Balance characteristics of soil organic carbon under different long-term fertilization models in the upland fluvo-aquic soil of North China

【Objectives】 Investigating soil organic carbon fractions is important for better understanding the stabilization and fertility mechanism of soil organic carbon. This study was conducted in order to estimate the effect of long-term fertilization on variation of soil organic carbon and discuss fertility,maintaining mode for sustainable land use. 【Methods】 The soil organic contents in a 34-years?? long-term fertilization experiment in the upland fluvo-aquic soil were investigated, and their balance characteristics, the relationship with C input from different treatments were calculated. The fertilization models include: no fertilizer control (CK), inorganic nitrogen (N), inorganic nitrogen and phosphorus combination (NP), inorganic nitrogen, phosphorus and potassium combination (NPK), inorganic nitrogen and organic manure combination (NM), inorganic nitrogen and straw combination (NS). 【Results】 After 34 years of fertilization, the soil organic carbon content in the combined application of organic manure and chemical fertilizers (NM, NS), the chemical fertilizers (N, NP, NPK) and CK treatment are averagely increased by 52.9%, 29.0% and 11.1%, respectively. SOC surplus was significantly different among different fertilization models. The SOC surplus in NM treatment was 6.61 t/hm2, which was significantly higher than others, that in NS treatment was 4.68 t/hm2, in CK was only 0.21 t/hm2. There was a strong positive linear correlation between sequestered amount of soil organic carbon and carbon input (r =0.86**), and the calculated annual transform rate of input carbon is about 8.5% in the fluvo-aquic soil, the least carbon input for maintaining the initial organic carbon level is about 0.04 t/(hm2a). There was a positive linear correlation between the yields of wheat (r=0.37**)and maize (r=0.42**)and the soil organic carbon levels in all treatments. 【Conclusions】The soil organic carbon content has been increasing in the 34 years of experiment, including CK treatment, while the combined manure and chemical fertilizer application has the highest increase in the soil organic carbon content. The linear equation shows that the soil organic carbon in the upland fluvo-aquic is still not in balanced state, has the potential to sequester more carbon in the future. Therefore, the combined application of chemical fertilizer with manure is recommended in the tested area.