Microbial Colonization and Decomposition of Corn, Wheat, and Soybean Residue

Decomposition studies were conducted on corn (Zea mays L.), wheat (Triticum aestivum L.), and soybean (Glycine max L.) residue with emphasis on quantitative determination of the microbial populations colonizing the residues and identification of the predominant fungi isolated from the residues. The study was conducted over a 2‐yr period in a Mexico silt loam (Udollic Ochraqualf) at Sanborn Field in Columbia, MO. Plant residues were placed in a porous net and buried in the field. At intervals these sacks were exhumed, washed, and the washings were plated on rose bengal and soil extract agars. The residue was dried and weighed to determine decomposition rates. The predominant fungi were isolated and identified. Microbial determinations were conducted on soil samples taken from the same plots. Of the three residues, soybean had the most rapid rate of decomposition, losing 68% of its total organic matter over the course of 32 d compared with 42 and 47% for corn and wheat residue, respectively. Despite a more rapid rate of decomposition in wheat in comparison with corn residue at 32 d, after 756 d in the soil corn retained 15% of its original organic matter in comparison with 18% for wheat. Bacterial and actinomycete populations were consistently higher on soybean residue in comparison with corn and wheat residue; however, fungal populations were consistently highest on corn residue and lowest on wheat residue. Size of microbial populations from soil were similar for the three plots over the course of the study. Aspergillus, Penicillium, and Trichoderma were the three predominant genera isolated from the residues. Aspergillus niger was consistently isolated at high levels from corn residue, and A. clavatus was isolated frequently, but at low levels from wheat residue. Penicillium spp. were isolated from residues mainly during drought conditions and during the latter stages of decomposition. Trichoderma spp. were isolated at their highest levels during the second summer of the decomposition study, but were found at substantially lower levels during the third summer. Myrothecium spp. was isolated consistently from soybean residue but only during the latter stages of the decay process.