Use of shallow samples to estimate the total carbon storage in pastoral soils

Using data from pastoral soils sampled by horizon at 56 locations across New Zealand, we conducted a meta-analysis. On average, the total depth sampled was 0.93 +/- 0.026 m (+/- SEM), and on a volumetric basis, the total C storage averaged 26.9 +/- 1.8, 13.9 +/- 0.6 and 9.2 +/- 1.4 kg C m2 for allophanic (n=12), non-allophanic (n=40) and pumice soils (n=4), respectively. We estimated the total C storage, and quantified the uncertainty, using the data for samples taken from the uppermost A-horizon whose depth averaged 0.1 +/- 0.003 m. For A-horizon samples of the allophanic soils, the mean C content was 108 +/- 6 g C kg1 and the bulk density was 772 +/- 29 kg m3, for non-allophanic soils they were 51 +/- 4 g C kg1 and 1055 +/- 29 kg m3, and for pumice soils they were 68 +/- 9 g C kg1 and 715 +/- 45 kg m3. The C densitya product of the C content and bulk densityof the A-horizon samples was proportional to their air-dried water content, a proxy measure for the mineral surface area. By linear regression with C density of the A-horizon, the total C storage could be estimated with a standard error of 3.1 kg C m2, 19% of the overall mean.