Data from: Temporal variability is a major source of uncertainty in soil carbon measurements

This dataset contains five sets of monthly samples extending up to three years each that were collected from field experiments at four locations representative of dryland farming on silt-loam soils in Oregon and Washington, USA. 

Two experiments conducted at the Columbia Plateau Conservation Research Center near Adams, OR, USA (45.767°, -118.563°, 401 mm yr‑1 precipitation) involved monthly sample collection for three years. The first experiment (“Adams-tillage”) examined annual winter wheat and included three tillage treatments: 1) no-till, 2) surface residue incorporated with tillage, and 3) surface residue removed and then replaced after tillage (Wuest, 2014). The soil was a Walla Walla silt loam (coarse-silty, mixed, superactive, mesic Typic Haploxeroll) on a level, uniform field. Plots were 3.6 by 53 m replicated in four randomized, complete blocks, for a total of 12 experimental units. Each month from 2001 to 2005 three 50-mm diameter soil cores from each plot were combined, one from the crop row, one from the center between crop rows, the third between the crop row and the center sample. Depths were six 5-cm increments to 30-cm depth. Each month’s sample was 30 cm further down a crop row from the previous month’s sample. Samples were dried at 40° C starting within an hour of collection. Soil organic carbon was measured with dry combustion analysis. Surface soil pH averaged 5.3 and lab tests indicate that carbonates were not present in the surface soil. Since the soil samples in this study were taken and chemically analyzed by length increments from the soil surface, the analytical values based on equivalent dry soil mass were calculated by interpolating between the cumulative dry mass of each incremental sample. Weather data were obtained from a station located 400 m from the plots. 

The second experiment near Adams (“Adams-residue”) examined residue management in annual winter wheat from May 2016 to Aug 2019, with three treatments: 1) no wheat residue removed, 2) all chaff and straw removed, and 3) removal of chaff only. Plots were 6 by 60 m, with four replicates in randomized complete blocks for a total of 12 plots. The field was level with a slight SOC gradient from block to block. The soil type was the same as the above Adams-tillage site. Each plot was sampled 29 times, monthly except for a few winter months due to either snow-covered or wet conditions. Three 25-mm diameter intact cores were taken from the center of each plot within 30 cm of the previous month’s sample, wrapped in paper to maintain the core intact, dried, and the top 250 kg m‑2 of soil from each core removed for cleaning (removing visible plant matter) and analysis. After weighing the mass-depths, samples from the three cores were pooled for dry combustion analysis. This site was 300 m from the weather station. 

A third experiment involved monthly samples in three sets of long-term plots near the townsites of Echo, OR, Moro, OR, and Ritzville, WA (Wuest, Schillinger, et al., 2023). These were two- or three-year-rotation studies including a fallow year. 

At the Echo site (45.7303, -119.0569, 265 mm yr-1 precipitation) the soil was Ritzville silt loam (coarse-silty, mixed, superactive, mesic Calcidic Haploxerolls) with 5.7 pH and containing <15% sand, ≥75% silt and 5 to 10% clay (Soil-Survey-Staff, 2022). Treatments were a comparison of no-till versus tilled winter wheat—fallow rotations. The two tillage treatments and two rotation entry points were randomized in four complete blocks for a total of 16 plots. Plot size was 7 m by 55 m. A weather station was located within 200 m of the plots. 

At Moro (45.4833, -120.7184, 269 mm yr⁻1 precipitation) the soil was Walla Walla silt loam (coarse, silty, mixed, superactive, mesic Typic Haploxeroll) with 5.7 pH. Soil texture was 14% fine sand, 72% silt, and 14% clay (Soil-Survey-Staff, 2022). Treatments examined winter wheat--fallow comparing no-till and minimum till, with both rotation phases represented each year. These four treatment plots were replicated in each of three randomized complete blocks for a total of 12 plots sampled monthly. Two of the blocks were on a backslope and the third on a less productive shoulder slope. The weather station was within 400 m of the plots. 

At Ritzville (47.1437, -118.4693, 296 mm yr-1 precipitation) the soil was Ritzville silt loam (coarse-silty, mixed, superactive, mesic Calcidic Haploxerolls) >2 m deep with no rocks or restrictive layers and slopes were <1%. Soil texture is 30% fine sand, 59% silt, and 11% clay (Soil-Survey-Staff, 2022). Soil pH in the surface was 6.5. Treatments compared two three-year rotations: spring wheat, fallow, and winter triticale using no-till, versus spring wheat, fallow, and winter wheat using conservation tillage. Experimental design was a randomized complete block with four replications. Individual plot size was 9 by 150 m. All rotation phases were present every year (total = 24 plots). Weather data were taken from a station near the city of Ritzville, 7 km from the plots. 

The Echo site was terminated after 18 samples in 19 months. The Moro and Ritzville sites were sampled 29 and 31 times over 33 months, missing some winter months due to snow-cover or excessive wetness. At all three sites a single 25-mm core was taken from each plot within a few meters of the same location each month. The intact core was wrapped in paper, dried, and the top 250 kg m‑2 (approximately 20 cm) removed for analysis. This sample depth is deeper than the influence of current tillage and has proven to contain the major changes in SOC in these cropping systems. As above, dry combustion analysis was used to determine SOC. 

At all four research sites lab measurements have demonstrated no significant inorganic carbon in the surface 30 cm, so total C was considered equal to SOC. Samples were processed monthly by the same lab using the same standards and procedures throughout the experiment. Our objective was to examine and characterize SOC temporal variability measured at five sites where high-frequency soil sampling campaigns were carried out in a semiarid dryland agriculture environment.

References

Soil-Survey-Staff. 2022. Natural Resources Conservation Service, United States Department of Agriculture. Official Soil Series Descriptions. Accessed 3/14/2023. Available online.

Wuest, S. 2014. Seasonal variation in soil organic carbon. Soil Sci. Soc. Am. J. 78: 1442-1447. doi:10.2136/sssaj2013.10.0447.

Wuest, S.B., W.F. Schillinger and S. Machado. 2023. Variation in soil organic carbon over time in no-till versus minimum tillage dryland wheat-fallow. Soil Tillage Res. 229: 105677. doi:https://doi.org/10.1016/j.still.2023.105677.