Document: SEA-3-59-16

Nitrogen dynamics in arid ecosystems in response to carbon and nitrogen addition.

SCHAEFFER, S.M.*, S.A.BILLINGS and R.D.EVANS

University of Arkansas, AR, 72701,USA ¹

Abstract:
Nitrogen (N) is thought to be the second most limiting resource controlling net primary production (NPP) next to water in arid and semi-arid ecosystems. Nitrogen cycling by soil microbes may be limited by carbon (C) available to soil microbes. We investigated the effects of carbon and N additions on ammonia (NH³) volatilization, ammonification, nitrification, and denitrification in a Mojave desert ecosystem in order to assess potential limitations of C or N to these processes. All combinations of C and N additions were made at approximately four times ambient levels at four types of locations: Under the canopies of a C₃ evergreen shrub, C₄ perennial bunchgrass, and a C₃ deciduous shrub, as well as in the interspaces. Addition of C reduced NH³ volatilization by 53% (from 8.49 +/- 1.10 g N-NH₃ m^-2 hr^-1 to 4.50 +/- 0.74 g m^-2 hr^-1 for plots with added C and no added C respectively) while added N, in the form of KNO₃, had no effect on volatilization. After 24 hours, ammonium (NH₄) levels were lower with C addition compared to controls (2.12 +/- 0.39, 0.25 +/- .25, 2.91 +/- 0.71, and 0.99 +/- 0.35, g g dry soil^-1 for control, +C, +N, and +C+N plots respectively). Soil CO₂ fluxes indicated relatively greater microbial activity with added C compared to controls (32.40 +/- 7.61, 67.64 +/- 13.70, 21.36 +/- 6.01, and 83.18 +/- 9.12, g C-CO₂ m^-2 hr^-1 for control, +C, +N, and +C+N plots respectively). Potential denitrification rates were relatively greater under plant canopies compared to interspaces (2.99 +/- 2.23, 30.34 +/- 7.13, 11.95 +/- 5.33, and 21.03 +/- 7.15, ng N-N₂O g dry soil ^-1 for interspace, C3 evergreen, C4 bunchgrass, and C3 deciduous plots respectively). Decreases in NH₃ volatilization, NH₄ levels, and increases in soil CO₂ flux with added C suggest an increase in microbial immobilization of inorganic N, and further indicate a potential C limitation for microbial activity in this arid ecosystem. By characterizing the response of these ecosystem processes to potential changes in C availability we can gain an improved understanding of the structure and function of arid ecosystems and how they may respond to various types of perturbations.

Keywords:

This abstract is being presented at: 11:30 AM in session:
Oral Session #41: N Dynamics: Additions, Retention and Transformations.