Respiration, available N and microbial biomass N in soil amended with mixes of organic materials differing in C/N ratio and decomposition stage

The effect of C/N ratio on decomposition after amendment with individual litters and their mixes has been studied extensively. But less is known about the effect of highly decomposed organic materials such as manures on interactions in mixes. In this study, a sandy clay loam was amended with finely ground young faba bean shoots (FB, C/N 9), sheep manure (SM, C/N 6) and mature wheat straw (W, C/N 82) either individually (referred to as 100FB, 100SM, 100SM) or as mixes (e.g. 75FB-25SM, 50FB-50SM, 25FB-75SM, where the value represents the weight percentage of the organic materials). Soil was sampled on days 16, 32 and 48. Cumulative respiration after 48 days was similar with 100FB and 100 W, where it was about seven-fold higher than in 100SM. It decreased with percentage SM in the mixes and was about two-fold higher in 25SM-75 W than in 75SM-25 W. Available N was low with 100 W and microbial biomass N (MBN) was low with 100SM (five-fold lower than with FB). In mixes of W with FB or SM, available N was between two and 40-fold lower than expected with greater differences between measured and expected values FB-W than with SM-W. In FB-W and SM-W mixes, MBN was between 50% and two-fold higher than expected. In mixes of FB and SM, MBN on day 16 was 50% higher than expected in 75FB-25SM, but 30-50% lower than expected in mixes with ≥50% SM. We conclude that mixing of W with FB can provide plants with N, but also reduce N loss via leaching or denitrification.