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Plant phenology effects on soil microbial C and N cycling in a semiarid ecosystem.
Hooker, Toby*,1, Stark, John1, Norton, Urszula1, 1 Department of Biology / Ecology Center, Logan, UT, USA
ABSTRACT- Invasion and dominance of winter-annual cheatgrass (Bromus tectorum) into perennial dominated shrub-steppe ecosystems may alter seasonal patterns of microbial C and N cycling via shifts in timing of plant detrital C inputs to soil. Previous work in Rush Valley, Utah, found large increases in dry-season soil NO3- concentration under cheatgrass over three consecutive years, occurring 6-8 weeks after plant senescence, with no large NO3- accumulation observed under perennial dominated vegetation types. Both plants and soil microbes respond to infrequent and unpredictable summertime pulses of rainfall and inorganic N, though little evidence exists that this uptake can be related back to net C gain for plants. A herbicide study was initiated to examine whether plant N uptake maintained low soil NO3- levels under perennial vegetation. Glyphosate was added to plots in annual cheatgrass and perennial crested wheatgrass vegetation types in late May 2001, the time of cheatgrass senescence. Measurements of field soil inorganic N concentrations were collected monthly, with laboratory potential C and net N mineralization rates, and organic C and N contents from soil organic matter density fractions. Soil moisture was slightly higher under cheatgrass, and herbicide treatment differences were small for the first year of measurements. Cheatgrass had greater potentially mineralizable C than crested wheatgrass across the growing season, however there was no effect of herbicide treatment for either vegetation type. The pattern of field soil NO3- accumulation was similar among herbicide and control plots for cheatgrass. Soil NO3- concentration increased after four months in crested wheatgrass herbicide plots, and remained elevated the next growing season under continued herbicide treatment, relative to control plots. It remains unclear whether the observed NO3- accumulation in herbicide treated crested wheatgrass plots, and in the annual grass system, is due to lack of plant N uptake, or C limitation of microbial biomass due to the reduction in belowground C inputs.
Key words: microbial C and N cycling, semiarid ecosystems, Bromus tectorum, plant life-form