George N. Furey and David Tilman

PNAS December 7, 2021 118 (49) e2111321118

ECOLOGY

Significance

Both plant biodiversity and soil fertility are in decline. We find that restoration of plant biodiversity on a nutrient-poor, unfertilized soil led to greater increases in soil fertility than occurred when these same plant species grew in monocultures. The plant species in this biodiversity experiment fell along a trade-off surface in their nutrient content traits, precluding any one species, or any one type of species, from markedly increasing soil fertility. Our results have implications for degraded agroecosystems, suggesting that increasing plant functional biodiversity may help restore their soil fertility. Creative applications of our findings to pastures, cover crops, and intercropping systems may provide greenhouse gas benefits from soil carbon storage and reduce the amounts of fertilizers needed for optimal yields.

Abstract

Fertile soils have been an essential resource for humanity for 10,000 y, but the ecological mechanisms involved in the creation and restoration of fertile soils, and especially the role of plant diversity, are poorly understood. Here we use results of a long-term, unfertilized plant biodiversity experiment to determine whether biodiversity, especially plant functional biodiversity, impacted the regeneration of fertility on a degraded sandy soil. After 23 y, plots containing 16 perennial grassland plant species had, relative to monocultures of these same species, ∼30 to 90% greater increases in soil nitrogen, potassium, calcium, magnesium, cation exchange capacity, and carbon and had ∼150 to 370% greater amounts of N, K, Ca, and Mg in plant biomass. Our results suggest that biodiversity, likely in combination with the increased plant productivity caused by higher biodiversity, led to greater soil fertility. Moreover, plots with high plant functional diversity, those containing grasses, legumes, and forbs, accumulated significantly greater N, K, Ca, and Mg in the total nutrient pool (plant biomass and soil) than did plots containing just one of these three functional groups. Plant species in these functional groups had trade-offs between their tissue N content, tissue K content, and root mass, suggesting why species from all three functional groups were essential for regenerating soil fertility. Our findings suggest that efforts to regenerate soil C stores and soil fertility may be aided by creative uses of plant diversity.

See: https://www.pnas.org/content/118/49/e2111321118

Fig. 1.

Soil chemistry vs. plant diversity. Mean ±1 SE of soil chemistry (0- to 20-cm depth) before planting in 1994 in green (diamond) and in 2017 in orange (circle) of (A) total carbon, (B) total nitrogen, (C) exchangeable potassium, (D) exchangeable calcium, (E) exchangeable magnesium, (F) CEC, (G) soil pH, and (H) extractable Bray phosphorus versus number of planted species (1, 2, 4, 8, or 16; log scale). Lines are linear regressions ± 1 SE (n = 154 plots). The quantity (grams per square meter) for C, N, P, K, Ca, and Mg were calculated using soil bulk density. Sample sizes for each diversity level (1 to 16 species) are 1 species = 32 plots; 2 = 28; 4 = 29; 8 = 30; and 16 = 35.