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Ontario Grain Farmer Magazine

Phosphorus loss mitigation: cover crop species and soil P interactions

Principal Investigator

Ivan O’Halloran

Research Institution

University of Guelph

Project Start

April 2016

Project End

October 2017

Objectives

To evaluate cover crop species with respect to their:

  • Biomass production, P uptake and subsequent release after freeze/thaw cycles under different levels of P fertility;
  • Ability to reduce soil erosion under conditions of varying soil P fertility, snowmelt with freeze-thaw cycles and rainfall simulation; and
  • Influence on proportions and timing of soluble and total P losses under conditions of varying soil P fertility, snowmelt with freeze-thaw cycles and rainfall simulation.

Impacts

  • The identification of conditions where cover crops are an effective best management practice to reduce agricultural phosphorus (P) loss will allow farmers to confidently integrate cover crops into their cropping system to reduce their environmental impact.
  • The identification of soil test P ranges where additional mitigating practices may be warranted to reduce P losses in surface runoff.

Scientific Summary

Algal blooms in Lake Erie have again raised questions around phosphorus (P) management and ways to limit P entering surface waters. Agricultural P losses occur as particulate-P (P bound to soil particles) and as reactive, soluble-P that is readily biologically available.  Recent increases in soluble-P loading to Lake Erie have led us to reconsider management practices used primarily to control particulate-P losses. Cover crops, while effective to deal with erosion, have had mixed impacts on P losses, especially soluble forms of P loss. Some studies show that cover crops can be a significant source of soluble-P, especially after freeze-thaw cycles, while others found little concern for cover crop soluble-P losses. Differences likely reflect environmental conditions and cover crop species grown. The ability of cover crops to help with P loss is season specific and may be region specific. When plants get rained on, P is leached out. If a plant is killed over the winter, P will stay in the plant; however, when living plant material freezes there is evidence that more phosphorus leaches out of plant material. If P is leached out when soil is frozen, then soluble P has a greater chance of running off the soil, leading to increased losses. With the freeze-thaw cycles that we generally see in Ontario, the soil may be able to interact with the P so less is lost when plants thaw out. As soil test P stays higher, then there is a greater risk of losing P. Cover crops could have a net benefit when you consider the protections they offer to reduce P that is lost through freezing. Water extractable P (WEP) is the dissolved P which feeds algae blooms. Cover crops hold soil and stop erosion, therefore holding particulate P (it is not as bioavailable as soluble P).  If we are to meet the proposed total and soluble P targeted reductions of 40% proposed in the Ontario government’s phosphorus reduction strategies, we need to understand the impact that cover cropping practices have on P losses.

The goal of this project was to identify conditions where we can confidently consider cover crops to be an effective best management practice for mitigating P loss. The type and amount of P lost under cover cropping systems depends upon key factors, such as soil type, soil test P levels, erosion potential, cover crop age and species, and occurrence of freeze-thaw cycles in relation to runoff events. A simulation of a freeze-thaw cycle was conducted to determine how easily P was lost from the cover crop, taking into consideration its age and whether it overwinters. We examined the growth, P uptake and P release from plant biomass for four plants species (oat, rye, mustard and red clover) grown on soils with low, medium and high soil test P at 8 and 12 weeks after planting. Higher soil test P did not affect biomass at 8 weeks, but did increase biomass at 12 weeks. Higher soil test P levels increased WEP (in kg P/ha) only for oat and rye at 8 weeks. WEP is used to estimate the soluble phosphorus that could potentially be released from the plant material. The amount of WEP ranged between 0.2 kg P/ha for red clover to 2.4 kg P/ha for oat. Subjecting the 8-week biomass to four freeze-thaw cycles substantially increased (~ 1.6 to 16 times) the amount of P released from the biomass. The grasses were less affected by freeze-thaw cycles (~ 4 times more) than mustard and red clover (~ 12 times more). The WEP released from red clover was the lowest (1.8 kg P/ha) and was not affected by soil test P, and oat the highest (4.4 kg P/ha) with WEP increasing with soil test P level. For 12-week biomass, soil test P did not affect the amount of WEP on either fresh or freeze-thaw biomass. Red clover had the lowest amounts of WEP reflecting the amount of biomass produced. Of importance was the WEP at this stage was lower than the previous stage, indicating that the older plant material would be less susceptible to WEP losses.  By this stage of sampling, the impact of freeze-thaw cycles increased the amount of WEP by only 1.5 to 2.8 times, showing a reduced risk of WEP loss the older the cover crop was prior to the freeze-thaw cycle. The results to date indicate that soil test P and stage of development can affect the release of WEP from the cover crop. How this relates to net P loss in runoff is still under investigation.

External Funding Partners

OMAFRA/University of Guelph funding partnership

This project was funded in part through Growing Forward 2 (GF2), a federal-provincial-territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario.