Spring Nitrogen Fertilization of Ryegrass–Bermudagrass for Phytoremediation of Phosphorus-Enriched Soils

Published in Agron. J. 104:908–916 (2012) Posted online [DATE] doi:10.2134/agronj2011.0385 Copyright © 2012 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. I the southeastern United States, the hay and pasture fields near broiler chicken (Gallus gallus domesticus) farms often have soils with high residual fertility, particularly P, due to repeated or heavy application of broiler litter (a mixture of manure, wasted feed, and bedding materials) (Sharpley, 2003; Read et al., 2009). Soils with elevated P concentrations may represent a risk for nutrient transport via runoff or leaching and ultimately contribute to eutrophication of surface waters (Pierson et al., 2001; DeLaune et al., 2004). Growing high biomass yielding forage crops provides a means of removing P from soils and reducing the rate and extent of P accumulation. The nutrients taken up by the plants are removed when the hay is harvested. This process, termed phytoremediation, is slow, largely because concentrations of P in forages are relatively low (Brink et al., 2001; Pant et al., 2004). Bermudagrass, a warm-season perennial forage, is preferentially used in these fields because it has a high N requirement; however, annual uptake of P is strongly attenuated by its winter dormancy (Novak and Chan, 2002). Overseeding annual ryegrass in dormant bermudagrass extends the period of active crop growth into the autumn and spring, providing opportunity for increased dry matter (DM) production and utilization of manure nutrients (Brink et al., 2001; Read et al., 2007). Annual ryegrass is the primary species used due to its adaptability to a broad range of soil and climatic conditions, ease of establishment, late maturity compared with small grains, excellent forage quality, and tolerance of close, continuous stocking (Brink et al., 2006; Lemus, 2009). Because ryegrass is a cool-season plant and bermudagrass is a warm-season plant, the two can grow at the same site with little competition and can provide year round phytoremediation of P-enriched soil. Mississippi has adopted the P-index (PI) risk assessment approach to estimate P transport potential from fields receiving animal manure (Osmond et al., 2006; Oldham, 2007). The Mississippi version of the PI provides a site-specific qualitative rating based on three transport factors (erosion, runoff, and distance to water) and five P-source characteristics. The soil test P category used in this PI is determined from measurements of P concentration in 0to 15-cm soil depth (STP) using the Mississippi Soil Test method, described by Cox (2001). In an evaluation of the PI approach for pastures, DeLaune et al. (2004) concluded PI-based broiler litter application allows more management options, such as the rate or method of litter application and soluble P application rate, as compared to litter application based on a STP threshold value. They found significant correlation between STP (by Mehlich-3 extraction) and concentration of soluble reactive P in runoff water, which suggests knowledge of STP can be used to assess potential for P runoff from the land before manure is applied, and thereby help a grower be proactive about when to resume manure applications. For high P transport risk sites where broiler litter use is proposed, producers are encouraged to eschew litter use and/or apply at rates that satisfy crop P requirements, as opposed to N requirements (Oldham, 2007). Where litter use is curtailed or lessened in these cases, STP will remain high unless the added P is either tied-up through ABSTRACT Because annual ryegrass (Lolium multiflorum L.) exhibits increases in herbage P with increasing soil P, biomass response to N should influence phytoremediation of P-enriched soils. Field plot (each 2 by 5 m) studies were conducted to determine the effect of six N schemes that provided 112 or 168 kg N ha–1 on uptake of P and soil test P (STP, 0–15-cm depth) of double-cropped bermudagrass [Cynodon dactylon L. (Pers.)] and ryegrass. Bermudagrass was previously fertilized with 9 and 18 Mg broiler litter ha–1 in 2004–2007, no litter was applied after 2007. At past litter rate of 9 Mg ha–1, total uptake of P averaged 48.5 kg ha–1 in 2008 and 51.7 kg ha–1 in 2009. Based on initial STP levels in 2007, results in 2009 indicated STP levels decreased by approximately 50 kg ha–1 yr–1, with reductions of 44% at 9 Mg litter ha–1 (104–58 mg kg–1) and 38% at 18 Mg litter ha–1 (173–108 mg kg–1). Averaged across years and litter rates, uptake of P in ryegrass was greater at 168 than 112 kg N ha–1 (35.4 vs. 29.4 kg ha–1, P < 0.01) and ranged from 35.8 kg ha–1 in scheme N5 to 26.7 kg ha–1 in scheme N1. Schemes N5 and N6, with a total rate of 168 kg N ha–1 in March and May, stimulated uptake of P in ryegrass and ryegrass–bermudagrass, but STP levels in 2009 were not lower than a single application of 112 kg N ha–1 in January and March.