Elevated [CO2] effects on herbage production and soil carbon and nitrogen pools and mineralization in a species-rich, grazed pasture on a seasonally dry sand

Rising concentrations of atmospheric [CO2] in a multi-species ecosystem can influence species composition and increase plant productivity, but have a less predictable effect on soil C storage and nutrient availability. Using a free-air [CO2]-enriched (FACE) system and seasonal sampling over a 5-year period, we examined the influence of elevated atmospheric [CO2] (475 muL L-1) on soil C and N pools and mineralization in a fertilized (P, K, S), sheep-grazed pasture of mixed grass, clover, and forb species on a seasonally dry sand (Mollic Psammaquent). Annual yields of herbage dry matter ranged from about 300 to 1600 g m(-2). Total yields did not increase significantly under elevated [CO2], but the proportions of clovers and forbs increased markedly. Most properties in 0-50 mm-depth soil differed significantly (P < 0.05) with year of sampling, but [CO2]-treatment effects were non-significant (P > 0.10) for moisture, pH, total C and N, extractable C and organic N, microbial C, and mineral-N. However, microbial N, CO2-C production (0-14 days) in field-moist soil, and net mineral-N production (14-56 days) in soil at 60% of water-holding capacity were significantly higher (per unit weight of soil) in the elevated-[CO2] treatment (P = 0.071, 0.063, 0.003, respectively); the degree of these treatment differences was roughly similar when values were also expressed on a total C or N basis. Relationships with soil moisture were mainly non-significant for microbial C and N, but mainly significant (P < 0.05) for net mineral-N production in field-moist soil, and highly significant (P < 0.001) for CO2-C production. Overall, the data tend to suggest greater soil metabolic activity, but little if any change in soil C pools, after 5 years' exposure of the pasture to elevated [CO2]. They do, however, suggest increased availability of N, probably because of increased inputs from N-fixing clovers.