Differential Grain Yields and Soil Organic Carbon Levels between Maize and Rice Systems of Subtropical Red Soil in Response to Long-Term Fertilizer Treatments

Higher grain yields may be attained by increasing soil organic carbon (SOC). However, yield response to increasing amounts of SOC vary based on land-use types. A fertilization experiment was conducted in upland and paddy agricultural fields of subtropical soil in Jiangxi Province, China. For 33 years, one of four different fertilization regimes, (1) no fertilizer (CK); (2) chemical fertilizers of nitrogen, phosphorus, and potassium (NPK); (3) double the rates of NPK (DNPK); and (4) manure combined with NPK (MNPK), were seasonally applied to upland and paddy soils beginning in 1986 and 1981, respectively. Data on SOC concentration and stock from soil samples and grain yield from crops were analyzed to determine relationships among grain yield, SOC, soil types, and fertilizer treatments. Among all treatments, the growth rates of SOC concentration and stock in the MNPK treatment after 33 years were highest and with higher rates observed from paddy soil than from upland soil. For most years, the annual grain yields of MNPK were higher than yields of other treatments in the two soils. Moreover, the MNPK treatment increased yield stability with less variation. Linear equations of the relationships between grain yield and SOC stock suggest that yield could be improved by 695 and 1507 kg/(ha yr) in upland and paddy soils, respectively, when SOC stock increased by 1 t/ha. Furthermore, in-depth analysis showed that grain yield and SOC concentration and stock were regulated by land-use types and fertilizer application through carbon inputs. Furthermore, fertilization had positive effects on carbon input, and its influence was greater than that of land-use type. Therefore, SOC and grain yield may be more effectively increased by the continuous application of manure with chemical fertilizers. Among the variables in our model, the greater increase of grain yield in upland soil than in paddy soil was primarily due to carbon inputs.