Animal manure and anhydrous ammonia amendment alter microbial carbon use efficiency, microbial biomass, and activities of dehydrogenase and amidohydrolases in semiarid agroecosystems

The potential negative impact of agricultural practices on soil and water quality is of environmental concern. The associated nutrient transformations and movements that lead to environmental concerns are inseparable from microbial and biochemical activities. Therefore, biochemical and microbiological parameters directing nitrogen (N) transformations in soils amended with different animal manures or inorganic N fertilizers were investigated. Soils under continuous corn cultivation were treated with N annually for 5 years at 56, 168, and 504 kg N ha−1 in the form of swine effluent, beef manure, or anhydrous ammonia. Animal manure treatments increased dehydrogenase activity, microbial biomass carbon (Cmic) and N (Nmic) contents, and activities of amidohydrolases, including l-asparaginase, urease, l-glutaminase, amidase, and β-glucosaminidase. Soils receiving anhydrous ammonia demonstrated increased nitrate contents, but reduced microbiological and biochemical activities. All treatments decreased Cmic:organic C (Corg) ratios compared with the control, indicating reduced microbial C use efficiency and disturbance of C equilibrium in these soil environments. Activities of all enzymes tested were significantly correlated with soil Corg contents (P < 0.001, n = 108), but little correlation (r = 0.03, n = 36) was detected between Cmic and Corg. Activities of amidase and β-glucosaminidase were dominated by accumulated enzymes that were free of microbial cells, while activities of asparaginase and glutaminase were originated predominately from intracellular enzymes. Results indicated that soil microbial and biochemical activities are sensitive indicators of processes involved in N flow and C use efficiency in semiarid agroecosystems.