Release of Phosphorus from Crop Residue and Cover Crops Over Winter in Southwestern Ontario

English Abstract:

Maintaining sufficient crop residue on fields over the non-growing season as a means to improve soil health and reduce soil erosion remains a well-implemented agricultural beneficial management practice (BMP). As the concern for sustainable agriculture increases, there has been an emphasis on the use of cover crops to improve soil health through the retention of soluble nutrients, mainly nitrogen. However, in northern climates, crop residue and cover crops exposed to freeze-thaw cycles (FTC) have been recognized as a potential source of dissolved phosphorus to runoff; yet, there are limited field studies to support these findings. The objective of this study was to quantify the changes in water extractable phosphorus (WEP) from winter wheat (Triticum aestivum), red clover (Trifolium pretense) and oats (Avena sativa) over the non-growing season as the result of changes in hydroclimatic conditions such as FTC. WEP was determined from crop residue and cover crops placed in litterbags (20cm x 20cm) at different topographic locations (upland, midslope, low slope) on two fields. WEP in vegetation material was determined on fresh material, dead material, and material following FTC. Concentrations of WEP in cover crops were larger than those in the wheat residue, and, differences in WEP were also found with topographic position in the field. Concentrations of WEP varied with time, and were highest in freshly harvested wheat residue in comparison to residue that had been left on the field or exposed to FTC. In contrast, WEP in cover crops increased following FTC. This study provides insight into the timing of soluble phosphorus losses from crop residue and cover crops as well as discusses their potential contribution to winter phosphorus mobilization in Ontario.