Have we overlooked the importance of soil micronutrients for global grassland biomass production?
Dr. Dajana Radujković & dr. Sara Vicca
University of Antwerp, Belgium
Figure 1 The distribution of 72 NutNet grassland sites along the precipitation gradient. White points indicate the location of different sites and different sizes of pink circles correspond to the amount of aboveground biomass per site (Radujković et al., 2021).
Grasslands are one of the largest terrestrial systems, occupying about 40% of the global land surface. Given that they account for up to one-third of the net primary productivity on land, they play an important role in global carbon sequestration as well as in the provision of multiple other ecosystem services to human society, including biodiversity, fodder production and nutrient cycling. Biomass production is a key property of grasslands that determines their capacity to take up and store carbon, controls which plant species can co-exist and influences the diversity of numerous animals that depend on plants for food and habitat.
We have long known that, besides water and temperature, grassland biomass production is limited by the availability of the macronutrients nitrogen and phosphorus. This has been confirmed by numerous experiments which often observed an increase in biomass production with the addition of N and P. More recently, it has been indicated that other, less-studied nutrients (potassium, calcium, magnesium, sulphur) and elements found in trace amounts in plants – micronutrients (iron, boron, copper, manganese, zinc) limit plant growth in some grasslands. Even though micronutrients are needed in much smaller quantities by plants than N and P are, they are part of important enzyme complexes and they indirectly influence biomass production by regulating aspects of plant defence and reproduction. While agronomists have long understood the potentially subtle but significant role of nutrients other than N and P for crops, their importance for plant production in non-agricultural grasslands has been mostly an unexplored frontier.
Figure 2 Path analysis depicting the direct (blue lines - positive and red lines - negative) and indirect (grey lines) influence of different predictors of biomass production. Micronutrients (represented by Zn and Fe) have the highest direct influence on biomass after climate (precipitation), followed by soil physicochemical properties (soil pcp), N deposition and soil carbon to nitrogen ratio (C:N; Radujković et al., 2021).
Our study used a global dataset comprising 72 sites that are part of the Nutrient Network (Figure 1) to investigate which of 16 soil factors that shape nutrient availability (including the concentrations of various macro- and micronutrients) associate most strongly with variation in grassland aboveground biomass. Climate and atmospheric N deposition were also considered. We found that soil micronutrients (particularly Zn and Fe) were important predictors of biomass (Figure 2) and, together with soil physicochemical properties and carbon to nitrogen ratio, they explained more unique variation (32%) than climate and N deposition (24%). We also found that the relationship between micronutrient availability and biomass was present only in grasslands with no signs of limitations by N and P suggesting that when soils contain a sufficient amount of N and P, there might be an increased demand for micronutrients which become limiting for plant growth. These results provide support for serial co-limitation, in which the response of plants to additional micronutrient resources occurs only after N and P are added.
The findings of this study highlight the potentially undervalued role of micronutrients in global grassland biomass production while motivating future experiments. Such manipulation experiments should focus on micronutrient (especially Zn) additions, alone and in combination with NP, particularly in the grasslands that are likely to be prone to micronutrient deficiencies. According to our results, the grasslands located in the regions with higher temperatures, with predominantly sandy soils poor in organic matter might be particularly prone to Zn and Fe deficiencies. Our dataset included only a few grasslands in arid regions with alkaline (calcareous) soils but, based on the studies in arable fields, micronutrients deficiency in such grasslands might be even more pronounced. We thus recommend measuring different soil properties, including micronutrients (both in plants and soil) in studies investigating grassland productivity to gain a better understanding of the extent and importance of nutrient deficiencies in grasslands.
Publication: Radujković, D., Verbruggen, E., Seabloom, E. W., Bahn, M., Biederman, L. A., Borer, E. T., Boughton, E. H., Catford, J. A., Campioli, M., Donohue, I., Ebeling, A., Eskelinen, A., Fay, P. A., Hansart, A., Knops, J. M. H., MacDougall, A. S., Ohlert, T., Olde Venterink, H., Raynaud, X., Risch A.C., Roscher, C., Martin Schütz, M., Silveira M.L., Stevens C.J., Van Sundert, K., Virtanen R., Wardle G.M., Wragg P.D, Vicca, S. (2021). Soil properties as key predictors of global grassland production: Have we overlooked micronutrients? Ecology Letters 24(12), 2713-2725. DOI: https://doi.org/10.1111/ele.13894