The mass water content of paddy soil after harvest is strongly associated with the accumulation of organic matter as the source of available nitrogen

Paddy soils are the dominant source of nitrogen for rice via mineralization, and knowledge about the available nitrogen (AN) in paddy soil is important for rice cultivation. Although the relationship between AN and soil physicochemical parameters has been studied, no systematic studies of the relationships between AN and the soil moisture content of various soil types in Japan have been studied. Therefore, we examined the impacts of soil types and treatment of organic matter application or paddy–upland rotation on these relationships. We sampled topsoils from 30 paddy fields throughout Japan. We compared the relationships between AN and soil physicochemical properties, such as total nitrogen (TN), cation exchange capacity, soil water-holding capacity (WHC), mass water content after harvest (MWH), and water content after harvest (WCH). The scatter diagram revealed that AN and either MWH or WCH (p < 0.01) had the highest positive linear correlation coefficient; the pattern became clearer after separating Andosols and non-Andosols. We analyzed the relationship between AN and either TN or MWH in 100 topsoils, including soils from treatment plots with successive organic matter application or paddy-upland rotation. A significantly positive correlation between AN and TN was observed, although the regression slopes for volcanic or non-volcanic soils differed. The decrease in AN due to frequent paddy–upland rotation was greater than that for TN. On the other hand, a highly positive correlation between AN and MWH was observed, and the coefficient of determination was higher than that for AN and TN. TN and the WHC increased when the accumulation of organic matter in paddy soil was greater. As there was a positive correlation between WHC and MWH, a positive correlation was also observed between AN and MWH. Additionally, we investigated year-to-year variation in MWH using topsoil samples collected before autumn tillage, for 3–5 consecutive years in 24 plots, for various soil types throughout Japan. In order to eliminate the effects of precipitation and to minimize the fluctuation of MWH in paddy topsoil, it was desirable to collect topsoil samples within 24–96 hours of the last precipitation of 1 mm h⁻¹ or more, for the measurement of moisture content.