Microbial decomposition of corn residue in two Iowa Mollisols

Decomposition agent, Requate®, formulated by the Chinese company Chendu Hecheng Biological Technology CO., LTD, was examined for efficacy in agricultural plant-residue decomposition. We measured carbon dioxide (CO2) evolution over 21 days to determine the plant-residue decomposition rate. A Clarion soil (Typic Hapludoll) and Zea mays L. (corn) residue were characterized for several traits, including total C, total N, and C:N ratio. Our experiment included six treatments: no soil, no residue, no product (b); soil, no residue, no product (soil); soil, residue, no product (soilr); soil, no residue, product (soilp); soil, residue, product (soilrp); soil, residue, sterilized product (soilrsp). Our mesocosms simulated no-tillage with and without corn residue addition. Requate® product was applied to our treatments at the recommended rate of 86.6 g/2,221 L per hectare. Our sterile Requate® factor served to identify whether the product had a carrier that facilitated decomposition via stimulating the native microbiota. One soil moisture potential, -33 kPa, was used in this experiment. Univariate statistical analyses found no significant differences in corn-residue decomposition among our three treatments that included corn residue. We concluded that the Requate® product did not increase the rate of corn-residue decomposition in our Clarion soil. Microbial growth efficiency (MGE) was analyzed for isolated soil bacteria at three pH’s. In a stressed environment, we expected a greater level of C partitioned for cell maintenance than for microbial biomass C (MBC). Three Bacillus genus bacteria were isolated: B. pasteurii, B. circulans, and B. sphaericus. A characterized Webster soil (Typic Endoaquoll) was amended with Ca(OH)2 (calcium hydroxide) and Al2(SO4)3 (aluminum sulfate) to alter a soil pH of 6.5, to 7.5 and 5.5, respectively. Characterized corn residue was added and mixed into 100 g dry-weight Webster soil and placed inside a standard 0.47 L (pint) Mason jar (mesocosm). Corn residue (0.45 g) was added and mixed into the soil and then sterilized with high pressure saturated steam inside the mesocosm for 1 hr. The mesocosm was brought to room temperature (20-25°C) and then inoculated with a known quantity of our individual bacteria. Carbon dioxide and MBC were measured throughout a 35-day incubation period on days 3, 7, 14, 21, and 35. Our analysis determined that there was a significant difference in the rate of CO2 evolution (RCO2) by treatment and day. In general, RCO2 from all treatments was greater at day 3 and declined throughout the study; we measured significant differences in RCO2 on all days. B. pasteurii degraded the corn residue most rapidly, with the rate of decomposition being greater at pH 7.5 than pH 6.5 or 5.5. All other treatments followed a similar pattern, having greater rates of decomposition at pH 7.5. Microbial biomass C was greatest at pH 7.5 for all treatments and increased to a peak on day 21, where MBC was significantly different. The lowest MBC values were observed at pH 5.5 with B. circulans, followed by B. sphaericus, and then B. pasteurii. The least amount of MBC was observed when RCO2 was at its greatest. However, we could not determine the ratio of RCO2 per unit MBC produced because of measured negative MBC values. Thus, our data provided weak evidence that stressed cells required more metabolized C for cell maintenance but differences existed among the three isolates and an overall generalization could not be made