Plant inputs of carbon to metal-contaminated soil and effects on the soil microbial biomass

The amounts of soil microbial biomass in metal-contaminated (high-metal) soils of the Woburn Market Garden Experiment in the U.K. are now about half those in similar uncontaminated (low-metal) soils from the same experiment. The metal-contamination was caused by applications of metal-rich sewage-sludge which ceased about 30 yr ago. Soil metal concentrations in the high-metal soils are now at, or a little above, current European Community limits. This work was designed to see if heavy metals decreased soil microbial biomass by decreasing the input of plant material to the soil or if the synthesis of microbial biomass is less efficient in the presence of heavy metals. Either or both of these mechanisms could explain the effects of heavy metals on microbial biomass in the Woburn experiment. Sunflower (Helianthus annus L., cultivar Sunbred 246) seedlings were grown for 31 days under controlled conditions (12 h day at 20 degrees C, 12 h night at 17 degrees C) in a low-metal or a high-metal soil from the Woburn experiment. From days 21 to 31 of growth the plants were supplied with 14C-labelled CO2 on alternate days. The final dry matter yield (shoots plus roots) of the plants grown on the low-metal soil was about 30% greater than that of the plants grown on the high-metal soil. The distribution of total C and 14C-labelled C between the various plant and soil compartments viz. plant shoots, roots, soil microbial biomass and bulk soil, were measured 2 days after the final 14(CO)2 labelling. The percentage distribution of 14C within shoots, roots, soil microbial biomass and bulk soil was quite similar in both soils. Plant-derived 14C-labelled organic C inputs into the high-metal soil were about 20% less than in the low-metal soil. About 35% less of this 14C-labelled C was in the microbial biomass in the high-metal soil than the low-metal soil at harvest. The plants caused an increase in total biomass C of about 22 and 42 micrograms C g-1 soil respectively in the high-metal and low-metal soil at harvest, about half of which was 14C-labelled in both cases. These increases in biomass were thus in the same ratio as those of the biomass in high-metal and low-metal soils taken directly from the field. These results suggest that both mechanisms (i.e. decreased inputs of C from plants to the soil and decreased efficiency of conversion of this C into new biomass C) operate in causing smaller biomasses in metal-contaminated soils of the Woburn experiment. The latter mechanism would appear more important than the former.