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

Abstract

Summary The growth and chemical composition of most plants are influenced by elevated CO 2 , 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 CO 2 spring (Hakanoa) in Northland, New Zealand, at which the surrounding vegetation has been exposed to elevated CO 2 for at least several decades. The mean annual temperature at this site is ≈ 15.5 °C and rainfall ≈ 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 CO 2 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 °C, average rates of CO 2 ‐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 CO 2 concentrations, but not with soil moisture (except for CO 2 ‐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 CO 2 , and may include an appreciable labile fraction in mineral soil with an adequate nutrient supply.