Soil Processes Affected by Sixteen Grassland Species Grown under Different Environmental Conditions

Plant species, and their interactions with the environment, determine both the quantity and chemistry of organic matter inputs to soils. Indeed, countless studies have linked the quality of organic matter inputs to litter decomposition rates. However, few studies have examined how variation in the quantity and chemistry of plant inputs, caused by either interspecific differences or changing environmental conditions, influences the dynamics of soil organic matter. We studied the effects of 16 grassland species from 4 functional groups (C3 and C4 grasses, forbs, and legumes) growing under ambient and elevated CO2 (560 ppm) and N inputs (4 g m yr) on soil carbon (C) and nitrogen (N) dynamics after 4 yr in a grassland monoculture experiment in Minnesota, USA. Specifically, we related soil C and N dynamics to variation among species and their responses to the CO2 and N treatments in plant biomass and chemistry of roots, the dominant detrital input in the system. The 16 species caused much larger variation in plant litter inputs and chemistry, as well as soil C and N dynamics, than the CO2 and N treatment. Not surprising, variation in the quantity of plant inputs to soils contributed to up to a two-fold variation in microbial biomass and amount of respired nonlabile soil C. Root N concentration (across species and CO2 and N treatments) was significantly negatively related to decomposition of nonlabile soil C and positively related to net Nmineralization. Greater labile C inputs decreased rates of net N mineralization, likely because of greater N immobilization. Thus, of the traits examined, plant productivity, tissue N concentration, and labile C production such as from rhizodeposition were most important in causing variation in soil C and N dynamics among species and in response to altered atmospheric CO2 and N supply. INCREASEDATMOSPHERIC CO2 concentration andN deposition are potentially altering the structure and functioning of many terrestrial ecosystems (Vitousek, 1994). These global change factors influence the amount and chemistry of primary productivity as well as cause shifts in the relative abundances of species and functional groups that themselves differ from one another in their productivity and chemical composition (Reich et al., 2001a). The amount (i.e., dry mass) and chemistry of plant litter inputs have long been known to affect fresh litter decomposition and nutrient release (e.g., Hobbie, 1992; Mack and D’Antonio, 2003; Melillo et al., 1982). Differences among and changes in plant communities have been shown to affect soil organic matter pools and dynamics through interspecific differences in litter quantity and chemistry (Eviner and Chapin, 2004; Finzi et al., 1998; Knicker et al., 2000), but also through interspecific differences in rhizodeposition of labile C compounds (Cheng et al., 2003; Fu and Cheng, 2002; Reid and Goss, 1982). Similarly, interspecific differences in litter quantity and chemistry and labile C production through rhizodeposition play an important role in soil N dynamics (Eviner and Chapin, 2004; Wedin and Tilman, 1990), but their relative importance remains unclear. It is also unclear how important species-specific changes in litter quantity, chemistry and labile C production caused by elevated atmospheric CO2 and N supply are on soil organic matter and N dynamics, compared with plant species or community effects. Although elevated atmospheric CO2 andN supply can alter litter quantity, chemistry, and labile C production (e.g., Cheng and Johnson, 1998;Hobbie andVitousek, 2000), their effect on soil organic matter and N dynamics may be limited compared with plant community effects (Aerts et al., 2003; Finzi and Schlesinger, 2002). Nonetheless, litter quantity and chemistry (including both nutrient and C chemistry) and labile C production may serve to integrate influences of species trait differences (both within and among functional groups) as well as trait responses to atmospheric CO2 and N supply on soil organic matter dynamics. The aim of this study was to assess how soil microbial biomass, soil organic matter, and N dynamics are affected by 16 grassland species grown under ambient and elevated atmospheric CO2 (560 ppm) with 0 or 4 g m yr of N fertilizer. We also examined the degree to which the quantity and chemistry of litter inputs and labile C production influence soil C and N dynamics under the different treatments. In a companion paper, we have reported the main effects of CO2, N, plant species richness, and their interactions on microbial biomass and activity (Dijkstra et al., 2005). Whereas the previous paper compared the monoculture plots to more speciesrich plots to determine how increasing species richness (along with elevated CO2 and N) affects soil processes, here we focus on comparisons among the different monocultures themselves (i.e., among different species) to determine the relationship between plant traits and aspects of soil C and N dynamics such as C and N mineralization and microbial C and N, under ambient and elevated CO2, with and without N fertilization. MATERIALS AND METHODS