Quantifying plant contributions to root‐zone available C during the growing season is important for better understanding of microbial processes regulating N cycling. However, it is difficult to determine root‐derived available C under conditions that better reflect plant growth in the field. We proposed an approach using 13 C natural abundance in soil with a history of C 3 vegetation, which is planted to a C 4 plant, to estimate the contribution of C 4 root‐derived C to soil microbial biomass by measuring the δ 13 C change in microbial biomass C during crop growth. In a pilot greenhouse study using this approach with large‐diameter (30.5 cm) repacked soil cores, we estimated that 48 and 121 kg C ha −1 of root‐derived available C was produced, which accounted for 16 and 13% of total C 4 ‐C entering the soil during 5 and 10 wk growth of corn ( Zea mays L.), respectively. Similarly, 88, 283, and 402 kg C ha −1 of root‐derived available C from corn were estimated at 4, 8, and 16 wk, respectively, in a repeated experiment. Thus, the 13 C natural abundance signals in microbial biomass can provide a practical and reliable way to assess available C released from plant roots during growth.
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Qian et al. (1996) studied this question.