How does winter affect nutrient losses?

Wednesday, November 5, 2014

It probably depends where you are in Ontario and what type of winter it is. But anything you can do to minimize the soluble nutrients in the soil from late winter through until after the spring thaw is going to directly reduce the amount of loss

by KEITH REID


While some of you, particularly if you ski or snowmobile, are hoping for a repeat of last year, most of us are looking forward to a shorter winter with a bit less snow. No matter what type of winter we have, it will influence the nutrients that we lose in ways that our friends to the south don't have to worry about. Understanding the impacts that winter has will help us to manage nutrients more effectively.

The first and most obvious impact of winter conditions is the accumulation of snow, which stores most of the precipitation over the winter and releases it as runoff at spring thaw. The extent of this delay depends on the severity of winter conditions.

In the Canadian Prairies, over 80 per cent of the annual runoff can occur during spring thaw and most of this is over frozen soil. In milder areas, like southern Ontario, the bottom of the snow pack can be gradually melting all winter and soaking into the soil, so the proportion running off in the spring is much less.  

Dr. Xiaoyuan Geng at Agriculture and Agri-Food Canada in Ottawa has been studying the distribution of snow across the landscape to predict where most of the meltwater will come from during the spring thaw. Early indications are that, as we would expect, the distribution of snow across a field is not uniform, with more accumulating in lower areas than higher ones. The tops of the knolls may be exposed to freezing conditions, but the areas adjacent to streams and ditches have a much greater depth of snow to insulate the soil.  

The second impact is the freezing of both the soil and any plants growing in the soil. During this process, ice crystals form within living cells, which pierce the cell walls and allow the nutrients held within those cells to escape.

This can be good or bad, depending where those nutrients end up. Held in the soil, they form a readily available pool of nutrients for crops the following spring. Carried from the field in runoff, they can contribute to algae blooms in streams and rivers. Several recent papers have implicated phosphorus losses from forages and cover crops in the Prairies for the algae blooms in Lake Winnipeg. This may be true for the Prairies, but how relevant is it to Ontario?

The short answer is that it probably depends where you are in Ontario and what type of winter we have. If the weather gets cold enough that the ground freezes, blocking infiltration into the soil, then losses of nutrients from above-ground plant parts will be significant. In this situation, we could expect losses of dissolved phosphorus and nitrogen to be greater from a field with a cover crop than with bare soil.

Under more normal winter conditions for southern Ontario, the nutrients will be released from the plant parts as they are killed by winter conditions, but they will be gradually carried into the soil by the melting and infiltration of the bottom of the snow pack. In this case, the presence of forages or cover crops is not likely to result in increased nutrient loading during spring runoff.

In fact, two factors could reduce the risk of nutrient runoff. The presence of growing plants will increase the porosity of the soil, allowing more infiltration, and the top growth of the plants could trap more snow to insulate the soil surface and keep it from freezing.  Tilling to incorporate the cover crop isn't likely to reduce the amount of dissolved nutrients leaving the field, but it will increase the risk of soil erosion. In general, the farther south you are, the less risk there is of nutrient losses from winter cover and the greater the potential benefits.

Along with runoff from the surface, winter is also the season when most of the replenishment of soil moisture occurs, including the deep aquifers. This deep drainage can carry dissolved nutrients, particularly nitrate, below the rooting zone for plants. Not only is this a loss of available nitrogen for the next crop, but it can adversely affect drinking water quality at high enough concentrations.

What does this mean for management? It is clear that, in any area that has a true winter, the greatest risk for nutrient losses is going to be from late winter through until the ground has dried out after the spring thaw. Anything you can do to minimize the soluble nutrients in the soil during that time is going to directly reduce the amount of loss.

For nitrogen, that means delaying applications until spring. If you must apply in the fall (to empty a manure storage, for example), delaying until the soil cools down will help to keep the nitrogen in organic or ammonium forms, which do not move as easily. Cover crops can help to soak up leftover mineral nitrogen from an early maturing crop, although you will want something that doesn't die at the first hint of frost so it can hold on to the nitrogen through the winter.

Guidelines for phosphorus (P) management are similar, but there is more flexibility for fall application as long as the material is worked into the soil. The greatest risk for P losses will be from manure or fertilizer that is sitting on the soil surface over winter. If you are broadcasting phosphorus onto forages, where incorporation is not possible, it would be better to spread in the spring or early summer, so the phosphorus has time to bind to the soil particles. BF

Keith Reid is Soil Scientist, Agriculture and Agri-Food Canada, Guelph.