Relationships of plant traits and soil biota to soil functions change as nitrogen fertiliser rates increase in an intensively managed agricultural system

1. Because plant and soil systems are strongly inter-linked, manipulating plant traits in intensively managed agricultural systems could be used to improve soil functioning and sustainability. However, we have little understanding of whether the impacts of plant traits on soil systems are modified by other management practices, such as fertiliser use.2. We tested whether relationships among plant traits, soil biota (microbes and nematodes) and soil functions (carbon and nitrogen (N) cycling) change with N fertiliser regime in a field-based, intensively managed experiment with high biomass removal. The experiment consisted of seven plant species compositions crossed with six N fertiliser rates (0–500 kg N ha⁻¹ year⁻¹).3. Relationships among plant traits, soil biota and soil functions were often consistent across N fertiliser rates. However, the relationship of percentage N2 fixer to qCO2 shifted from negative to neutral as N rates increased, and the slope of several relationships of plant traits to N cycling indicators declined when &gt;200 kg N ha⁻¹ yea^r−1 was added. The negative relationship of the fungal: bacterial ratio to N cycling indicators also became neutral when &gt; 200 kg N ha⁻¹ year⁻¹ was added, and the relationships of bacterivore abundance and the plant parasitic index to respiration changed direction as N inputs increased.4. Some relationships of plant traits and soil biota to soil functions were in unexpected directions. This was sometimes associated with species-specific effects and inconsistent trait trade-offs within species. In general, conservative plant traits and fungal dominance were associated with low N cycling and cellulose paper decomposition rates, but high respiration rates. Nematode-based variables were better predictors of some functions than microbial ones; their incorporation into plant trait research could improve predictive power and system understanding.5. <i>Synthesis and applications</i>. Manipulating plant community traits can modify soil functions in intensively managed systems, but may result in larger changes where fertiliser nitrogen inputs are relatively low. Improved modelling that integrates how plant community traits, species-specific effects and management practices interact to determine soil functions will be required before managers can confidently predict the consequences of changing plant community traits.