Cascading effects of N fertilization activate biologically driven mechanisms promoting P availability in a semi-arid grassland ecosystem

Cui, Haiying; Sun, Wei; Delgado-Baquerizo, Manuel; Song, Wenzheng; Ma, Jian-Ying; Wang, Keying; Ling, Xiaoli

Structural equation model of the effects of N addition on multiple cascading events and the P cycle. Numbers at arrows are standardised path coefficients. Percentage values indicate the variance explained by the model (R2). Arrow widths indicate the strength of the relationship. Avai N – Available N, Avai P – Available P, AGB – Aboveground biomass, PR – Plant richness, phoD – log10-transformed phoD gene abundance, MBP – Microbial biomass P

The understanding of the effects of nitrogen (N) fertilization on the structure and function of terrestrial ecosystems has been one of the major fields of study for ecologists over the last century. A growing number of studies demonstrate that N enrichment accelerates phosphorus (P) cycling (i.e., increase in phosphatase activity, soil organic P mineralization rate, and plant P resorption efficiency). Despite the known importance of N enrichment in influencing diverse ecosystem attributes of the P cycle, these potential cascading effects on the ecosystem P cycle, regulated by plant and soil interactions, remain poorly understood under global change scenarios. Most terrestrial ecosystems are known to have experienced excessive N inputs including atmospheric N deposition, N in manure from livestock grazing, and large N fertilization load, and this is especially true for semiarid grassland ecosystems in northern China.

Here, we conducted a 3-year field experiment to evaluate the direct and indirect effects of a gradient of N additions on a wide range of structural and functional attributes of ecosystems associated with over 20 plant, soil, and microbial variables in semiarid grassland. We found that N fertilization can have multiple cascading effects on more than 20 attributes of ecosystem structure and function. These cascading events ultimately result in the activation of

multiple biologically driven mechanisms that promote P availability (i.e., increased soil organic P mineralization, plant phosphorus resorption, and enzymatic and genetic processes associated with increased phosphatase activity).

Our results also indicate that excessive N is likely to result in the decoupling of key symbiotic plant–fungal relationships, and shifts in the carbon-use strategies of plants and microbes (probably from biomass to enzyme production). These findings can help us to better understand and predict how multiple plant, soil and microbial ecosystem attributes and mechanisms (e.g., plant and microbial carbon allocation strategy and P acquisition mechanisms) simultaneously respond to elevated N fertilization in terrestrial ecosystems.

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