Sensitivity of carbon sequestration costs to soil carbon rates.

Modifying current agricultural management practices as a means of sequestering carbon has been shown to be a relatively low cost way to offset greenhouse gas emissions. In this paper we examine the sensitivity of the estimates of the amount of soil carbon sequestered and the implied costs of sequestering a tonne of carbon to changes in the rates of soil carbon sequestered for alternative production practices. An application is made to the dryland grain production systems of the US Northern Plains where the marginal costs of soil C range from $20 to $100 per MT. We show that the resulting changes in the marginal costs quantities of C sequestered are not a monotonic transformation of the changes in the soil carbon rates. These results underscore the importance of using a linked economic and biophysical simulation model to assess the economic potential for sequestering carbon in agricultural soils.

Trophic interactions in soils as they affect energy and nutrient dynamics. III. Biotic interactions of bacteria, amoebae, and nematodes.

Bacteria (Pseudomonas), amoebae (Acanthamoeba), and nematodes (Mesodiplogaster) were raised in soil microcosms with and without glucose additions. Nematode and amoebal grazing on bacteria significantly reduced bacterial populations by the end of a 24-day incubation period. Amoebal numbers decreased in the presence of nematodes with a corresponding increase in nematode numbers which reached a maximum of 230 nematodes/g of soil in the treatment with amoebae and glucose additions. After 24 days the nematode populations in the treatments without carbon additions were dominated by resistant dauer larvae indicating the unavailability of food. Although larval numbers were high in the treatments with glucose additions, the adult component of the population was still increasing at the end of the 24-day experiment. The effect of the presence of amoebae on nematode abundance was of the same magnitude as addition of 600Μg glucose-C.

Trophic interactions in soils as they affect energy and nutrient dynamics. IV. Flows of metabolic and biomass carbon.

Flows of biomass and respiratory carbon were studied in a series of propylene-oxide sterilized soil microcosms. One-half of the microcosms received three pulsed additions of 200 ppm glucose-carbon to mimic rhizosphere carbon inputs. Biotic variables were: bacteria (Pseudomonas) alone, or amoebae (Acanthamoeba) and nematodes (Mesodiplogaster) singly, or both combined in the presence of bacteria.Over the 24-day experiment, respiration was significantly higher in the microcosms containing the bacterial grazers. Biomass accumulation by amoebae was significantly higher than that by nematodes. The nematodes respired up to 30-fold more CO2 per unit biomass than did amoebae. Similar amounts of carbon flowed into both respiratory and biomass carbon in microcosms with fauna, compared with the bacteria-alone microcosms. However, partitioning of available carbon by the microfauna varied considerably, with little biomass production and relatively more CO2-C produced in the nematode-containing microcosms. The amoebae, in contrast, allocated more carbon to tissue production (about 40% assimilation efficiency) and correspondingly less to CO2.

Trophic interactions in soils as they affect energy and nutrient dynamics. V. Phosphorus transformations.

Regeneration of nutrients from relatively nutrient-poor organic residues is essential for overall operation of an ecosystem. Nutrients thus released are, however, inadequate for the needs of the decomposer populations, and a much faster nutrient turnover involving bacterial immobilization and release occurs concurrently. Evidence from aquatic ecosystems indicates that bacteria release little phosphorus, for which they have high demand, whereas bacterial grazers play an important role in regeneration of bacterial phosphorus. Our studies extend these relationships to terrestrial ecosystems. We studied phosphorus immobilization and mineralization in soil incubations, simulating rhizospheres with combinations of bacterial, amoebal, and nematode populations. Bacteria quickly assimilated and retained much of the labile inorganic phosphorus as carbon substrates were metabolized. Most of this bacterial phosphorus was mineralized and returned to the inorganic phosphorus pool by the amoebae. Nematode effects on phosphorus mineralization were small, except for indirect effects on amoebal activity. The observed remineralization may reflect direct excretion by the amoebae, physiological effects on the bacterial populations, or both. These results suggest a major role of microfauna in nutrient cycling.