Carbon decomposition and nitrogen mineralization of foxtail and milk vetch incorporated into paddy soils for different durations of organic farming

Abstract

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.