Budgets for root-derived C and litter-derived C: comparison between conventional tillage and no tillage soils K.W. Kisselle a, * , C.J. Garrett a,1 , S. Fu a,2 , P.F. Hendrix a , D.A. Crossley Jr. a , D.C. Coleman a , R.L. Potter a,b a Institute of Ecology, University of Georgia, Athens, GA 30602, USA b Department of Crop And Soil Sciences, University of Georgia, Athens, GA 30602, USA Received 30 August 1999; received in revised form 16 August 2000; accepted 15 November 2000 Abstract Placement of plant residues in conventional tillage (CT) and no-tillage (NT) soils affects organic matter accumulation and the organization of the associated soil food webs. Root-derived C inputs can be considerable and may also in¯uence soil organic matter dynamics and soil food web organization. In order to differentiate and quantify C contributions from either roots or litter in CT and NT soils, a 14 C tracer method was used. To follow root-derived C, maize plants growing in the ®eld were 14 C pulse-labeled, while the plant litter in those plots remained unlabeled. The 14 C was measured in NT and CT soils for the different C pools (shoots, roots, soil, soil respiration, microbial biomass). Litter-derived C was followed by applying 14 C labeled maize litter to plots which had previously grown unlabeled maize plants. The 14 C pools measured for the litter-derived CT and NT plots included organic matter, microbial biomass, soil respiration, and soil organic C. Of the applied label in the root-derived C plots, 35±55, 6±8, 3, 1.6, and 0.4±2.4% was recovered in the shoots, roots, soil, cumulative soil respiration, and microbial biomass, respectively. The 14 C recovered in these pools did not differ between CT and NT treatments, supporting the hypothesis that the rhizosphere microbial biomass in NT and CT may be similar in utilization of root-derived C. Root exudates were estimated to be 8±13% of the applied label. In litter-derived C plots, the percentage of applied label recovered in the particulate organic matter (3.2±82%), microbial biomass (4±6%), or cumulative soil respiration (12.5±14.7%) was the same for CT and NT soils. But the percentage of 14 C recovered in CT soil organic C (18±69%) was higher than that in NT (12±43%), suggesting that particulate organic matter (POM) leaching and decomposition occurred at a higher rate in CT than in NT. Results indicate faster turnover of litter-derived C in the CT plots. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: No tillage; Root exudate; Rhizosphere; Soil organic matter; Microbial biomass; Carbon allocation 1. Introduction Under conventional tillage (CT) management, plant resi- dues are incorporated into the soil, whereas under no-tillage (NT) management, they are placed on the soil surface. Placement of plant residues, more than tillage itself, may affect decomposition of the residues and the organization of associated soil food webs (Holland and Coleman, 1987; Beare et al., 1992). Studies of soil food webs associated with buried litter and surface litter have indicated that buried litter systems are more bacteria-dominated, while surface litter systems are more fungi-dominated (Hendrix et al., 1986; Holland and Coleman, 1987; Beare et al., 1992, 1995). This is expected to result in faster turnover of inputs in CT with greater stabilization of inputs into organic matter in NT soil. Beare et al. (1997) found that fungi helped increase water-stable aggregates in NT, and that organic matter in macroaggregates was less microbially-processed than organic matter in microaggregates. In a simulated NT lab study, Gale and Cambardella (2000) found that 42% of root-derived C was in the soil, while only 16% of litter- derived C was in the soil after 360 days. They concluded the bene®ts of NT management are mainly from the increased retention of root-derived C in the soil. The importance of root-derived C inputs to soil organic matter and soil food webs may be considerable. Campbell et al. (1991) found no differences in soil organic matter Soil Biology & Biochemistry 33 (2001) 1067±1075 0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(01)00012-8 www.elsevier.com/locate/soilbio * Corresponding author. Present address: US Environmental Protection Agency, 960 College Station Road, Athens, GA 30605-2700, USA. Tel.: 11-706-355-8126; fax: 11-706-355-8104. E-mail address: kisselle.keith@epa.gov (K.W. Kisselle). 1 Current address: 1371 County Road 389, Opelika, AL 36804, USA. 2 Current address: Department of Agricultural and Social Sciences, University of California at Santa Cruz, Santa Cruz, CA 95064, USA.