Effect Of Long-term Fertilizer Application On Soil Organic Carbon Stability And Phosphorus Fractions In A Rice-wheat Rotation System

Management practices, such as crop residue return and long-term manure application, can not only improve soil fertility, but also benefit soil organic carbon(SOC) accumulation. In this study, the effect of long-term fertilization practices on the content and chemical structure of SOC of a rice-wheat rotation soil was investigated in a long term field experiment in Zhejiang Province, China. Soil phosphorus(P) dynamics and soil P availability in this agricultural system were also studied. The experiment had six treatments with three replications. The fertilizer treatments included CK(control, no fertilizer applied), NPK(NPK fertilizers), RS(rice straw alone), OM(organic manure alone), NPKRS(NPK fertilizers and rice straw), and NPKOM(NPK fertilizers and organic manure). Topsoil(0-20 cm) samples were collected after 17 years of fertilizer treatments. Main results are as follows.(1) Soil samples were analyzed to evaluate the effect of fertilization practices on distribution of aggregates and aggregate-associated organic carbon in the paddy soil. The wet-sieving method was used for evaluation of physical stability of soil aggregates and their particle-size composition. Fourier transform infrared spectroscopy(FTIR) was employed to characterize inherent chemical composition of soil organic carbon(SOC) at the molecular level in the 2~0.25 mm and <0.053 mm aggregates. In comparison with the CK, NPKOM, OM, NPKRS and RS treatments significantly(p<0.05) increased the proportion of >2 mm and 2~0.25 mm water stable aggregates, and mean diameter of the soil aggregates, which could enhance the physical protection of the SOC in the macroaggregates. The SOC content of the bulk soil and all aggregate fractions, as well as the contribution of macroaggregate-associated(i.e., >2 mm and 2~0.25 mm) SOC to total soil organic C in the NPKOM and NPKRS treatments were significantly higher than that in the CK. However, no significant difference was observed in accumulation of SOC between CK and NPK or RS treatments. The 2~0.25 mm fraction of aggregates contained SOC, accounting for 34.2%~48.6% of the total SOC in the soils in all the treatments, suggesting that 2~0.25 mm aggregates were the main carriers of SOC in the soil. The FTIR spectra of the 2~0.25 mm and <0.053 mm aggregates showed that the ratios of aromatic-C to total SOC in NPKOM, OM, NPKRS and RS treatments were 29.9%~45.2% higher than that in the CK, and 22.3%~36.6% higher than that in the NPK treatment. The highest ratio was observed in the NPKOM treatment. Our results indicate that both enhanced physical protection of SOC by macroaggregates and increased proportion of chemically recalcitrant organic compounds may contribute to carbon accumulation in the paddy soil treated with organic amendments, with the most prominent effect being observed in the NPKOM treatment.(2) Effect of fertilization practices on soil organic carbon(SOC) content in particle size fractions of paddy soil was investigated and the inherent chemical composition of silt- and clay-associated SOC was evaluated with solid-state 13C-NMR spectroscopy. Compared to the CK, the NPKRS, NPKOM, NPK and OM treatments significantly(p<0.05) increased the SOC content of sand-(2~0.02 mm), silt-(0.02~0.002 mm) and clay-sized(<0.002 mm) fractions. However, no significant difference was observed in accumulation of silt- and clay-associated SOC between the CK and RS treatments. Besides, in comparison with the plots applied with NPK fertilizers alone, application of organic amendments and NPK fertilizers combined facilitated storage of newly sequestered SOC in the silt- and clay-sized fractions, which could be more conducive to the stability of SOC. Based on 13C-NMR spectra, both silt and clay fractions composed of Alkyl-C, O-alkyl-C, Aromatic-C and carbonyl-C. Changes of the relative proportion of different C species were observed between the silt and clay fractions. The clay fraction had relatively more Alkyl-C, carbonyl-C and less O-alkyl-C, Aromatic-C than did the silt fraction. The spectra also showed that relative proportion of different C species was modified by fertilization practices. In comparison with the organic amendments alone, the relative proportion of Alkyl-C was decreased by 9.1%~11.9% and 13.7%~19.9% under combined application of organic amendments and chemical fertilizers, respectively, for silt and clay, and that of O-alkyl-C was increased by 2.9%~6.3% and 13.4%~22.1%, respectively. These results indicate that the NPKOM and NPKRS treatments had a relatively low decomposition rate of SOC. The aromaticity, hydrophobicity and, hence, chemical recalcitrance of silt- and clay-associated SOC in the NPK treatment were lower than those of the organic amendment and unfertilized treatments, indicating decreased recalcitrance of SOC against decomposition in the NPK treatment.(3) Application of the organic manure and inorganic fertilizers either alone or in combination with other types of fertilizer phosphorous(P) significantly(p<0.05) increased in total soil P(TP), Olsen P and M3 P level. The NPKOM treatment had the highest increase in the P levels. Application of inorganic fertilizer P and organic manure not only facilitated fertilizer P recovery by crops, but also reduced the percentage of fertilizer P unaccounted for. Besides, we have identified that fertilization practices caused the redistribution of P among fractions during the length of study by sequential extraction modified from the Hedley’s fractionation procedure. According to the results of P fractionation, labile P(NaHCO3-Pi + NaHCO3-Po) constituted the least soil P fractions(2.8%~8.4% of the total P), while the sparingly labile forms of P accounted for 62.2%~82.2% of the TP, indicating low P availability of the soil. The NPKOM treatment not only facilitated P accumulation in the labile, moderately labile and sparingly labile forms of P, but also decreased the relative proportion of sparingly labile P fractions to total P and prevented the conversion of applied fertilizer P into recalcitrant P forms. These results demonstrate that the combined application of organic manure and inorganic fertilizers is an effective practice for enhancing the availability of soil P.