Plant diversity effects on soil food webs are stronger than those of elevated CO2 and N deposition in a long-term grassland experiment

Abstract

Recent metaanalyses suggest biodiversity loss affects the functioning of ecosystems to a similar extent as other global environmental change agents. However, the abundance and functioning of soil organisms have been hypothesized to be much less responsive to such changes, particularly in plant diversity, than aboveground variables, although tests of this hypothesis are extremely rare. We examined the responses of soil food webs (soil microorganisms, nematodes, microarthropods) to 13-y manipulation of multiple environmental factors that are changing at global scales--specifically plant species richness, atmospheric CO2, and N deposition--in a grassland experiment in Minnesota. Plant diversity was a strong driver of the structure and functioning of soil food webs through several bottom-up (resource control) effects, whereas CO2 and N only had modest effects. We found few interactions between plant diversity and CO2 and N, likely because of weak interactive effects of those factors on resource availability (e.g., root biomass). Plant diversity effects likely were large because high plant diversity promoted the accumulation of soil organic matter in the site's sandy, organic matter-poor soils. Plant diversity effects were not explained by the presence of certain plant functional groups. Our results underline the prime importance of plant diversity loss cascading to soil food webs (density and diversity of soil organisms) and functions. Because the present results suggest prevailing plant diversity effects and few interactions with other global change drivers, protecting plant diversity may be of high priority to maintain the biodiversity and functioning of soils in a changing world.

Fig. 1.

(A) Effects of plant species richness and atmospheric CO₂ concentrations on soil microbial biomass C (plant species richness × CO₂ interaction: F_1,209 = 4.15, P = 0.043). Effect of plant species richness on (B) basal respiration, (C) metabolic quotient, (D) bacterial feeding nematodes, (E) nematode predators, (F) nematode omnivores, (G) Oribatida, (H) Astigmata and Prostigmata, and (I) Thysanoptera. Means with SDs.

Fig. 2.

Effect of plant species richness on soil biodiversity, represented by the taxa richness of soil microfauna (nematodes) and soil mesofauna (Table 1). Means with SDs.

Fig. 3.

Path analysis models of plant diversity effects on soil biota. (A) Causal influences of plant diversity (exogenous variable; green rectangle) on soil pH, soil N concentration, soil water content, root biomass productivity (endogenous explanatory variables; white rectangles), microbial biomass, and abundance of soil animals (endogenous variables; brown rectangles; = 2.43, P = 0.93). Numbers on arrows are standardized path coefficients. Solid line arrows indicate significant paths (P < 0.05); dotted lines indicate nonsignificant arrows (black = positive, red = negative). Percentages indicate the variance explained by the model. (B) Causal influences of plant diversity on the soil nematode community ( = 11.54, P = 0.24).