Soil biological response to multi-species cover crops in the Northern Great Plains

Abstract

Farmers in semi-arid regions are experimenting with cover crop mixtures as partial summer fallow replacement, with the goal of increasing soil biological activity. We compared extracellular enzyme activity, microbial biomass, potentially mineralizable nitrogen (PMN) and mycorrhizal colonization levels after growing cover crops comprised of one or four functional groups. We asked whether cover crops can alter those soil processes and components relative to fallow during the cash crop growing season, and whether a cover crop mixture (CCM) will have a greater impact than a single-species legume green manure (LGM) after one and two rotations with wheat. Cover crops were planted at two sites in Montana April 2012 and May 2014, and at two additional sites in May 2013, and April 2015, terminated at each site approximately 60 days after seeding. Wheat was the cash crop for the following growing season. Cover crop treatments included a fallow control, a sole pea LGM, and a FULL mix consisting of eight species, with two species of each of four functional groups. Four additional treatments of single functional groups were compared following the second rotation of cover crops. The geometric mean of soil enzyme activity increased after cover crops relative to fallow (1.1–1.3 fold) at two sites after the first rotation, and by 1.1–1.4 fold after the second rotation at three sites. At the fourth site, the geometric mean of the FULL mix was greater than LGM or fallow by 1.4 and 1.5 fold. The response of individual enzymes was less consistent. Microbial biomass and PMN increased after LGM and/or FULL at three of eight site-years, by 1.2–1.7 for microbial biomass and 1.6–2.1 for PMN. Mycorrhizal colonization of wheat increased in at least one cover crop treatment relative to fallow at four of six site-years, and decreased in cover crops relative to fallow at one site. There were no differences in soil biological traits measured among single-functional-group treatments. Soil biological parameters were correlated with cover crop biomass in 7 of 16 comparisons at two sites after one rotation. After two rotations, 5 of 8 soil parameters were correlated with cover crop biomass at one site, with no correlations for the remaining sites. Precipitation declined 30–50 % from the first rotations at those three sites. Positive effects of cover crops on soil biological parameters in semi-arid systems may be measurable only when environmental conditions do not limit growth.