Decomposition and nutrient release from fresh and dried pine roots under two fertilizer regimes

Root decomposition and nutrient release are typically estimated from dried root tissues; however, it is unlikely that roots dehydrate prior to decomposing. Soil fertility and root diameter may also affect the rate of decomposition.. This study monitored mass loss and nutrient concentrations of dried and fresh roots of two size classes (<2 and 2-5 rnrn) over a 12-month period in fertilized and control plots in a 13-year-old loblolly pine (Pinus taeda L.) plantation. Nutrient content was calculated and used to assess the effects of fertilization, root size, and initial condition (hydration) on nutrient release rates. Roots that grew and decomposed in fertilized plots had higher concentrations and greater total release of N, P, K, and Mg than roots in control plots, but C concentrations and mass loss rate were not significantly different between roots in fertilized plots and those in control plots. Very fine roots (<2 mm) had higher concentrations of N, P, and Ca and faster release rates for C, N, and K than fine roots (2-5 mm), resulting in greater total release of C and N. Roots dried prior to decomposition decayed and released C, K, Ca, and Mg at a faster rate than fiesh roots. Results indicate that using dried root tissues will overestimate fme root decomposition and nutrient cycling rates. RCsume' : La dCcomposition des racines et la liberation des nutriments sont typiquement estimkes ?i partir de tissus racinaires sCchCs. Cependant, il est peu probable que les racines se dbshydratent avant de se dCcomposer. La fertilite du sol et le diam6tre des racines peuvent aussi affecter le taux de decomposition. Cette Ctude a suivi 17Cvolution de la perte de masse et de la concentration des nutriments de racines fraiches et sCchees appartenant B deux classes de dimension (<2 et 2-5 m). L'Ctude s'est poursuivie pendant une p6riode de 12 mois dans des parcelles fertilis6es et t6moins Ctablies dans une plantation de pin B encens (Pinus taeda L.) BgCe de 13 ans. Le contenu en nutriment a CtC calculC et utilisd pour Cvaluer les effets de la fertilisation et de la dimension et de I7Ctat initial (hydratation) des racines sur le taux de liberation des nutriments. La concentration et la liberation totale de N, P, K et Mg Ctaient plus &levees chez les racines qui se sont dCveloppCes et dCcomposCes dans les parcelles fertilisCes cornparativement aux racines dans les parcelles tCmoins mais aucune difference significative n'a CtC observCe dans la concentration de C ou le taux de perte de masse. Les racines trhs fines (<2 mm) avaient des concentrations de N, P et Ca et des taux de liberation plus ClevCs pour C, N et K que les racines fines (2-5 mm), ce qui se traduisait par une 1iMration totale plus ClevCe de C et N. Les racines sCchCes avant leur dCcomposition se sont dCcompos&s et ont IibdrC C, K, Ca et Mg plus rapidement que les racines frakhes. Les rCsultats indiquent que l'utilisation de tissus racinaires sCchCs entralne la surestimation des taux de dCcomposition et de recyclage des nutriments chez les racines fines. [Traduit par la RCdaction] Introduction ents from roots upon senescence (Nambiar 1987; Aerts 1990; Nutrient inputs from the atmosphere and soil weathering are key to long-term sod development and ecosystem sustainability, but on an annual basis, nutrient recycling within an ecosystem provides the major source of nutrients for plants (Joslin and Henderson 1987; Fahey et al. 1988). Although fine roots and foliage account for a small percentage of total mature pine stand biomass, they contain roughly a quarter of the stand's nitrogen (N) and phosphorus (P) (Van Lear and Kapeluck 1995). Because fine roots often have higher concentrations of nutrients than current foliage (Meier et al. 1985), and because there is minimal retranslocation of nutriNambiar and ~ i f e 1991), decomposing fine roots are an important source of nutrient recycling (Wells et al. 1975; Persson 1979; Raich and Nadelhoffer 1989; Vogt et al. 1991). Calculations of N returns to soil from decomposing hardwood fine roots have been estimated to be 54 to 72 kg-hal -ye~ ' (Hendrick and Pregitzer 1993). Using a global data set, Silva and Miya (2001) determined that root chemistry (particularly Ca and C/N) appeared to be the primary controller of root decomposition rate, while climate and environmental factors played secondary roles. In a study by Fahey et al. (1988), initially high rates of N and P release were observed for fine roots, whereas those nutrients were effectively retained in decaying woody roots. A strong Received 14 February 2005. Accepted 27 September 2005. decline in nutrient concentration with an increase in diamePublished on the NRC Research Press Web site at ter of fine roots has been found for N and P in an Abies http://cjfr,nrc.ca on 19 January 2006. amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes stand (Vogt I K.H. ~udovici' and L.W. Kress. USDA Forest Service, et al. 1982), for N in a range of hardwood and pine forests Southern Research Station, 3041 Cornwallis Road, Research (McClaugherty et a]. 1982; Gordon and Jackson 2000), and Triangle Park, NC 27709, USA. for N, P, K, Ca, and Mg in Pseudotsuga rnenziesii (Mirb.) Franco (Fogel and Hunt 1983). The inverse relationship be'corresponding author (e-mail: [email protected]). tween N, P, and Ca concentration and root diameter was subCan. J. For. Res. 36: 105-1 11 (2006) doi: 10.1 139K05-227 63 2006 NRC Canada Can. J. For. Res. Vol. 36, 2006 Fig. 1. Effects of fertilization on (A) root mass remaining, (B) nitrogen concentration, and (C) phosphorus concentration in loblolly pine roots decomposing in situ over a 12-month period. Root size and initial condition have been averaged by fertilization treatment. Error bars represen{ one standard deviation, and each data point represents n = 12. + Contrd -b Fertilized O-' 1 -AControl -AFertilied 0.2 -