Cover crop mixtures as partial summerfallow replacement in the semi-arid northern Great Plains
Tallman, Susan Marie
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Farmers in the semi-arid northern Great Plains are currently experimenting with multi-species cover crops, or cover crop mixtures (CCMs), as a partial summerfallow replacement and conservation practice, in response to the anecdotal claim that CCMs provide more ecosystem services than their single-species legume green manure (LGM) counterparts. This is in the absence of any published data. We conducted a 2-yr plot scale study from 2012 to 2013 at four on-farm locations in Montana to compare fallow with a pea LGM and nine CCM treatments comprised of four plant functional groups, including nitrogen fixers, fibrous roots, tap roots, and brassicas. Agronomic factors reported include cover crop biomass yield, biomass N yield, soil water and nitrate-N use, and subsequent wheat yield and quality. In addition, soil biological factors measured included; microbial respiration rate, soil enzyme activity, potentially mineralizable nitrogen, and mycorrhizal colonization. Mean cover crop biomass by site ranged from 0.4 Mg ha -1 in a record dry year, to 3.7 Mg ha -1 in a record wet year. Cover crop C:N differed between single-species Pea and an eight-species Full mix only at one site-year with 13.4 and 16.7 measured for each treatment, respectively. Soil water after cover crop treatments was less than Fallow at the time of cover crop termination at three site-years, and was strongly correlated with decreased subsequent wheat yield at three N fertility levels. Surface soils were 5 to 10 °C cooler with Pea and Full cover crops than Fallow from the time of cover crop canopy closure until six to eight weeks later. Soils following cover crops had increased microbial respiration rate at one site, however, no differences in six measured soil enzymes activities were found. Mycorrhizal colonization of wheat increased at one site from 11 to 22% following cover crops when compared with Fallow. Very few differences were observed between the pea LGM and the CCM treatments in all measured factors, indicating little advantage of CCMs over LGMs after one cover crop cycle. However, field observations indicate CCMs may have potential to provide biological pest control and this topic is recommended for further study.