Asynchronous Response of Plant and Nematode Communities to Decadal Nitrogen Enrichment in a Meadow Steppe

Description of the data and file structureThe above-ground vegetation from each plot was harvested in August, using a 1 m × 1 m quadrat. After harvest, the vegetation was sorted to species, oven-dried at 65 ℃ for 48 hours, and then weighed to determine above-ground biomass. Plants were categorized into grasses, forbs, and sedges based on their life forms.We collected five soil cores (5 cm diameter) from the top 0-10 cm of each plot, sieved through a 2-mm mesh. The composite soil sample was separated into two parts: One part of the fresh soil was stored in a refrigerator at 4 ℃ for measurements of soil inorganic N, soil moisture and nematode community structure. Soil moisture was measured after drying at 105 ℃ for 24 hours.The soil inorganic N (NH₄⁺+NO₃^-) was extracted with 2 M KCl and quantified with a 2300 Kjeltec Analyzer Unit. Another part of the fresh soil was air-dried for analyzing the soil pH in a 2.5 : 1 (water:soil) suspension.Nematodes were extracted from 100 g of fresh soil using the Baermann funnel method. After heat-killing in a 60°C water bath for approximately 3 minutes, the specimens were preserved in 4% formalin and then transferred to the laboratory for microscopy analysis.Nematode counts were conducted under a stereoscopic microscope, identifying up to the first 100 individuals or the entire sample if fewer. Classification into trophic groups (bacterivores, fungivores, herbivores, omnivores-predators) was based on feeding habits and morphological traits.Code/softwareAll statistical analyses were conducted using R version 4.3.1 (R Development Core team, 2023):The R ColorBrewer package was used to examine precipitation intensity from 1957 to 2023, focusing on the years 2015, 2019, and 2023, to investigate if variations in N addition effects on nematodes and plants could be attributed to precipitation patterns.We employed the lme4 package to examine the effects of varying N addition levels and addition years on nematodes and plants, including their respective functional groups, as well as the dominance and maturity indices (MI).We used the AGROREG package for determining the threshold values of the responses of nematode genus richness, functional groups, plant species, and their functional groups, alongside with dominance and maturity indices to N addition levels.The CODYN R package was used to calculate turnover, we now define that number of species appearing only at the later stage of the experiment but not in the early stage are considered “species gained”, whereas the number of species present only in the early stage but not in the later stage of the experiment are denoted as “species lost”. The number of all species observed at both sampling times are defined as the “total species observed in both timepoints”.