Abstract. Crop residues are valuable soil amendments in terms of
the carbon and other nutrients they contain, but the incorporation of residues
does not always translate into increases in nutrient availability, soil
organic matter (SOM), soil structure, and overall soil fertility. Studies
have demonstrated accelerated decomposition rates of chemically
heterogeneous litter mixtures, compared to the decomposition of individual
litters, in forest and grassland systems. Mixing high C:N ratio with low C:N ratio amendments may result in greater carbon use efficiency (CUE) and
nonadditive benefits in soil properties. We hypothesised that nonadditive benefits would accrue from mixtures of
low-quality (straw or woodchips) and high-quality (vegetable waste compost)
residues applied before lettuce planting in a full factorial field
experiment. Properties indicative of soil structure and nutrient cycling
were used to assess the benefits from residue mixtures, including soil
respiration, aggregate stability, bulk density, SOM, available N, potentially mineralisable N, available P, K, and Mg, and crop yield. Soil organic matter and mineral N levels were significantly and
nonadditively greater in the straw–compost mixture compared to individual
residues, which mitigated the N immobilisation occurring with straw-only
applications. The addition of compost significantly increased available N, K, and Mg levels. Together, these observations suggest that greater nutrient
availability improved the ability of decomposer organisms to degrade straw
in the straw–compost mixture. We demonstrate that mixtures of crop residues can influence soil properties
nonadditively. Thus, greater benefits may be achieved by removing, mixing,
and reapplying crop residues than by simply returning them to the soils
in situ.
The Influence of Nutrient Availability on Soil Organic Matter Turnover Estimated by Incubations and Radiocarbon Modeling