Assessing strategies to mitigate phosphorus leaching from drained clay soils

Hydrologic Extremes and Legacy Sources Can Override Efforts to Mitigate Nutrient and Sediment Losses at the Catchment Scale

Combating eutrophication requires changes in land and water management in agricultural catchments and implementation of mitigation measures to reduce phosphorus (P), nitrogen (N) and suspended sediment (SS) losses. To date, such mitigation measures have been built in many agricultural catchments, but there is a lack of studies evaluating their effectiveness. Here we evaluated the effectiveness of mitigation measures in a clay soil-dominated headwater catchment by combining the evaluation of long-term and high-frequency data with punctual measurements upstream and downstream of three mitigation measures: lime-filter drains, a two-stage ditch, and a sedimentation pond. Long-term hydrochemical data at the catchment outlet showed a significant decrease in P (-15%) and SS (-28%) and an increase in nitrate nitrogen (NO-N, +13%) concentrations. High-frequency (hourly) measurements with a wet-chemistry analyzer (total and reactive P) and optical sensor (NO-N and SS) showed that the catchment is an abundant source of nutrients and sediments and that their transport is exacerbated by prolonged drought and resuspension of stream sediments during storm events. Lime-filter drains showed a decrease in SS by 76% and total P by 80% and an increase in NO-N by 45% compared with traditional drains, potentially indicating pollution swapping. The effectiveness of two-stage ditch and sedimentation pond was less evident and depended on the prevalent hydrometeorological conditions that drove the resuspension of bed sediments and associated sediment-bound P transport. These results suggest that increased frequency of prolonged drought due to changing weather patterns and resuspension of SS and sediment-bound P during storm events can override the generally positive effect of mitigation measures.

Potential Phosphorus Export in Snowmelt as Influenced by Fertilizer Placement Method in the Canadian Prairies

Placement strategies for P fertilizer can affect P availability to crops and influence the amounts and forms of P removed from soil in runoff, contributing to eutrophication. On the Canadian prairies, most runoff occurs during snowmelt. Two adjacent farm fields in Saskatchewan, Canada, were used to assess the effects of spring P fertilizer placement on crop P uptake, residual soil P, and potential P export in simulated snowmelt. One was in conventional tillage (CT) with no history of P fertilization, and the other was in a no-till (NT) system with multiyear P fertilization at recommended rates. Fertilization (monoammonium phosphate) treatments were no P fertilizer (control); seed placed, deep banded, and broadcast and incorporated at 20 kg PO ha; and broadcast treatments at 20, 40, and 80 kg PO ha. Yield and P uptake were not affected by placement method. Water-extractable P at the soil surface after harvest was unaffected by placement or rate at either site but increased below the 5-cm depth at the NT site in 2016. Broadcast treatments increased P in runoff relative to in-soil P placement for the 20- and 80-kg PO ha treatments at the CT site and for the 80-kg PO ha treatment at the NT site. Thus, in-soil application of P fertilizer appears to be an effective strategy to reduce the risk of P export in snowmelt runoff.

Permeable Reactive Barriers for Preventing Water Bodies from a Phosphorus-Polluted Agricultural Runoff-Column Experiment

This paper aims to examine the potential of permeable reactive barriers (PRBs) as an in-situ removal approach for phosphate polluted agricultural runoff. Four different reactive materials (RMs) of: autoclaved aerated concrete (AAC), Polonite®, zeolite and limestone were tested. The study was conducted as a column experiment with a sandy loam soil type charging underlying RM layers with phosphorus (P) and a soil column without RM as a reference. The experiment was carried out over 90 days. During this time the P-PO4 load from the reference column equaled 6.393 mg and corresponds to 3.87 kg/ha. Tested RMs are characterized by high P-PO4 retention equaling 99, 98, 88 and 65% for Polonite®, AAC, zeolite and limestone, respectively. At common annual P loss rates of 1 kg/ha from intensively used agricultural soils, the PRB volume ranged from 48 to 67 m3 would reduce the load between 65 and 99% for the RMs tested in this study.

Handling the phosphorus paradox in agriculture and natural ecosystems: Scarcity, necessity, and burden of P

This special issue of Ambio compiles a series of contributions made at the 8th International Phosphorus Workshop (IPW8), held in September 2016 in Rostock, Germany. The introducing overview article summarizes major published scientific findings in the time period from IPW7 (2015) until recently, including presentations from IPW8. The P issue was subdivided into four themes along the logical sequence of P utilization in production, environmental, and societal systems: (1) Sufficiency and efficiency of P utilization, especially in animal husbandry and crop production; (2) P recycling: technologies and product applications; (3) P fluxes and cycling in the environment; and (4) P governance. The latter two themes had separate sessions for the first time in the International Phosphorus Workshops series; thus, this overview presents a scene-setting rather than an overview of the latest research for these themes. In summary, this paper details new findings in agricultural and environmental P research, which indicate reduced P inputs, improved management options, and provide translations into governance options for a more sustainable P use.