Thompson, Karen A

In Ontario, the dominant cash crop rotations consist of soybean (SB), which is a leguminous crop grown in rotation with maize (MZ) and winter wheat (WW). In addition to these crops, some farmers integrate cover crops (CC) into crop rotation, especially during the fallow period and winter seasons, to reduce nitrogen (N) losses via nitrate (NO3-) leaching and emission of N2 and the greenhouse gas nitrous oxide (N2O). This thesis focused on understanding the impact of crop phases in a MZ-(SB-WW)-CC rotation on the abundance of N-cycling bacterial communities that mediate nitrification and denitrification pathways. In addition, the influence of CCs on soil cytokinin (CK) profiles, which are plant growth-promoting hormones, were studied in a greenhouse trial to assess their potential impacts when integrating CCs into crop rotations. In particular, the relationship between traditional soil health parameters and the soil CK profiles was studied to understand how CKs might reflect biotic interactions and soil vitality. Results indicate N fertilizer application mono ammonium phosphate (MAP) and starter N:P: K (24:6:24) during WW planting in fall largely supported nitrifying bacterial communities (amoA) and potentially contributed to NO3- leaching. Management of MZ, which included spring-applied MAP resulted in larger denitrifying (nirK) bacterial communities, increasing the potential risk of N-loss via emission of dinitrogen gas (N2) and greenhouse gas N2O. However, CC soils had significantly lower nirK than MZ, reflecting the importance of strong and deep root systems of CCs, which have a higher ability to scavenge the substrates for denitrifying communities (NO3-). This highlights the importance of growing CCs in reducing the potential risk for N-loss via leaching and denitrification. Additionally, in the greenhouse trial, the ability of CCs to affect CK was detected, highlighting the importance of integrating CC in crop rotations. This is particularly noteworthy, given that total CK profiles showed strong associations with traditional soil health parameters such as labile or active carbon and soil microbial community diversity. It was concluded that total soil CK can be used as a novel and dynamic soil health measure. Future research on quantifying N2O fluxes and levels of NO3- in leachates would provide a more precise understanding of the impact of different crop rotation phases on N-dynamics in these fields. Further studies on single or combined measures of soil CKs are warranted to develop its potential as a practical and effective soil health parameter.

Author Keywords: Cover crops, Crop rotations, Cytokinin hormone, Nitrogen Cycle, qPCR, Soil health