Abstract
The number of spots to monitor to evaluate soil respiration (Rs) is often chosen on an empirical or conventional basis. To obtain an insight into the necessary number of spots to account for Rs variability in a Mediterranean pine-dominated mixed forest, we measured Rs all year long on sixteen dates with a portable gas-analyser in 50 spots per date within an area 1/3 ha wide. Linear mixed-effects models with soil temperature and litter moisture as descriptors, were fitted to the collected data and then evaluated in a Monte Carlo simulation on a progressively decreasing number of spots to identify the minimum number required to estimate Rs with a given confidence interval. We found that monitoring less than 14 spots would have resulted in a 10% probability of not fitting the model, while monitoring 20 spots would have reduced the same probability to about 5% and was the best compromise between field efforts and quality of the results. A simple rainfall index functional to select sampling dates during the summer drought is proposed.
Highlights
The number of spots to monitor to evaluate soil respiration (Rs) is often chosen on an empirical or conventional basis
There is an extensive number of studies which have measured Rs in many ecosystem types; very few of them were concerned about how many spots should be measured in that specific environment to properly account for the intrinsic spatial variability of soil
Soil respiration was highly variable during the year and between the spots (Supplementary Tab. 1), ranging from a minimum of 0.01 g CO2 m−2 h−1 – measured in May, July, October, and August, when the highest recorded values were instead 1.15. 1.05. 0.57 and 0.48 g CO2 m−2 h−1, respectively – to a maximum of 3.58 g CO2 m−2 h−1 in April, when the lowest recorded value was 0.31 g CO2 m−2 h−1 (Supplementary Tab. 2)
Summary
The number of spots to monitor to evaluate soil respiration (Rs) is often chosen on an empirical or conventional basis. There is an extensive number of studies which have measured Rs in many ecosystem types (see the updated global dataset by Bond-Lambert and Thomson10); very few of them were concerned about how many spots should be measured in that specific environment to properly account for the intrinsic spatial variability of soil One of these is the experiment set up by Saiz et al.[11], who in a first rotation Sitka spruce chronosequence composed of four age classes in Ireland, first assessed that coefficients of variation in Rs varied largely during the year – being lowest during periods with highest Rs – determined that on average the sampling strategy of 30 sampling spots per stand (of unspecified area) was adequate to obtain a Rs within 20% of its actual value with p = 0.05. Measuring Rs on a regular grid covering an area of 2400 m2 in a mature plantation of Cryptomeria japonica in Japan, Lee and Koizumi[12] assessed that the spots required to produce a sample mean within ±10%
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