Enzyme activities and microbial nutrient limitations in response to digestate and compost additions in organic matter poor soils in the Marches, Italy

Digestate and its derived composts are valuable organic fertilizers for growing vegetables and field crops. However, the impacts on enzymatic activities and microbial resources acquisition pattern are not well addressed. This study investigated ecoenzymatic activities and microbial resource acquisition patterns following these organic fertilizations for sunflower cultivation. The experiment involved applying digestate, five compost types derived from digestate, and inorganic N fertilizer (NINORG), with a control plot having no treatment. Activities of nine enzymes related to C-, N-, P-, and S- acquiring were measured at three development stages: seedling, pre-blooming, and harvest. Across all treatments, resource allocations, especially for C-, N-, and P- acquisitions, increased with plant development, correlating positively with microbial biomass, soil organic carbon, and soil nutrients contents, showing the heightened energy and nutrient sources stimulated microbial growth, prompting increased resource allocation to meet elevated demands, particularly in later crop developmental stages. Stoichiometric analysis revealed greater investments in P compared to N and C, persisting throughout the study period, with P acquisitions at least four-fold greater than N and at least seven times greater than C. This pattern was greatly impacted at harvest for all treatments except NINORG and control, with a reduced investment rate for P- acquisition compared to that of C-, N-, albeit the C: N:P ratio was still far short of the global mean 1:1:1 ratio. The strong negative correlation between available P and vector angle (p<0.001) indicated that the addition of bioavailable P through organic amendments helped reduce the gap towards the ideal ratio. In summary, on soils with imbalanced microbial nutrient and energy demands, organic amendments intensified microbial growth, resulting in heightened nutrient acquisition. However, the declining trend in P acquisition rates compared to C and N over the cropping period suggests possible adjustments with the help of organic fertilizations. Organic fertilizers, by addressing energy-nutrient imbalances, can contribute to a healthier soil ecosystem under conventional farming systems.