Effects of organic materials on soil carbon pool management index in urban greenspaces

We conducted a 475-d urban field experiment using a nylon mech bag method, combined with the solid-state ¹³C-nuclear magnetic resonance spectroscopy, to explore the effects of organic materials application on soil organic carbon components and carbon pool management index (C_PMI) and their relationships with the chemical compositions of organic materials. There were seven treatments with the addition of different organic materials: green waste (GW), green waste compost (GWC), biogas residue (BR), biogas residue compost (BRC), peat (PT), biochar (BC), and no organic material (CK). Results showed that organic materials addition increased total soil organic carbon and labile organic carbon content by 34.1%-87.0% and 11.1%-39.5%, respectively. The positive effects of organic materials addition on those two variables in the PT, BC, and GWC treatments were significantly higher than that in the BR and BRC treatments. Soil microbial biomass carbon and dissolved organic carbon contents were increased dramatically in the BR and GW treatments compared to the CK, while PT and BC treatments had no effect on soil microbial biomass carbon and dissolved organic carbon contents. The addition of organic materials increased the C_PMI, with all values exceeding 100%, being the highest in BC treatment and the lowest in BR treatment. C_PMI was significantly negatively correlated with the total nitrogen of organic materials, but not with the total organic carbon of organic materials. Moreover, the aromaticity index of organic material was linearly correlated with the stabilization rate of organic material-derived carbon and C_PMI. In conclusion, applying organic materials might increase soil organic carbon and labile organic carbon contents, as well as the C_PMI in urban greenspaces. The C_PMI was mainly regulated by carbon structure rather than the quantity of added organic material-derived carbon. Adding organic materials with high aromatic carbon groups was beneficial to improving the urban soil quality of the carbon pool.