Cover crops can provide alternatives to chemical weed control and nutrient management, particularly in organic farm systems. While cover crops can be planted as monocultures, they can also be combined into multispecies “cocktails” in order to provide multiple benefits. In this study, we quantified the effects of different cover crop combinations on weed suppression and soil nitrogen dynamics at two different organic farms. We planted six cover crops (fava bean, field pea, oats, spring wheat, Kodiak mustard, and purple top turnips) as monocultures and as all possible five-species combinations, and we estimated the biomass of each species after six weeks of establishment. We incorporated the plant residues into the soil and collected soil samples on days 3, 7, and 32 post-incorporation. We used seed germination bioassays to estimate weed suppression potential of these soils, and we also measured soil ammonium and nitrate concentrations and potentially mineralizable nitrogen. We used classification and regression trees to investigate how weed suppression and soil nitrogen responded to different cover crop combinations.
Results/Conclusions
In general, we found that the effects of some cover crops were strongest immediately following incorporation, while others played a stronger role later on. For example, the germination rate of field pennycress was inhibited by small amounts of spring wheat and purple top turnips in the first week after incorporation, but thereafter field pennycress became sensitive to field pea. However, mustard consistently suppressed the germination rates of velvetleaf and lettuce. We found soil ammonium levels to be highest in all plots immediately following cover crop termination, particularly when plots contained high biomass of mustard. In contrast, we found that the presence of high oat biomass consistently lowered soil nitrate levels. At one of our farms, high levels of any plant biomass enhanced potentially mineralizable nitrogen, while at the other, only mustard, field pea, and oats elevated this nitrogen pool. Our results suggest that cover crop effects on the soil may be variable over time and may vary from place to place. However, some cover crops, such as mustard and field pea, may play outsized roles in weed suppression and nitrogen dynamics.