Impact of Compost Application during 5 Years on Crop Production, Soil Microbial Activity, Carbon Fraction, and Humification Process

Compost amendment is considered as a practical tool to increase the soil organic matter (SOM), which contributes to agricultural sustainability. The objective of the present work was to evaluate the impacts of organic soil management over 5 years on orchard prune production (Prunus salicina), microbial activity, soil carbon (C) fraction, and stabilization degrees of soil humification. Plot experiment was designed with two different soil managements: i) for minimizing anthropogenic disturbances, only mulching of orchard residues derived from prune tree plot area was applied to soil surface (S + V); and ii) the amendment of composted manure was annually practiced in addition to the utilization of orchard residues inside the plot area (S + V + C). After 5 years, the soil with the continuous compost application (S + V + C) showed higher productivity of Prunus salicina (21.4%), greater fruit diameter (7.8%), and heavier fruit weight (22.4%) than the soil without compost application (S + V). Nutrient content in foliar analysis showed no difference between the two treatments (S + V and S + V + C). By contrast, the amended soil by compost (S + V + C) increased the SOM and water-soluble C fraction in parallel with the increase of microbial parameters (microbial biomass C, adenosine triphosphate (ATP), basal respiration, and dehydrogenase). Analyzing soil humic acid character by chemical spectra techniques of Fourier-transform infrared (FT-IR) spectra and ¹³C nuclear magnetic resonance (¹³C-NMR), gradual reformation of a more stabilized structure was shown in both soils (S + V and S + V + C), due to the selective biodegradation and humification process after the amendments over 5 years. Especially, in the soil treated with compost application (S + V + C), the increase of functional C groups (aromatic and carboxylic groups), which reinforce the recalcitrant character of soil humified fraction, was clearly observed. The continuous application of composted manure for the duration of 5 years improved the orchard soil fertility as well as productivity. Abbreviations: ATP, adenosine triphosphate; CPMAS, cross-polarization magic angle spinning; EC, electrical conductivity; FT-IR, Fourier-transform infrared; HA, humic acid; HS, humic substance; INTF, iodonitrotetrazolium formazan; K, Potassium; LSD, least significant differences, N, nitrogen; NMR, nuclear magnetic resonance; O, oxygen; OM, organic matter; MBC, microbial biomass C; P, phosphorous; SD, standard deviation; SE, standard error; SOM, soil organic matter; TOC, total organic carbon; WSC, water-soluble C; WS-Ch, water-soluble carbohydrate