Unveiling nematode responses to afforestation from distributions of body size in a subalpine ecosystem

Understanding soil nematode responses is crucial for assessing and predicting the effects of afforestation on soil food webs. While we possess good knowledge of the nematode taxonomic indicators (e.g., abundance and richness), the response of nematode trait distribution (e.g., body size) to afforestation, offering insights into community assembly, remains poorly understood.

We investigated the influence of afforestation (19-, 11- and 3-year-old forests and unplanted meadows) on soil nematode structure and trait distribution in a subalpine ecosystem.

Taxonomically, afforestation did not enhance nematode community performance, e.g., nematode abundance in 11- and 3-year-old forests was lower than in unplanted meadows. However, afforestation significantly impacted distributions of body size. In 11- and 19-year-old forests, nematodes tended to be large-bodied, with body sizes more evenly distributed and less skewed, suggesting niche differentiation. However, in 3-year-old forests and unplanted meadows, nematodes were highly concentrated around smaller sizes, with only a few large-bodied individuals, indicating environmental filtering. According to redundancy analysis, we found that small-bodied nematodes in 3-year-old forests and unplanted meadows related to poor soil fertility with high carbon: nitrogen ratio, while large-bodied nematodes in 11- and 19-year-old forests were associated with fertile soils, as indicated by high nitrate and available phosphorus.

Our study reveals assembly rules for soil nematodes: afforestation leads to the replacement of smaller nematodes in meadows with larger ones in older forests, mainly due to increased niche differentiation. This research highlights the importance of trait distribution in understanding afforestation’s ecological impacts on soil food webs.