Soil aggregate stratification of nematodes and microbial communities affects the metabolic quotient in an acid soil

The addition of fresh organic matter is known to modify both soil aggregation and soil biotic community composition. We hypothesized that fertilization alters the composition of soil nematode and microbial communities in soil aggregates, and the interaction between nematodes and microbes can stimulate or inhibit microbial activity. We used a field experiment with 9 years of manure application to investigate changes in nematodes and microbial communities among aggregate size fractions in an acid soil planted with maize in subtropical China. Nematodes, microbial communities, and metabolic quotient (qCO2) were examined within three aggregate size fractions from soils under four fertilization regimes. Three aggregate fractions include large macroaggregates (>2000 μm; LA), small macroaggregates (250–2000 μm; SA), and inter-aggregate soil and space (<250 μm; IA). Four manure treatments include no manure (M0), low-rate manure with 150 kg N ha−1 y−1 (M1), high-rate manure with 600 kg N ha−1 y−1 (M2), and high-rate manure with 600 kg N ha−1 y−1 and lime at 3000 kg Ca(OH)2 ha−1 3 y−1 (M3). Fertilization influenced the proportion of the aggregate size fractions. The proportion of the LA fraction significantly increased under M2 and M3 treatments compared to M0 and M1 treatments, while the SA fraction significantly decreased. Aggregate fractions significantly affected the total number of nematodes and the abundance of bacterivorous nematodes (dominant genus Protorhabditis) and plant parasitic nematodes (dominant genus Pratylenchus), with values following the trend of LA > SA > IA. A high value for the nematode structure index (SI) in the LA fraction suggested a complex community structure with many linkages in the food web. Aggregate fractions also influenced microbial biomass and diversity. PLFA signature analysis revealed that microbial biomass and diversity (Shannon index) increased with decreasing aggregate size. However, the SA fraction had a significantly higher soil metabolic quotient (qCO2) than the IA fraction. Only fertilization had a significant effect on the compositions of nematode groups, while both fertilization and aggregate fractions significantly affected microbial community composition. The variations in the composition of nematode and microbial communities could be explained independently by fertilization treatments (44% and 48%, respectively) and aggregate size (6.0% and 21%, respectively). Aggregated boosted trees (ABT) analysis indicated that total C exerted the strongest influence on microbial biomass, while pH influenced the total number of nematodes. The abundance of bacterivores showed a significant positive association with bacterial biomass across fertilization treatments and aggregate fractions (r2 = 0.17, P = 0.026), which could partly explain the significant negative correlation between the total number of nematodes and qCO2 (r2 = 0.25, P = 0.002). The grazing on microbes by microbivores may decrease microbial activity.