Factors affecting the distribution and dynamics of14C in two soils cropped to barley

The distribution of¹²C and¹⁴C and dynamics of¹⁴C in barley plots were studied after pulse labelling with¹⁴CO₂. Barley was grown in microplots on a Black soil at Ellerslie and a Gray Luvisol at Breton, Alberta and was sampled on four dates between July 31 and October 20. The quantity of¹²C in shoots, microbial biomass, and soil was greater at Ellerslie than Breton. Root¹²C did not differ between sites. There were no significant differences over time in the quantity of¹²C in any of the pools. On the first sampling date 18.59% of the¹⁴C was recovered in the soil-plant system at Ellerslie and 60.82% was recovered at Breton. At Ellerslie 20.35 MBq were recovered in shoots, 0.50 in roots, 0.08 in microbial C and 0.77 in soil while at Breton 27.92 MBq were recovered in shoots, 2.70 in roots, 0.22 in microbial C and 39.3 in soil. The difference between sites in soil¹⁴C was due to higher water filled porosity at Breton than at Ellerslie at the time of labelling. Soluble C (gg^?1 root C), used as a measure of root exuded C, increased above 70% water filled porosity. There were no significant differences over time in the quantity of¹⁴C in any of the pools or in the specific activity of¹⁴CO₂ released from soil during 10-day laboratory incubation. This indicated that the belowground system was in steady state with a continuous input of¹⁴C from labelled root matrial. Differences in specific activity of the various belowground pools revealed that an average of 17% of the microbial C was active at Ellerslie while 43% was active at Breton. Active microbial C (gm^?2) was the same at both sites because total microbial C was lower at Breton than at Ellerslie. At Breton some of the¹⁴C released under conditions of high water filled porosity at the time of labelling appeared to be stabilized against microbial turnover.