Soil microbial respiration and biomass C over time
Soil microbial respiration and biomass increased significantly with plant species richness four years after establishment of the experiment (Fig. 3A–B), and this positive relationship persisted until the last measurement of the present time series in 2014. Furthermore, the slope of the relationship between microbial respiration and plant species richness was highest seven years after the establishment of the experiment, while the slope of the relationship between microbial biomass C and plant species richness peaked only after 11 years (Fig. 4).
Figure 3. Soil microbial respiration (A) and biomass C (B) as affected by plant species richness from 2003 to 2014 (except 2005). Regression lines (black) with 95% confidence intervals (grey). Asterisks indicate significant effects of plant species richness within years (*, p ≤ 0.05;**, p ≤ 0.01; ***, p ≤ 0.001; n.s., not significant).
Figure 4. Slopes of the relationships between microbial basal respiration (BR) and plant species richness (SR), and between microbial biomass C (Cmic) and plant species richness from 2003 to 2014. Dotted lines are used where measurements (in 2005) were missing.
Spatial stability of soil microbial properties
Plant species richness and functional group richness did not significantly affect spatial stability of soil microbial parameters, but increased the means of both soil microbial respiration and biomass (Table 1). In contrast, plant functional group richness significantly increased the standard deviation of soil
microbial biomass. Among plant functional groups, legumes significantly reduced the spatial stability of both microbial respiration and biomass (Fig. 5A, G). Legumes significantly increased the mean of microbial biomass, but even more they increased the standard deviations of both microbial
respiration and biomass, indicating destabilization of microbial communities in space (Fig. 5H, C, I). In addition, tall herbs significantly reduced the spatial stability of microbial respiration (Fig. 5D) and in trend microbial biomass. Further, tall herbs significantly increased the mean of soil microbial biomass. In the presence of grasses, the spatial stability of soil microbial biomass increased significantly but mean microbial biomass remained unaffected (Fig. 5K, L). Spatial stability of mi-crobial respiration did not significantly respond to the presence of grasses, while the mean increased significantly. The presence of small herbs did not significantly affect the spatial stability of microbial properties, but significantly increased the means of both microbial respiration and biomass.
Figure 5. Spatial stability (A), mean value (B), and standard deviation (C) of microbial respiration (BR) in the absence (0) and in the presence (1) of legumes. (D–F) Same parameters in the absence and presence of tall herbs. (G–I) Spatial stability, mean value, and standard deviation of microbial biomass C (Cmic) in absence and presence of legumes, and (K–M) same parameters in the absence and presence of grasses. Means with standard error bars. Asterisks indicate significant differences (*, p 0.05; **, p 0.01; ***, p 0.001; n.s. , not significant).
Table 1. GLM (type I sum of squares) table of F-values for effects of block, log-transformed plant species richness (SR), plant functional group richness (FGR), presence of legumes (LEG), grasses (GR), small herbs (SH), and tall herbs (TH) on the spatial stability, mean and standard deviation of soil microbial basal respiration (BR, log-transformed) and microbial biomass C (Cmic).
F-values refer to those where the respective factor was fitted first. DF = degrees of freedom; F = F-value; p = p-value. ↓/↑ = increase/decrease with increasing diversity level or in the presence of the respective plant functional group. Significant effects (p 0.05) and marginally significant effects (p 0.1) are given in bold.
Temporal stability of soil microbial properties
Plant diversity (plant species richness and functional group richness) did not significantly affect the temporal stability of microbial respiration in any of the three time phases (Table 2, Fig. 6A for plant species richness), while the means and the standard deviations of microbial respiration significantly increased with plant species richness in most phases (Fig. 6B–C).
During phase 1, plant functional group richness tended to enhance the temporal stability of microbial respiration. Simultaneously, plant functional group richness significantly increased mean microbial respiration. The presence of grasses significantly enhanced the temporal stability of microbial respiration. Additionally, the mean microbial respiration tended to increase in the presence of
Table 2.GLM (type I sum of squares) table of F-values for effects of block, log-transformed plant species richness (SR), plant functional group richness (FGR), presence of legumes (LEG), grasses (GR), small herbs (SH), and tall herbs (TH) on the temporal stability, mean, and standard deviation of soil microbial basal respiration (BR) and microbial biomass C (Cmic) for three subsequent time phases.
F-values refer to those where the respective factor was fitted first. DF = degrees of freedom; F = F-value; p = p-value. ↑/↓ = increase/decrease with increasing diversity level or in the presence of the respective plant functional group. Significant effects (p0.05) and marginally significant effects (p 0.1) are given in bold.
grasses. None of the other plant functional groups significantly influenced the stability of soil microbial respiration.
In general, the temporal stability of microbial biomass responded more strongly to plant community properties than that of microbial respiration. During phase 1, the temporal stability of microbial biomass significantly decreased with increasing plant species richness (Fig. 6D) and functional group richness. Although the mean of soil microbial biomass significantly increased with increasing plant diversity (Fig. 6E), the concurrent increase in standard deviation with increasing plant diversity (Fig.
6F) was much more pronounced and caused the destabilizing effect of plant diversity during the phase 1. Also, the presence of legumes and small herbs reduced the temporal stability of microbial biomass during the phase 1.
During phase 2, plant community properties did not significantly influence the temporal stability of microbial respiration, while associated means significantly increased with plant diversity as well as in the presence of grasses, small herbs, and tall herbs. In contrast, the temporal stability of microbial biomass significantly decreased with increasing plant species richness during phase 2, while plant functional groups did not significantly affect the temporal stability of microbial biomass. The means of microbial biomass increased in response to the presence of all plant functional groups (only by trend for grasses).
During phase 3, plant community properties did not significantly affect the temporal stability of microbial respiration, but means of microbial respiration significantly increased with plant species richness and functional group richness. Also, the means of microbial respiration increased in presence of each of the four plant functional groups, with plant functional group effects getting stronger in later phases. As for mean soil microbial biomass in phases 2 and 3, these results have to be treated with caution as testing the presence of plant functional groups before plant diversity may have captured some of the plant diversity effects.
Temporal stability of microbial biomass did not significantly vary with plant community properties during phase 3, i.e. the initial destabilizing effect of plant diversity vanished with time. The mean of microbial biomass significantly increased in response to increasing plant diversity and in the presence of the different plant functional groups. The effects of plant community properties on the mean microbial biomass got stronger from phase to phase and were strongest for plant diversity.
Importantly, the temporal stability of microbial biomass increased with time (as the number of plant community properties that reduced stability decreased with time).
Testing for potential relationships between spatial and temporal stability (Eisenhauer et al. 2011c) of microbial basal respiration and biomass revealed no significant correlations (all p > 0.1).
Figure 6. Temporal stability, mean value, and standard deviation of (A–C) microbial respiration (BR) and (D–F) microbial biomass C (Cmic) as affected by plant species richness in phases 1, 2 and 3. Regression lines (black) with 95% confidence intervals (grey). Asterisks indicate significant differences (*, p 0.05; **, p 0.01; ***, p 0.001; n.s. = not significant).