Biomass and root production of two plant life forms in a semiarid Mexican scrub:: responses to soil nitrogen availability

The contribution of cacti and shrubs to root biomass and fine-root production was described in a semiarid Mexican scrub. Both life forms were evaluated for fine-root production variation in relation to changes of nitrogen in the soil, with a fertilization experiment. Cacti represented 78% of the total mean root biomass (660 +/- 70 g(.)m(-2) (mean +/- SE)) in the complete soil profile (50 cm in depth). In both life forms, root biomass was higher near the surface of the soil. Roots < 3amm in diameter represented 92.5% for cactus root biomass and 69.4% for shrubs. Monthly root biomass varied significantly between months, and significant differences were obtained between plant life forms. Fine and very fine root production was estimated as 3.76 Mg(.)ha(-1.)year(-1), and cactus contribution to total root production was 81.2%. Significant differences were obtained between life forms. It was clear that a low concentration in the soil nitrogen diminishes fine-root production, supporting the hypothesis that in arid ecosystems nitrogen is a limiting factor for primary production.