Background and Aims Previous studies have clearly shown substantial increases of soil organic carbon (SOC) in agricultural soils of Yellow River reaches. Those soils did not receive organic fertilizer input, but did receive chemical fertilizer inputs. Thus, to investigate the hypothesis that the observed SOC increases were driven by chemical fertilizer additions, a maize pot experiment was conducted using a Fluvisol that developed under C-3 vegetation in the Yellow River reaches. Methods Using the natural C-13 abundance method we calculated the SOC renewal ratio (C-renewal), and separated total soil organic carbon (TOC) into maize-derived soil organic carbon (SOCmaize) and original soil organic carbon (SOCoriginal). Carbon dioxide fluxes and microbial biomass carbon (MBC) were determined by closed chamber method and fumigation-extraction method, respectively. The experiment included five treatments: (1) NPK: application of chemical fertilizer NPK; (2) NP, application of chemical fertilizer NP; (3) PK: application of chemical fertilizer PK; (4) NK, application of chemical fertilizer NK; and (5) CK: unfertilized control. Results Fertilization increased maize biomass (including grain, straw and root), TOC, C-renewal, SOCmaize, maize-derived carbon (MDC: including SOCmaize, and root and stubble biomass carbon) and MBC, and these values among the treatments ranked NPK>NP>PK>NK>CK. The C-renewal was 5.548.50% across the treatments. Fertilization also increased soil CO2 emission (including root respiration and SOCoriginal decomposition), while the SOCoriginal decomposition during the maize growing season only amounted to 74.0-93.4 and 33.5-46.1% of SOCmaize and MDC among the treatments, respectively. Thus input was larger than export, and led to SOC increase. Maize grain and straw biomass were positively and significantly correlated with soil delta C-13, TOC, C-renewal, SOCmaize, MDC and MBC. Conclusions The study suggests that chemical fertilizer application could increase C-renewal by increasing crop-derived C and accelerating original SOC decomposition, and that as long as a certain level of crop yield or aboveground biomass can be achieved, application of chemical fertilizer alone can maintain or increase SOC level in Fluvisol in the Yellow River reaches.
