The use of C14 labeled plant material to measure the turnover of organic residues in soil
Biederbeck, Volkmar Otto
This investigation was designed to measure, using C14, the dissimilation of added plant tissue in soil and to study the effect of these additions upon the mineralisation of native soil organic matter. Oat plants were uniformly labeled by growth in an atmosphere enriched with C14O2. A biosynthesis chamber, built from ordinary laboratory equipment, was used for the labeling of plants. To facilitate uniform labeling, the net assimilation of C14O2 was measured by tracing the changes in radioactivity in the atmosphere of the chamber. At the end of the 35 day labeling period, 98.7% of the released radioactivity was recovered in the plant and soil system. Eighty six percent of this was retained in the above-ground parts of the plants. Dry sieving and hand picking removed 54% of the labeled root material from the soil. Subsequent incubation of this soil showed a rapid mineralisation (86%) of the unrecovered labeled material. The addition of 1% (22.6 tons/ha or 10 tons/acre) labeled oat straw to a Chernozemic (Oxbow) and to a Grey Wooded (Waitville) soil resulted in a markedly accelerated mineralisation of native organic matter ("priming effect"), especially during the initial two weeks of incubation. The amount of organic matter lost due to priming, after 222 days of incubation, was quite considerable. Thus, 34% and 58% more native soil carbon was mineralized from the amended Oxbow and Waitville soils than from the respective unamended soils. The priming effect tapered off earlier in the Waitville than in the Oxbow soil. This difference in duration of priming and in the susceptibility towards priming is evidence of a more rapid turnover of humus in the Grey Wooded soil. Isotopic measurements showed that the extent of plant tissue mineralisation was essentially equal in both soils. A comparison of tracer and non-tracer techniques revealed that non-isotopic methods of evaluation considerably over-estimated (in some cases up to 27%) the extent of plant residue mineralisation in soil. A radioactivity balance sheet for the incubated soils showed that either the direct measurement of C14O2 evolved or the indirect determination by measuring the amount of C14 retained in soil, were reliable in estimating the amount of plant residues decomposed in soil after extended incubation periods. The large quantities of mineral nitrogen, measured after 65 days of incubation, indicated that most of the humus-N mineralized during the initial period of strong priming was derived from the nitrogen-rich fraction of hydrolysable humus. It was concluded that, on a short-term basis, the dynamics of soil organic matter turnover in an organically amended soil represent in essence an exchange of carbon in the hydrolysable or "active" soil humus. The large amount of soil carbon lost initially due to priming was offset by the occurrence of a plant carbon 11 "sparing" effect during the later stages of incubation. Thus, the priming effect of added plant tissues did not result in a net loss of total soil carbon.