Effects of fertilization and straw return methods on the soil carbon pool and CO2 emission in a reclaimed mine spoil in Shanxi Province, China

Reclaimed soil is similar to an "empty cup" with a large carbon (C) sequestration potential. Agricultural management practices strongly influence C storage and soil carbon dioxide (CO2) emission. The objective of this study was to identify the effects of fertilization and straw return methods on the soil C pool and CO2 emission in a reclaimed mine spoil. Thus, we studied the effects of four fertilization treatments [chemical fertilizer (NP), manure (M), 50% M plus 50% NP (NPM), and unfertilized control (CK)] and three straw return methods [no straw return (no straw), return of straw mixed with soil (straw mixing), and return of straw to the surface of the soil (straw mulching)] in a mine reclamation region by examining changes in the soil C pool and CO2 emission. The soil C pool was evaluated by acid hydrolysis, considering three pools: (a) labile pool I (LP I), obtained by hydrolysis with 5 N H2SO4 at 105 degrees C for 30 min; (b) labile pool II (LP II), obtained by hydrolysis with 26 N H2SO4 at room temperature overnight, followed by 2 N H2SO4 at 105 degrees C for 3 h; and (c) the recalcitrant pool, measured as the unhydrolyzed residue. The CO2 emission/C sequestration and CO2 emission/grain yield ratios were used as indicators of C emission. Compared with the CK, the NPM treatment was the most efficient among the fertilization treatments in increasing grain yield (136%) and C sequestration (311%) and reducing the CO2 emission/C sequestration (62%) and CO2 emission/grain yield (32%) ratios. The straw mixing treatment had more soil organic carbon (SOC), a larger LP I and LP II, and more C sequestration than the other straw return treatments. The addition of manure resulted in a higher distribution rate of new organic C to the labile C pool (73-78%) than the addition of inorganic fertilizer (45%). Soil C emissions were mainly concentrated in the maize growing season, accounting for more than 80% of the total annual C emissions. LP I, LP II and CO2 emission were significantly positive correlated with SOC and yield. Therefore, soil C pools and CO2 emissions are significantly influenced by fertilization regimes and straw return methods, which should be used to important indicators to estimate the C balance in agro-ecosystems during the process of mine spoil reclamation.