Microbial activity in pig slurry-amended soils under semiarid conditions

Soil amendment with manures from intensive animal industries is nowadays a common practice that may favorably or adversely affect several soil properties, including soil microbial activity. In this work, the effect of consecutive annual additions of pig slurry (PS) at rates of 30, 60, 90, 120 and 150 m3 ha-1 y-1 over a 4-year period on soil chemical properties and microbial activity was investigated and compared to that of an inorganic fertilization and a control (without amendment). Field plot experiment conducted under a continuous barley monoculture and semiarid conditions were used. Eight months after the fourth yearly PS and mineral fertilizer application (i.e. soon after the fourth barley harvest), surface soil samples (Ap horizon, 0-15 cm depth) from control and amended soils were collected and analysed for pH, electrical conductivity (EC), contents of total organic C, total N, available P and K, microbial biomass C, basal respiration and different enzymatic activities. The control soil had a slightly acidic pH (6.0), a small EC (0.07 dS m-1), adequate levels of total N (1.2 g kg-1) and available K (483 mg kg-1) for barley growth, and small contents of total organic C (13.2 g kg-1) and available P (52 mg kg-1). With respect to the control and mineral fertilized soils, the PS-amended soils had greater pH values (around neutrality or slightly alkaline), electrical conductivities (still low) and contents of available P and K, and slightly larger total N contents. A significant decrease of total organic C was observed in soils amended at high slurry rate (12.3 g kg-1). Compared with the control and mineral treatments, which produced almost similar results, the PS-amended soils were characterized by a higher microbial biomass C content (from 311 to 442 g kg-1), microbial biomass C/total organic C ratio (from 2.3 to 3.6%) and dehydrogenase (from 35 to 173 microgram INTF g-1), catalase (from 5 to 24 micromol O2 g-1 min-1), BAA-protease (from 0.7 to 1.9 micromol NH4+ g-1 h-1) and beta-glucosidase (from 117 to 269 micromol PNP g-1 h-1) activities, similar basal respirations (from 48 to 77 microgram C-CO2 g-1 d-1) and urease activities (from 1.5 to 2.2 micromol NH4+ g-1 h-1), and smaller metabolic quotients (from 6.4 to 7.7 ng C-CO2 microgram-1 biomass C h-1) and phosphatese activities (from 374 to 159 micromol PNP g-1 h-1). For example, statistical analysis of experimental data showed that, with the exception of metabolic quotient and total organic C content, these effects generally increased with increasing cumulative amount of PS. In conclusion, cumulative PS application to soil over time under semiarid conditions may produce not only beneficial effects but also adverse effects on soil properties, such us the partial mineralization of soil organic C through extended microbial oxidation. Thus, PS should not be considered as a mature organic amendment and should be treated appropriately before it is applied to soil, so as to enhance its potential as a soil organic fertilizer.