Fast production of wheat straw aerobic compost through regulating C/N ratio with urea ammonium nitrate solution

Objectives Composting is one of the most effective methods in plant straw recycling attributing to the accelerated degradation and concentrated available nutrients. In this study, urea ammonium nitrate solution (UAN) was attempted as a pollution-free nitrogen conditioner to regulating the C/N ratio for rapid production of wheat straw compost.

Methods Four C/N ratio levels of 15, 20, 25 and 30 were designed in wheat straw composting mixture by supplementing UAN. The temperature, pH value, electrical conductivity (EC), total organic carbon (TOC), ammonium-N, nitrate-N and germination index (GI) were continuously measured during the 50 days’ composting process.

Results The maximum temperature in treatments of C/N25 and C/N30 were 63.4℃ and 65.9℃, and the thermophilic period (≥ 50℃) lasted for 7 d and 8 d, respectively. While the treatments of C/N15 and C/N20 generated temperature not high enough for the hygienic requirements for harmless disposal of plant straw. At the initial, the pH value of compost mixture increased with the increase of C/N, ranging from 6.79 to 7.94, and became close after 50 days composting, ranging from 7.63 to 7.89. Among all treatments, there was 8.29%–13.5% of decrease in TOC and 53.3%–83.7% of increase in the total N content after composting. C/N25 and C/N30 treatments showed significantly higher organic matter degradation rate than the other twos (P < 0.05). Polymer components of lignocellulose in wheat straw showed the tendency of cellulose > hemicellulose > lignin in concentration. After composting, concentration of hemicelluloses, cellulose and lignin decreased by 30.5%–50.9%, 42.4%–55.8% and 15.3%–29.4%. The nitrogen volatile losses after composting process were 34.9% (in C/N15), 29.0% (in C/N20), 22.1% (in C/N25) and 7.37% (in C/N30), respectively. Transformation of inorganic nitrogen to organic nitrogen was promoted during the process. Content of inorganic nitrogen decreased from 52.4%–75.8% (before composting) to 25.4%–63.1% (after composting). The germination indexes of C/N15, C/N20, C/N25 and C/N30 treatments were 3.55%, 51.6%, 81.5% and 109%, respectively.

Conclusions UAN is an effective nitrogen regulator to adjust the C/N ratio in wheat straw composting. The C/N ratio of 30 produces the most rapid composting process.