Optimized Initial Moisture Content Enhances Microbial Biomass and Nutrient Cycling During Aerobic Composting of Corn Straw and Cow Manure

Optimizing the initial moisture content during composting is an important phenomenon for promoting microbial processes and nutrient dynamics in compost. Therefore, a study was carried out at Gansu Agricultural University, China (36°02′N, 104°25′E; 2400 m a.s.l.) to assess how an initially optimized moisture content (45% MC = MC₁, 55% MC = MC₂ and 65% MC = MC₃) affects the decomposition of corn straw and cow manure mixture in a bioreactor system during winter. The results obtained show that all treatments, except MC₁, entered the thermophilic phase (>50 °C) on day 3, with peak temperatures observed on day 7 for 55% MC (57.8 °C), day 9 for 45% MC (51.5 °C), and day 11 for 65% MC (56.7 °C). Despite the initial differences in moisture, the electrical conductivity (EC) values stabilized in the final phase of composting and were between 2.96 and 3.01 mS/cm for all treatments. The 65% MC outperformed the other treatments in microbial metrics. It had the highest microbial biomass carbon (MBC =254.81 mg/kg) and nitrogen (MBN = 26.38 mg/kg), corresponding to 6.7% and 6.9% increases over the 55% MC, and 14.8% and 26.4% over the 45% MC, respectively. The dissolved organic nitrogen (DON) content followed the trend 21.2 mg/g (MC₃) > 19.6 mg/g (MC₂) > 14.4 mg/g (MC₁), with MC₃ showing an increase of 47.2% and 8.2% over the MC₁ and MC₂, respectively. The total phosphorus (TP) content increased by 14.9%, 27.4%, and 35.2% for 45% MC, 55% MC, and 65% MC, respectively, compared to the baseline values. The extent of moisture loss was inversely proportional to the initial moisture content, with MC₁ (44%) showing the highest losses, while MC₃ showed the lowest (23.4%). These results suggest that optimizing the initial moisture content of corn straw and cow manure mixture to 65% significantly improved the stimulation of microbial biomass, nutrient retention and overall quality of the compost.