Carbon and Nitrogen Cycling under Warm Season Turfgrasses

Abstract

Despite its large-scale presence in the urban ecosystem, the role of turfgrasses in C and N cycling in urban soils in the southeastern United States has not been documented, and in particular C and N cycling studies in warm-season turfgrasses are lacking. The objectives of the proposed study were: 1) determine C sequestration under three major warm-season turfgrass species including: bermudagrass (Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy), centipedegrass (Erecholmoa ophroides Munroe Hack.), and zoysiagrass (Zoysia japonica Stued.), 2) determine CO2 flux from soil as affected by N applied to bermudagrass, and, 3) determine decomposition rates and C and N release of warm and cool season turfgrass clippings. The first objective was initiated in the winter of 2012 and conducted for two years on a Marvyn loamy sand (fine-loamy, kaolinitic, thermic Typic Kanhapludult). Eighteen lawns were sampled twice per year, six lawns of each grass species, with the harvested grasses separated into stems (rhizomes and/or stolons), above ground biomass (verdure), belowground roots, and belowground soil. Results showed an accumulation of organic C in the top 20 cm of the soil profile, with C sequestration ranging from 2.3±0.2 (bermudagrass) to 4.3±0.4 (zoysiagrass) Mg ha-1 yr-1. The second objective was initiated in March, 2012 on eight-year-old ‘Tifway’ hybrid bermudagrass plots on a Marvyn loamy sand. The experimental design was a randomized complete block with four N rates of 24, 49, 98, and 196 kg N ha-1 yr-1, replicated three times. Carbon dioxide flux was measured weekly for 95 weeks using an automated soil CO2 flux system. Results showed strong correlation between CO2 flux and N rate, and CO2 flux during the study period significantly increased from 107±4.5 to 144±4.4 Mg ha-1 as N rate increased from 24 to 196 kg N ha-1. The third objective was initiated on May 17, 2012 and conducted for 46 weeks at the Auburn University Turfgrass Research Unit. Litter from five turfgrasses was selected for this study including: bermudagrass, centipedegrass, St. Augustinegrass (Stenatophrum secumdatum L.), tall fescue (Lolium arundinaceum S.J. Darbyshire), and zoysiagrass. Litter was placed into nylon bags measuring 10 × 20 cm with 50 to 60 m openings based on an oven dry rate of 3.6 Mg ha¬-1. Litter bags were retrieved from the field after 0, 1, 2, 4, 8, 16, 24, 32, and 46 weeks, and retrieved bags were analyzed for total C and N. A double, four-parameter exponential decay model was used to describe mass, C, and N loss. Results indicated that tall fescue decomposition occurred more rapidly compared to other turfgrasses. In all, established turfgrasses are significant C sequestrators, although the degree of this C storage is affected by grass species and N fertilization.