Near‐Surface Soils as a Source of Phosphorus in Snowmelt Runoff from Cropland

Abstract

In northern regions, a high proportion of annual runoff and phosphorus (P) export from cropland occurs with snowmelt. In this study, we analyze 57 site-years of field-scale snowmelt runoff data from 16 small watersheds draining fine-textured soils (clay or clay loam) in Manitoba, Canada. These fields were selected across gradients of soil P (2.4 to 26.7 mg kg −1 , 0-to 15-cm Olsen P), tillage intensity (high frequency to long-term no-till), and fertilizer input. The strongest predictor of flow-weighted mean concentrations of total dissolved P (TDP) in snowmelt runoff was Olsen P in the top 5 cm of soil (r 2 = 0.45, p < 0.01). Residual variation in this relationship related positively to volumetric soil moisture and negatively to water yield. Although Olsen P levels were relatively consistent from year to year, suggesting control by long-term fertilization and tillage history, Olsen P stratification (ratio of 0-5/0-15 cm) increased with rates of fertilizer application. Particulate P (PP) comprised <34% of total P on average, and concentrations were not well predicted by soil or management characteristics. Loads of PP and TDP exported during snowmelt were primarily a function of water yield and size of accumulated snowpack; however, residual variation in the TDP relationship correlated positively with both soil moisture and Olsen P. Retention of runoff water on the landscape could reduce loads, but careful management of near-surface soil P is required to prevent snowmelt runoff losses of P at the source and to reduce the potential for the eutrophication of downstream aquatic ecosystems.

Agricultural Water Quality in Cold Climates: Processes, Drivers, Management Options, and Research Needs

“…Across 26 fields in Wisconsin and Minnesota, USA, Zopp et al (2019) observe that the flow‐weighted mean total P concentrations in runoff from mixed precipitation events (rainfall and snow) are dominated by dissolved P forms when the soil is frozen, and for nonfrozen soil conditions, total P is frequently dominated by particulate P forms. The results are supported by Good et al (2019), who emphasize that runoff over frozen soils or partially frozen soils is particularly important for the loss of total dissolved P. However, Wilson et al (2019b) report that although the P in snowmelt runoff is frequently dominated by dissolved forms in Manitoba, Canada, high rates of particulate P loss can result from erosion of frozen soils in some circumstances, necessitating further research to elucidate the drivers of this erosional P loss from frozen soils during snowmelt (Wilson et al, 2019b).…”

“…Nutrient rate management Implementing soil drawdown practice by reducing P input in combination with crop removal reduces total dissolved P concentrations in both snowmelt and rainfall runoff without affecting crop yields (Liu et al, 2019a). Maintaining low or moderate soil test P to reduce P concentrations and loads in snowmelt runoff (Wilson et al, 2019b;Zopp et al, 2019). Decreasing N application rates reduces nitrate leaching in tile drainage (Vetsch et al, 2019).…”

Surface runoff and nutrient dynamics in cover crop–soybean systems in the Upper Midwest

“…During the intercrop phase, concern for increased loss of soil nutrients resulting from spring fertilizer application was driven by previous findings that indicated soil NO 3 − –N and available P (0–30 cm) increased between pre‐fertilization (beginning of intercrop phase) and oilseed harvest soil samples (beginning of soybean phase) in all overwintering cover treatments but did not change in fallows (Weyers et al., 2019). Specifically, the greater concern has been for loss of P through surface and subsurface drainage waters, which was correlated to greater soil test P levels (King et al., 2015; Liu et al., 2019a, 2019b; Wilson, Elliott, Macrae, & Glenn, 2019), although spring versus fall application of P‐based fertilizers poses less risk (King et al., 2015). However, the current data suggested only a slight relationship between runoff mineral N and soil NO 3 − –N and no relationship between runoff SRP and soil available P. On one hand, pre‐fertilization soil NO 3 − –N was significantly greater in chisel‐till and f.rad (averaging 28 kg ha −1 ) compared with overwintering cover treatments (averaging 11 kg ha −1 ), and intercrop phase runoff total N was elevated in chisel‐till and f.rad treatments but was not significantly different among treatments.…”

Options for Improved Phosphorus Cycling and Use in Agriculture at the Field and Regional Scales

“…In the Cerrado region of Brazil, Rodrigues et al (2016) found that no‐till management increased soil organic C and P o in the soil surface (0–10 cm) and supported that no‐till, through greater organic P cycling, could increase PUE. However, no‐till management also increases the potential for dissolved P losses in surface runoff due to stratification (Jarvie et al, 2017; Wilson et al, 2019), so the benefits and potential negative impacts of no‐till on PUE need to be considered, taking into account regional nutrient management priorities.…”