PARENT SESSION
Poster Session #75: Nutrient Cycling II.
Friday, August 9. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

How does summer precipitation affect soil N and C cycling? Differences between cheatgrass and sagebrush soils.

STARK, JOHN^*,1, SAETRE, PETER¹, ¹ Utah State University, Logan, Utah

ABSTRACT- Semiarid ecosystems are characterized by pulses in resource availability that are driven by sporadic rainfall events. Summer rainfall can provide enough soil moisture to drastically increase CO₂ efflux and rates of soil N turnover. However, on some occasions rainfall causes net release of plant available N; whereas on other occasions it causes net N immobilization. We examined changes in soil respiration, microbial and microfaunal populations, and gross and net rates of ammonium and nitrate production and consumption during the 10 d following wetting of dry soils collected from two Great Basin plant communities: native big sagebrush (Artemisia tridentata, subsp. wyomingensis, and adjacent stands converted by wildfire to nearly monocultures of the exotic annual cheatgrass (Bromus tectorum). Soil CO₂ production increased more than ten-fold during the 17 h immediately following wetting. The labile organic C pool was almost completely respired within 2-3 d, and was nearly three times as large in sagebrush soils as in cheatgrass. In spite of larger labile C pools in sagebrush soils, microbial and microfaunal populations were nearly equal in the two soils. Bacterial and fungal growth coincided with depletion of labile C, and populations peaked in both soils 2 d after wetting. Protozoan populations, whose biomass was 1000-fold lower than bacteria and fungi, also peaked after 2 to 4 d. Gross mineralization and nitrification rates were both faster in cheatgrass soils than in sagebrush soils, and caused greater nitrate accumulation in cheatgrass soils. Grazing of bacteria and fungi by protozoans and nematodes could explain neither temporal trends in N-mineralization rates nor differences between soil types; however, a mass balance model indicated that differences in rates of N cycling could be explained by differences in the C:N of microbial substrates that were released by soil wetting.

KEY WORDS: vegetation change, N mineralization, nitrification, carbon dioxide flux