The effects of nitrogen, phosphorus, and potassium deficiency and the continuous application of organic matter on paddy rice yield were examined in relation to the nutrient balance and changes in soil chemistry that were recorded over a period of 95 years, i.e., since 1926. The experiment consisted of the following nine treatment plots: NPK (complete), NF (non-fertilized), -N (non-nitrogen), -P (non-phosphorus), -K (non-potassium), +RSC750 (NPK+rice straw compost, 750 g m⁻²), +RSC2250 (NPK+rice straw compost, 2250 g m⁻²), NPK-Ca (NPK without slaked lime), and NF-Ca (non-fertilized and without slaked lime).

The rice grain yield among various treatment plots was arranged in the following order: (NF-Ca, NF, -P)<-N<(-K, NPK-Ca, NPK)<+RSC750<+RSC2250. The yield ratios of the NF, -P, -N, and -K plots to the NPK plot were 0.27, 0.33, 0.44, and 0.94, respectively. Rice straw compost application increased the grain yield 1.17 times for +RSC750 and 1.31 times for +RSC2250 as compared with the NPK plot. Potassium balance was negative in the NF, NF-Ca, -K, NPK, NPK-Ca, and +RSC750 plots. Total soil potassium, exchangeable potassium, and nonexchangeable potassium concentrations were stable for the last 45 years (since 1976), despite the negative potassium balance. Although total potassium concentration in each experimental plot was almost equal, exchangeable potassium and nonexchangeable potassium concentrations differed depending on the treatment, reflecting potassium balance. These findings suggested that potassium dynamics in this paddy field have already reached equilibrium and that weathering of soil minerals is an important source of potassium for crop yield. In addition, potassium fertility in this paddy field is considered to be very persistent because the decrease in yield in the -K plot was lower than that in the NPK plot over 95 years.

Fly ash is an effective material for ameliorating soil acidity. Soil acidity amelioration has been considered to influence the priming effect by affecting the microbial carbon mineralization activity. However, there is little quantitative information available concerning the relationship between soil acidity amelioration by fly ash application and the priming effect. Thus, this study aimed to investigate the effect of fly ash on soil pH and the priming effect by applying ¹³C-labeled maize residue to red-yellow cropland soil (Acrisol, WRB) from Okinawa, Japan. A 60-day incubation experiment with five treatments (soil only, control [C]; ¹³C-labeled maize residue only, [R]; and ¹³C-labeled maize residue plus 5, 10, and 20％ (w/w) fly ash [R+FA5, R+FA10, and R+FA20, respectively]) was conducted. During incubation, CO₂ emission, δ¹³C of released CO₂ (to calculate primed carbon), and soil pH were periodically measured. After 60 days, the amount of primed carbon was −31.8, −56.9, −43.9, and 8.4 mg C kg⁻¹ in the R, R+FA5, R+FA10, and R+FA20 plots, respectively. Soil pH after 60 days was 4.5, 4.6, 5.0, 5.3, and 5.9 in the C, R, R+FA5, R+FA10, and R+FA20 plots, respectively. These results indicated that 20％ (w/w) fly ash application significantly ameliorated soil pH, led to a positive priming effect, and promoted the decomposition of native soil organic carbon. On the other hand, 5 and 10％ (w/w) fly ash applications only slightly, but significantly, ameliorated soil pH from 4.6 to 5.0 and 5.3, respectively, causing a negative priming effect similar to that observed in the R plot.

To predict present-day soil distribution in Japan, it is necessary to clarify the effects of landform, land improvement, and land-use changes on soil class because many agricultural fields have experienced anthropogenic activities since the original agricultural soil maps were created. For this purpose, we compared the previous soil profiles of a paddy field in Shiga prefecture with the results of a present-day simple soil profile survey. The results of this survey revealed that Gley lowland soils were distributed in a valley bottom plain, regardless of soil class, before land improvement and associated soil movement due to normal water addition from a nearby forested area. Although there was no relationship between soil classes in the delta landform, the soil class changed after the introduction of a drainage system and subsequent land-use changes. Gley lowland soils that occurred in the delta before land improvement were subsequently altered to other soil classes. Some soils provided good drainage in the delta before the land became a lowland-paddy soil area by paddy soil-forming processes over more than 50 years. In conclusion, although land improvement and land-use changes had a significant impact on the agricultural field surveyed, we are still able to predict modern soil distribution by considering current landforms and past soil conditions.