LAUSR

Microbial biomass (MB) plays a critical role in residue decomposition and soil organic matter (SOM) turnover. We investigated the effects of the initial size of soil MB pool and nitrogen (N) availability on the incorporation of ryegrass residue carbon (C) into microbial biomass C (MBC) and dissolved organic C (DOC). Soils from a row crop system (CS) and an adjacent grass sod (GS) were preincubated with (i.e., increased MB) and without (i.e., unchanged MB) glucose to produce soils with two levels of initial MB size before residue additions. Ammonium sulfate was added to test the effect of N availability. Residue addition increased DOC production in both soils, regardless of initial MB size and N availability, contributing 9 to ∼22% to the total pool. Residue quality was observed to affect the incorporation of residue C into DOC, which depended on soil type and initial MB size. However, the assimilation of residue C into MBC was not affected by the quality of ryegrass residue. Compared with the control (which did not have residue and N additions), a significant increase in SOM-derived MBC by residue addition was observed in CS without glucose preincubation but not in the glucose preincubated CS and in GS. This indicated that the primed MBC by residue addition depended on initial MB size and soil type. The assimilation of residue C into MB was marginally inhibited by the preincubation with glucose in GS but was promoted in CS. With the addition of extra N, higher ryegrass-derived MBC was observed in CS with glucose preincubation compared with the corresponding treatments without preincubation, which was not found in GS. These results suggested that residue-derived MBC was not only regulated by initial MB size and N availability but also was affected by soil management history. However, N addition reduced ryegrass-derived DOC production in GS without glucose preincubation. Apparently, both soil MB size and N availability largely affected the assimilation and incorporation of residue C in soil labile pools (i.e., DOC and MBC), and the extent of this relationship varied between two agricultural soils.