Long-Term Effects of Nitrogen and Lime Application on Plant–Microbial Interactions and Soil Carbon Stability in a Semi-Arid Grassland

This study investigated the long-term (70 years) effects of N fertilisation (ammonium nitrate [AN], ammonium sulphate [AS]) at 70 and 211 kg N kg/ha, and liming (L) on plant-microbial interaction and soil carbon stability in a semi-arid grassland in South Africa. Aboveground biomass increased with N addition, particularly AN211, showing a 119% increase compared to the control, while both liming and N applications increased belowground biomass. Nitrogen addition significantly altered plant stoichiometric ratios, with root N ratios showing greater treatment-induced variation (12.7-51.3) than shoot N ratios (10.2-16.8). Microbial biomass carbon peaked with AN70 treatment, while dehydrogenase activity was highest in lime-only plots but suppressed in non-limed N treatments. Conversely, urease activity was highest in the control group and suppressed in most fertilised treatments. Despite increased biomass production, SOC remained unchanged across treatments (49.7-57.6 g/kg), whereas soil pH was lowest (<3.5) and highest (>6) under N fertilisation and lime, respectively. PCA revealed distinct clustering of treatments, with N forms differentially affecting plant allocation patterns and microbial parameters. This study demonstrates that plant-soil-microbe stoichiometric imbalances and pH-induced limitations on microbial function explain the disconnect between plant productivity and carbon sequestration in this semi-arid grassland ecosystem.