Search

Monitoring changes in soil organic carbon over time is important to many agricultural and environmental goals. Despite decades of measurements, temporal variability in soil carbon measurements has not been studied extensively. In this report, we examine five sets of monthly samples extending up to 3 years each that were collected from field experiments at four locations representative of dryland farming in the Pacific Northwest. The variance from month‐to‐month was 15%–32% of the random error, averaging 20%. This was often greater than the variance between replicate experimental units (2%–42%, averaging 17%). At certain sites, sequential samples were found to be temporally autocorrelated, but no consistent trend patterned on seasonal factors like precipitation was found. This suggests that a single point‐in‐time sample can deviate substantially from the long‐term average soil carbon at the site. We illustrate this problem with the results of repeated soil samples taken from 12 commercial farm fields. We recommend that confidence intervals for soil organic carbon estimates should include variance based on a large population of samples rather than from a single sample set at one timepoint. Core Ideas: Soil organic carbon measurements demonstrate significant variability over time.The variability does not appear to be related to season, rainfall, or temperature.Carbon quantification requires an accounting for temporal variation. [ABSTRACT FROM AUTHOR]

Soil organic carbon was measured after surface application of equal C amounts from various C sources to fallow soil or a winter wheat crop for five consecutive years. Municipal biosolid produced the largest gain, followed by manure and alfalfa (Medicago sativa L.) foliage. Wheat (Triticum aestivum L.) straw, sucrose, and wood sawdust were not different than no addition. These effects were additive to and independent from the effect of fallow soil versus cropped soil. Soil cropped to winter wheat increased in C comparable to the biosolid application or to plots planted to perennial grass. Measurements 4, 6, and 12 years after a return to normal farming practices produced no change in relative or quantitative differences. These results illuminate the durability of above‐ and below‐ground contributions to soil C. Core Ideas: Increased soil organic carbon (SOC) produced by crop roots or amendments is still present 12 years later.The effect of wheat straw and sucrose on SOC was not significant, but biosolids, manure, and alfalfa increased SOC.Five years of wheat or perennial grass produced a substantial gain in SOC compared to fallow. [ABSTRACT FROM AUTHOR]

A 2-yr winter wheat (Triticum aestivum L.)-fallow rotation continues to be the most common cropping system in much of the U.S. Pacific Northwest. The sustainability of soils in the region depends on our ability to halt or greatly reduce wind and water erosion. An incomplete understanding of how tilled summer fallow preserves seed-zone moisture for timely fall crop establishment has slowed efforts to optimize tillage techniques for creating profitable and erosion- resistant systems. This 2-yr study created a series of soil mulches at two sites representing major soils in the region. It was found that timing and depth of mulch creation had consistent effects at all four site-years. Tillage performed in mid-June to depths of 10 and 15 cm preserved up to 0.01 kg[kg.sup.-1] greater water content than no or 5-cm tillage, an amount of water that can make substantial differences in the germination of winter wheat. The later or shallower tillage treatments produced water contents similar to zero tillage below the 15-cm depth. Temperature profiles at 1-cm resolution demonstrated different shapes under different mulch treatments, which may prove useful in making quick mulch performance comparisons in the field. To optimize the timing and depth of summer fallow tillage, it will be necessary to characterize spring water storage plus the potential for end-of-summer water storage for each soil type. doi: 10.2136/sssaj2010.0046

This study compared linear depth from the soil surface vs. equivalent sample mass for interpreting soil water content and concentration data. Our first experiment sampled soils that differed only in surface bulk density. A second experiment compared different soil sampling tools. In both cases, analysis by mass instead of depth corrected water content discrepancies caused by bulk density or sample compaction differences. In a third experiment, samples collected from an on-farm test resulted in many significant differences in soil moisture between tillage practices for linear depth data that did not exist when analyzed using equivalent sample mass. When it is important to avoid confounding bulk density, depth from the surface, or sampling method with quantitative measurements, sampling by mass instead of volume is more accurate and precise than many quantitative methods currently in use, and represents an important advance in our ability to make comparative measurements across time, treatments, locations, and equipment.

Mixed solid-water-gas media such as soil can supply water to a dry object as both liquid and vapor. Modelers and agricultural engineers have commonly assumed liquid transport dominates imbibition by seed. This study tests the hypothesis that vapor is the major source of water for imbibition under normal soil moisture conditions. Water uptake was measured under varying degrees of seed-soil contact. Wheat (Triticum aestivum L.) seeds were suspended in holes in blocks of moist soil (average water potential -0.16 MPa). Hole diameter varied from 13 to 0 mm (no hole), so that the distance between the moist soil and the seed varied from a maximum of 5 mm to intimate seed-soil contact. The amount of water absorbed by seed in 24 h increased from 15 to 17 mg [seed.sup.-1] as seed-soil contact increased from none to maximum contact, indicating that at least 85% of the water absorbed by seed in intimate seed-soil contact could be attributed to vapor alone. Reevaluation of the role of vapor transport should result in improved multiphase transport models in soil and other media.

Measurement of soil bulk density and volumetric water content in small ([less than or equal to]2-cm) depth increments is tedious and time consuming. Most methods require compositing several subsamples to control measurement error, and few are feasible with loose, dry soils. We developed and tested a sampler that uses an electric linear actuator to push an intact soil core out of the sampling tube. The soil core is maintained in an upright position and protected from fracturing by remaining inside the tube until sectioned with a cutting saw. Precise length increments and flat cuts are easily obtained, even in loose soils. Compared with existing incremental sampling technology developed 30 yr ago, the electric sampler reduced sample variability by half. The electric sampler requires only 7 min to collect a 26-cm core and section it into 2-cm increments, compared with 20 min per core with the older sampler.