Long-term Effects of Mineral Fertilizers on Soil Microorganisms

Increasing nutrient inputs into terrestrial ecosystems affect not only plant communities but also the associated soil microbial ones. Studies carried out in predominantly unmanaged ecosystems have found that increasing nitrogen (N) inputs generally decrease soil microbial biomass while less is known about their long-term impacts on managed systems such as agroecosystems. The objective of this study was to analyze the responses of soil microorganisms to mineral fertilizers using data from long-term fertilization trials in cropping systems. A meta-analysis based on 107 datasets from 64 long-term trials from around the world revealed that mineral fertilizer application led to a 15.1% increase in the microbial biomass (Cmic) above the levels observed in unfertilized control treatments. Mineral fertilization also increased soil organic carbon (Corg) content, suggesting that Corg is a major contributor to the overall increase in Cmic under mineral fertilization. The magnitude of the effect of fertilization on Cmic was found to be pH-dependent. While fertilization tended to reduce Cmic in soils with a pH below 5 in the fertilized treatment, it had a significantly positive effect at higher soil pH values. Duration of the trial also affected the response of Cmic to fertilization, with increases in Cmic most pronounced in studies with a duration of at least 20 years. The input of N per se does not seem to negatively affect Cmic in cropping systems. Application of urea and ammonia fertilizers can, however, temporarily increase pH, osmotic potential, and ammonia concentrations to levels inhibitory to microbial communities. Even though impacts of fertilizers are spatially limited, they may strongly affect soil microbial biomass and community composition in the short term. Long-term repeated mineral N applications may alter microbial community composition even with small changes in pH. The way specific microbial groups respond to repeated applications of mineral fertilizers, however, varies considerably and seems to depend on environmental and crop management related factors.