Changes of soil organic carbon and its structure in rhizosphere of black soil under long-term fertilization

Abstract: A large amount of nitrogen fertilizer and manure can be input into the arable land of black soil in agricultural production, thus resulting in a serious negative impact on the soil's organic carbon pool. Nearly 40% of plant primary products are transferred to the soil as substantial rhizodeposits. There is also an important carbon source for the soil microorganism surrounding the root of the plant. The rhizosphere soil can be often the zone of the highest soil microbial activity, further regulating the soil carbon cycle and nutrient transformation in terrestrial ecosystems. Therefore, it is necessary to clarify the responses of soil organic carbon content and structure to the intensification of nitrogen fertilizer and manure. In this study, a 38-year (1979-2017) long-term fertilization experiment of wheat-soybean-maize rotation was conducted at Harbin County, Heilongjiang Province, China. The rhizosphere soil of soybean was collected from nine treatments, including no fertilization (CK), nitrogen fertilizer (N), two times of nitrogen fertilizer (N2), horse manure (M), two times of horse manure (M2), horse manure plus nitrogen fertilizer (MN), and two times of horse manure plus nitrogen fertilizer (M2N2). Each treatment was performed on the 36 m2 (4 m×9 m) plot area. The soil organic carbon and labile organic carbon were also characterized in the 0-20 cm profile soil. A carbon-13 (C13) nuclear magnetic resonance (13C-NMR) spectroscopy was used to analyze the spectral pattern of the organic carbon in the rhizosphere soil. The results showed that the organic carbon content of rhizosphere soil was higher than that in the bulk soil for most treatments. About 18.3% and 26.7% of exogenous organic carbon contents were supplemented in the MN and M2N2 treatments, respectively. The 13C-NMR spectroscopy showed that there were a higher proportion of O-alkyl C and the lower proportion of aromatic C than those in the bulk soil, indicating that the soil organic carbon structures were changed by the rhizosphere effect. Furthermore, the organic carbon content of rhizosphere soil increased significantly by fertilization, especially the highest organic carbon content under M2N2 treatments, compared with the CK. The organic carbon contents increased by 3.21 and 1.93 g/kg in the 0-20 cm profile soil of M2N2 treatments, respectively, compared with the N2 and M2. There was also an increased proportion of alkyl C, aromatic C, the ratios of alkyl C to O-alkyl C, and the ratios of aromatic C to total C in rhizosphere soil under the M2 and M2N2 treatments. There was an increase in the proportion of alkyl C, O-alkyl C, and the ratios of alkyl C to O-alkyl C under the MN treatment, where the formation of aggregates was promoted for the stability of soil particle structure. Nevertheless, there was a decrease in the proportion of alkyl C, aromatic C, and O-alkyl C in the rhizosphere soil under the N2 treatment, particularly for the less stability of aggregates, which did not benefit the soil carbon sequestration. Correspondingly, the MN/M2N2 treatment can be widely expected to significantly promote the organic carbon content of rhizosphere soil, the alkyl C, the ratios of alkyl C to O-alkyl C in the rhizosphere soil, and the O-alkyl C in the bulk soil, particularly for the formation of aggregates and the stability of soil particle structure in the bulk soil. Anyway, the mixed application of horse manure and nitrogen fertilizer can greatly contribute to enhancing the soil's organic carbon retention. The excessive application of nitrogen fertilizer (N2) can reduce the alkyl C, aromatic C in the rhizosphere soil, and the O-alkyl C in the bulk soil, leading to a threat to the environment from the less stability of aggregates once after the autumn harvest. At the same time, the 13C-NMR spectrum combined with the semi-quantitative analysis can be widely used to comprehensively characterize the structures of functional groups, and further better understand the stability mechanism of organic carbon in the rhizosphere soil.