U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


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Published in Soil Science 2007;172:189-208.


Soil organic matter is the earth's largest terrestrial reservoir of carbon (C). Thus, it serves as a major control on atmospheric carbon dioxide (CO2) levels. To better understand these controls, decreases in soil organic C (SOC), soil microbial biomass (SMB) C, and the role of SMB as a source of mineralizable C were measured during a long-term incubation (853 days) without added substrate. The 2 soils used were a Weld loam (fine montmorillonitic, mesic, Aridic Paleustoll) from near Akron, Colorado, and a Duroc loam (fine silty, mixed mesic Pachic Haplustoll) from near Sidney, Nebraska. The Akron soil was uniformly cropped to small grain crop-fallow rotations until 1989 when wheat (Triticum aestivum L.) in conventional (stubble mulch) till-fallow, reduced till-fallow, and no-till fallow treatments were adopted. On additional rotation plots, continuous corn (Zea mays L.) or no-till corn, fallow, wheat, and no-till corn in a 4-year rotation were grown. The Sidney soil was broken from native sod in 1970 and planted to wheat-fallow with no-till, plow-tillage, and sod-plot treatments. Moist soil samples were collected and refrigerated until plant material removal by sieving and picking. The SOC and SMB-C decreased during incubation and rates of loss measured. The results from this study allow insights into contributions of SMB and changes in soil isotope C ratios not previously available.

Soil microbial biomass C contributed an average of 31% of the evolved C02-C across all treatments between day 10 and day 79 of incubation and an average of about 20% during the more extended times between later measurements thereafter. Until day 160, evolution of 13C02 during incubation indicated that evolved C came from plant residues and was soil derived thereafter, including from the native grassland SOC. Where corn was grown, evolution of evolved C is hypothesized to have had a less negative 13C02 isotope signature from days 630 to 720 of the incubation because of the delayed microbial breakdown of the cob materials. After 853 days of incubation and across all plots, the SOC remaining averaged 67% and was similar to the amount of observed hydrolysis residue C. Acid hydrolysis and 14C dating were also used to characterize the resistant SOC fraction and showed increased 14C age with hydrolysis but not with long-term incubation.