User:Alandmanson/Bracken fern and soil fertility

 Bracken fern and soil fertility in South African grasslands 

The productivity of many grasslands is limited by N availability, and many respond strongly to a combination of N and P fertilizers. . Grass growth responses in South African grasslands are no exception, but changes in species composition within a few years are generally considered undesirable. This is because most natural South African grasslands are highly productive without fertilizer, and both plant biomass and fodder quality generally decline with the change in species.

The N used by grasses is mainly in the form of NO3-, and NH4+. Microorganisms are strong competitors for this N, but research reported since 2000 has increasingly shown that plants can modify soil NO3- and NH4+ availability by modulating nitrification and denitrification, sometimes through the release of specific compounds by roots. This control of N cycling processes greatly contributes to the survival of particular species and the stability of plant communities, particularly in ecosystems with low soil N availability.

In acid soils, NH4+ is often the predominant plant-available N form and plants growing on acid soils often develop a preference for NH4+, although some also assimilate NO3-. In these soils, the biological control of key N cycle processes such as mineralization, nitrification, and denitrification to increase soil NO3- availability would give a nitrate-loving species a competitive advantage.

Bracken fern (Pteridium aquilinum) is not known to respond to fertilizer application (is this true?), but bracken invasion of SA grasslands is of concern. Stands of bracken are agriculturally unproductive, and have low value in terms of biodiversity conservation; they are also persistent and difficult to eradicate. Online reviews of the biology and ecology of Pteridium aquilinum are available as are reviews of control measures.

From Bardon, C., B. Misery, F. Piola, F. Poly, and X. Le Roux. 2018. Control of soil N cycle processes by Pteridium aquilinum and Erica cinerea in heathlands along a pH gradient. Ecosphere 9(9):e02426. 10.1002/ecs2.2426

"Nitrogen (N) is a major factor that limits plant growth in many terrestrial ecosystems (Vitousek and Howarth 1991, LeBauer and Treseder 2008). Plants depend heavily on the N availability in soils (Bothe et al. 2006). The N used by plants is mainly in the form of amino acids, NO3-, and NH4+ (Boudsocq et al. 2012), and the competition between roots and microorganisms for these N forms is intense (Hodge et al. 2000, Kuzyakov and Xu 2013). Microorganisms are often considered as better competitors than plant roots for this resource (Hodge et al. 2000, Kuzyakov and Xu 2013). However, during the last twenty years, it has been increasingly reported that plants can modify soil NO3- and NH4+ availability by modulating nitrification and denitrification (Patraet al. 2006, Le Roux et al. 2013), sometimes through the release of specific compounds by roots (Subbarao et al. 2009, DeLuca et al. 2013, Dietz et al. 2013, Bardon et al. 2014, Srikanthasamy et al. 2018). These strategies to control N cycling processes greatly contribute to plant survival or capacity to colonize adverse environments, particularly in ecosystems with low soil N availability (Ishikawa et al. 2003, Bardon et al. 2014). In acid soils, selection pressure for NO3- is intense because of its low levels, and NH4+ and amino acids are considered as the predominant N forms (Havill et al. 1974, Troelstra et al. 1995). Plants growing on acid soils often develop a preference for NH4+ and high abilities to assimilate N from amino acids, but some also assimilate NO3- (Rorison 1986, Raven et al. 1992, Troelstra et al. 1995, Jones et al. 2005). In these soils, selection pressure could also have promoted strategies for biological control of key N cycle processes such as mineralization, nitrification, and denitrification to increase soil NO3- availability and allow a significant uptake of  NO3-  by plants, but this largely remains to be explored."

"Nitrification is an aerobic process, which consists in the oxidation of NH4+ to NO3- (Bothe et al. 2006), and is principally driven by abiotic factors such as NH4+ availability, pH, and temperature, along with the abundance of nitrifiers (Bothe et al. 2006, Le Roux et al. 2008). Nitrification is highly sensitive to soil acidity. Below pH 5, nitrification, and hence  NO3-  production, often remains very low (De Boer and Kowalchuk 2001, Yao et al. 2011). On the other hand, denitrification is a reductive process, which transforms water-soluble NO3-  into the gaseous forms NO, N2O, and N2 (Zumft 1997), and is a major pathway for the loss of NO3-  (Bothe et al. 2006). Denitrification is mainly driven by abiotic factors such as the availability of NO3- , carbon substrate, O2, pH, and temperature, along with the abundance of denitrifiers (Zumft 1997, Wallenstein et al. 2006). Denitrification is less affected than nitrification by low pH (Bothe et al. 2006), which also contributes to the decrease in NO3-  availability in acid soils (Slmek and Cooper 2002). Developed on acid soils, heathland is an ecosystem of great conservation value because of the important cultural and ecological services it provides, such as water purification, carbon sequestration, and biodiversity conservation (Gimingham 1972, Fagundez 2013). French Atlantic heathlands are dominated by ericaceous dwarf shrubs (heathers) such as Erica cinerea (bell heather; Gallet and Roze 2001). They are characterized by NO3- -poor soils (De Boer et al. 1990, Troelstra et al. 1990). In many heathlands, the heather cover is threatened by the invasion of Pteridium aquilinum (Mitchell et al. 1999, Pakeman et al. 2002, DeLuca et al. 2013). Native in temperate ecosystems, P. aquilinum is one of the most widespread plant species around the world and is highly tolerant to marked soil acidity (Marrs et al. 2000). Pteridium aquilinum invasion of heathlands has been attributed to its fast growth, high biomass production, and ability to outgrow ericaceous shrubs despite the low soil pH (Marrs et al. 2000, DeLuca et al. 2013). Different strategies have been proposed for bracken control in heathlands (Milligan et al. 2016). However, a major limitation for vegetation management and restoration ecology is the ability to predict the outcome of proposed treatments across a range of sites, which is essential to meet the objectives of both policy-makers and practitioners implementing management schemes. In the case of bracken control in heathlands, meta-analyses have demonstrated that the effectiveness of control strategies varies between sites (Stewart et al. 2008) and there is a need to ascertain why the outcomes of different strategies are so variable (Pakeman et al. 2002)."

"Actually, bracken control strategies often rely on aerial spraying of herbicide and/or cutting (Pakeman et al. 2002, Stewart et al. 2008) and they have largely disregarded the specificities of belowground processes associated with this plant species (but see Milligan et al. 2018). It has been reported that NO3- concentration is up to 10 times higher in soil under P. aquilinum than in surrounding soil in heathland (Mitchell et al. 1999, DeLuca et al. 2013). Moreover, Stams and Lutke Schipholt (1990) reported that in a woodland acid soil under NH4+ fertilization, P. aquilinum concentrates NO3- in its leaves up to 100 times more than other plant species. These results suggest that P. aquilinum has developed an efficient strategy to increase NO3- availability in acid soils."

"Recently, Bardon et al. (2016) demonstrated that soil denitrification is reduced in response to the release of dimeric to tetrameric B-type procyanidins by roots of some plant species. This phenomenon is called biological denitrification inhibition (BDI). Norris et al. (2011) and Bardon et al. (2017) reported that, in contrast to different condensed tannin mixtures, purified B-type procyanidins with a chain length below 4 do not inhibit soil nitrification and N mineralization. The addition of these compounds to a soil can increase soil NO3- content, likely through denitrification inhibition (Norris et al. 2011, Bardon et al. 2017), which can limit NO3- losses from soils and increase N availability to plants (Bardon et al. 2014). However, both P. aquilinum and E. cinerea are known to produce B-type procyanidins (Wang et al. 1976, Hofland-Zijlstra and Berendse 2009) and thus may inhibit denitrification and limit NO3- losses. Differences in procyanidin production (in terms of type and quantity) may explain differences in NO3- availability in soils under P. aquilinum and E. cinerea."

"In addition, some plant species stimulate nitrification (Lata et al. 2000, Hawkes et al. 2005), which increases soil NO3- availability. This is another explanation (non-exclusive to BDI) for the different NO3- levels reported for soils under the two species. As soil pH is a major factor that drives NO3- production in heathland (Troelstra et al. 1990), we explored the control strategy of N cycle processes that drive NO3- availability in P. aquilinum and E. cinerea stands in Atlantic heathland sites along a soil pH gradient. We hypothesized that both species could influence nitrification and denitrification, but that P. aquilinum would have a more efficient strategy than E. cinerea to increase soil NO3- availability particularly at lower pH. Our objectives were (1) to test whether both species can induce BDI—in particular, we studied how each plant species influences denitrification according to the quantity and type of procyanidins produced; (2) to assess whether the influence of P. aquilinum and/or E. cinerea on nitrification can also contribute to increase soil NO3- availability; and (3) to discuss the possible implications of the control of denitrification and nitrification by P. aquilinum and E. cinerea for the performance of these species and the invasive capacity of P. aquilinum in Atlantic heathland."