PLANT-PRODUCTION AND SOIL-MICROORGANISMS IN LATE-SUCCESSIONAL ECOSYSTEMS - A CONTINENTAL-SCALE STUDY

Annual C inputs from plant production in terrestrial ecosystems only m eet the maintenance energy requirements of soil microorganisms, allowi ng for little or no net annual increase in their biomass. Because micr obial growth within soil is limited by C availability, we reasoned tha t plant production should, in part, control the biomass of soil microo rganisms. We also reasoned that soil texture should further modify the influence of plant production on soil C availability because fine-tex tured soils typically support more microbial biomass than coarse-textu red soils. To test these ideas, we quantified the relationship between aboveground net primary production (ANPP) and soil microbial biomass in late-successional ecosystems distributed along a continent-wide gra dient in North America. We also measured labile pools of C and N withi n the soil because they represent potential substrate for microbial ac tivity. Ecosystems ranged from a Douglas-fir forest in the western Uni ted States to the grasslands of the mid-continent to the hardwood fore sts in the eastern U,S. Estimates of ANPP obtained from the literature ranged from 82 to 1460 g.m(-2).yr(-1). Microbial biomass C and N were estimated by the fumigation-incubation technique. Labile soil pools o f C and N and first-order rate constants for microbial respiration and net N mineralization were estimated using a long-term (32 wk) laborat ory incubation. Regression analyses were used to relate ANPP and soil texture with microbial biomass and labile soil C and N pools. Microbia l biomass carbon ranged from 2 g/m(2) in the desert grassland to 134 g /m(2) in the tallgrass prairie; microbial N displayed a similar trend among ecosystems. Labile C pools, derived from a first-order rate equa tion, ranged from 115 g/m2 in the desert grassland to 491 g/m2 in the southern hardwood forest. First-order rate constants for microbial res piration (k) fell within a narrow range of values (0.180 to 0.357 wk(- 1)), suggesting that labile C pools were chemically similar among this diverse set of ecosystems. Potential net N mineralization rates over the 32-wk incubation were linear in most ecosystems with first-order r esponses only in the alpine tundra, tallgrass prairie, and forests. Mi crobial biomass C displayed a positive,linear relationship with ANPP ( r(2) = 0.51), but was not significantly related to soil texture. Labil e C also was linearly related to ANPP (r(2) = 0.32) and to soil textur e (r(2) = 0.33), Results indicate that microbial biomass and labile or ganic matter pools change predictably across broad gradients of ANPP, supporting the idea that microbial growth in soil is constrained by C availability,