Phytotoxic effect of Barley, oat, and wheat-straw mulches in eastern quebec forest plantations 2. Effects on nitrification and black spruce ( Picea mariana) seedling growth

Abstract

Red raspberry ( Rubus idaeus L.), a common forest weed especially in clearcut areas of northeastern America, is known to possess definite nitrophilous habit. In a companion paper we showed that barley, oat, and wheat straws used as cover mulches inhibited red-raspberry establishment and N nutrition after clearcutting and site preparation. As part of the mechanisms involved, we hypothesized that an inhibition of nitrification would prevent the establishment of red raspberry. Effects of barley, oat, and wheat-straw cover-mulches were evaluated under field conditions. We monitored soil NO 3-N production, the abundance of NH 4 +-oxidizers, and nitrate reductase activity (NRA) in R. idaeus foliage as an indication of the availability of nitrate in soil. Over two consecutive years, the use of straw mulches significantly reduced NO 3-N soil production and NH 4 +-oxidizers. The five phenolic acids, p-coumaric, sytingic, vanillic, ferulic and p-hydroxybenzoic, responsible for the toxicity of the straws, significantly inhibited NH 4 +-oxidizers at concentrations ranging from 10 −3 M to 10 −6 M in a laboratory experiment. The NRA measured during the 1987 growing-season in red-raspberry foliage confirmed the results obtained for NO 3-N content of soil. The results suggest that the inhibition of NO 3-N production under straw mulching restricted, to some extent, the establishment of red-raspberry seedling. Other mechanisms could also be partially responsible for the inhibition observed, and are discussed. The effect of straw mulches was also evaluated on shoot growth and basal stem-diameter growth and on mineral nutrition (total N, P, K, Ca, Mg and Mn) of black-spruce ( Picea mariana (Mill) BSP) seedlings over the two successive growing-seasons (1986 and 1987) after straw application. The straw treatments enhanced shoot and stem-diameter growth of P. mariana seedlings. Treated black-spruce seedlings showed a significantly higher foliar N-content than control seedlings. There were no consistant significant differences between treated and control seedlings for the other nutrients analysed. The promotion of black-spruce seedling growth and foliar N-content is associated with a reduction in competitive vegetation and also an inhibition of nitrification from the straws.