Long-term fertilization has significant and durable effects on soil enzyme activity.
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Manure increased soil nutrients, pH, and carbon and nitrogen-cycling enzyme activity.
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Treatment trends in enzyme activity were similar between moist and oven-dried soils.
Abstract
Long-term nitrogen fertilization imparts significant effects on the soil environment and soil microbial communities relevant to nutrient cycling. Understanding the complex interactions between soil biology, management practices, and the soil environment is an important step toward improving soil health. The Crop Residue study of the Long-Term Plots at Pendleton, Oregon USA demonstrate declines in soil quality with acidification and reduced soil carbon under a wheat-fallow cropping system. Soil enzyme activity (β-glucosidase, β-glucosaminidase, acid and alkaline phosphatase, arylsulfatase, and arylamidase) and ammonia oxidation was measured in plots managed with three different fertilizer treatments (no fertilization, urea-ammonium nitrate, and manure) at two stages of crop growth. The fertilizer treatments had a strong impact on total carbon and nitrogen and produced a broad range of soil pH from 4.8 (urea-ammonium nitrate) to 5.7 (no fertilizer) and 6.7 (manure). Overall, potential nutrient cycling activity (hydrolytic enzymes and ammonia oxidation) was enhanced in manure-fertilized soils except for acid phosphatase which was greater in the low pH soil. Treatment trends in the proportional geometric mean (an index of soil nutrient cycling capacity) were generally consistent over a 32-year transect excluding acid phosphatase. All activities were highly correlated to total carbon, nitrogen, and sulfur. Except for β-glucosaminidase, all enzymes were also highly correlated to pH. The only enzyme to vary by crop phase was β-glucosidase which was greater at boot than tillering. Soils remaining after the analyses were oven-dried at 40 °C and stored in a non-climate-controlled warehouse for archiving. After 2-years of storage, the oven-dried soils were re-assessed for hydrolytic enzyme activity to determine whether the ability to distinguish treatment differences is retained in archived soils. The oven-dried archived samples showed similar contrasts as the soils analyzed moist between manure and the other fertilizer treatments, although the level of activity was significantly reduced. Overall, long-term fertilization with manure slowed the decline in soil acidity and enhanced soil function relevant to nutrient cycling and organic matter dynamics in the wheat-fallow cropping system.
