Estimates of White Clover Nitrogen Fixation within Maintained Bermudagrass Turf.

Efforts to decrease supplemental nitrogen (N) applications to turfgrass justify alternative fertility strategies such as legume inclusion. Legumes such as clovers (Trifolium spp.) are present within many turfgrass scenarios. Legume persistence is partly due to an ability to biologically fix atmospheric N, which is incorporated into the plant as proteins and other compounds. N is subsequently shared with associated grasses through the decomposition of legume -roots and -foliage. For this reason, turf health is often improved rather than diminished. Our research estimates biological N fixation within mixed bermudagrass (Cynodon spp.) -white clover (T. repens) swards.

Research was conducted within an existing bermudagrass (C. dactylon x C. transvaalensis) lawn at Auburn University’s Turfgrass Research Unit in Auburn, AL. Treatment factors were clover inclusion and supplemental N rate (0 to 8 g N m^-2) applied as CaNO₃, with Ca applied to uniformity via CaSO₄. White clover was seed-established in October of 2010 and 2011 (3 g m^-2). Supplemental N was applied monthly, April to August, during 2011 and 2012. Clover population densities were quantified via subsample plant counts. Plots were harvested monthly during active bermudagrass growth (April to October). Clippings were air-dried, weighed, and representative samples were partitioned into their constituent grass or clover parts. Grass-only and grass-plus-clover mixtures were analyzed for N and carbon (C) composition. N fixation was calculated using the difference method, by subtracting N-yield of grass-alone plots from the total N-yield of grass-plus-clover mixtures. Furthermore, the apparent N transfer was estimated as the difference between grass-alone N-yield of mixtures and that of grass monocultures. Means were separated using mixed models analysis. Adjusted 95% confidence intervals were used to detect significant differences between treatments. Data analyzed to present are from 2011 only.

Unlike previous research, N fixation did not differ due to supplemental N rates. With 95% certainty, apparent N fixation was between 18.9 and 23.1 g m^-2 across all supplemental N rates during the seven month long harvest cycle. N transfer between clover and bermudagrass was between 2.8 and 10.5 g m^-2 across all supplemental N rates, which equates to roughly 30% N transfer to the grass portion of the sward during active bermudagrass growth. Results also confirm that fixation is dependent upon clover population density and time of year. Clover populations were generally largest in early spring but quickly waned during summer months. N fixation followed this same decreasing trend. Clover populations were slightly decreased by the highest supplemental N rates, likely due to competition from the associated bermudagrass sward.

These results and others provide evidence of clover’s utility as an amenity plant within bermudagrass swards. Ongoing research evaluates N contributions of foliage deposited during mowing occurrences and soil C change as a result of mixed grass-clover swards. Future research may evaluate isotopic signatures of sequestered C. Further analysis of N fate within mixed swards is required in order to better assign nutrient credits to the practice of clover inclusion.