Soil water content determined at saturation and water holding capacity and calculated at 50% water filled pore space
Data files
Aug 28, 2020 version files 61.19 KB
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AEL-2020-07-0041-data-v2.xlsx
61.19 KB
Abstract
Optimizing soil microbial activity requires an equal balance between water- and air-filled porosity, i.e. 50% water filled pore space (WFPS). However, many soil biological investigations report water as some fraction of water-holding capacity (WHC). This study was conducted to fill a quantitative gap between WFPS and WHC. Soil samples (n=198) from 10 eastern U.S. states and one state in Brazil provided a wide distribution of clay (0.064-0.487 kg kg-1) and soil organic C (SOC, 5.2-52.0 g kg-1) concentrations (5-95% range). Gravimetric soil water content (SWC) was determined at WHC and at saturation. Both clay and SOC concentrations strongly influenced SWC; the effect of SOC was strongest and non-linear. To achieve 50% WFPS, gravimetric SWC was 0.69+0.10 times that of WHC and 0.59+0.03 times that of saturation. For soil biological assays, 50% WFPS could be reasonably accurately and simply achieved with calculations using gravimetric SWC at saturation multiplied by 0.59.
| Total C and N were determined from soil subsamples on a Leco TruMac CN analyzer and assumed as organic since pH was acidic for all samples (5.6-6.6). |
| Sand and clay concentrations were predicted from near infrared spectroscopy (Model 5000 with WinISI version 1.5 software, Foss North America, Inc., Eden Prairie, MN) of pulverized soil (ball milled for 2 min). |
| Calibration was from 278 samples that were selected using ‘H’ statistic of 0.6. |
| Standard error of calibration was 0.156 and 0.060 kg kg-1 for sand and clay, respectively. |
| The middle 90% of calibration observations was 0.160 to 0.744 kg kg-1 for sand and 0.078 to 0.480 kg kg-1 for clay. |
| Statistical distribution of samples in this study at 5, 50, and 95 percentile limits was 0.064, 0.259, and 0.487 kg kg-1, respectively, for clay and 5.2, 19.7, and 52.0 g kg-1, respectively, for SOC. |
| Soil passing a screen with 4.75-mm openings was heaped into a cylindrical metal container with total volume of 80 mL (54 mm diameter, 35 mm height). |
| Containers had five 1-mm diameter holes punched into the bottom. |
| The container was tapped vigorously 10 times to allow soil to settle, then a metal blade was used to shear off excess soil to exactly 80 mL volume. |
| Containers of soil were dried in a forced-air oven at 55 °C for 9 h before immersing into a pan of deionized water ~10 mm deep. |
| Water was allowed to wick up into soil for 14 h, at which time containers were wiped of outside water and weighed immediately to calculate SWC at saturation. |
| Containers were placed onto a paper towel, lid placed loosely on top to avoid evaporation, and allowed to drain freely for 9 h, at which time containers were weighed to calculate SWC at WHC. |
| Containers of soil were dried in an oven at 55 °C for 3 d and then further dried at 105 °C for 16 h and mass recorded. |
| Density of sieved soil in the metal container was calculated as mass (variable among samples) per volume (80 mL fixed). |
| Total porosity was derived from density, assuming particle density of 2.65 Mg m-3 (Total porosity = 1 – Density / 2.65). |
| All other calculations were derivations from soil mass at 105 °C, gravimetric SWC at saturation and at WHC, and density of soil. |