Harris, D., T. Willson, E.A. Paul, G.P. Robertson, A.M. Smucker
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
The Below Ground Productivity (BGP) study was initiated in 1995 to take advantage of 3 long term agricultural experiments at KBS: the LTER main site (LTER), the Agroecosystem Interaction Site (AIS), and the Living Field Laboratory (LFL). In 1995, these experiments offered otherwise similar treatments with contrasting histories with respect to the cultivation of C-3 and C-4 crop species. In treatment 6 at the LTER, alfalfa was being grown for the 7th consecutive year following a year in soybeans (all C-3), whereas at the AIS, alfalfa was planted following 9 years of continuous corn (C-3 on C-4), and at the LFL, corn was planted following at least 8 years in alfalfa, soybean and wheat (C-4 on C-3).Our plan was to use the relative 13C enrichment of C-4 plants as a tracer to differentiate new production (associated with 1995 crop roots) from previous production, and to differentiate the signal produced by the past decade of management from the signal produced by all previous vegetation. We were particularly interested in the possibility using delta 13C analysis to differentiate between the root and microbial components of soil respiration.To achieve these objectives, we recruited a multidisciplinary team of researchers to measure the total production of, and C isotope ratio in, roots, soil organic matter, soil microbes, and soil CO2. Root analyses included minirhizotron measurements throughout the 1995 season, and destructive samples for root length density and root mass taken at five dates from spring 1995 to spring 1996. Soil and microbial analyses were performed in conjunction with these destructive samples, and included total C, 320 day C mineralization kinetics, and microbial biomass using both chloroform fumigation and direct microscopy. Soil respiration measurements were planned at the soil surface and at depth throughout the season, although the depth samples proved unsuccessful.Data analyzed in time for this report include root mass and delta 13C , microbial biomass C, and mineralization kinetics for the May 1995 soil sample. The delta 13C dynamics of the CO2 collected during the May 320 day incubation are presented graphically below with the delta 13C measurement of each soil provided as a reference.All of the soils are enriched in 13C relative to C-3 plant residue, which indicates a previous history of C-4 vegetation, most likely from corn cultivation. The recent history of corn cultivation at AIS produced a strong C-4 signal throughout the incubation. Combining the delta 13C measurements for the last three sample periods (80-200d), 46% of the CO2 produced by the AIS soils can be attributed to corn production, compared to 31% for the LFL and 25% for the LTER sites. See (Figure 1).
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