J. T. Hill and John Rogers

USING PLANTS to absorb the over-supply of nutrients produced by animal waste is not a new concept, but determining which plants provide optimum uptake is an ongoing challenge for Alabama livestock producers. Auburn researchers recently determined that plants in the Phragmites species, including common reeds, provide excellent uptake of nutrients and subsequently produce high levels of biomass.

Constructed wetlands are being studied in Alabama and nationally as a pre-treatment for many kinds of agricultural waste, including livestock lagoon effluent, prior to its application to pastures and cropland. Not final disposal sites, these units are treatment processes which offer a method of reducing the nutrients in waste and providing for reuse of the nutrients.

Constructed wetlands are new to agricultural operations, being seriously considered as an alternative treatment process since the late 1980s. Hence, a study was conducted at the Swine Nutrition Unit on the Auburn campus to provide much needed data and knowledge to help Alabama producers make intelligent management decisions as to the potential and practicality of applying these filtered nutrients to common agricultural practices.

In this study, the effluent from the second cell of a two-cell anaerobic lagoon system (see figure) was used to treat waste from a swine house. This effluent was used as the input to five model wetland ponds, each with a monoculture of five different plant species, including Sagittari latifolia, arrowhead; Phragmites australis, common reed; Scirpus acutus, bulrush; Typha latifolia, cattail; and Juncus roemerianus, common rush. All of these plants are commonly found in wetlands.

Each model wetland pond was in the same environmental condition and provided the same influent from the second cell of the anaerobic lagoon for three months. The ponds were operated using a 12-day liquid detention time. During this period, three replications were made by harvesting the plants and measuring the dry biomass produced.

The nitrogen content of all five wetland plant species during this study was essentially the same, which demonstrates that nutrient uptake by the different species was approximately identical for the same plant mass. This means that the species which provides the greatest dry matter production will also provide the greatest nutrient removal when harvested. This is because nitrogen removal is proportional to dry matter production regardless of species.

The species ranking in dry matter production was: common reed, 20.4 grams per square meter per day; cattail and bulrush, 9.0; arrowhead, 7.5; and common rush, 0.9. From a management and environmental standpoint, common reed was by far the preferred species. Cattail, bulrush, and arrowhead were essentially the same, but less than half as productive as common reed, and common rush was the least competitive. This information is useful in selecting the best wetlands plant species for management of nutrients and environmental pollution abatement for agricultural operations that choose to use constructed wetlands for livestock production waste management.

Hill is a Professor and Rogers is a former Research Assistant in Agricultural Engineering