Overgrazing-induced legacy effects may permit Leymus chinensis to cope with herbivory

Concurrent and legacy effects of sheep trampling on soil organic carbon stocks in a typical steppe, China

Grazing plays a key role in ecosystem biogeochemistry, particularly soil carbon (C) pools. The non-trophic interactions between herbivores and soil processes through herbivore trampling have recently attracted extensive attention. However, their concurrent and legacy effects on the ecosystem properties and processes are still not clear, due to their effects being hard to separate via field experiments. In this study, we conducted a 2-year simulated-sheep-trampling experiment with four trampling intensity treatments (i.e., T0, T40, T80, and T120 for 0, 40, 80, and 120 hoofprints m-2, respectively) in a typical steppe to explore the concurrent and legacy effects of trampling on grassland ecosystem properties and processing. In 2017 (trampling treatment year), we found that trampling decreased aboveground biomass (AGB) of plant community and community-weighted mean shoot C concentration (CWM C), soil available nitrogen (N) and available phosphorus (P), but did not affect plant species diversity and belowground biomass (BGB). We show that compared with T0, trampling increased soil bulk density (BD) at T80, and decreased soil organic carbon (SOC) stocks. After the cessation of trampling for two years (i.e., in 2019), previous trampling increased plant diversity and BGB, reaching the highest values at T80, but decreased soil available N and available P. Compared with T0, previous trampling significantly increased soil BD at T120, while significantly decreased CWM C at T80 and T120, and reduced SOC stocks at T80. Compared with 2017, the trampling negative legacy effects amplified at T80 but weakened at T40 and T120. We also show that trampling-induced decreases in soil available N, AGB of Fabaceae and CWM C were the main predictors of decreasing SOC stocks in 2017, while previous trampling-induced legacy effects on soil available P, AGB of Poaceae and CWM C contributed to the variations of SOC stocks in 2019. Taken together, short-term trampling with low intensity could maintain most plant functions, while previous trampling with low intensity was beneficial to most plant and soil functions. The results of this study show that T40 caused by sheep managed at a stocking rate of 2.7 sheep ha-1 is most suitable for grassland adaptive management in the typical steppe. The ecosystem functions can be maintained under a high stocking rate through the process of providing enough time to rebuild sufficient vegetation cover and restore soil through measures such as regional rotational grazing and seasonal grazing.

DNA methylation mediates overgrazing-induced clonal transgenerational plasticity

Overgrazing generally induces dwarfism in grassland plants, and these phenotypic traits could be transmitted to clonal offspring even when overgrazing is excluded. However, the dwarfism-transmitted mechanism remains largely unknown, despite generally thought to be enabled by epigenetic modification. To clarify the potential role of DNA methylation on clonal transgenerational effects, we conducted a greenhouse experiment with Leymus chinensis clonal offspring from different cattle/sheep overgrazing histories via the demethylating agent 5-azacytidine. The results showed that clonal offspring from overgrazed (by cattle or sheep) parents were dwarfed and the auxin content of leaves significantly decreased compared to offspring from no-grazed parents'. The 5-azaC application generally increased the auxin content and promoted the growth of overgrazed offspring while inhibited no-grazed offspring growth. Meanwhile, there were similar trends in the expression level of genes related to auxin-responsive target genes (ARF7, ARF19), and signal transduction gene (AZF2). These results suggest that DNA methylation leads to overgrazing-induced plant transgenerational dwarfism via inhibiting auxin signal pathway.