Phase change of soil water of vegetable field transformed from paddy field by low field nuclear magnetic resonance

Abstract: Soil pore water is the important part of terrestrial water resource and the basis for plant survival. Furthermore, it is in charge of heat and solute transportation throughout soil profile. This study aimed to identify the variations of soil moisture under land use change from paddy field to greenhouse vegetable field at different time frameworks. To do so, soil samples were taken at the saturated condition of moisture capacity from both paddy and greenhouse plots. The selected greenhouse vegetable site was located in the urban area of Nanjing, China. In this region the main soil type is paddy soil coming from many-year rice-wheat rotation. A new method called low field nuclear magnetic resonance (LF-NMR) technology was adopted to acquire soil samples' transverse relaxation time (T2) spectrum. Based on this 4 treatments were chosen, which included a long-term paddy field and 3 greenhouse vegetable sites with different transformation time. Accordingly, the paddy field was supposed to be beginning point (0 year) (PS0), and the conversion ages of the greenhouse vegetable fields were 2, 6, and 8 years (GVS2, GVS6 and GVS8, respectively). Based on the previous survey, greenhouse field in the pakchoi (Brassica campestris L. ssp. chinensis) plantation was selected as the research site in this study. Results of soil profile survey indicated that paddy soil profile (0-30 cm) was divided into 2 layers: a plough horizon (0-15 cm) and plow pan (>15-30 cm). Since a new firm structural layer was observed in the lower part of the plough layer, we divided greenhouse vegetable soils' profile (0-30 cm) into 3 layers: plough horizon, new plow pan, and old plow pan. Soil physicochemical properties including soil organic matter, bulk density, pH value and particle size composition were measured for selected sampling sites. The results showed that: 1) Water flow in topsoil macro pores decreased from 29.7% in PS0 to 14.9% in GVS6. In contrast, slight changes occurred in water flow across deep layers; 2) the average annual rate of organic fertilization in PS0, GVS2, GVS6 and GVS8 were 0, 46.5, 36 and 144 t/hm2, respectively. The organic fertilizer added to GVS8 was 3.1 and 4 times that of GVS2 and GVS6, respectively. Therefore, it suggests that an improvement in plough layer quality caused by heavy application of organic fertilizer can increase water flow and prevent further soil degradation; 3) Land use transformation from paddy field to greenhouse vegetable field for 2 years decreased water holding capacity due to the emergence of new plow pan. In conclusion, the findings of this study confirm that LF-NMR method can be more directly and accurately used to monitor changes in soil moisture content of each phase instantly, and hence it may provide new theoretical and technical support for scientific management of facility agriculture.