Effect of elevated tropospheric ozone on soil carbon and nitrogen: a meta-analysis

Elevated tropospheric ozone concentration ([O-3]) may substantially influence the belowground processes of the terrestrial ecosystem. Nevertheless, a comprehensive and quantitative understanding of the responses of soil C and N dynamics to elevated [O-3] remains elusive. In this study, the results of 41 peer-reviewed studies were synthesized using meta-analytic techniques, to quantify the impact of O-3 on ten variables associated with soil C and N, i.e. total C (TC, including soil organic C), total N (TN), dissolved organic C (DOC), ammonia N (NH4 (+)), nitrate N (NO3 (-)), microbial biomass C (MBC) and N (MBN), rates of nitrification (NTF) and denitrification (DNF), as well as C/N ratio. The results depicted that all these variables showed significant changes (P < 0.05) with [O-3] increased by 27.6 +/- 18.7 nl l(-1) (mean +/- SD), including decreases in TC, DOC, TN, NH4 (+), MBC, MBN and NTF, and increases in C/N, NO3 (-) and DNF. The effect sizes of TN, NTF, and DNF were significantly correlated with O-3 fumigation levels and experimental duration (P < 0.05). Soil pH and climate were essential in analyses of O-3 impacts on soil C and N. However, the responses of most variables to elevated [O-3] were generally independent of the O-3 fumigation method, terrestrial ecosystem type, and additional [CO2] exposure. The altered soil C and N dynamics under elevated [O-3] may reduce its C sink capacity, and change soil N availability and thus, impact plant growth and enhance soil N losses.