ABSTRACT

Foxtail (Alopecurus aequalis) and milk vetch (Astragalus sinicus) are typical winter weeds grown in single-cropping rice fields during the fallow season in central Japan. These winter weeds are generally incorporated into the soil as green manure during plowing, and before rice transplanting, to improve soil fertility and rice yield. To understand the carbon (C) decomposition and nitrogen (N) mineralization behavior of fallow weeds in organic rice farming, we carried out two experiments. Firstly, we examined the effect of adding 1% foxtail to soils that were sampled from fields that were farmed conventionally or organically for different durations (4–12 years). The results showed that net N mineralization of incorporated foxtail was not affected by the duration of organic farming, although the N mineralization of indigenous soil increased with organic farming duration. Secondly, 1% pulverized young and old foxtail (YF and OF; C/N ratios of 47.0 and 54.9, respectively), and milk vetch (YM and OM; C/N ratios of 13.0 and 14.6, respectively), were mixed with soil samples from fields that had been farmed organically for 15 years to determine the role of each plant. More than half of weed C was decomposed during 4 weeks of anaerobic incubation at 30°C. In addition, the net weed C decomposition potentials were significantly higher in YF and YM than those in OF and OM. The lower C/N ratio of milk vetch was contributed to a net N mineralization rate that was 8.4 times higher than that of foxtail. The percentage of N mineralized from milk vetch was significantly higher (about 2.3 times) than that from foxtail. Even though foxtail incorporation can contribute to the growth of organic rice as an N nutrient source, it should be noted that net N immobilization occurred for 2 weeks of incubation at 30°C, and sometimes extended to 4 weeks before net N mineralization became tangible. Considering this early net N immobilization following incorporation of foxtail, it is recommended to wait at least 4 weeks between the start of flooding after weed incorporation and transplanting rice to facilitate initial growth and avoid N exhaustion.

摘要

狐尾 ( Alopecurus aequalis ) 和紫云英 ( Astragalus sinicus)) 是日本中部休耕季节在单季稻田中生长的典型冬季杂草。这些冬季杂草通常在耕作期间和水稻移栽前作为绿肥掺入土壤中，以提高土壤肥力和水稻产量。为了了解有机水稻种植中休闲杂草的碳 (C) 分解和氮 (N) 矿化行为，我们进行了两个实验。首先，我们检查了在从常规耕作或有机耕作不同持续时间（4-12 年）的田地中取样的土壤中添加 1% 狐尾草的效果。结果表明，混入狐尾的净N矿化不受有机耕作时间的影响，尽管土着土壤的N矿化随着有机耕作时间的增加而增加。其次，将 1% 的幼老狐尾粉（YF 和 OF；C/N 比分别为 47.0 和 54.9）和紫云英（YM 和 OM；C/N 比分别为 13.0 和 14.6）与来自已经有机耕种了 15 年的田地，以确定每种植物的作用。在 30°C 的 4 周厌氧培养过程中，超过一半的杂草 C 被分解。此外，YF 和 YM 的净杂草 C 分解电位显着高于 OF 和 OM。紫云英较低的 C/N 比导致净 N 矿化率比狐尾草高 8.4 倍。紫云英中N的矿化百分比显着高于狐尾（约2.3倍）。尽管加入狐尾草可以促进有机大米作为 N 营养源的生长，应该注意的是，净氮固定发生在 30°C 孵育 2 周，有时会延长至 4 周，然后净氮矿化才变得明显。考虑到掺入狐尾草后的早期净氮固定，建议在杂草掺入后开始淹水和移栽水稻之间至少等待 4 周，以促进初始生长并避免氮耗尽。