Mineralization dynamics and biochemical properties during initial decomposition of plant and animal residues in soil

The use of organic residues as soil amendments or fertilisers may represent a valuable recycling strategy. In this study, a series of laboratory assays was performed to study the effects of the application of organic residues on C and N mineralization and biochemical properties in a Mediterranean agricultural soil. Two crop residues (straw and cotton) and two animal by-products (meat bone meal and blood meal) were added at three rates (5, 10 and 20 mg g^?1 on dry weight basis) to a moist (40% water holding capacity) sandy soil and incubated at 20 °C for 28 days. Each residue underwent a different mineralization pattern depending on the nature and complexity of its chemical constituents. In all cases, the addition of the waste produced, after a short lag-phase, an exponential increase in the soil respiration rate, reflecting the growth of microbial biomass. The amount of total extra CO₂-C evolved after 28 days, expressed as % in respect to added C, differed significantly (P < 0.005) among application doses: 5 > 10 > 20 mg g^?1 and residue type: meat bone meal > blood meal > cotton cardings > wheat straw. Plant residues led to a rapid immobilisation of N that affected microbial size and activity and further mineralization. Animal by-products produced an immediate and remarkable increase of mineral N in the soil. However, the large amounts of NH₄⁺ released in the soil at high rates of animal residues led, in some cases, to temporary adverse effects on microbial biomass growth and nitrification. All residues produced a significant increase in soil microbial biomass size and activity, being the intensity of the response related to their chemical properties.