Changes in light fraction soil organic matter and in organic carbon and nitrogen in compost amended soils

Summary

Organic vegetable growers can use compost to supply crops with nutrients and increase soil organic matter (OM), but little information is available about the transformations of compost OM over time in organically managed systems. This study examined light fraction soil organic matter (LFSOM) and organic carbon (C) and nitrogen (N) in organically and conventionally managed snap bean cropping systems (continuous beans (CB) and a fully phased beans/fall rye rotation (BR)) following three years of fertility treatments (1x and 3x compost, and 1x synthetic fertiliser, where 1x provided 50 kg ha-1 N). Light fraction C and N increased with compost amendment, with the C:N ratio significantly lower in composted plots than in synthetically fertilised plots. Rotation and weeding method played no role in the composition of LFSOM, or the percentage of LFSOM making up whole soil organic C or N. Light fraction N and C roughly doubled in the 1x compost plots over the three years, compared with synthetically fertilised plots, but was only 2.5 times higher in 3x compost plots. While the addition of 13 t ha-1 C increased whole soil C by 5.6 Mg ha-1, tripling the amount of added C raised whole soil C by 9.9 Mg ha-1. 1x and 3x compost increased whole soil N by 20 and 33%, respectively, compared with the 1x synthetically fertilised plots. The 3x compost treatment only, by reducing bulk density, improved soil physical properties.