Nutrient Management Research Database

http://www.sciencedirect.com/science/article/pii/S0038071714001187

Increasing nutrient inputs into terrestrial ecosystems affect not only plant communities but also associated soil microbial communities. Studies carried out in predominantly unmanaged ecosystems have found that increasing nitrogen (N) inputs generally decrease soil microbial biomass; less is known about long-term impacts in managed systems such as agroecosystems. The objective of this paper was to analyze the responses of soil microorganisms to mineral fertilizer 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 (C_mic) above levels in unfertilized control treatments. Mineral fertilization also increased soil organic carbon (C_org) content and our results suggest that C_org is a major factor contributing to the overall increase in C_mic with mineral fertilization. The magnitude of the effect of fertilization on C_mic was pH dependent. While fertilization tended to reduce C_mic 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 C_mic to fertilization, with increases in C_mic most pronounced in studies with a duration of at least 20 years. The input of N per se does not seem to negatively affect C_mic in cropping systems. The application of urea and ammonia fertilizers, however, can 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 when pH changes are small. How specific microbial groups respond to repeated applications of mineral fertilizers, however, varies considerably and seems to depend on environmental and crop management related factors.

• Analyzed results from peer-reviewed studies in a meta-analysis
• Applied the following criteria to select studies: Annual crops; trials have been initiated at least 5 years prior to soil sampling; study reported microbial biomass and organic carbon from an unfertilized control and a treatment with mineral N fertilization (including urea)
• Looked at levels of microbial biomass and organic carbon in topsoil
• Studies ranged from 5 to 130 years
• Included studies that also applied P and K, thus some observed effects may have been affected by P and K, rather than N alone
• Analyzed 107 datasets from 64 long-term trials

-          Analyzed results from peer-reviewed studies in a meta-analysis

-          Applied the following criteria to select studies

o   Annual crops

o   Trials have been initiated at least 5 years prior to soil sampling

o   Study reported microbial biomass and organic carbon from an unfertilized control and a treatment with mineral N fertilization (including urea)

-          Looked at levels of microbial biomass and organic carbon in topsoil

-          Studies ranged from 5 to 130 years

-          Included studies that also applied P and K, thus some observed effects may have been affected by P and K, rather than N alone

-          Analyzed 107 datasets from 64 long-term trials

• The addition or mineral fertilizer significantly increased organic carbon compared to the unfertilized control by an average of 12.8%.
• Fertilization significantly increased microbial biomass by 15.1% percent.
• pH was an important factor: Microbial biomass was reduced by mineral N fertilization in soils with pH lower than 5. However in soils where the pH was 7 or higher, there was an increase in microbial biomass in the fertilized treatments.
• Duration of study was an important factor studies: Increases in microbial biomass from fertilization were highest in longer-term studies (20 years or more), while studies less than 10 years duration saw a decreased in microbial biomass in the fertilized treatments.
• Type of fertilizer also played a role: microbial biomass was significantly lower in anhydrous ammonia than urea treatments.
• Urea and ammonium inputs can considerably lower the soil pH over time, whil nitrate fertilizers do not.
• The trials analyzed varied considerably with respect to duration, crops grown, fertilizer type, fertilizer rate, soil types, complexity of crop rotation and soil pH.
• Results suggest that higher organic carbon found in fertilized treatments is a major factor contributing to the overall increases in microbial biomass and microbial enzyme activity. Thus is it suggested that fertilization leads to higher crop productivity which increases organic carbon, thereby increasing microbial biomass.
• The increased microbial biomass observed in fertilized soils contrasts with studies of unmanaged ecosystems, where N fertilization reduced microbial biomass.