Carbon and nitrogen pools and mineralization in a grassland organic soil at a New Zealand carbon dioxide spring

Recent evidence from naturally occurring CO2 springs suggests that soil org anic C levels can increase under prolonged exposure to elevated atmospheric CO2. Here we describe C and N pools and metabolism in a strongly acid orga nic soil under different concentrations of atmospheric CO2 (averaging 543-7 29 mul l(-1) ca. 20 cm above ground level at four sampling locations and 12 20-3900 mul l(-1) at 10 others) in an unfertilized, ungrazed grassland at H akanoa Springs in Northland, New Zealand. In 0-5 cm depth soil, organic C ( mean 274 g kg(-1)) was similar in these two sets of samples, whereas total N was slightly higher (P > 0.05 < 0.10) in the lower CO2 grouping. Extracta ble and microbial C and N, CO2-C and net mineral-N production, and their ra tios with organic C or total N were also similar in the two sets of samples . These ratios were, however, all significantly higher than those in an adj acent gley soil under elevated CO2 at this site. Although organic C concent rations in this organic soil are unrelated to current concentrations of atm ospheric CO2, the soil organic matter does appear to be enriched in labile C and N components. (C) 2001 Elsevier Science Ltd. All rights reserved.