Carbon and nitrogen pools and mineralization in a grassland gley soil under elevated carbon dioxide at a natural CO2 spring

The growth and chemical composition of most plants are influenced by elevated CO2, but accompanying effects on soil organic matter pools and mineralization are less clearly defined, partly because of the short-term nature of most studies. Herein we describe soil properties from a naturally occurring cold CO2 spring (Hakanoa) in Northland, New Zealand, at which the surrounding vegetation has been exposed to elevated CO2 for at least several decades. The mean annual temperature at this site is approximate to 15.5 degrees C and rainfall approximate to 1550 mm. The site was unfertilized and ungrazed, with a vegetation of mainly C3 and C4 grasses, and had moderate levels of 'available' P. Two soils were present - a gley soil and an organic soil - but only the gley soil is examined here. Average atmospheric CO2 concentrations at 17 sampling locations in the gley soil area ranged from 372 to 670 ppmv. In samples at 0-5 cm depth, pH averaged 5.4; average values for organic C were 150 g, total N 11 g, microbial C 3.50 g, and microbial N 0.65 g kg(-1), respectively. Under standardized moisture conditions at 25 degrees C, average rates of CO2-C production (7-14 days) were 5.4 mg kg(-1) h(-1) and of net mineral-N production (14 -42 days) 0.40 mg kg(-1) h(-1). These properties were all correlated positively and significantly (P < 0.10) with atmospheric CO2 concentrations, but not with soil moisture (except for CO2-C production) or with clay content; they were, however, correlated negatively and mainly significantly with soil pH. In spite of uncertainties associated with the uncontrolled environment of naturally occurring springs, we conclude that storage of C and N can increase under prolonged exposure to elevated CO2, and may include an appreciable labile fraction in mineral soil with an adequate nutrient supply.