Effects of Tree Root-Derived Substrates and Inorganic Nutrients on Pyrene Mineralization in Rhizosphere and Bulk Soil Kevin E. Mueller and Jodi R. Shann* ABSTRACT This study investigated the effects of organic and inorganic nu- trients on the microbial degradation of the common soil contami- nant pyrene. The material used in this investigation was collected from potted trees that had been growing for over a year in a soil artificially contaminated with polycyclic aromatic hydrocarbons. Soil was removed from the nonroot (bulk) and root (rhizosphere) zones of these pots and used in mineralization studies that tracked mi- crobial degradation of 14C-pyrene. The factors influencing degra- dation in these zones were then tested by amendment with essential inorganic nutrients or with root-derived materials. As expected, pyrene mineralization was greater in soil removed from the rhi- zosphere than in bulk soil. The rate of mineralization in rhizosphere soil was inhibited by inorganic nutrient amendment, whereas nu- trients stimulated mineralization in the bulk soil. Pyrene mineraliza- tion in bulk soil was also increased by the addition of root extracts intended to mimic exudation by living roots. However, amendment with excised fine roots that were allowed to decay over time in soil initially inhibited mineralization. With time, the rate of mineraliza- tion increased, eventually exceeding that of unamended bulk soil. Combined, the initial inhibition and subsequent stimulation produced a zero net impact of decaying fine roots on bulk soil mineralization. Our results, in conjunction with known temporal patterns of fine root dynamics in natural systems, support the idea that seasonal variations in nutrient and substrate availability may influence the long-term effect of plants on organic degradation in soil, possibly reducing or negating the beneficial effects of vegetation that are often observed in short-term studies. ROOT-DERIVED CARBON SUBSTRATES, released during exudation and during the decay of fine roots, are important sources of plant influence on microbial com- munities in the rhizosphere. These root-derived sub- strates are often implicated in explaining enhanced microbial degradation of persistent organic pollutants (POPs), including polycyclic aromatic hydrocarbons (PAHs), in soil. Indirect support for this comes from findings that PAH dissipation and counts of microbial PAH degraders increase with proximity to plant roots (Joner and Leyval, 2003). In addition, POP-degrading microorganisms are capable of utilizing root-derived carbon substrates as a sole energy source (Leigh et al., 2002; Rentz et al., 2004), potentially increasing their ability to colonize and function in the rhizosphere. Relatively little work has been done to directly test the effect of root exudates or fine root decay on micro- Department of Biological Sciences, University of Cincinnati, Cincin- nati, OH 45221-0006. Received I Apr. 2006. *Corresponding author (Jodi.shann@uc.edu). Published in J. Environ. Qual. 36:1204127 (2007). Technical Reports: Organic Compounds in the Environment doi: 10.21134/jeq2006.0130 © ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA bial degradation of PAHs. While several studies have shown that root exudates (or compounds that mimic exudates) can enhance mineralization or dissipation of PAHs (Yoshitomi and Shann, 2001; Joner et al., 2002), others report no effect (Wetzel et al., 1997) or even inhibition (Kamath et al., 2004; Qiu et al., 2004; Rentz et al., 2004). The impact of fine root decay on mi- crobial degradation of PAHs has rarely been tested (Parrish et al., 2005). If both root exudation and decay increase the microbial degradation of PAHs, the peren- nial nature and size of tree root systems would be ad- vantageous for phytoremediation of PAH-contaminated soils. However, seasonal root dynamics exhibited by temperate trees (Tierney et al., 2003; Mueller and Shann, 2006) could lead to variable effects on PAH degrada- tion in soil. The few experiments aimed at using trees to remediate PAH contamination have had mixed re- sults (Liste and Alexander, 2000; Qiu and Loehr, 2002; Genney et al., 2004; Tang et al., 2004; Mueller and Shann, 2006). Other gradients between bulk soil and the rhizosphere may also impact POP dissipation. Concentrations of ni- trogen, phosphorus, and other essential inorganic nu- trients can differ widely between rhizosphere and bulk soils, with rhizosphere soil often being relatively nutrient- depleted (Wang et al., 2001; Wang et al., 2004). Since microbes and plants are known to compete for soil nu- trients such as nitrogen (Hodge et al., 2000), it is clear that these gradients may also be important in determining the outcome of phytoremediation trials. For example, in a field trial, mineralization of phenanthrene (a common PAH) in planted soil was increased relative to unvege- tated soil only after amendment with nitrogen and phos- phorus (Siciliano et al., 2003). This was attributed to nutrient limitation of microbial PAH degraders in the planted soil. Even in the absence of plants, PAH deg- radation is often limited by inorganic nutrient availabil- ity (Breedveld and Sparrevik, 2000; Joner et al., 2002) although this is not always the case (Carmichael and Pfaender, 1997; Johnson and Scow, 1999). Despite our minimal understanding regarding the effect of inorganic soil nutrients on POP dissipation in planted soils, planted and unplanted treatments in phytoremediation trials are often fertilized. The purpose of this study was to investigate the in- fluence of living and decaying fine roots of red maple (Acer rubrum) on microbial degradation of a model PAH, pyrene. Specifically, we tested: (1) the ability of microbial communities in rhizosphere and bulk soils to mineralize freshly added pyrene, and (2) the impacts of Abbreviations: POPs, persistent organic pollutants; PAHs, polycyclic aromatic hydrocarbons. 120