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 organic C levels can increase under prolonged exposure to elevated atmospheric CO2. Here we describe C and N pools and metabolism in a strongly acid organic soil under different concentrations of atmospheric CO2 (averaging 543–729 μl l−1 ca. 20 cm above ground level at four sampling locations and 1220–3900 μl l−1 at 10 others) in an unfertilized, ungrazed grassland at Hakanoa 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. Extractable and microbial C and N, CO2-C and net mineral-N production, and their ratios 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 adjacent gley soil under elevated CO2 at this site. Although organic C concentrations in this organic soil are unrelated to current concentrations of atmospheric CO2, the soil organic matter does appear to be enriched in labile C and N components.