Patterns of nitrogen fluxes in watersheds of the H.J. Andrews Experimental Forest, OR

Seasonal and annual patterns of N fluxes and concentrations in streamwater in six conifer-dominated watersheds at the H.J. Andrews Experimental Forest, OR, were studied to gain insight into the factors that influence N retention in this ecosystem. Processes affecting N flux in streamwater differed between organic (DON) and inorganic (DIN; NO3-N and NH4-N) forms of N. Annual DON flux increased with increasing annual discharge in all watersheds, implying that regional-level climatic fluctuations may influence DON retention. Annual DIN flux, in contrast, was not consistently related to annual stream discharge. DON concentrations in stream water peaked in winter before the peak in the hydrograph, suggesting that DON may be flushed from the soil into the stream. Concentrations of DIN were relatively constant throughout the year.
Annual and seasonal patterns of N retention were calculated using three separate estimates of N inputs to this ecosystem: (1) atmospheric DIN alone, (2) atmospheric DIN + DON, and (3) atmospheric DIN + DON + inputs from biological fixation. Vegetation demand for N had little effect on annual DIN retention, perhaps because inputs of DIN to the Andrews Forest and periods of forest growth are asynchronous. High inputs of biologically-fixed N did not result in increased nitrate leaching, suggesting that biologically-fixed N is efficiently retained in this ecosystem.
Forest harvest at the Andrews resulted in very small losses of N from the soil relative to other sites. The high C:N ratio of soil at the Andrews Forest probably promotes immobilization of N after harvest, and little of the N released by decomposing roots or made available via reduced vegetation demand escapes the soil microbial community.