Biofuels Agronomy Journal  Volume 101, Issue 6  2009 1363 Published in Agron. J. 101:1363–1371 (2009) Published online 26 Aug. 2009 doi:10.2134/agronj2008.0225x Copyright © 2009 by the American Society of Agronomy, 677 South Segoe 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 in- formation storage and retrieval system, without permission in writing from the publisher. R ecent attention has focused on the use of perennial, warm-season grasses as renewable energy crops (McLaugh- lin et al., 2002; Greene et al., 2004; Sims et al., 2006). Tese species are considered to be particularly well-suited for the production of biomass for energy applications because they utilize C4 photosynthesis and are perennial (Heaton et al., 2004). Grasses that employ the C4 photosynthetic pathway typically use water, N, and solar radiation more efciently than plants having the C3 pathway (Brown, 1999), and therefore are generally more productive per unit land area and resource input relative to other potential energy crops (Black, 1971; Lewandowski and Schmidt, 2006). Perenniality also confers important advantages to energy crops, including the ability to cycle nutrients seasonally between shoots and roots (Clark, 1977; Heckathorn and Delucia, 1996), thus improv- ing feedstock quality and minimizing fertilizer require- ments for sustained biomass production (Muir et al., 2001; McLaughlin and Kszos, 2005). Despite high N-use efciency by grasses possessing both C4 photosynthesis and a perennial lifecycle, numerous studies have demonstrated that signifcant fertilizer N inputs are required to optimize biomass production by these species when managed as forage crops (Brejda, 2000). Although relatively few studies have assessed the efect of N fertilization on yield of peren- nial, warm-season grasses managed specifcally as bioenergy feedstocks (Ma et al., 2001; Muir et al., 2001; Tomason et al., 2004; Lemus et al., 2008a), there is emerging consensus that N fertilization requirements should be reduced for single-harvest feedstock management systems relative to multiharvest forage systems (Parrish and Fike, 2005; McLaughlin and Kszos, 2005), as the latter are characterized by greater N removal as a result of harvest of immature, N-rich biomass (Reynolds et al., 2000). Nonetheless, information regarding yield optimal N requirements for perennial, warm-season grasses managed as energy crops is limited in the central United States, particu- larly for species other than switchgrass. Additionally, relatively little is known regarding the efects of N fertilization on other important attributes of perennial, warm-season grasses, includ- ing partitioning of biomass and nutrients to crowns and roots for remobilization in subsequent growing seasons. Allocation of plant nutrients to roots before crop harvest is a desirable trait for energy crops, as a high mineral concentra- tion negatively afects biomass quality for bioenergy applica- tions, especially thermochemical processes (McKendry, 2002). Additionally, nutrients retained in roots can be recycled by the crop for future growth, thus reducing long-term fertilization requirements (Clark, 1977; Beale and Long, 1997). Previous research on switchgrass and eastern gamagrass indicated that N fertilization can result in increased shoot N concentra- tions, and therefore increased N removal with biomass harvest (Brejda et al., 1996; Vogel et al., 2002). However, the efect of ABSTRACT Recent attention has focused on the use of perennial, warm-season grasses as renewable energy crops. Te objective of this study was to assess the efects of N fertilization on partitioning of biomass and nutrients between above- and belowground plant components by four warm-season grass species in Iowa. In 2006–2007, established stands of big bluestem (Andropogon geradii Vitman), switchgrass (Panicum virgatum L.), indiangrass [Sorghastrum nutans (L.) Nash], and eastern gamagrass [ Tripsacum dactyloides (L.) L.] were fertilized with 0, 65, 140, or 220 kg N ha -1 in the spring and harvested following frost in the fall. Dependent on grass species and year, yield response to N was linear or quadratic. Optimum yield afer 2 yr was 13.5 Mg ha -1 at 140 kg N ha -1 for all grasses except eastern gamagrass, which demonstrated lower yield and a consistent linear N response. Nitrogen inputs had pronounced but grass-specifc efects on root biomass and nutrient partitioning. For big bluestem and switchgrass, 140 kg N ha -1 maximized root biomass and favored allocation of nutrients to roots over shoots. In contrast, for indiangrass and eastern gamagrass, root biomass and root nutrient allocation were adversely afected by N inputs. For all grasses, 220 kg N ha -1 shifed allocation of nutrients to shoots over roots. Selection of crops and management practices that optimize yield, and maintain a high level of resource partitioning to roots at low to intermediate N input rates will promote the develop- ment of productive and efcient bioenergy systems. A.H. Heggenstaller, Midwest Research Institute, 425 Volker Blvd., Kansas City, MO 64110; K.J. Moore and M. Liebman, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011; R.P. Anex, Dep. of Agriculture and Biosystems Engineering, NSRIC Buiding, Iowa State Univ., Ames, IA 50011. Received 12 Dec. 2008. *Corresponding author (aheggenstaller@mriresearch.org). Abbreviations: OPT, optimal partitioning theory; PLS, pure live seed; RSR, root–shoot ratio. Nitrogen Influences Biomass and Nutrient Partitioning by Perennial, Warm-Season Grasses Andrew H. Heggenstaller,* Kenneth J. Moore, Matt Liebman, and Robert P. Anex Published November, 2009