PARENT SESSION
Oral Session # 19: Biogeochemistry II: Grasslands.
Presiding: JT Lennon
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 105.

Plant carbon inputs regulate timing of nitrogen pulses in semiarid soils.

Stark, John¹, ¹ Department of Biology, Logan, UT, USA

ABSTRACT- Little is known about the timing and magnitude of nutrient pulses in semiarid ecosystems, and even less is known about the factors that regulate these pulses. We studied a big sagebrush community in southern Idaho to answer the questions: Are pulses of water and nitrogen correlated with one another? Do these pulses coincide with plant demand? And what are the mechanisms regulating these pulses? We monitored soil moisture, temperature, inorganic N, net N mineralization rates, gross N mineralization and immobilization rates, and CO₂ flux in soils beneath sagebrush and crested wheatgrass plants at 2-wk intervals throughout the growing seasons of two years. Interestingly, water pulses did not always cause a pulse of N mineralization, as is usually assumed. During mid- to late summer of both years, water addition actually stimulated a phase of net N immobilization. A mass-balance model linking C and N dynamics suggests that this immobilization phase results from the microbial community shifting from low to high C:N substrates during late July. The shift in substrate C:N appears to coincide with senescence of plant foliage and possibly near surface roots, and thus may result from a large release of plant biomass into the labile soil organic matter pool. During the subsequent few months, the C:N of this pool gradually declines, presumably due to microbial degradation and CO₂ release, and by the time the fall rains stimulate plant growth, a new phase of mineralization has begun. Therefore, while water and nitrogen pulses are not always correlated, water does appear to stimulate N mineralization during seasons when plants are actively taking up N.

Key words: sagebrush, soil moisture, N mineralization, microbial substrates