Mineral Nitrogen Input Increases Microbial Biomass in Soil Under Annual Crops but Not Grasslands.

Increasing nutrient inputs into terrestrial ecosystems affects both plant and soil microbial communities. A meta-analysis based on 107 observations from 64 long-term trials with annual crops revealed that mineral nitrogen application led to a 15.1% increase in the microbial biomass compared with unfertilized control treatments. The positive effect of mineral fertilization on the microbial biomass was surprising as studies carried out in predominantly unmanaged ecosystems have found that nitrogen inputs generally decrease soil microbial biomass.

To further explore the factors contributing to differences in natural versus agricultural systems, we conducted a second meta-analysis comparing short- and long-term effects of mineral fertilizer inputs on soil microorganisms from field trials in permanent grassland (n = 54) and annual crops (n = 150). Similar to long-term trials, mineral fertilizer had a positive effect on the microbial biomass in shorter-term trials with annual crops. In contrast, mineral nitrogen additions reduced the microbial biomass in both natural and managed grassland by an average of 12%. In both systems, the effect became more pronounced over time.  Pronounced shifts among major microbial groups from N additions were not evident in our analyses.

We found the best support for the hypothesis that higher crop productivity in annual cropping systems brought on by fertilization increases plant residue inputs, which in turn increases microbial biomass. In the short term, N inputs may also increase primary production in permanent grassland. However, N inputs gradually reduce grassland plant species richness which likely leads to the observed decrease in microbial biomass. Over time, the effect of plant species richness on microbial biomass becomes more pronounced.

Our analysis reveals that management is an important consideration in predicting how ecosystems respond to environmental change and highlights the complexity and diversity of plant-microbial interactions, and underscores the immense value of long-term field trials.