Organic Matter Decomposition in a Soil Heavily Amended with Poultry Manure

A Davidson clay loam (Rhodic Paleudult) was planted to maize annually and amended with poultry manure at the rates of 0, 27, 54, 85, and 110 metric ton ha⁻¹ year⁻¹ for 5 years. In the fifth year soil organic matter content, microbial respiration, and leaf decomposition were studied to determine how these applications of poultry manure had affected the organic matter-assimilating capacity of the soil. In early May soil organic matter in the top 8 cm of the unmanured controls and the most heavily manured plots were 2.8 and 7.25, respectively. By November the latter value had dropped to 5.4% while the former remained unchanged. At the 28- to 36-cm depth, well below the depth of tillage, organic matter averaged over all treatments increased from 0.70% in April to 1.16% in September, possibly due to leaching and earthworm activity. Microbial respiration, as indicated by CO₂ evolved from soil samples taken in November and incubated at 28°C, increased from 0.02 mg C g⁻¹ in the unmanured soil to 0.101 mg C g⁻¹ day⁻¹ at the 85 metric tons ha⁻¹ year⁻¹ rate. However, at the highest rate of manure soil respiration dropped to 0.069 mg C g⁻¹ day⁻¹. Disappearance of maize leaf pieces from litter bags with mesh openings of 0.02, 0.1, 1.0, and 7.0 mm was initially retarded in the manured plots. However, the total amount of leaf disappearance was generally greater in the 1.0- and 7.0-mm mesh bags than in the 0.02- and 0.1-mm mesh bags, indicating a significant contribution by the meso- and macrofauna in the soil. The manure treatments increased the numbers of mites (Acari) and springtails (Collembola) by two orders of magnitude, but greatly reduced the diversity of the soil arthropod community. The overall ability of the soil to accept and dispose of the poultry manure did not appear to be seriously impaired even at high rates of manure application for it was estimated that only 7% of the 393 metric tons of organic matter added per ha remained in the plowlayer after the 5-year period.