Spatial and temporal effects on plant-microbial competition for inorganic nitrogen in a california annual grassland

Abstract

The changes in N-dynamics which occur after the start of autumn rains, following an extended summer drought, were examined in a California annual grassland. Competition for NH + 4and NO − 3 between plants and microbes, and the role of spatial compartmentalization. were studied using short-term ( < 24 h) experiments using 15N pool dilution and tracer techniques. Temporal dynamics of mineralization, nitrification and simultaneous plant and microbial uptake of NH 4 + and NO − 3 were assessed in intact soil microcosms periodically watered to simulate autumn rains. During the first week after initial soil wetting, both mineralization and immobilization rates increased; mineralization rate continued to increase during the next 6 weeks but immobilization rate remained constant. After 6 weeks of simulated wet-season, microbes consumed more of both NH + 4 and no − 3 than did plants in 8 h diurnal 15N tracer experiments. Though ambient NO − 3 concentrations were low, nitrification accounted for about 1 3 of the N mineralized. Plants competed better for NO − 3 than for NH 4 +. This suggests that nitrification benefitted plants by increasing accessible N. Spatial factors were critical in controlling N-dynamics. Microsites free of NH + 4 were presumably responsible for the extensive microbial NO 3 − uptake. Of the activities measured in the top 9cm of the soil, the surface 5mm accounted for almost half of mineralization and plant NH 4 +-uptake, but only 11% of microbial NH 4 +-uptake.