Effects of Freeze-thaw Cycles and Organic Matter on Sediment and Phosphorus Losses from Silt Loam Soils

Abstract

In cold regions, snow cover, freeze-thaw (FT) cycling, and land management practices affect sediment and sediment-bound nutrient losses and contribute to water quality degradation (Wang et al., 2012). FT- cycles can expand (frost heave) and shrink soils causing damage to soil aggregates. Soil that has undergone FT-cycles may be more susceptible to erosion, particularly when rain occurs during and after thaw. In addition, soil organic matter can also affect sediment losses because soil aggregate structure depends on organic matter. Combined, soil FT action and lower organic matter may intensify soil erosion. This study aims to understand the effects of soil organic matter and FT action on rainfall erosion from thawed silt loam soil through laboratory experiments. Treatments included two silt loam soils with different organic matter and two temperature conditions replicated in triplicate. Soil was collected in southern Wisconsin from fields with similar management practices at Arlington (AR) and Lancaster (LC) Agricultural Research Stations, and they have 1.9% and 1.2% organic matter (OM), respectively. Temperature treatments were a control (CT) and FT treatment. <fig><graphic xlink:href=23068_files/23068-00.jpg id=E1DC277D-B7E0-4928-8C63-0AE59AC279C9></graphic></fig> The freezing temperature (-3±0.5°C), duration of FT (48 hr freezing, 48 hr thawing) and number of FT cycles (11) for the FT treatment were decided by analyzing December to March soil temperatures (2016-2019) from experimental plots in south central Wisconsin using an R-programming package (FTC-Quant; Boswell et al., 2020). Field collected soil was mixed, and soil boxes (47cm x 22cm x 14cm) were packed uniformly to a bulk density of 1.4-1.5 g cm^-3 and brought to a moisture content of 0.28 m³ m^-3 (average pre-freezing moisture in experimental plots) using distilled water. CT boxes were placed in a cold room (15-18°C) throughout the experiment, while FT boxes were placed in a chest freezer and subjected to FT cycles. After 48 hr of freezing, FT soil boxes were placed in a cold room for 48 hr thawing, and this procedure was repeated for 11 cycles. After completion of FT-cycles, rainfall was simulated at an intensity of 7.6 cm hr^-1 for 30 min after runoff began on all boxes using an indoor rainfall simulator. Time to runoff, runoff rate, and runoff depths were measured, and at the end of the simulation samples were analyzed for total solids (TS), dissolved reactive phosphorus (DRP) and total phosphorus (TP) losses. Treatments and their interaction effects on all measured variables were statistically analyzed (α=0.10) using R-software. Soil type, temperature and their interactions were not statistically significant on runoff depth and DRP losses (Table 1). The average (all treatments) time to runoff and runoff rate were 2.5 min and 0.14 L m^-2 min^-1,respectively. Runoff rate in all treatments increased up to about 15 minutes and then stabilized. Irrespective of temperature, soil type significantly affected TS and TP losses (p = 0.02 to 0.05). AR soil had up to two times greater TS and TP losses than LC soil. Runoff nutrient loads depend on both runoff volumes and nutrient concentrations. The runoff depth was similar between AR and LC soils, but the TS and TP concentrations (p=0.01; data not shown) were significantly higher in AR than LC. Therefore, greater TS and TP loads from AR can be attributed to greater organic matter and associated phosphorus content. While results indicate that FT action did not influence runoff, TS, and phosphorus losses, this may be valid only for the conditions (soil moisture, freezing temperature and number of cycles) of this study. Also, the difference in OM (0.7%) between soil types may not be great enough to produce differences in runoff parameters due to FT-action. Overall, results indicate that higher OM may influence TS and TP losses in rainfall-runoff for freeze-thaw soils. Future work should consider soils with greater OM differences, and long-term soil temperature and moisture data to effectively simulate FT cycles and understand the combined effect of organic matter and FT action on runoff, sediment and associated nutrient losses.