Influence of nitrogen availability on soil respiration kinetics

Background/Question/Methods

Investigation of soil respiration (R) under the anthropogenic influence of nitrogen deposition is necessary to advance our ability to predict terrestrial feedbacks to climate change.  We conducted a series of microcosm experiments to identify how parameters of the Arrhenius temperature sensitivity model and the Michaelis-Menton substrate kinetics model for soil R respond to nitrogen enrichment.  We hypothesized that nitrogen addition would influence soil R and Q10 based on soil C:N stoichiometry.  Soils were collected from three sites in southern California: a desert site, a subalpine site and an agricultural field.  Soils were incubated at 40% water holding capacity at approximately 25°C, and microcosm flux responses were measured from 13-31°C.  Arrhenius parameters were determined for four treatments: control (no addition), nitrogen amendment (100 µg NH₄NO₃ g^-1), dextrose addition (90 g L^-1), and nitrogen plus dextrose enrichment across 120 days of incubation.  A second experiment was performed to determine the respiration Michaelis-Menton parameters and their temperature sensitivity (Q10) at 5 levels of dextrose addition (0 to 30 mg g^-1) with and without nitrogen enrichment (200 µg g^-1) on Day 1 of incubation.

Results/Conclusions

In the long term incubation experiment, soil CO₂ flux for the control treatment was not significantly different from the nitrogen treatment, while the dextrose treatment was significantly greater than the control and nitrogen and less than the combined nitrogen and dextrose treatment for all soil samples from all sites on Day1.  For our desert and subalpine soils, Q10 for dextrose and dextrose plus nitrogen was greater than the control soils.  However, nitrogen additions alone did not increase temperature sensitivity relative to controls.  There was a significant increase in the Michaelis-Menton parameters Vmax and Km with nitrogen amendment for all soils from all sites.  Vmax and Km were temperature-dependent, which also increased under nitrogen enrichment for all soils.  These results suggest that the soils were capable of adjusting enzyme kinetics and enzyme production rates in response to nutrient additions.  In order to predict how soil R will change in response to N deposition and climate change, models must take into account the initial C and N limitation of soils, as these limitations influence both the rate and temperature sensitivity of soil R.