Long term multispecies cover crops in semi-arid Montana: soil response and aboveground biomass

Abstract

Low and variable annual precipitation (250-350 mm) make management strategies that conserve soil moisture imperative for wheat producers in semi-arid Montana. A wheat-fallow rotation was historically the most common dryland cropping system in semi-arid Montana, due to its ability to conserve soil water; however, summer fallow has negative environmental impacts (Campbell et al., 1991). There is interest to incorporate cover crops into a rotation as a partial replacement for summer fallow to enhance soil quality. An eight-yr study explored the effect of cover crops on biological soil properties through aboveground biomass inputs of four plant functional groups: brassica (BC), fibrous root (FR), tap root (TR), and nitrogen fixers (NF) grown as two-species mixes, six-species mixes (three functional groups), a full eight-species mix, and two controls--chemical fallow and sole pea. Cover crops grew for about 60 days, were terminated with glyphosate, then soil samples were taken nine months after termination at wheat seeding. The only difference in biological parameters based on functional group was that mycorrhizal colonization in wheat was higher following FR than BC at one site. Potentially mineralizable nitrogen (PMN) was 1.6-1.7 times higher and microbial biomass was 1.4 times higher in soils from cover crop treatments relative to fallow at one of two sites. PMN was 1.2-1.3 times higher in soils from six-species mixes than two-species mixes at both sites, and six-species mixes produced 1.4 times more biomass at one site. Nitrogen fixers had the lowest C:N ratio of the functional groups at both sites, while FR had the highest at one site. In a second study of cover crop termination, cover crops were grown about 90 days and terminated with one of three strategies: chemically, grazing, or haying. Soils were sampled nine months after termination at the time of wheat seeding. Few enzyme differences and no PMN differences or meaningful patterns were discovered among termination strategies. Minimal differences in biological parameters, even when shoot biomass was removed, may mean grazing or haying could improve net revenue without detracting from soil health. In semi-arid annual systems, water limitations may be the main concern with growing cover crops.