PS 28-153 - Microbial communities under winter annual cover crops in the subtropical Mid-South United States

Tuesday, August 8, 2017
Exhibit Hall, Oregon Convention Center


Lisa Fultz, Louisiana State University AgCenter; Ina I Sanchez, Louisiana State University AgCenter; Kathleen Bridges, Louisiana State University AgCenter; Autumn Acree, Louisiana State University AgCenter


The ‘Dead Zone’ along the Gulf Coast is attributed, in part, to agricultural production and its use of commercial fertilizers and intensive management practices. Use of fallow period cover crops has increased in response to the degradation of soil health and water quality commonly associated with intensive production systems. In the sub-tropical Mid-South, however, cover crops pose a unique situation when combined with warmer winters and increased soil moisture. These conditions support active growing conditions above- and below-ground that may stimulate microbial communities and production to benefit producers or promote the growth of disease and delay germination in the spring. To study the effects cover crops on soil microbial activity and community structure a split-plot field trial with cover crops (cereal rye [Secale cereal L.], forage radish [Raphanus sativus var. longipinnatus], berseem clover [Trifolium alexandrinum], crimson clover [Trifolium incarnatum L.], winter pea [Pisium sativum L.], hairy vetch [Vicia villosaRoth] as main plots and N-rates (0, 235, 269 and 303 kg/ha) as sub-plots was established at LSU Agcenter’s Macon Ridge Research Station in northeast Louisiana. Soil samples (0-7.5 cm) were collected prior to cover crop establishment (October 2014 & 2015) and after chemical burn-down (February 2015 & 2016). Soil were sieved to 4.75 mm and stored at 4⁰ C for Ester-linked Fatty Acid Methyl Ester profiles or air dried for β-glucosidase and β-glucosaminidase assays.


Significant increase in β-glucosidase activity was observed after cover crop termination and in the presence of monoculture cover crops relative to a grass/brassica mixture and fallow treatments (61.1 and 50.8 mg p-nitrophenol g-1 h-1, respectively). β-glucosaminidase activity increased 46% from fall 2014 to fall 2015, but was lowest following leguminous cover crops. This increase in C-cycling activity may indicate increased influxes of biomass and its subsequent degradation and incorporation into the soil. Decreased N-cycling activity may be hindered by application of N-fertilizers during the growing season, thus reducing need of N-cycling enzymes where N is no longer limited. Total microbial biomass increased in fall samples and plots seeded with cereal rye when compared to all other cover crop treatments (140 and 126 nmol g-1, respectively). Soils collected prior to the first cover crop seeding were dominated my arbuscular mycorrhizae shifting to populations dominated by bacteria and saprophytic fungi in subsequent samples. Microbial activity, sensitive to changes in management practices, and nutrient cycling, benefited from the integration of cover crops in production systems in the Mid-South.