Seasonal variation in root growth of three grass species under varying cutting treatments

Abstract

There are few data describing full seasonal variations in root mass fluctuations of perennial grasses. This experiment documented the seasonal changes in root weight density of three grass species under different management regimes. Kentucky bluegrass (Poa pratensis L.) and tall fescue (Festuca arundinacea Schreb.) received one annual cut, twice yearly forage harvest, and mown turf treatments. Orchardgrass (Dactylis glomerata L.) received one or two yearly cutting treatments. Soil cores were collected to a 15-cm depth and divided into two 7.5-cm increments. Roots and mat were recovered from the cores, washed, dried and weighed. Differences in year, season, and soil depth significantly influenced root weight density. Mean seasonal root weight densities in the top 7.5 cm during the cool, wet growing season of 1992 were 4.81,4.36, and 3.67 mg cm-3 in spring, summer, and fall, respectively. In the lower 7.5-cm horizon the mean root weight densities were 1.38,1.28, and 0.93 mg cm-3. Mean seasonal root densities in the top 7.5 cm for the hotter, drier season of 1993 were 3.25,2.74, and 2.71 mg cm-3 for spring, summer, and fall, respectively. In the deeper horizon the mean root densities were 0.99, 0.95, and 0.93 mg cm-3. Root weight density differences attributable to grass species and cutting treatments were also significant. Differences among seasons, grass species, and cutting treatments significantly influenced mat weight density. Mean mat densities for combined species and treatments were 66.8, 53.3 and 71.6 mg cm-3 for spring, summer, and fall, respectively. In all species and treatments, new root growth was heavy in spring and ceased at the beginning of summer, with the lowest root weight densities occurring in late August. The less frequently cut grasses resumed new growth earlier in the fall than did regularly mown turf. An awareness of the seasonal fluctuations in root growth is necessary when researching or managing the rhizosphere. The quantity of roots present, cycles of growth and senescence, and subsequent root interaction with soil micro-organisms all exhibit seasonal differences with unique environmental consequences.