Budgets for root-derived C and litter-derived C: comparison between conventional tillage and no tillage soils

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 influence 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 C-14 tracer method was used. To follow root-derived C, maize plants growing in the field were C-14 pulse-labeled, while the plant litter in those plots remained unlabeled. The C-14 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 C-14 labeled maize litter to plots which had previously grown unlabeled maize plants. The C-14 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 C-14 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 C-14 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. (C) 2001 Elsevier Science Ltd. All rights reserved.