One-time omission of puddling improves soil structure and post-rice soybean yield in clay-rich fields within paddy–soybean rotation systems in central Hokkaido, Japan

A one-time omission of puddling was noted to increase post-rice soybean yields within the paddy–soybean rotation systems in central Hokkaido, Japan. We analyzed the physical, chemical, and biological properties of soil via an onsite survey in 47 farmers’ soybean fields where either puddled transplanted rice (PT), nonpuddled transplanted rice (NT), or dry direct-seeded rice (DS) was cultivated in the previous year (referred to as one-time PT, NT, and DS treatments, respectively). The objective of the study was to determine the key soil parameters that could explain the higher post-rice soybean yield following the one-time omission of puddling (NT and DS treatments). In addition, soybean yield and soil properties were surveyed in an experimental field experiment where PT and NT treatments were applied for 3 consecutive years with the expectation that the increases in post-rice soybean yield caused by the omission of puddling (NT treatment) would be more pronounced. In the survey of farmers’ field, soybean yields were comparable among the one-time PT, NT, and DS treatments when the clay content was < 40%. However, soybean yield decreased as the clay content increased only under the one-time PT treatment. The adverse effects of puddling (PT treatment) on clay-rich soil appeared in a few soil physical parameters but not on any soil chemical and biological parameters. In the 34 selected clay-rich farmers’ fields (clay content >40%), narrow macropore and mesopore volumes, which indicate total available water capacity, were significantly (P < 0.05) lower under the one-time PT treatment than under the other two treatments. At the same time, the narrow macropore volume was significantly (P < 0.05) correlated with soybean yield. The one-time omission of puddling (NT and DS treatments) adopted in the surveyed farmers’ fields did not affect the wide macropore volume. Three consecutive years of NT treatment improved several soil physical properties as well as soybean root development in the experimental field. However, these improvements did not significantly increase soybean yield in the experimental field where the clay content was < 40%. These findings suggest that the one-time omission of puddling (NT and DS treatments) prevents the decreasing in the yields of post-rice soybean in clay-rich soils in central Hokkaido. The stable soybean yield, irrespective of the clay content, after the one-time omission of puddling (NT and DS treatments) in fields was attributed to the increased plant-available water capacity owing to the increased narrow macropore and mesopore volumes without clear changes in the wide macropore volume. By contrast, the effects of the one-time and three-time omission of puddling (NT and DS treatments) were not obvious in non–clay-rich field conditions in central Hokkaido.