A Comparison of Landscape Mulches: Chemical, Allelopathic, and Decomposition Properties

Abstract

Abstract This study compared chemical, allelopathic, and decomposition properties of 6 mulches: cypress, eucalyptus, pine bark, pine needle, melaleuca, and a utilitytrimming mulch (GRU). Eucalyptus and GRU mulches had the highest decomposition after 1 year (21% and 32%), while only 3% to 7 % of the other mulches decayed. Lignin and lignin:nitrogen ratio were negatively correlated with decomposition; high values resulted in low decomposition. Winter respiration of both eucalyptus and GRU mulches was high, and respiration was positively correlated with decomposition. Pine-straw mulch subsided from 9 cm (3.5 in.) to 4 cm (1.6 in.) during the year, while the other mulches subsided approximately 2 cm (0.8 in). Nutrient composition of the mulches was significantly different, with GRU mulch having the highest levels of calcium (Ca), magnesium (Mg), nitrogen (N), phosphorus (P), and potassium (K). Pine straw was the next highest in N and P. Soils under the mulches were acidified most by pine straw (from a pH of 5.0 to 4.4), followed by pine bark and cypress. In a standard test of allelopathy, all fresh mulches inhibited germination of lettuce seed, and although variable in concentration, all mulches contained hydroxylated aromatic compounds that could have caused these allelopathic effects. After 9 months and 1 year, pine straw and GRU still exhibited allelopathic effects on germination. Cypress, melaleuca, and pine bark retained their color after 1 year, while the other 3 mulches changed to a pinkish gray.

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