Effect of temperature on below-ground N-dynamics in a weedy model ecosystem at ambient and elevated atmospheric CO2 levels

Model multispecies terrestrial communities composed of four trophic levels (plants, herbivores, parasitoids, decomposers) were established in the Ecot ron controlled environment facility. Two experimental runs enabled us to in vestigate the effects of enhanced temperature on below-ground microbial pro cesses (N-mineralisation. urease, arginine deaminase, protease activity and potential denitrification) in both ambient and elevated (ambient +200 ppm) CO2 atmospheres. The enzyme activities involved in nitrogen cycling showed weak responses to elevated temperature in both experimental runs. In the Ambient CO2 Run, pr otease and arginine deaminase values tended to be lower in elevated tempera ture; on the other hand, N-mineralisation, urease and denitrification enzym e activity (DEA) were higher. In the Elevated CO2 Run, all microbial variab les showed higher activities at elevated temperature, although only the res ults for DEA and arginine deaminase were statistically significant. The int eraction between higher temperature and elevated CO2 weakly affected root g rowth and tissue C:N ratio, limiting feedbacks into the microbial community . Besides temperature and CO2, substrate availability, water stress and succe ssional development regulated the response of the soil microbes. The supply of organic carbon and nitrogen in the soil allowed plant growth and mainte nance of the microbial population. Nitrogen competition between vegetation and microbes restricted net microbial growth. The increase of dissolved org anic carbon (DOC) at higher CO2, and temperature levels significantly favou red DEA. The high water regime in the soil also favoured DEA and inhibited oxidation of organic compounds. as indicated by low levels of enzyme activi ty. Additionally, water stress decreased rooting density in the soil; this resulted in negative feedback into microbial processes. We conclude that wa ter stress and soil nitrogen deficiency caused an early levelling-off of bo th microbial population growth and activity rates during the early part of the model ecosystem's development. (C) 2001 Elsevier Science Ltd. All right s reserved.