Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Henry, Hugh

2nd Supervisor

Wagner-Riddle, Claudia

Affiliation

Environmental Sciences, University of Guelph, Ontario

Co-Supervisor

Abstract

Cover crops, which are used to provide ground cover after the harvest of the grain crop, can potentially improve the sustainability of agroecosystems by reducing nutrient losses. However, few data are available to document the extent to which cover crops improve both the retention of soil nitrogen (N) and the transfer of this N to the grain crop. The efficiency of this N transfer may be further influenced by variation in winter soil temperature; for example, reduced snow cover can increase the frequency and intensity of soil freezing, which can affect the survival of cover crops and the timing of the decomposition of their residues. I quantified N transfer from cover crops (legumes, non-legumes and mixtures) to the subsequent corn crop using 15N tracer. Residue swapping was used to isolate the individual contributions of the aboveground and belowground N components of the cover crops. N transfer responses to soil temperature variability over the winter were examined via snow removal and pulsed warming from overhead heaters. My results revealed that the belowground N pool contributed substantially more to N retention and N transfer than the aboveground N pool. However, less than 10% of the 15N added to the soil was transferred to the corn, while the majority remained in the soil. In addition, increased soil freezing reduced the effectiveness of the cover crops in transferring N to the corn, with legumes being more susceptible to N losses than non-legumes. Year-to-year variability in cover crop establishment and productivity also had strong effects on the effectiveness of the cover crops in retaining and transferring N. Overall, my results reveal that while the scavenging of N by cover crops after harvest of the main crop may not provide a substantial contribution of N to the grain crop the following year, this scavenging may be important for reducing N losses to the surrounding environment.

Summary for Lay Audience

To meet the demand of increased food production required to feed the growing human population, fertilizers are used to increase nutrient inputs to agricultural fields. Nitrogen based fertilizers are often used to increase crop yield; however, the excess nitrogen not taken up by crops can become an environmental pollutant. In particular, farmers often leave their soils bare after grain crops are harvested, and bare soil is at risk of eroding and losing nutrients. Farmers can reduce these losses by using cover crops, which are planted after the harvest of the grain crop in the fall. Cover crops are not harvested but are planted to provide benefits to the soil in the absence of the grain crop. The cover crops are then terminated (killed), either by frost or from using chemical herbicide in the spring. When cover crops die, their decomposition releases any nutrients they have taken up back to the soil. Ideally, these nutrients will become available to the grain crop planted in spring, allowing farmers to use less fertilizer. However, winter conditions may affect the timing of when cover crops decompose and release nutrients. If there is reduced snow cover, soil and overwintering plants can be exposed to severe freezing, thereby reducing the positive benefits they could provide. My goals were to quantify where nitrogen taken up by cover crops ends up (e.g., in the soil or the main crop), and how different winter conditions affect the loss and transfer of nitrogen. I simulated soil freezing by reducing snow cover either by shoveling the snow off the plots or by using heaters to melt the snow. My data showed that cover crops are important at increasing the retention of nitrogen in the soil, and I also found that the effectiveness of cover crops is reduced by increased soil freezing. Overall, my results reveal that while nitrogen uptake by cover crops may not provide a substantial contribution of N to the grain crop the following year, this uptake may be important for reducing N losses to the surrounding environment.

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