EFFECTS OF PRIOR CROP RESIDUE MANAGEMENT ON MICROBIAL PROPERTIES AND CROP RESIDUE DECOMPOSITION

A litterbag decomposition experiment was conducted to test the hypothe sis that differences in crop residue management practices influence th e size and activity of the microbial community that regulates residue decomposition. Residues of wheat (Triticum aestivum), barley (Hordeum vulgare) and lupin (Lupinus albus) were incorporated into soils in whi ch cereal crop residues were previously managed by burning, removal or incorporation for three years. Litterbags ere collected periodically for analysis of mass loss and N content, total substrate-induced respi ration (SIR), and direct counts of fungi and bacteria. Mass loss from lupin residue (75%) was nearly twice that of either barley (42%) or wh eat (38%) straw after the first 90 days of decomposition. The differen ces in residue mass loss were also reflected in significantly higher r ates of SIR and greater populations of bacteria and fungal hyphae on r esidues of lupin than on wheat or barley straw. Temporal differences i n wheat and barley SIR were positively correlated to residue N concent rations. Across all treatments and residue types, SIR rate at 28 days of decay was a good predictor of residue mass loss over the first 90 d ays of decay. During the first 90 days of decay, wheat straw from the incorporated treatment had 33% greater mass loss and a 50% greater dec omposition rate than wheat from the burned or removed treatments. Cons istent with these differences, microbial activity, as measured by SIR, was nearly 30% higher on wheat straw from the incorporated treatment than that of the burned or removed treatments. Although the effects we re not significant, numbers of bacteria and fungi were also higher on wheat straw from the incorporated treatment. Prior management of resid ues also influenced the magnitude and timing of N fluxes from decaying wheat straw. Wheat from the incorporated treatment had the greatest n et N immobilisation in the early stages of decay (0-14 days) and the g reatest net N mineralisation in the later stages of decay (90-150 days ). There were no significant effects of prior residue management on mi crobial activity and decomposition of barley or lupin residues. This r esidue-specific response to management may be explained by a prior con ditioning of the soil microbial community to wheat straw which has a m ore recalcitrant chemical composition (eg > lignin:N ratio) than that of either barley or lupin residues. (C) 1998 Elsevier Science B.V.