A simple method for determining limiting nutrients for photosynthetic crusts

Abstract

Background: Photosynthetic crust communities are important to the functioning of many desert and early successional ecosystems. Little is known about the factors that limit the growth of these communities, especially during early stages of primary succession or following disturbance. Aims: Our main goal was to develop a method to study nutrient limitations of crust growth in laboratory microcosms. We used the new method to test the hypothesis that phosphorus limits the growth of crusts in newly deglaciated soils of the high Andes. Methods: We modified the point–intercept method used in plant ecology to quantify the spread of cyanobacteria, algae and mosses on the soil surface in response to additions of nitrogen and phosphorus. Results: Fertilization with phosphorus significantly increased the growth rate and final percentage cover, and decreased the lag time for growth of cyanobacterial and algal communities in recently deglaciated soils. By contrast, nitrogen additions had no significant effect on the growth of microbial phototrophs, and all nutrient additions suppressed the growth of early successional mosses. Conclusions: We propose that the method described here offers a valuable tool for assessing the nature of nutrient limitation of photosynthetic organisms in early successional and desert ecosystems. The information provided by using this approach can increase our understanding of the earliest stages of ecosystem development and may help inform strategies for the reclamation of disturbed arid ecosystems by identifying potential limiting nutrients.