Decomposition of straw from herbage seed production: Effects of species, nutrient amendment and straw placement on C and N net mineralization

The market for cereal cover crop straw and herbage seed straw has diminished in many seed-production areas due to there being less livestock. Seed growers therefore want to chop and return the straw both in the sowing year and in the seed-harvest years. The objectives of this research were (1) to compare decomposition rates of straw of barley and wheat cover crops and timothy (Phleum pratense L.), meadow fescue (Festuca pratensis Huds.), and red clover (Trifolium pratense L.) seed crops, (2) to study the effect on soil microbial activity of adding mineral nitrogen or fresh leaves from undersown seed crops, and (3) to evaluate the effect of straw placement/soil contact on straw-decomposition rates. Microbial activity/straw-decomposition rates were evaluated by regular measurement of CO2 production in four laboratory incubation experiments lasting for 150–161 days. Microbial activity was always enhanced by adding straw on the soil surface. Straw of barley resulted in higher CO2 production than did straw of wheat, while straw of red clover and meadow fescue resulted in higher CO2 production than with straw of timothy. Inclusion of fresh leaves of white clover and timothy increased CO2 production during the first and second half of the incubation experiments, respectively. Neither in the sowing year nor in the seed-harvest year was microbial activity stimulated by adding fertilizer nitrogen, but soil mineral nitrogen by the end of the experiments was higher after adding fertilizer or fresh leaves, and lower after adding grass seed straw. Presumably owing to higher humidity, straw decomposition in a field experiment was higher in timothy stubble than in wheat stubble and higher at 2 cm than at 10 cm above the soil surface. For practical seed production, growers are recommended to leave short stubble, to chop the straw as finely as possible, and to distribute the straw evenly on the surface in close contact with soil particles.