A hypothetical model that explains differing net effects of inorganic fertilization on biomass and/or abundance of soil biota.

Researchers recently proposed a model describing the trade-offs between the positive and negative effects of nitrogen (N) fertilization on biomass and/or abundance of soil biota. The positive effects presumably result from the bottom-up influences of fertilizer-enhanced plant growth and from the improved soil habitat for soil biota. The negative effects presumably result from degradation of soil physico-chemical properties (e.g., salt effect, aluminum toxicity, calcium or magnesium deficiency, soil pH decline, and soil carbon resource availability reduction). The magnitude of these effects determines the net effect (or trade-off) of N fertilization on the biomass and/or abundance of soil biota. Our understanding of how positive and negative effects generate different net effects with changes in the N fertilization level is inadequate. In this paper, we propose two patterns of positive and negative effects (i.e., S-shaped and linear curves when the effects are plotted on fertilization level) and consider the many possibilities for the trade-offs. Specifically, there were 7 possible trade-offs between S-shaped positive effects and linear negative effects, 9 possibilities of the trade-off between linear positive effects and linear negative effects, 20 possibilities of the trade-off between S-shaped positive effects and S-shaped negative effects, and 9 possibilities of the trade-off between linear positive effects and S-shaped negative effects. In addition, the net effect might change or remain neutral (±), positive (+), or negative (−) with increasing nitrogen application. The hypothetical model could help explain the inconsistent results of the impacts of fertilization on soil biota reported in previous studies and could increase our understanding of the responses of soil biota to fertilization and other environmental disturbances. [ABSTRACT FROM AUTHOR]