Changes in soil organic carbon pools after 10 years of continuous manuring combined with chemical fertilizer in a Mollisol in China

It is still unclear that whether substantial amounts of organic manure amendment could increase soil organic carbon (SOC) sequestration in a Chinese Mollisol with relatively high organic C content. Further, changes accompanied by different organic carbon (C) fractions are not well understood based on long-term experiment. The knowledge of this kind is important for assessing the potential for C sequestration of a high SOC soil and selecting effective management practices for increasing soil C sequestration and productivity in agroecosystem of northeastern China. This study was aimed to assess the effects of 10 years' organic manuring at various rates combined with chemical fertilizer on organic C sequestration in topsoil (0-20 cm) and to identify changes in different SOC (total, labile, and recalcitrant) pools. Five fertilization treatments were included: (1) CK, unfertilized control; (2) OM₀, only chemical fertilizer, no manure added; (3) OM₁, organic manure added at 7.5 Mg ha^-1 year^-1 plus chemical fertilizer; (4) OM₂, organic manure added at 15 Mg ha^-1 year^-1 plus chemical fertilizer; and (5) OM₃, organic manure added at 22.5 Mg ha^-1 year^-1 plus chemical fertilizer. Chemical fertilizer was supplied with the same rate in each fertilized treatment. We found that the application of graded rates of manure from OM₁ to OM₃ significantly enhanced total SOC, labile C pools, and recalcitrant C pool as compared with OM₀ and CK. The C storage (in top 20 cm) in the OM₀, OM₁, OM₂, and OM₃ was increased by 3.19%, 12.5%, 14.5%, and 18.2%, respectively, over the CK treatment, suggesting that the chemical fertilizer addition had less effects on C-sequestration in topsoil compared to manure plus mineral fertilizer amendment. Moreover, topsoil C-sequestration increased with organic manure addition rates with an order of OM₃ (10.5 Mg ha^-1) > OM₂ (8.4 Mg ha^-1) > OM₁ (7.2 Mg ha^-1). A positive relationship between C sequestration and organic manure input indicates that the soil has not reached its maximum capacity of C sequestration. Application of organic manure with chemical fertilizer was found to produce greater size of both labile and recalcitrant pools than application of mineral fertilizers alone. In most cases, the increases in these C fractions were greater when organic manure was supplied at higher rates. Moreover, increase in recalcitrant C (10.5-29.5%) was significantly higher than labile C (5.6-10.2%) in manure treated plots as compared with no amendment plot. This indicated that a majority of organic C sequestered due to C inputs was accumulated and stabilized in recalcitrant C pool. In general, organic manure combined with inorganic fertilizer exerted greater influence on topsoil C storage and crop yield than chemical fertilizer alone. Based on crop yield and soil C storage, applying organic manure at the rate of 22.5 Mg ha^-1 year^-1 was the most effective way to improve soil productivity and C sequestration in the agroecosystem of northeastern China. However, taking use efficiency of organic inputs and environmental factors into consideration, application of manure at moderate rate (about 15 Mg ha^-1 year^-1) may be more feasible in this region.