Belowground productivity accounts for 46% of total terrestrial C fixation
Dr. Laureano A. Gherardi
School of Life Sciences, Arizona State University - Tempe, USA
Figure 1. At the global scale, belowground net primary productivity increases with mean annual precipitation. Individual biomes have idiosyncratic responses of belowground production to precipitation increasing or decreasing with mean annual precipitation (Gherardi and Sala, 2020).
Every year, terrestrial ecosystems reabsorb approximately 20% of the carbon dioxide emitted to the atmosphere by human activities. This crucial ecosystem service is provided through net primary productivity that is the plant organic matter accumulation rate exceeding plant respiratory efflux during a period of measurement. Plants allocate a significant fraction of total net primary production belowground, primarily to roots. Carbon entering the soil strongly affects the global carbon cycle as root growth and rhizodeposition are the main sources of soil-organic carbon. Functionally, roots are the main belowground resource acquisition organs and therefore play an important role in the uptake of water and nutrients. In addition, belowground productivity is the energy source supporting soil microbial and faunal life and a vast assemblage of key ecological processes.
Belowground net primary production (BNPP) is the fraction of total primary productivity occurring in the soil and is not quantifiable in a strict sense. Conceptually, BNPP equals the sum of the positive change in belowground biomass, the amount of biomass consumed by herbivores, the amount of biomass lost to death and the amount of rhizodeposits all during a time interval. Here, we used a combination of satellite and field observations to estimate BNPP at the global scale. This very parsimonious approach allowed us to assess the major climatic controls of BNPP among different biomes.
Figure 2. Global F-BNPP patterns across major biomes (Gherardi and Sala, 2020).
Our estimates indicated that long-term mean global BNPP totaled 24.7 ± 5.7 Pg C yr-1 representing 46% of total terrestrial net primary productivity. BNPP increased from arid to humid sites non-linearly (Figure 2) where the increase in BNPP for each unit increase in precipitation decreased going from arid to humid ecosystems. This is evidence of a strong water limitation of BNPP in arid ecosystems that gets weaker as rainfall increases to a point where BNPP showed very small increments with further precipitation increase. At this point BNPP seems to be limited by resources other than water such as nutrients or light.
Another important variable is the fraction of total productivity allocated belowground (F-BNPP). A reliable estimation of F-BNPP allows the partitioning of modelled total productivity into aboveground and belowground components. Our study indicated that F-BNPP decreased logarithmically with increasing mean annual precipitation at the global scale ranging from ~70% in arid ecosystems to ~35% in humid ecosystems. Water limited ecosystems such as deserts, grasslands and shrublands allocate a larger fraction of fixed carbon belowground. Forest ecosystems transition from co-limitation by below- and above-ground resources due to temperature-limited nutrient mineralization in boreal and temperate forests to limitation by aboveground resources in tropical ecosystems. Lastly, croplands allocate the smallest fraction of productivity to belowground organs probably due to historical crop breeding aimed at increasing allocation to harvestable products and to grow under artificial fertilization and irrigation conditions.
Figure 3. Shortgrass roots. Illustration by Courtney M. Currier.
This work is among the first attempts to estimate global belowground productivity. Although the results are promising, there are limitations and a lot of work left to do. Spatial patterns of BNPP and their correlation with precipitation gradients provide hints about the consequences of projected changes in precipitation patterns due to ongoing climate change. However, the scarcity of long-term BNPP and F-BNPP data, particularly in forest ecosystems, limited our ability to assess responses to precipitation change in one location over time. Experimental field studies of BNPP considering its interactions with soil processes and organisms are the next key milestone. Such experiments should manipulate global change stressors whereas other drivers are kept constant in order to identify cause-effect relationships. These studies would complement the more abundant observational studies included in this work and will allow to accurately estimate the effects of global change stressors on the fate of belowground carbon fluxes and stocks.
Publication: Gherardi LA and Sala OE (2020) Global patterns and climatic controls of belowground net carbon fixation. PNAS 117(33), 20038-20043. DOI: 10.1073/pnas.2006715117