Crop residue is an important source of N₂O emission from agricultural fields, although it is a less-studied N₂O source compared with fertilizer application. The aims of this study were to investigate the temporal change in N₂O emission from cabbage and lettuce residues and to elucidate the relationship between N₂O emission derived from crop residue and WFPS (water-filled pore space). Cabbage and lettuce crop residue were placed onto the surface of repacked soil cores with a soil moisture content of 55％ or 70％ WFPS. N₂O flux was measured for 64 days. Approximately 50％ of the cumulative N₂O emission was emitted for over 20–27 days after lettuce residue application, whereas it was emitted for over 38–45 days after the cabbage residue application. These results may be attributed to the difference in decomposition rates between the crop residues. Moreover, our results suggested that WFPS affected N₂O emission after crop residue input through change in contributions of nitrification and denitrification to N₂O production. Crop residue type and soil water content were the major factors affecting N₂O emission after the crop residue input in agricultural fields.

A long-term fertilizer experiment has been conducted since 1973 in the apple ‘Jonathan’ orchard in Fukushima prefecture, Japan. The soil type in the orchard is a brown forest soil. This fertilizer experiment is equipped with a 0N plot (0 kg N 10 a⁻¹) and a 4N plot (20 kg N 10 a⁻¹ as annual application of ammonium nitrate). In this field of study, we evaluated the effects of long-term nitrogen (N) fertilization on the changes in the yield, fruit quality, and mineral contents of apple trees, and soil chemical properties using previous (1974–1994) and our recent data (2014–2016). We also collected soil samples in the year 2014, 41 years after the experiment began, and evaluated the effects of long-term nitrogen application on the changes of the contents of different forms of soil N (NO₃⁻, exchangeable NH₄⁺, total fixed NH₄⁺, and organic N). The results were summarized as follows.

1) The long-term nitrogen fertilization decreased the fruit acidity and increased the N contents of the fruit and leaves. On the other hand, the yield, fruit number, fruit weight, fruit firmness, and brix sugar were not affected by nitrogen application and annually fluctuated.

2) Nitrogen fertilization decreased the pH of the surface soil (0–20 cm). The soil acidification was accompanied by a significant decrease in the contents of soil exchangeable Ca, Mg, and K and a significant increase in the content of soil exchangeable Al.

3) In the 0N and 4N plots, the content of Ca in the leaves in the year 2016 was lower than the average content in the years 1975, 1976, and 1984, which corresponded to the remarkable decrease of the content of soil exchangeable Ca during these three decades.

4) The contents of total N and total C in the surface soil (0–5 cm) of both plots gradually increased during the course of the experiment. The increase in total N and total C in the 4N plot was greater than that in the 0N plot.

5) On comparing the forms of soil N in the 0N and 4N plots in the year 2014, it was observed that the contents of NO₃⁻ and total fixed NH₄⁺ were not significantly different at all soil depths. The content of exchangeable NH₄⁺ was not significantly different except in the 70–90 cm soil layer. The content of organic N in the 0–5 cm and 70–90 cm layers was significantly higher in the 4N plot. More than 80％ of the soil N existed in an organic form, regardless of nitrogen application and soil depth.

6) It was suggested that most of the applied N was lost by leaching of NO₃⁻ and was hardly accumulated in the soil. However, it was also suggested that a part of the applied nitrogen was absorbed by the undergrowth plants, and was accumulated in the surface soil (0–5 cm) after their decomposition. The leaching of NO₃⁻ was accompanied with the leaching of Ca²⁺.

We monitored soil erosion in two sloping fields that were contaminated with ¹³⁷Cs to test the effect of cover crops on ¹³⁷Cs runoff control as a countermeasure for pollutant diffusion caused by rain erosion. In the first field in the Tohoku Agricultural Research Center (slope angle: 4 deg., Low-humic Allophanic Andosols), we established three test plots: the first was sown with Festulolium (×Festulolium Aschers. et Graebn.), the second with soybean (Glycine max) cultivation, and the third was a plowed plot. Then, we monitored soil erosion from June 2013 for 17 months. In the second field (slope angle: 6–7 deg., Allophanic Andosols), in the Yamakiya district of Kawamata Town, decontamination work (removing the top 5 cm of soil and dressing with decomposed granite soil) had been conducted. Here, we established two kinds of cover crop plots and a plowed plot, and it was then monitored from October 2015 for 13 months. In the first cover crop plot, perennial ryegrass (Lolium perenne L.), Kentucky bluegrass (Poa pratensis L.), and white clover (Trifolium repens L.) were sown (Pe+Kb+Wc plot) as a combination to cover the ground quickly and persistently with minimal management effort. In the second plot, hairy vetch (Vicia villosa Roth.) was sown as a green manure (Vetch plot). In both fields, cover crops reduced sediment and ¹³⁷Cs runoff by one order of magnitude than that in the plowed plots and soybean plot. Compared with the Pe+Kb+Wc plot, the Vetch plot had a lower coverage for several months after sowing in September, and consequently, had an 8 times higher ¹³⁷Cs runoff in December. Thereafter, the differences in coverage and runoff between the plots decreased. Thus, we demonstrated a large-scale runoff control regardless of the grass species that was cultivated.