A reinterpretation of the earliest quantification of global plant productivity by von Liebig (1862)
Photosynthetic assimilation of atmospheric carbon dioxide (CO2) is a fundamental process of the biosphere that is capable of affecting atmospheric composition and climate. According to an historical review by Lieth (1975), the first global-scale quantification of plant productivity was carried out in Germany in 1862 by Justus von Liebig, one of the founders of organic chemistry and agronomy. On p. 263 of his book ‘Der chemische Process der Ernährung der Vegetabilien’, von Liebig (1862) stated that ‘if the total surface of the earth were one coherent meadow from which annually 100 centner hay per ha could be gained [1 German centner = 50 kg], the meadow plant would reap within 21–22 yr all CO2 from the atmosphere and all life on earth would end’ (translation in Lieth, 1978). Lieth (1975) considered this statement to be one of the milestones in primary production research, similar to the first speculation by Aristotle and the worldwide survey by the International Biological Programme (IBP). von Liebig did not provide an absolute value for productivity, but Lieth (1975) calculated it to be 230–240 × 1012 kg CO2 yr−1 (equivalent to 62.7–65.5 × 1012 kg C yr−1). Despite the very simple assumptions, this value is surprisingly close to later estimations (Table 1) and is regarded as the precedent (e.g. Smil, 2000; Binkley, 2002). However, I found an inconsistency in the interpretation: the global plant productivity calculated by Lieth (1975) is much higher than that required to exhaust all atmospheric CO2 in 21–22 yr. Ice-core analyses (e.g. Neftel et al., 1985) indicate that atmospheric CO2 in the mid-19th century contained about 620 × 1012 kg of carbon. If this value is correct, plant productivity of 65 × 1012 kg C yr−1 would consume all atmospheric CO2 within 10 yr, assuming no replenishment. In this Letter, I provide a reinterpretation of the idea proposed by von Liebig, the originator of quantitative primary productivity research.
First, we must understand the state of natural science in von Liebig's era. At that time, most biologists did not clearly distinguish between photosynthetic CO2 assimilation and respiration (Sachs, 1882). Typically, in his book published in 1840, von Liebig stated that plant CO2 emissions do not occur by metabolic respiration but rather by some nonmetabolic chemical reaction. This may raise a question concerning the definition of plant productivity, but the original phrase ‘gain of hay’ may reasonably allow us to regard it as net primary production (NPP: production of new dry matter), irrespective of the confusion of respiratory consumption in the carbon budget.
Second, I examined the amount of atmospheric CO2 used in von Liebig's calculation. On pp. 18–19 of von Liebig (1862), the volumes of atmosphere and atmospheric CO2 were calculated as 9 307 500 meilen3 and 3862.7 meilen3, respectively [1 German meile = 7532.5 m]. The total volume of the atmosphere was calculated as a homogeneous layer with a thickness of 1 meile. Here, the volume and concentration of atmospheric CO2 (415 ppmv) were overestimated, probably because of deficiencies in instruments and global data sets. Assuming that the global average air pressure and temperature are 1 atm and 10–25°C, respectively, I calculated that the amount of carbon in atmospheric CO2 assumed in von Liebig (1862) was 769–852 × 1012 kg C (Table 2), which is much higher than the plausible value of 620 × 1012 kg C based on ice-core analyses. Although this overestimation of atmospheric CO2 volume led to a longer CO2 exhaustion time (c. 12 yr) in the reinterpretation, the time is still less than the proposed 21–22 yr.
Table 2. Summary of the reinterpretation of the first estimation of global plant productivity by von Liebig (1862)
|Atmospheric CO2 amount|
| Atmospheric volume*||9 307 500||meilen3|
| Atmospheric CO2 volume*||3862.7||meilen3|
| Atmospheric CO2 concentration*||415||ppmv|
| Atmospheric CO2-C weight (10–25°C)||769–852||1012 kg C|
| von Liebig (1862)||100||centner ha−1 yr−1|
| This study, in dry weight||5000||kg d. wt ha−1 yr−1|
|Land area (± 3% range)||145–155||108 ha|
|Total productivity of land area|
| This study, in dry weight||72.5–77.5||1012 kg d. wt yr−1|
| (C in dry matter = 50%)||36.3–38.8||1012 kg C yr−1|
|Atm. CO2 duration for exhaustion|
| von Liebig (1862)||21–22||yr|
| This study||21.2–23.5 ||yr|
Finally, I reconsidered the definition of plant production. To date, the statement in von Liebig (1862) has been interpreted as follows ‘the entire earth surface is covered by coherent meadow, in which annually 5000 kg CO2 are assimilated’. Thus, global plant productivity of 230–240 × 1012 kg CO2 yr−1 was obtained. Here, I suggest another interpretation: ‘the entire land surface is covered by coherent meadow, in which annually 5000 kg dry matter are produced’. The German word Erde, which is used in the original text, means both terrestrial ground and the planet Earth. Considering the double meaning and the status of geography at that time, it is reasonable to assume that von Liebig applied the meadow analogy to terrestrial surfaces only. In addition, hay production can be interpreted as the production of dry matter (i.e. NPP), based on the reason already mentioned. Table 2 summarizes the reinterpretation of the idea proposed in von Liebig (1862); global terrestrial NPP was calculated as 36.3–38.8 × 1012 kg C yr−1. This value is about 60% of the former interpretation, but is still close to the present estimation among those made before the IBP (Table 1). This value is also lower than the currently plausible value (60 × 1012 kg C yr−1; Prentice, 2001), probably because von Liebig did not take account of below-ground production (H. Lieth, University of Osnabrück, pers. comm.). A recent synthesis by Saugier et al. (2001) suggests that the root fraction represents 13–67% of plant production. Thus, if we assume that the below-ground fraction contains one-third of plant production, the corrected NPP (c. 56 × 1012 kg C yr−1) comes close to the present estimation. This supports the assumption in von Liebig (1862) that the meadow has average productivity. Also, we should note that environmental changes during the last 150 years could have increased plant productivity (Binkley, 2002). Importantly, the reinterpretation explains consistently both the productivity and the CO2 exhaustion time in von Liebig (1862), such that vegetation would consume all atmospheric CO2 in 21.2–23.5 yr (Table 2). Although we cannot come to an unequivocal result, this reinterpretation may help us to understand von Liebig's reasoning.
In this Letter, I have reinterpreted the earliest quantification of global plant productivity made about 140 years ago. This paleographic attempt is not intended to improve the current estimation of global plant productivity, for which large amounts of data and new tools are utilized. Moreover, we now acknowledge that not only NPP but also heterotrophic respiration should be included to evaluate net ecosystem CO2 exchange with the atmosphere. However, von Liebig provided one of the earliest insights into the atmosphere–biosphere interaction, and this is worthy of special note. This attempt is also intended to enhance our historical point of view concerning primary production research (e.g. Lieth, 1975). Hopefully, this Letter provides researchers the opportunity to look back on how global plant productivity has been investigated, and I welcome different opinions, counterarguments and suggestions.