[Heterogeneity and optimal sampe size of soil physicochemical properties in subtropical forest.]

Selecting the optimal sample size accurately is important for improving the measurement precision of soil physicochemical properties. In this study, we examined the within-site heterogeneity of soil physical and chemical properties in an evergreen broad-leaved forest and a Chinese fir (Cunninghamia lanceolata) plantation in subtropical China. We applied both traditional and bootstrapping technique to determine the optimal sample size, the number of observation (n), for predicting soil nutrient content with an allowable error of 10% at a 95% confidence level for both forest types. The results showed that soil pH and C/N had low variation, while the soil nutrients had a moderate variation in both forests. The coefficients of variation (CV) of total element concentration in the evergreen broad-leaved forest, such as total carbon and total nitrogen, were significantly larger than those in the Chinese fir plantation, while the CVs of available nutrients were similar between two forests. The evaluation error of all soil properties in two forests showed a stable trend after a sharp decline with the increase of sample size. The mean values also changed from violent fluctuation to stable, and extravagant sample could not effectively improve the accuracy of mean value measurement. The sample sizes of soil water content, total carbon, total nitrogen, nitrate (NO₃^--N), total potassium, total calcium, and total magnesium in the evergreen broad-leaved forest were signifi-cantly larger than those in the Chinese fir plantation, while the sample size for ammonium (NH₄⁺-N) showed opposite pattern, and the others had no significant difference. In addition, the sample sizes of different soil physicochemical properties varied significantly in the same forest. The available elements, such as NH₄⁺-N and NO₃^--N, needed a larger sample size than total elements in both forests. Accordingly, we proposed that the heterogeneity and sample size of soil physicochemical properties should be considered before soil survey. An adequate sample size may be varied according to the purpose of sample and the site-specific variation of soil characters.