Species existence and coexistence under nutrient enrichment in the Park Grass

Abstract

Numerous studies have shown that nutrient enrichment causes the loss of plant diversity in different grasslands across the globe. Thus far, three main hypotheses (niche dimension, competitive asymmetry, and soil acidification) have been proposed to account for this general phenomenon, but our knowledge of the underlying mechanisms remains rather vague. To reveal the cryptic mechanisms, we analyzed the famous long-term Park Grass Experiment (1856-) under modern coexistence theory by fitting Lotka-Volterra competition models with time-series data from the different treatments (15 different combinations of nutrient addition fully crossed with four levels of soil pH) to quantitatively test the three competing hypotheses. Supportive of the competitive asymmetry and soil acidification hypotheses, both nutrient addition and soil acidification overall decreased intrinsic population growth rates (r) and intensified competitive differences or asymmetries dramatically, which mostly favored grasses over forbs and legumes (and also forbs over legumes). These changes in r and competitive differences or asymmetries are generally consistent with the abundance changes of different functional groups following the various treatments. Moreover, the altered r (determining species existence) and competitive differences (affecting species coexistence) effectively explained the diversity loss and recovery (after nitrogen addition was withheld). However, while nutrient addition significantly decreased per-capita intra- and inter-specific competition (which indicates that belowground competition becomes less intense when soil nutrients are more abundant), it did not decrease niche differences as predicted, poorly supporting the niche dimension hypothesis. These findings advance our understanding of fundamental mechanisms driving the response of plant communities to nutrient deposition in nature.

Density‐dependent and independent mechanisms jointly reduce species performance under nitrogen enrichment

“…However, all the above studies do not provide direct evidence for density‐independent mechanisms. For example, N‐induced acidification can also change competitive interactions (Yao and Feng 2022). The same reasoning applies to the soil microbiome, affecting plant performance in density‐dependent and independent manners (Ke and Wan 2020).…”