Effects of [CO2] and nitrogen fertilization on soils planted with ponderosa pine

The effects of six years treatment with elevated [CO2] (350, 525, and 700 mu l l(-1)) and nitrogen (N) (0, 10, and 20 g N m(-2) yr(-1)) on soils, soil solution, and CO2 efflux in an open-top chamber study with ponderosa pine (Pinus ponderosa Laws.) are described. The clearest [CO2] effect was in year 6, when a pattern of lower soil N concentration and higher C/N ratio with elevated [CO2] emerged. Statistically significant effects of elevated [CO2] on soil total C, extractable P, exchangeable Mg2+, exchangeable Ca2+, base saturation, and soil solution HCO3- and NO3- were also found in various treatment combinations and at various times; however, these effects were inconsistent among treatments and years, and in many cases (P, Mg2+, Ca2+, base saturation) reflected pre-treatment differences. The use of homogenized buried soil bags did not improve the power to detect changes in soil C and N or help resolve the inconsistencies in soil C patterns. Nitrogen fertilization had the expected negative effects on exchangeable Ca2+, K+, and Mg2+ in year 6, presumably because of increased NO3- leaching, but had no consistent effect on soil C, N, or extractable P.