Legacy effect of grazing intensity mediates the bottom‐up controls of resource addition on soil food webs

Abstract

Large‐scale studies have demonstrated that nitrogen (N) and water (W) availabilities greatly affect terrestrial ecosystems world‐wide, and this is especially true for the resource‐poor semi‐arid grasslands. Yet, experimental evidence is lacking for how N and W availabilities affect soil food webs across historical grazing intensity‐altered environments at a local scale. Here, we included N‐ and W‐addition treatments in an 8‐year grazing experiment (with four grazing intensities) to determine how the legacy effects of grazing intensity mediate the responses of key components of soil food webs (plants, micro‐organisms and nematodes) to resource addition in a semi‐arid grassland. After 4 years of N‐ and W‐addition treatments (with no grazing during that 4‐year period), we found that a legacy of grazing, even light grazing, had significant negative effects on the components of plant community and soil food webs. Both N and W addition increased above‐ and below‐ground plant biomass, especially under moderate and heavy grazing. N addition had negative effects on the biomass of bacteria under no grazing, while W addition increased the biomass of actinomycetes under light grazing. N addition decreased the abundance of omnivorous + carnivorous nematodes under light and heavy grazing, while W addition increased their abundance under heavy grazing. Overall, the effects of resource addition on soil food webs progressively decreased from the lowest trophic level (primary producers, i.e. plants), to intermediate trophic levels (micro‐organisms and root‐feeding nematodes), to higher trophic levels (microbial‐feeding nematodes and omnivorous + carnivorous nematodes). Synthesis and applications. Our results, which are the first data concerning the effects of resource addition on key components of soil food webs across a historical grazing‐induced environmental gradient, show that the strong bottom‐up controls of resource addition on soil food webs are mediated by the legacy of grazing intensity. These findings should be useful for predicting the responses of grassland ecosystems to future climate change and suggest that the recovery of degraded grasslands will require more than restoration measure of resource inputs alone.

Even short‐term revegetation complicates soil food webs and strengthens their links with ecosystem functions

“…Plant species richness is an important indicator of the effectiveness of revegetation in degraded ecosystems (Devoto et al, 2012; Guiden et al, 2021). The substantial increase in plant species richness induced by short‐term revegetation in the current study is inconsistent with previous studies, which showed that revegetation did not alter species diversity or even reduced species diversity due to the limited availability of soil nutrients and water content in degraded grasslands (Wang et al, 2021; Wu et al, 2015; Wu et al, 2020). Our findings for plant productivity, species richness and community structure at both degradation intensities indicate that revegetation is effective for the restoration of degraded grasslands.…”

“…Our results do not support the ‘Field of Dreams Hypothesis’, which predicts that restoring plant diversity leads to the passive recovery of the biodiversity of other organisms (Devoto et al, 2012; Guiden et al, 2021). There are at least two possible explanations for the failure of revegetation to increase soil biodiversity in the current study: (a) at the outset of our study, the soil may have mainly contained soil organisms that are resistant to degradation stress and that are not altered by revegetation (Rousk et al, 2010; Wu et al, 2020; Xiao et al, 2020); and (b) the increases in plant diversity were insufficient to cause increases in the diversity of soil organisms (Scherber et al, 2010; Wang et al, 2021). Our VPA analyses confirmed that revegetation‐induced increases in soil bacterial and fungal biomass and changes in soil bacterial and fungal community structure were mostly associated with increases in plant variables (plant productivity and community structure).…”

Direct and indirect effects of nitrogen enrichment and grazing on grassland productivity through intraspecific trait variability

“…This may be because the addition of N to the N‐limited systems directly increases the soil available N (Chen et al., 2019), which can be rapidly absorbed and utilized by plants, thus promoting plant growth and ecosystem productivity. Grazing, however, generally affects vegetation and soil through livestock feeding, trampling and excretion (Cingolani et al., 2005; Lemaire et al., 2000), so there may be an indirect lag effects on grassland (Wang et al., 2021). Previous studies have shown that grazing increases soil bulk density, decreases soil porosity and penetrability, and reduces soil nutrient and water availability (Eldridge et al., 2016; Shan et al., 2011), thus inhibiting plant growth.…”

Exploring the optimal grazing intensity in desert steppe based on soil nematode community and function

“…We believe that the increase in the proportion of PP nematodes may be because the aboveground loss in lightly grazed ecosystems is lower compared to the underground compensation. At the same time, the increase in root biomass provides a parasitic habitat for PP nematodes, and the increase in root exudates provides an increase in food resources (Chen et al, 2013; Wang et al, 2021). Under moderate grazing conditions, the ratio of FF to OP nematodes in the surface soil layer increased, whereas the ratio of BF nematodes decreased.…”