Minoshima H, Jackson LE.

Reducing or eliminating tillage causes spatial heterogeneity in the distribution of plant residue in soil. The fates of 13C-labeled cowpea shoots were investigated to determine if the presence of decomposing cowpea roots and their associated biota affect the decomposition of cowpea shoots. Intact soil cores were taken from the plant row and the center of the beds in no-till (NT) and from the middle of the beds and plant rows in standard tillage (ST) in a California vegetable-legume rotation. Cores taken from the plant rows included cowpea roots; however, cores taken from the center of the beds included only small amount of cowpea roots. Aboveground biomass was removed from cores. Cowpea shoots with lower 13C values were placed on the soil surface in NT cores or incorporated in ST cores. Also, cowpea residue (without soil) was compared. Cumulative CO2 emission tended to be higher in NT regardless of presence or absence of roots than in ST with residue. Surprisingly, a relatively large amount of CO2 was emitted from the cowpea treatment without soil. Cumulative CO2 emission was higher in NT, and delta 13C from cowpeas in MBC was much more negative in ST than in NT. This indicates that plant residue in ST was readily decomposed and C from residue was immediately assimilated by soil microbes. Contrastingly, residue on the soil surface in NT was not decomposed and little C from residue was assimilated by soil microbes in this short 4 week period. The finding that ST caused greater retention of plant residue C in soil that did NT should be considered in determining how tillage management affects C storage.