Soil multifunctionality and drought resistance are determined by plant structural traits in restoring grassland

Abstract

It is increasingly recognized that belowground responses to vegetation change are closely linked to plant functional traits. However, our understanding is limited concerning the relative importance of different plant traits for soil functions and of the mechanisms by which traits influence soil properties in the real world. Here we test the hypothesis that taller species, or those with complex rooting structures, are associated with high rates of nutrient and carbon (C) cycling in grassland. We further hypothesized that communities dominated by species with deeper roots may be more resilient to drought. These hypotheses were tested in a 3-yr grassland restoration experiment on degraded ex-arable land in southern England. We sowed three trait-based plant functional groups, assembled using database derived values of plant traits, and their combinations into bare soil. This formed a range of plant trait syndromes onto which we superimposed a simulated drought 2 yr after initial establishment. We found strong evidence that community weighted mean (CWM) of plant height is negatively associated with soil nitrogen cycling and availability and soil multifunctionality. We propose that this was due to an exploitative resource capture strategy that was inappropriate in shallow chalk soils. Further, complexity of root architecture was positively related to soil multifunctionality throughout the season, with fine fibrous roots being associated with greater rates of nutrient cycling. Drought resistance of soil functions including ecosystem respiration, mineralization, and nitrification were positively related to functional divergence of rooting depth, indicating that, in shallow chalk soils, a range of water capture strategies is necessary to maintain functions. Finally, after 3 yr of the experiment, we did not detect any links between the plant traits and microbial communities, supporting the finding that traits based on plant structure and resource foraging capacity are the main variables driving soil function in the early years of grassland conversion. We suggest that screening recently restored grassland communities for potential soil multifunctionality and drought resilience may be possible based on rooting architecture and plant height. These results indicate that informed assembly of plant communities based on plant traits could aid in the restoration of functioning in degraded soil.

Keywords: aboveground-belowground interactions; carbon cycling; functional traits; plant-soil (belowground) interactions; resilience; restoration; root traits; soil microorganisms.

Figure 1

Nonmetric multidimensional scaling (NMDS) ordination of species composition in July 2015. Ellipses are standard deviation from the mean. Each ellipse and color represents a different functional group combination, and each dot represents a subplot.

Figure 2

(a–f) Realized community weighted trait means (CWM) and (g–l) functional divergence (FDvar) of each functional group treatment in July 2015. Treatments are seven functional group combinations. Error bars are SEM. *P < 0.05; **P < 0.01; ***P < 0.001; n.s., not significant.

Figure 3

Repeated‐measures analyses of the effect of plant functional traits on soil functions in May, July, and September 2015. Soil functions have been standardized and data shown are predicted values derived from linear mixed effects models. When the effect of month is non‐significant, one line of best fit is used for all data. Mycorrhizal affinity values are bounded between 1 (never mycorrhizal) and 3 (always mycorrhizal). FDvar is a measure of functional divergence, and is bounded between 0 (no variation in the trait) and 1 (every value is different). DOC, dissolved organic carbon.

Figure 4

Effects of soil moisture and plant functional traits on soil multifunctionality, derived from threshold values of each function calculated using the metric presented by Manning et al. (2018). Graphs represent predicted values based on significant model fits in (a, b) May, (c) July, and (d) September 2015. Root architectural class is a categorical variable ranging from 1 (fine fibrous roots [very complex]) to 8 (tap roots [very simple]). In July, best fit lines represent differing effects of the roof treatment, where yellow is roofed control, gray is drought, and blue is unroofed control. The shaded polygons represent standard error of the mean.

Figure 5

Effects of plant functional traits on resistance of soil functions to drought. Resistance is calculated using the metric of Orwin and Wardle (2004). A resistance value of 1 represents full resistance or no difference from the control when drought ends in July, 0 denotes a 100% change, while negative numbers indicate that the droughted value is higher than the control value. Root depth class is bounded between 1 (shallow) and 3 (deep).

Figure 6

Effects of plant functional traits on resilience of soil functions to drought eight weeks after roofs were removed. Resilience is calculated using the metric of Orwin and Wardle (2004). A resilience value of 1 represents full resilience, or no difference from the control after eight weeks of recovery. 0 denotes a 100% change, while negative numbers indicate that the droughted value is higher than the control value.