This study investigated the effect of C/N ratio, placement of plant residues, and leaching amounts on soil respiration, microbial biomass, and nutrient availability within four weeks after amendment. Young faba bean shoots (FB, C/N 7) and mature wheat straw (WH, C/N 80) were used as low and high C/N residue, respectively. Soil was unamended, mulched with FB or WH only, or mulched with one residue and mixed with the other residue. Leaching with 5 or 25 mL water was carried out on days 4, 12, and 20. Cumulative respiration and microbial biomass N were higher with 25 than 5 mL only in treatments with two residues. WH under FB mulch reduced N availability compared to FB mulch alone, whereas FB under WH increased N availability compared to WH mulch alone. When soils were leached with 25 mL water, available N in FB mulch over WH was lower on day 12, but higher later, compared to WH mulch over FB. In contrast, in WH mulch over FB microbial biomass N increased over time whereas available N decreased. In conclusion, the effect of C/N ratio of the mulch on soil available and microbial biomass N was greater with the higher leaching amount.