Triplicate soil samples were taken from each of the eight sampling locations (n = 24 samples) for further characterization. Samples were collected from the surficial 5 cm of soil in July 2013 and chilled (4°C) until they were subsampled for analyses. Three subsamples per sample (n = 72) were dried and analyzed for total carbon and nitrogen content using a vario MAX cube CN analyzer (Elementar Americas, Mt. Laurel, USA) with helium as a carrier gas. Additional subsamples (n = 72) were dried, lightly ground, and analyzed for the size distribution of aggregates and fine particles via laser diffraction (LS-13-320, Beckman Coulter, Indianapolis, USA). Aggregate stability was analyzed using a wet sieving apparatus (Eijkelkamp Soil & Water, Gelderland, The Netherlands) containing four sieves (2.0-, 1.0-, 0.25-, and 0.053-mm openings). Samples were measured in duplicate (n = 16 per sieve size), although one sample with anomalously high passage was excluded from the statistical analysis (n = 63 in total). An additional soil block (2000 to 2500 cm3) was sampled from each plot for measurements of total porosity. Blocks were scanned using multistripe laser triangulation (40) to determine volumes (n = 8). Dry masses were determined in triplicate subsamples taken from each block and averaged.

The frequency of crack formation in soils experiencing control versus supplemental precipitation was determined in two steps. First, the relationship between crack development in the soil surface as a function of decreasing θv was characterized. This was done using three soil cores with known surface areas (50 cm2) when saturated; cores were from the Clime-Sogn complex but not from the ITE. Color images and core mass measurements were taken at 7 to 11 time points while cores were dried under ambient laboratory conditions over the course of several weeks. Core mass was used to calculate θg, which was translated to θv using final measurements of core bulk density. The relationship between mean crack width and θv was then characterized using smoothing splines fitted to bootstrapped samples of the data (n = 1000 iterations; smoothing parameter = 0.7) (fig. S3). Second, the resulting functions were applied to mean time series of θv measured via time domain reflectometry (TDR) at 30-min intervals during the 2008–2015 growing seasons. TDR data were collected at three locations in each combination of water regime and landscape position (n = 12), with probes extending vertically through the 15 cm of soil nearest the surface.

CROSS (cation ratio of structural stability) was used to assess the potential for the groundwater used in irrigation to alter soil structure by dispersing or flocculating clays (28). CROSS values for groundwater were compared to those for rainwater and published relationships between CROSS and clay dispersion (28, 29). CROSS was calculated asCROSS=[Na]+0.56[K][Ca]+0.60[Mg]2(2)where concentrations are in mmolc liter−1. The dataset on groundwater chemistry included measurements spanning 1991–2014 from a set of wells approximately 3 km from the source of irrigation water (41). The dataset on rainwater chemistry came from a National Atmospheric Deposition Program station (NTN site KS31) approximately 0.25 km from the experimental site; data for the same time span were extracted. In both cases, the mean and SD of all measurements taken within a given year (n = 14 to 107 per year for groundwater and n = 20 to 54 per year for rainwater) were determined after averaging replicate measurements, if present. The dispersive capacity of cations adsorbed to soils at the study site was assessed using the EDPEDP=[Na]+0.556[K]+0.037[Mg]CEC×100(3)where CEC is the cation exchange capacity and concentrations are in cmolc kg−1 (29). Cations were measured from ammonium acetate extracts of each soil sample (n = 24) and analyzed using an iCAP-7000 ICP-OES system (Thermo Fisher, Waltham, USA) with a liquid argon carrier. The CEC value used was the mean of those reported for the Clime and Sogn series by the National Resource Conservation Service (33.6 cmolc kg−1). This value was similar to the sum of [Na], [K], [Ca], and [Mg] in our samples (31.9 cmolc kg−1), confirming that methodological differences did not affect the comparability of measurements.

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