Response of pore structures to long-term fertilization by a combination of synchrotron radiation X-ray microcomputed tomography and a pore network model

Pore structure is sensitive to land management, fertilization and tillage. The response of pore structure to long-term fertilizer application is crucial for understanding the mechanisms of various soil processes involved. In this study, the effect of chemical fertilizer and organic manure on microscale pore structure and the pore network in soil was quantified by synchrotron-based X-ray microcomputed tomography (SR-mCT) and a pore network model. Macroaggregates of 5-7mm in diameter were collected from a paddy soil (Typic Haplustalf) with a long-term fertilization experiment established in 1996 under an annual rice-wheat crop rotation. The treatments were control (CK), chemical fertilizers (NPK), rice straw (RS), pig manure (PM), chemical fertilizer plus straw (NPK+RS) and chemical fertilizer plus pig manure (NPK+PM). Results showed that the application of NPK greatly reduced the C/I ratio (ratio of connected porosity and isolated porosity) of the macroaggregates compared with all the other treatments. The pore network model revealed that application of NPK reduced the complexity of the pore network system. Heatmap analyses showed that NPK increased the isolated porosity of macroaggregates and that straw markedly changed the pore shape by increasing the proportion of elongated pores, whereas the PM increased the connected porosity of macroaggregates. From our results, we believe a good pore structure' comprised a moderate total porosity and C/I ratio, which can balance the air, water exchange and moisture, and nutrient retention. We considered that total porosity and C/I ratio were the crucial pore variables to evaluate the effect of fertilization on pore structures at the microscale level. Therefore, we proposed that total porosity and the C/I ratio in macroaggregates could be used as effective indicators to evaluate the response of pore structure to fertilization. Application of a balanced approach of chemical fertilizer together with manure (NPK+PM) or straw (NPK+RS) improved pore structure. This suggested that fertilization management should include the integrated use of both mineral fertilizer and organic manure or straw to maintain a suitable pore environment in soil.