Impact of turning frequency on the transformation and bioavailability of nutrient elements and heavy metals in large-scale sheep manure composting on the Qinghai–Tibet Plateau

A comprehensive understanding of the transformation mechanisms of nutrient elements and heavy metals, as well as their bioavailability during composting, is critical to ensuring the safety of compost products and improving their nutritional quality. This study explored the transformation mechanisms of heavy metals, nitrogen, and phosphorus forms in industrial-scale composting of sheep manure on Qinghai-Tibet Plateau (QTP) under three turning frequencies: once every two days (T1), four days (T2), and six days (T3). Results showed that T1 increased the maximum composting temperature by 6.6 °C compared to T3 and increased the humification index by 15.1 %, respectively. T1 increased the contents of available nitrogen and labile phosphorus by 12.1 % and 12.8 %, respectively, enhancing nitrogen and phosphorus availability for plant seedlings. T1 significantly decreased the proportions of exchangeable Pb, Cr, and Mn in the compost products by 13.7 %, 11.8 %, and 17.1 %, respectively ( P < 0.05), while significantly increasing the proportions of residual Mn, Cu, Pb, and Cr, thereby reducing plant uptake of these heavy metals. More frequent turning suppressed denitrification while enhancing nitrite ammonification. This contributes to reduced nitrogen loss and enhanced nitrogen availability. Additionally, more frequent turning enhanced organic P mineralization, inorganic P solubilization, and polyphosphate degradation, thereby improving phosphorus bioavailability. Moreover, more frequent turning can influence heavy metal forms by modulating maturity, moisture content, and bacterial communities. These findings provide scientific support for the effective resource utilization of livestock waste on QTP. • Frequent turning every two days (T1) can enhance the maturation process of compost. • T1 increased available N and labile P contents by 12.1 % and 12.8 %, respectively. • Frequent turning inhibited denitrification and enhanced nitrite ammonification. • Frequent turning enhanced organic P mineralization and inorganic P solubilization. • T1 decreased the proportions of exchangeable Cr, Pb, and Mn by 11.8 %, 13.7 %, 17.1 %.