Global meta-analysis reveals agro-grassland productivity varies based on species diversity over time

Authors: Ashworth, Amanda; TOLER, HEATHER - University Of Tennessee; ALLEN, FRED - University Of Tennessee; AUGE, ROBERT - University Of Tennessee

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2018
Publication Date: 7/10/2018
Citation: Ashworth, A.J., Toler, H.D., Allen, F.L., Auge, R.M. 2018. Global meta-analysis reveals agro-grassland productivity varies based on species diversity over time. PLoS One. 13(7):e0200274. https://doi.org/10.1371/journal.pone.0200274.
DOI: https://doi.org/10.1371/journal.pone.0200274

Interpretive Summary: In order to continue to feed the earth’s expected 9 billion population in 2050, it will be necessary to sustainably intensify agricultural production. Positive legume-grass relationships have been reported previously in pasture and agroecosystems owing to nitrogen ‘fixation’ from legumes, although specific factors affecting productivity of diverse mixtures are not well defined. Systematic, quantitative reviews, or “meta analyses” are often carried out to ascertain cumulative treatment impacts of a large, multi-study datasets. Such systematic reviews of a response variable provide a global synthesis of research, and are a promising analytical technique for assessing agronomic performance spatially and temporally. Our meta-analysis identified 44% yield increases for diverse mixtures compared to monoculture grasslands. Diverse plant mixture responses also varied based on environment and was greatest in less extreme environments. In addition, legume intercropping benefits varied based on soil type, and over the past 50 years. Forage quality also improved with greater number of species in a mixture. Specifically, crude protein increased 32% compared to sole crops. These meta-analysis results demonstrate that grass-legume diversity improves productivity and forage quality for animal production and can improve agroecosystem sustainability. The framework provided in this study may help land managers and scientists optimize cropping schemes and develop the ‘right kind of diversity’ to deliver ecosystem services while improving productivity with fewer fertilizer inputs.

Technical Abstract: Ecological research suggests increased diversity may improve ecosystem services, as well as yield stability; however, such theories are sometimes disproven by agronomic research, particularly at higher diversity levels. We conducted a meta-analysis on 2,753 trials in 48 articles published over the last 53 years to test: if biological N2 fixation (BNF) supplies adequate nitrogen (N) for plant growth relative to conventional systems; synergistic/antagonistic effects of multiple species within a pasture agroecosystem; how biculture crop physiological traits affect legume-grass symbiosis; and, how cultural practices affect BNF over a range of soils and climates overtime. Globally, primary productivity (PP) increased 44% via legume associations relative to sole stands. Several moderating variables affected symbiosis efficacy including: (i) photosynthetic pathway (mixtures of C3 grasses resulted in a 57% increase from grasslands without diversity, whereas mixtures of C4 grasses resulted in a 31% increase; similarly cool-season legumes increased PP 52% compared to a 27% increase for warm-season legumes); (ii) legume life cycle [biculture PP response for perennial legumes was 50% greater than sole controls (with or without fertilizer), followed by a 28% increase for biennial, and a 0% increase for annual legumes)]; and, (iii) species richness (one leguminous species in a grassland agroecosystem resulted in 52% increase in PP, whereas >2 legumes resulted in only 6% increases). Temporal and spatial effect sizes also influenced BNF efficacy, considering BNF was greatest (114% change) in Mediterranean climates followed by oceanic (84%), and tropical savanna (65%) environments; conversely, semi-arid and subartic systems had lowest Rhizobium-induced changes (5 and 0% change, respectively). Rhizobium associations were affected by soil texture. For example, a 122% PP increase was observed in silt clay soils compared to 14% for silt loam soils. Niche complementarity effects were greatest prior to 1971 (61% change), compared to recent studies (2011-2016; -7% change), likely owing to reduced global sulfur deposition and increased atmospheric CO2 and annual temperatures overtime. These unambiguous trends suggest a great potential for BNF to displace inorganic-N and sustainably intensify global PP. Results provide a framework for ecologists and agronomists to improve crop diversification systems, refine research goals, and heighten BNF capacities in agro-grasslands.