No abstract

Field-scale relationships between soil test phosphorus (STP) and flow-weighted mean concentrations (FWMCs) of dissolved reactive phosphorus (DRP) and total phosphorus (TP) in runoff are essential for modeling phosphorus losses, but are lacking. The objectives of this study were (i) to determine the relationships between soil phosphorus (STP and degree of phosphorus saturation (DPS)) and runoff phosphorus (TP and DRP) from field-sized catchments under spring snowmelt and summer rainfall conditions, and (ii) to determine whether a variety of depths and spatial representations of STP improved the prediction of phosphorus losses. Runoff was monitored from eight field-scale microwatersheds (2 to 248 ha) for 3 yr. Soil test phosphorus was determined for three layers (0 to 2.5 cm, 0 to 5 cm, and 0 to 15 cm) in spring and fall and the DPS was determined for the surface layer. Average STP (0 to 15 cm) ranged from 3 to 512 mg kg(-1), and DPS (0 to 2.5 cm) ranged from 5 to 91%. Seasonal FWMCs ranged from 0.01 to 7.4 mg L(-1) DRP and from 0.1 to 8.0 mg L(-1) TP. Strong linear relationships (r2=0.87 to 0.89) were found between the site mean STP and the FWMCs of DRP and TP. The relationships had similar extraction coefficients, intercepts, and predictive power among all three soil layers. Extraction coefficients (0.013 to 0.014) were similar to those reported for other Alberta studies, but were greater than those reported for rainfall simulation studies. The curvilinear DPS relationship showed similar predictive ability to STP. The field-scale STP relationships derived from natural conditions in this study should provide the basis for modeling phosphorus in Alberta.

The risk of P losses from agricultural land to surface and ground water generally increases as the degree of soil P saturation increases. A single‐point soil P sorption index (PSI) was validated with adsorption isotherm data for determination of the P sorption status of Alberta soils. Soil P thresholds (change points) were then examined for two agricultural soils after eight annual applications of different rates of cattle manure and for three agricultural soils after one application of different rates of cattle manure. Linear relationships were found between soil‐test P (STP) levels up to 1000 mg kg−1 and desorbed P in the five Alberta soils. Weak linear relationships were also found between STP and runoff dissolved reactive phosphorus (DRP) in three of these soils. Change points for the degree of P saturation (DPS) were detected in four of the five soils at 3 to 44% for water‐extractable P (WEP) and at 11 to 51% for CaCl2–extractable P (CaCl2–P). Change points were not found for DPS or runoff DRP. Overall DPS thresholds for the five soils combined were 27% for WEP and 44% for CaCl2–P at a critical desorbable‐P value of 1 mg L−1 The corresponding STP levels (44 mg kg−1 for WEP and 71 mg kg−1 for CaCl2–P) are similar to agronomic thresholds for crops grown on Alberta soils. Soluble P losses in overland flow and leaching may be greater in soils with DPS values that exceed these thresholds than in soils with lower DPS values.

Estimating storm erosion with a rainfall simulator. Can. J. Soil Sci. 77: 669-676. Interpreting soil loss from rainfall simulators is complicated by the uncertain relationship between simulated and natural rainstorms. Our objective was to develop and test a method for estimating soil loss from natural rainfall using a portable rainfall simulator (1 m 2 plot size). Soil loss from 12 rainstorms was measured on 144-m 2 plots with barley residue in conventional tillage (CT), reduced tillage (RT) and zero tillage (ZT) conditions. A corresponding "simulated" soil loss was calculated by matching the simulator erosivity to each storm's erosivity. High (140 mm h -1 ) and low (60 mm h -1 ) simulation intensities were examined. The best agreement between simulated and natural soil loss occurred using the low intensity, after making three adjustments. The first was to compensate for the 38% lower kinetic energy of the simulator compared with natural rain. The second was for the smaller slope length of the simulator plot. The third was to begin calculating simulator erosivity only after runoff began. After these adjustments, the simulated soil loss over all storms was 99% of the natural soil loss for CT, 112% for RT and 95% for ZT. Our results show that rainfall simulators can successfully estimate soil loss from natural rainfall events. . Les déperditions correspondantes en conditions simulées étaient calculées en appariant les valeurs d'érosivité du simulateur au pouvoir érosif réel de chaque épisode pluvial. On utilisait une forte (140 mm h -1 ) et une basse (60 mm h -1 ) intensité de précipitations simulées. La meilleure concordance entre les déperditions de sol causées entre les pluies réelles et les pluies simulées s'observait au régime d'intensité pluviale inférieure, moyennant toutefois 3 corrections, la première, pour compenser la moindre (38 %) énergie cinétique du simulateur par rapport aux précipitations naturelles; le second pour tenir compte de la pente plus courte de la parcelle sous simulateur de pluie, tandis que la troisième consistait à ne commencer à calculer l'érosivité en régime simulé qu'après le début du ruissellement. Moyennant ces trois corrections, les déperditions de sol en simulation, tous épisodes pluviaux confondus, correspondaient à 99 % des déperditions par pluie naturelle observées en régime TC, à 112 % en régime TR et à 95 % en régime CST. Il semble que les simulateurs de pluie permettent parfaitement d'estimer les déperditions de sol causées par les épisodes de pluie naturelle.