Document: ROS-3-59-108

Nitrogen dynamics of gopher mounds and intermounds in a California annual grassland.

CANALS, R.M.*, D.J.HERMAN and M.K.FIRESTONE

University of California, Berkeley, CA 94720 USA ¹

Abstract:
The effects of disturbances by fossorial small mammals on soil grassland properties have been frequently studied. Studies consistently show higher amounts of inorganic nitrogen forms, especially nitrate, on the soils of the mounds compared to the intermounds. Since inorganic nitrogen is frequently the most limiting nutrient in natural grassland communities, the role played by small mammal activities may be relevant to the dynamics of these systems. Processes underlying the increase of the N-inorganic pool on mounds remain unclear. Although several hypotheses such as enhanced nitrification rates, absence of plant uptake in mounds, upward turnover of richer horizons have been suggested, they remain undemonstrated. In this study, we compared the N dynamics of disturbed and undisturbed sites and the processes derived from gopher digging activities in a California annual grassland. Nitrogen contents were determined at different soil depths and nitrogen transformations were assessed using both short-term ¹⁵N experiments and long-term field methods. Inorganic nitrogen pools and inorganic-N/organic-N ratios in the intermounds decreased significantly with depth suggesting that the upward turnover of horizons did not account for the pattern observed in mounds. Net mineralization rates determined from short-term experiments were significantly higher in the bottom of the mounds (3.54+1.37 mg N.kg-1 day-1) than in the rest of treatments. On the contrary, net nitrification rates revealed significant, higher rates on the top of the mounds (0.32+0.71 mg N.kg-1 day-1) than in their bottoms (-1.16+0.55 mg N.kg-1 day-1), or than in the intermounds (-1.95+0.33 mg N.kg-1 day-1). We suggest and discuss that most of the striking increase of the inorganic nitrogen in mounds (as a mean, 10 times more nitrate than in the intermounds) is derived from shifts and changes in the microbial processes caused by burrowing activities.

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This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.