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Microbial dynamics and carbon and nitrogen cycling following re-wetting of soils beneath two semi-arid plant species

  • Ecosystem Ecology
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Abstract

Sporadic summer rainfall in semi-arid ecosystems can provide enough soil moisture to drastically increase CO2 efflux and rates of soil N cycling. The magnitudes of C and N pulses are highly variable, however, and the factors regulating these pulses are poorly understood. We examined changes in soil respiration, bacterial, fungal and microfaunal populations, and gross rates of N mineralization, nitrification, and NH4+ and NO3− immobilization during the 10 days following wetting of dry soils collected from stands of big sagebrush (Artemisia tridentata) and cheatgrass (Bromus tectorum) in central Utah. Soil CO2 production increased more than tenfold during the 17 h immediately following wetting. The labile organic C pool released by wetting was almost completely respired within 2–3 days, and was nearly three times as large in sagebrush soil as in cheatgrass. In spite of larger labile C pools beneath sagebrush, 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 days after wetting. Protozoan populations, whose biomass was nearly 3,000-fold lower than bacteria and fungi, peaked after 2–4 days. Gross N mineralization and nitrification rates were both faster in cheatgrass soil than in sagebrush, and caused greater nitrate accumulation in cheatgrass soil. 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 the initial N pulse was associated with degradation of microbial substrates that were rich in N (C:N <8.3), and that microbes had shifted to substrates with lower N contents (C:N =15–25) by day 7 of the incubation. The model also suggested that the labile organic matter in cheatgrass soil had a lower C:N ratio than in sagebrush, and this promoted faster N cycling rates and greater N availability. This study provides evidence that the high N availability often associated with wetting of cheatgrass soils is a result of cheatgrass supplying substrates to microbes that are of high decomposability and N content.

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Acknowledgements

We thank M. Larson, A. Anderson, and E. Courtright-Risley for laboratory assistance, K. Sompi for quantifying the nematodes, and J. Norton for the use of her epifluorescent microscope. J. Moore, B. Sohlenius and J. Bloem provided valuable advice on methodologies. B. Mary provided the FLUAZ software and advice on how to run it. This research was funded by a grant from the National Science Foundation (DEB 9807097) and the Utah Agricultural Experiment Station.

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  1. Peter Saetre

    Present address: Department of Evolution, Genomics and Systematics, Uppsala University, 18D Norbyvägen, 75236, Uppsala, Sweden

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  1. Department of Biology, and the Ecology Center, Utah State University, Logan, UT, 84322-5305, USA

    Peter Saetre & John M. Stark

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Correspondence to John M. Stark.

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Saetre, P., Stark, J.M. Microbial dynamics and carbon and nitrogen cycling following re-wetting of soils beneath two semi-arid plant species. Oecologia 142, 247–260 (2005). https://doi.org/10.1007/s00442-004-1718-9

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