Activiteiten NewMob

Deskstudie Stikstof

 NewMob voorzitter Leonie Bosch Chapel heeft een aantal wetenschappelijke studies op een rij gezet die de een andere kant laten zien van minder beschikbare stikstof in onze ecosystemen en de mogelijk negatieve gevolgen daarvan voor bijvoorbeeld bossen en insecten.

1. Mason, J. Craine, Nina K. Lany, M. Jonard, S. Ollinger, P. Groffman, R. Fulweiler, J. Angerer, Q. Read, P. Reich, P. Templer, A. Elmore. Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems

• • The researchers studied a database of leaf chemistry of hundreds of species collected from all over the world from 1980-2017 and found a global trend in decreasing nitrogen availability.
  • They found that most terrestrial ecosystems, such as forests and land that has not been treated with additional fertilizers, are becoming more oligotrophic, which means too little nutrients are available.

• If nitrogen is less available, it has the potential to decrease the productivity of the forest.

• Nitrogen is essential for the growth and development of plants.

• On the forest floor, microbes break down organic matter such as fallen leaves and release nitrogen to the soil.
• The tree gets that nitrogen to build proteins and grow.
• As trees have access to more carbon, more and more microbes are becoming nitrogen-limited and releasing less nutrients to the trees.

Curtis, S. (2022). The decline in NITROGEN in could have dire consequences for nature. https://www.dailymail.co.uk/sciencetech/article-10719195/Decline-availability-NITROGEN-dire-consequences-plant-animal-growth.html

• • Simultaneous increases in carbon dioxide, along with other global changes, have increased the demand for nitrogen by plants and microbes.

• In many areas of the world that are not subject to input of nitrogen from people, nitrogen availability is declining, with important consequences for plant and animal growth.

• A new study suggests that some parts of the world are experiencing a dramatic decline in nitrogen availability.

• • Impacts of decreases in Nitrogen on Herbivores Reduction in forage quality (A and D) may result in reduced herbivore body size and/or development rate (B and E) and reproduction (C and F) because herbivore growth rates and populations are often limited by protein availability.

• The decreasing availability of nitrogen is likely to restrict the ability of plants to remove carbon dioxide from the atmosphere.

Morris, C. D., Kirkman, K. P., & Zacharias, P. J. (2022). Will The Grass Be Greener On The Other Side of Climate Change? Cold Spring Harbor Laboratory. https://doi.org/10.1101/2022.04.22.489016

• • Most likely, in a drier future climate, we predict that the quality of the food will be little altered in sweetveld where soil nutrients and temperature are not limiting but that the sour valley could become sourer because soil nutrients will be inadequate to match the higher plant production promoted by the warmer and longer growing seasons.

• The increase in atmospheric [CO2] stimulates photosynthesis and plant production, increasing the demand for nitrogen in relation to the supply of nitrogen in the soil, resulting in a decrease in nitrogen concentrations in leaves around the world.

• The effect of such oligotrophication on the forage quality of sweet, mixed and sour grasslands in South Africa, which support livestock production and native ungulates, is unknown.

Govaert, S., Vangansbeke, P., Blondeel, H., De Lombaerde, E., Verheyen, K., & De Frenne, P. (2021). Responses of forest understory plants to long‐term experimental warming, light and nitrogen addition. Plant Biology, 23(6), 1051–1062. https://doi.org/10.1111/plb.13330

• • Climate change, eutrophication, and intensified forest management are affecting forest understorey plants, a major component of forest biodiversity.

• The authors assessed the responses of 15 species occurring in the understorey of a deciduous temperate forest to experimental warming, light addition, and enhanced nitrogen inputs in permanent plots surveyed for 9 years.

• Eutrophication and intensified forest management affect forest under-storey plants, a major component of forest biodiversity.
• To a lesser extent, nitrogen addition had effects on cover and functional traits.
• Many species showed a directional change over time, and this change can be reinforced or weakened by treatments, indicating the importance of long-term monitoring. Against expectations, they observed few interactions between treatments. Species responses to treatments were related to ecological strategies (generalists versus forest specialist). 
• This might ultimately lead to biotic homogenization. Since the effects of light and warming treatment were additive, keeping the canopy closed will only mitigate, but not stop, the effects of global warming on the forest floor plants.
• Since the effects of light and warming treatment were additive, keeping the canopy closed will only mitigate, but not stop, the effects of global warming on the forest floor plants.

Craine, J. M. (2020). Looking back in time to reconstruct nitrogen availability trajectories. Global Change Biology, 26(10), 5353–5355. https://doi.org/10.1111/gcb.15222

• The prevailing paradigm for how the global nitrogen cycle has changed since the advent of industrial nitrogen fixation is one of eutrophication.

• • There has been an undercurrent of results that have questioned whether terrestrial N eutrophication is more prevalent than terrestrial oligotrophication: nutrients become more limiting over time.
  • N is not the only resource that has increased in availability.
  • Since the late 1800s, the availability of C to plants has roughly doubled relative to minimum requirements of the whole plant (Sage, 2004).

• This additional C has the potential to directly and indirectly cause N to be more limiting to plants (Luo et al., 2004).

• • Making matters more complex, elevated CO2 increases soil water availability as plants reduce transpiration (Fatichi et al., 2016).
  • Soil N mineralization has long been known to scale linearly with soil moisture (Stanford & Epstein, 1974), which could link elevated CO2 to decreases in N limitation.
  • There are too much theoretical uncertainties about the predominance of changes in resource limitation under elevated CO2 (Figure 1), whether the relative supply of N or C has increased for terrestrial ecosystems.
  • Elevated CO2 experiments have not definitively quantified consistent patterns of limitation changes (Liang, Qi, Souza, & Luo, 2016).

• Complementing this uncertainty, the scientific community has not invested in monitoring soil N availability, as it has other resources.

• • Despite networks monitoring wet and dry N deposition, there are few long-term direct measurements of soil N supply (Groffman et al., 2018), not less.
  • This is a commentary on Brookshire et al. 26, 5404–5413 routine quantification of proxies such as plant [N] or δ15N, which can serve as a proxy for N availability (Craine et al., 2015).
  • It is difficult to imagine trying to comprehend the pattern and consequences of global warming without a global network of weather stations or the global rise in elevated CO2 without Mauna Loa Observatory.

• The lack of N availability monitoring by the authors is tacitly accepted.

• • Within this backdrop of uncertainty, Brookshire, Stoy, Currey & Finney (2020) compile two datasets that plant a flag on changes in N availability for the Northern Great Plains.
  • The authors quantify increases in remotely sensed greenness from 2000 to 2018.
  • The authors further demonstrate that these increases were more prevalent in the hotter and drier regions of Montana, namely the shortgrass regions, while the forested and mountainous regions were more likely to experience declines in greenness.

• Analyzing the patterns of foliar δ13C, δ15N, and [N], the authors determined that increases in greenness have been associated with increases in plant water use efficiency, a link that is not unexpected.

• Although plants have experienced greater water, plant N availability appears to have been declining.

• The authors quantify that both foliar [N] and δ15N have declined by 6% to 18% since approximately the 1930s.

• These declines occurred despite a double of the N deposition rates.

• In the Northern Great Plains, the increase in N supply from deposition was not significant compared to the increase in C supply from eCO2.

• In summary of Brookshire et al.’s study, plants of the Northern Great Plains appear to have been experiencing greater C and water availability, yet lower N availability despite greater N deposition rates.
• No study can be considered definitive, but these patterns align closely with those observed in the central Great Plains some 1,500 km away in Kansas (McLauchlan, Ferguson, Wilson, Ocheltree, & Craine, 2010).
• For Kansas grasslands, the efficiency of plant has been steadily increasing, while foliar [N] and δ15N have been declining since approximately the 1930s.
• When these herbarium studies are paired with reconstructions of N availability from foliar, wood and sedimentary δ15N (McLauchlan et al., 2017), the general pattern in nonagricultural ecosystems in North America is not sustained eutrophication, but oligotrophication.
• This will have to include monitoring networks of soil N availability/supplies, plant N concentration, and isotopic composition, as well as more reconstructions of N availability.
• This research will have to be coupled with understanding the ecological consequences of any decline in N availability, which would affect everything from herbivore performance to ecosystem C balance to human food supply.