The plant nutrient and microbial enzyme concentrations in a soil, and the degree of soil aggregation reflect the quality of the soil’s chemistry, biology, and physical structure respectively. Each of these three measures responds to a soil’s long-term fertility management: long-term applications of either organic or mineral fertilizers will strongly influence both the levels of plant nutrients and microbial activity, and the soil physical structure. Insight into the soil quality may therefore also be gained from the relationship between these three measures. Our aim was investigating the influence of different fertilization regimes on nutrient and enzyme levels in soil aggregates, as well as the degree of aggregation itself.
We chose four treatments from the long-term fertilization experiment on the Loess Plateau at Northwest Science and Technology University of Agriculture and Forestry, Shaanxi, China, which was established in 1977. Treatments varied the fertilizer material——maize straw, manure, chemical fertilizer, and control (no fertilizer)——with fertilizer being applied after both plowing and planting. The experiment used a randomized complete block design with three blocks and 12 plots measuring 19.8 m2. A wheat-maize crop rotation in a year, commonly used in the region, was adopted for this experiment. The soil in the study site is a heavy loam Lou soil (Earth-cumuli-ortho anthrosols according to the Chinese classified system) with a average fertility level.
After 25 years of consistent crop rotation and fertilization, soil samples were collected in 2002 just before planting the winter wheat. Topsoil (0～20cm) samples were collected using a five-point method in each plot. Samples were air-dried, then machine-sieved into five aggregate size fractions: >5mm, 2～5mm, 1～2mm, 0.25～1mm, and <0.25mm. After passed through 1mm sieve, micro-aggregates were classified into two parts divided by 0.01mm by using physical shaking and sedimentation. Aggregate size fractions were then passed through 1mm and 0.25mm sieves prior to nutrient and enzyme analysis. Soil organic matter, total N, and total P were measured in duplicate using standard methods. Enzyme analysis was performed in triplicate: soil urease by the Hoffman method, invertase by the T.A.щербакова method, and alkaline phosphatase by the Ф.Х. Хазиев method.All fertilization increased the degree of aggregation and the quantity of aggregates in the soil, significantly improving soil structure over the control treatment. Mean weight diameter of soil aggregates, and C/F0.01 of micro-aggregates increased from the control to the fertilizer treatments. Long-term fertilization amplified the variance of soil chemical and biological properties within soil microenvironments. Within a given treatment, nutrient content and enzyme activity tended to decrease with increasing aggregate size, with the highest values generally occurring in the 0.01mm characteristic micro-aggregates. Nutrient and enzyme levels followed the pattern manure > maize straw > chemical fertilizer > control across all aggregate size fractions. Despite their lower concentrations of nutrients and enzymes, large aggregates accounted for most of the nutrients and enzymes in the topsoil. There was a significant positive correlation between nutrient levels, enzyme levels, and aggregate size, showing a consistent impact of fertility regime on soil quality.