Phosphorus Losses in the Non-Growing Season: Introduction and Overview.

Phosphorus losses from agriculture often occur when P sources (e.g., manure, soils, and fertilizers) are present in fields with a transport (i.e., hydrology) risk. The non-growing season, when commodity crops are not growing in the field, represents a challenging time for controlling P losses; historically, few studies evaluated P losses during this period. In this session, experts from North America and Europe will discuss both source and transport factors that influence P losses during the non-growing season. Manure and fertilizer applications during the growing and, in some cases, the non-growing season are long recognized as a major contributor to agricultural P losses. In many cropping systems, the timing of manure and fertilizer application is limited by either the growing crop (i.e., difficulties in applying to a standing corn crop), availability of labor (i.e., fall applications of manure or fertilizer to fallow land), or by climatic drivers (i.e., legislation prohibiting manure application to frozen or snow covered ground). In some regions, system hydrology represents the crux of the problem related to non-growing season P losses. Often, the growing season occurs during a period when evapotranspiration can exceed precipitation, thus resulting in less runoff than generated during the non-growing season for storms of similar precipitation rate/intensity. In snowmelt dominated systems, the only runoff events from a site may occur prior to the start of the growing season as the snow pack melts. Ultimately, traditional conservation practices fail to mitigate agricultural P losses during the non-growing season. In this symposium, experts will also explore novel concepts, such as altering the crop rotation to extend the growing season, to address P losses during the non-growing season.