Rhizosphere Effects on Decomposition: Controls of Plant Species, Phenology, and Fertilization

or a suppressive influence on SOM decomposition (Van Veen et al., 1991; Cheng, 1999). As a measure of main Plant species and soil fertility presumably control rhizosphere efenergy use for the acquisition of belowground resources fects on soil organic matter (SOM) decomposition, but qualitative (e.g., nutrients and water), rhizosphere respiration may and quantitative descriptions of such controls are still sparse. In this study, rhizosphere effects of soybean [Glycine max (L.) Merr.] and range from 30 to 80% of total belowground CO2 efflux spring wheat (Triticum aestivum L.) on SOM decomposition were (Rochette and Flanagan, 1997; Hanson et al., 2000) in investigated at four phenological stages under three levels of fertilizavarious ecosystems. Root-associated C fluxes represent tion in a greenhouse experiment using natural 13C tracers. The magnia major portion of the input to and the output from tude of the rhizosphere effect ranged from 0% to as high as 383% the belowground C pool (Schimel, 1995). However, the above the decomposition rate in the no-plant control, indicating that regulating mechanisms and the magnitude of the rhizothe rhizosphere priming can substantially intensify decomposition. sphere’s contribution to such fluxes have not been adeThe rhizosphere priming effect was responsible for a major portion quately addressed, particularly with respect to the ecoof the total soil C efflux. Cumulative soil C loss caused by rhizosphere logical linkages that functionally connect rhizosphere effects during the whole growing season equated to the amount of processes with soil fertility and SOM decomposition. root biomass C for the soybean treatment, and 71% of root biomass C for the wheat treatment. Different plant species produced significantly Although studies have indicated that input of labile different rhizosphere priming effects. The overall rhizosphere priming substrates in the rhizosphere may significantly enhance effect of soybean plants was significantly higher than for wheat plants. SOM decomposition as a result of the priming effect Plant phenology significantly influenced the rhizosphere priming ef(Helal and Sauerbeck, 1986; Liljeroth et al., 1994), rates fect. Little rhizosphere effect occurred in both wheat and soybean of SOM decomposition are commonly assessed by labotreatments initially. The priming effect of the wheat rhizosphere ratory incubations of soil samples with an assumption reached 287% above the no-plant control at the flowering stage and that rhizosphere processes have little impact on the redeclined significantly afterward. The priming effect of the soybean sults. This assumption has rarely been rigorously tested. rhizosphere peaked at 383% above the no-plant control during the Roots have been found to have both stimulatory and late vegetative stage and remained at high levels onward. Contrary inhibitory effects on SOM decomposition. Laboratory to many published reports, NPK fertilization did not significantly modify the rhizosphere priming effect. experiments have shown that when 14C-labeled plant material was decomposed in soils planted with maize (Zea mays L.), spring wheat, or barley (Hordeum vulgare L.), CO2 release from the soil was reduced comC all the pools and fluxes of C within pared with bare soil (Reid and Goss, 1982, 1983; Sparecosystems, C cycling belowground is increasingly ling et al., 1982). In contrast, a stimulatory effect has being recognized as one of the most significant compobeen reported in other laboratory experiments using nents of the global C cycle (Jackson et al., 1997), yet 14C-labeled litter (Helal and Sauerbeck, 1986; Cheng the least understood (Zak and Pregitzer, 1998). Beand Coleman, 1990). Furthermore, other research has lowground CO2 efflux can be partitioned into two disshown that SOM decomposition is dependent on the tinct processes: (i) rhizosphere respiration or rootlength of exposure to living roots. In a 2-yr study, roots derived CO2, including root respiration and microbial suppressed the decomposition of newly incorporated respiration utilizing materials released from live roots 14C-labeled plant material during the first 200 d but and, (ii) microbial decomposition of SOM, or soilstimulated the mineralization of the 14C-labeled materiderived CO2. Separating these two processes is necesals in the soil during the latter stage when compared sary for assessing how environmental changes may alter with bare soil (Sallih and Bottner, 1988). Recently we the balance of CO2 flux from belowground ecosystems have found that different combinations of plant species because the controls and responses of the two processes and soils resulted in different rhizosphere effects on are likely to respond distinctly to variable environments. SOM decomposition (Fu and Cheng, 2002). Several While the two processes act separately, they may also studies have also suggested that soil mineral nutrition be linked through rhizosphere interactions (Andrews et is an important modifier of rhizosphere effects (Merckx al., 1999), which may exert a stimulative (priming effect) et al., 1987; Liljeroth et al., 1994; Ehrenfeld et al., 1997); the direction of rhizosphere effects may change deWeixin Cheng, Dep. of Environmental Studies, Univ. of California, pending on the level of the soil mineral nutrients. HowSanta Cruz, CA 95064; Dale W. Johnson, Dep. of Environmental and ever, most studies have focused on the occurrence of Resource Sciences, Univ. of Nevada, Reno, NV 89557; and Shenglei rhizosphere effects, with little attention being paid to Fu, Dep. of Nematology, Univ. of California, Davis, CA 95616. Received 1 Sept. 2002. *Corresponding author ([email protected]). the multiple factors that may influence it. Published in Soil Sci. Soc. Am. J. 67:1418–1427 (2003).  Soil Science Society of America Abbreviations: DAP, days after planting; DI-H2O, deionized water; PVC, polyvinyl chloride; SOM, soil organic matter. 677 S. Segoe Rd., Madison, WI 53711 USA