility in N-saturated soils and further demonstrate that substrate quality alone is not necessarily a good predictor of soil N processing.