User:Alandmanson/Soil fertility and grasslands

Soil fertility and disturbance
Seabloom, E.W., Borer, E.T. and Tilman, D., 2020. Grassland ecosystem recovery after soil disturbance depends on nutrient supply rate. Ecology letters, 23(12), pp.1756-1765. Abstract Human disturbances alter the functioning and biodiversity of many ecosystems. These ecosystems may return to their pre-disturbance state after disturbance ceases; however, humans have altered the environment in ways that may change the rate or direction of this recovery. For example, human activities have increased supplies of biologically limiting nutrients, such as nitrogen (N) and phosphorus (P), which can reduce grassland diversity and increase productivity. We tracked the recovery of a grassland for two decades following an intensive agricultural disturbance under ambient and elevated nutrient conditions. Productivity returned to pre-disturbance levels quickly under ambient nutrient conditions, but nutrient addition slowed this recovery. In contrast, the effects of disturbance on diversity remained hidden for 15 years, at which point diversity began to increase in unfertilised plots. This work demonstrates that enrichment of terrestrial ecosystems by humans may alter the recovery of ecosystems and that disturbance effects may remain hidden for many years.

Nitrogen in grasslands
Grassland productivity worldwide is limited by nitrogen availability.

Boudsocq, S., Niboyet, A., Lata, J.C., Raynaud, X., Loeuille, N., Mathieu, J., Blouin, M., Abbadie, L. and Barot, S., 2012. Plant preference for ammonium versus nitrate: a neglected determinant of ecosystem functioning?. The American Naturalist, 180(1), pp.60-69. Abstract Although nitrogen (N) availability is a major determinant of ecosystem properties, little is known about the ecological importance of plants’ preference for ammonium versus nitrate (ẞ) for ecosystem functioning and the structure of communities. We modeled this preference for two contrasting ecosystems and showed that ẞ significantly affects ecosystem properties such as biomass, productivity, and N losses. A particular intermediate value of ẞ maximizes the primary productivity and minimizes mineral N losses. In addition, contrasting ẞ values between two plant types allow their coexistence, and the ability of one type to control nitrification modifies the patterns of coexistence with the other. We also show that species replacement dynamics do not lead to the minimization of the total mineral N pool nor the maximization of plant productivity, and consequently do not respect Tilman’s R* rule. Our results strongly suggest in the two contrasted ecosystems that ẞ has important consequences for ecosystem functioning and plant community structure.

Srikanthasamy, T., Leloup, J., N'Dri, A.B., Barot, S., Gervaix, J., Koné, A.W., Koffi, K.F., Le Roux, X., Raynaud, X. and Lata, J.C., 2018. Contrasting effects of grasses and trees on microbial N-cycling in an African humid savanna. Soil Biology and Biochemistry, 117, pp.153-163. PDF Abstract African humid savannas are highly productive ecosystems, despite very low soil fertility, where grasses and trees coexist. Earlier results showed that some perennial grass species are capable of biological nitrification inhibition (BNI) while trees likely influence differently on nitrogen cycling. Here we assessed the impact of the dominant grass and tree species of the Lamto savanna (Ivory Coast) on soil nitrifying and denitrifying enzyme activities (NEA and DEA, respectively) and on the abundances of archaeal and bacterial ammonia oxidizers (AOA and AOB, respectively) and nitrite reducers. This is one of the first studies linking nitrifying and denitrifying activities and the abundances of the involved groups of microorganisms in savanna soils. NEA was 72-times lower under grasses than under trees while AOA and AOB abundances were 34- and 3-times lower. This strongly suggests that all dominant grasses inhibit nitrification while trees stimulate nitrification, and that archaea are probably more involved in nitrification than bacteria in this savanna. While nitrite reducer abundances were similar between locations and dominated by nirS genes, DEA was 9-times lower under grasses than trees, which is likely explained by BNI decreasing nitrate availability under grasses. The nirS dominance could be due to the ferruginous characteristics of these soils as nirS and nirK genes require different metallic co-enzymes (Fe or Cu). Our results show that the coexistence of grasses and trees in this savanna creates a strong heterogeneity in soil nitrogen cycling that must be considered to understand savanna dynamics and functioning. These results will have to be taken into account to predict the feedbacks between climate changes, nitrogen cycling and tree/grass dynamics at a time when savannas face worldwide threats.

Wedin, D.A. and Tilman, D., 1990. Species effects on nitrogen cycling: a test with perennial grasses. Oecologia, 84(4), pp.433-441. Summary To test for differing effects of plant species on nitrogen dynamics, we planted monocultures of five perennial grasses (Agropyron repens, Agrostis scabra, Poa pratensis, Schizachyrium scoparium, and Andropogon gerardi) on a series of soils ranging from sand to black soil. In situ net N mineralization was measured in the monocultures for three years. By the third year, initially identical soils under different species had diverged up to 10-fold in annual net mineralization. This divergence corresponded to differences in the tissue N concentrations, belowground lignin concentrations, and belowground biomasses of the species. These results demonstrate the potential for strong feedbacks between the species composition of vegetation and N cycling. If individual plant species can affect N mineralization and N availability, then competition for N may lead to positive or negative feedbacks between the processes controlling species composition and ecosystem processes such as N and C cycling. These feedbacks create the potential for alternative stable states for the vegetation-soil system given the same initial abiotic conditions.

Nitrogen fixation in grasslands
Ritchie, M.E. and Raina, R., 2016. Effects of herbivores on nitrogen fixation by grass endophytes, legume symbionts and free-living soil surface bacteria in the Serengeti. Pedobiologia, 59(5-6), pp.233-241. Abstract Grass roots can harbor abundant endophytic N2-fixing microbes (diazotrophs), but their abundance and activity compared to those on legumes and in soil crusts is still unknown. Here, in a natural ecosystem, the Serengeti of East Africa, we explored whether herbivores and soil nutrients limited grass root endophyte diazotroph abundance and their root mass-specific and area-specific N2-fixation, as they often do for diazotrophs symbiotic with legumes and those free-living in soil. N2-fixation and copy number of the nitrogenase gene nifH was measured with stable isotope and molecular methods, respectively, for the dominant grass Themeda triandra, and legume, Indigofera volkensii, and in the top 5 cm of soil in a 16-year herbivore exclosure experiment across four sites that varied in mean annual rainfall and soil N, P, and moisture. T. triandra nifH gene copy number was highly variable across sites and individuals but often approached or exceeded that of I. volkensii roots and soils. T. triandra roots generally exhibited lower root mass-specific N2-fixation (activity), which was not reduced by herbivores and increased in drier soils. In contrast, I. volkensii activity was only reduced by herbivores and soil diazotrophs were mostly inactive. T. triandra exhibited greater area-specific N2-fixation than I. volkensii, due to its much greater root biomass, but this difference was reduced by herbivores. Grass-associated endophytic diazotrophs may fix far more N2 in natural systems than previously realized, and may be limited by different factors those affecting symbiotic legume and free-living soil diazotrophs. Soussana, J.F. and Hartwig, U.A., 1995. The effects of elevated CO 2 on symbiotic N 2 fixation: a link between the carbon and nitrogen cycles in grassland ecosystems. Plant and Soil, 187(2), pp.321-332. Abstract The response of plants to elevated CO2 is dependent on the availability of nutrients, especially nitrogen. It is generally accepted that an increase in the atmospheric CO2 concentration increases the C:N ratio of plant residues and exudates. This promotes temporary N-immobilization which might, in turn, reduce the availability of soil nitrogen. In addition, both a CO2 stimulated increase in plant growth (thus requiring more nitrogen) and an increased N demand for the decomposition of soil residues with a large C:N will result under elevated CO2 in a larger N-sink of the whole grassland ecosystem. One way to maintain the balance between the C and N cycles in elevated CO2 would be to increase N-import to the grassland ecosystem through symbiotic N2 fixation. Whether this might happen in the context of temperate ecosystems is discussed, by assessing the following hypothesis: i) symbiotic N2 fixation in legumes will be enhanced under elevated CO2, ii) this enhancement of N2 fixation will result in a larger N-input to the grassland ecosystem, and iii) a larger N-input will allow the sequestration of additional carbon, either above or below-ground, into the ecosystem. Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO2 under free-air or field-like conditions, supports the first two hypothesis, since: i) both the percentage and the amount of fixed N increases in white clover grown under elevated CO2, ii) the contribution of fixed N to the nitrogen nutrition of the mixed grass also increases in elevated CO2. Concerning the third hypothesis, an increased nitrogen input to the grassland ecosystem from N2 fixation usually promotes shoot growth (above-ground C storage) in elevated CO2. However, the consequences of this larger N input under elevated CO2 on the below-ground carbon fluxes are not fully understood. On one hand, the positive effect of elevated CO2 on the quantity of plant residues might be overwhelming and lead to an increased long-term below-ground C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over and, hence, limit the storage of below-ground carbon. Soussana, J.F. and Tallec, T., 2010. Can we understand and predict the regulation of biological N 2 fixation in grassland ecosystems?. Nutrient Cycling in Agroecosystems, 88(2), pp.197-213. Abstract We discuss results from controlled environment studies including mesocosms, grazing experiments and long term field experiments which show how biological N2 fixation in legume based systems is tightly coupled to the N demand at scales ranging from the individual plant to the grassland ecosystem. We further test the consequences of this hypothesis of a feedback regulation of biological N2 fixation by N demand with a mechanistic model linking plant community dynamics and ecosystem functioning. Results confirm the heuristic power of this hypothesis which accounts for a number of observations concerning changes in the relative abundance and N2 fixation rate of legumes in managed grasslands. Then we show how nitrogen and carbon fluxes are affected by plant-plant (e.g. competition and facilitation), plant-soil and plant-herbivore interactions and by climate and management changes.

Phosphorus in grasslands
Lambers, H., de Britto Costa, P., Cawthray, G.R., Denton, M.D., Finnegan, P.M., Hayes, P.E., Oliveira, R.S., Power, S.C., Ranathunge, K., Shen, Q. and Wang, X., 2022. Strategies to acquire and use phosphorus in phosphorus-impoverished and fire-prone environments. Plant and Soil, 476(1-2), pp.133-160.

Abstract Background: Unveiling the diversity of plant strategies to acquire and use phosphorus (P) is crucial to understand factors promoting their coexistence in hyperdiverse P-impoverished communities within fire-prone landscapes such as in cerrado (South America), fynbos (South Africa) and kwongan (Australia). Scope: We explore the diversity of P-acquisition strategies, highlighting one that has received little attention: acquisition of P following fires that temporarily enrich soil with P. This strategy is expressed by fire ephemerals as well as fast-resprouting perennial shrubs. A plant’s leaf manganese concentration ([Mn]) provides significant clues on P-acquisition strategies. High leaf [Mn] indicates carboxylate-releasing P-acquisition strategies, but other exudates may play the same role as carboxylates in P acquisition. Intermediate leaf [Mn] suggests facilitation of P acquisition by P-mobilising neighbours, through release of carboxylates or functionally similar compounds. Very low leaf [Mn] indicates that carboxylates play no immediate role in P acquisition. Release of phosphatases also represents a P-mining strategy, mobilising organic P. Some species may express multiple strategies, depending on time since germination or since fire, or on position in the landscape. In severely P-impoverished landscapes, photosynthetic P-use efficiency converges among species. Efficient species exhibit rapid rates of photosynthesis at low leaf P concentrations. A high P-remobilisation efficiency from senescing organs is another way to use P efficiently, as is extended longevity of plant organs. Conclusions: Many P-acquisition strategies coexist in P-impoverished landscapes, but P-use strategies tend to converge. Common strategies of which we know little are those expressed by ephemeral or perennial species that are the first to respond after a fire. We surmise that carboxylate-releasing P-mobilising strategies are far more widespread than envisaged so far, and likely expressed by species that accumulate metals, exemplified by Mn, metalloids, such as selenium, fluorine, in the form of fluoroacetate, or silicon. Some carboxylate-releasing strategies are likely important to consider when restoring sites in biodiverse regions as well as in cropping systems on P-impoverished or strongly P-sorbing soils, because some species may only be able to establish themselves next to neighbours that mobilise P.

Factors affecting the distribution of Fetuca costata and Themeda triandra
Aspect affects incident radiation and the distribution of F. costata and T. triandra in the Drakensberg (Granger and Schulze, 1977). On a global scale, nutrients and light interact to affect root/shoot resource limitation (Cleland et al., 2019).

Rainfall seasonality: Spring and autumn rainfall relative to summer rainfall favours temperate C3 grasses - summer droughts in the Mid-West resulted in C3 expansion (Knapp et al., 2020); this should favour C3 grasses in the southern Drakensberg if they receive more frontal rain in spring. Southern aspects may also have more favourable soil water content in spring and autumn?

Elevated atmospheric CO2 interacts with nitrogen in long-term C3-C4 competition (Reich et al., 2018)

Cleland, E.E., Lind, E.M., DeCrappeo, N.M., DeLorenze, E., Wilkins, R.A., Adler, P.B., Bakker, J.D., Brown, C.S., Davies, K.F., Esch, E., ... and Firn, J., 2019. Belowground biomass response to nutrient enrichment depends on light limitation across globally distributed grasslands. Ecosystems, 22(7), pp.1466-1477.

Granger, J.E. and Schulze, R.E., 1977. Incoming solar radiation patterns and vegetation response: examples from the Natal Drakensberg. Vegetatio, 35(1), pp.47-54.

Knapp, A.K., Chen, A., Griffin-Nolan, R.J., Baur, L.E., Carroll, C.J., Gray, J.E., Hoffman, A.M., Li, X., Post, A.K., Slette, I.J., ... and Collins, S.L., 2020. Resolving the Dust Bowl paradox of grassland responses to extreme drought. Proceedings of the National Academy of Sciences, 117(36), pp.22249-22255.

Reich, P.B., Hobbie, S.E., Lee, T.D. and Pastore, M.A., 2018. Unexpected reversal of C3 versus C4 grass response to elevated CO2 during a 20-year field experiment. Science, 360(6386), pp.317-320.

Bracken fern (Pteridium aquilinum) and soil fertility
Bardon, C., Misery, B., Piola, F., Poly, F. and Le Roux, X., 2018. Control of soil N cycle processes by Pteridium aquilinum and Erica cinerea in heathlands along a pH gradient. Ecosphere, 9(9), p.e02426. DOI PDF Abstract Nitrate is a limiting resource in heathland acid soils. Nitrate levels increase in heathland soils after Pteridium aquilinum invasions, and this species is assumed to biologically control nitrogen cycle processes, thus increasing nitrate availability. We compared how P. aquilinum (bracken) and Erica cinerea (bell heather) modify processes driving nitrate availability along a soil pH gradient in a Natura 2000 reserve facing bracken invasion. Soil nitrate and ammonium concentrations, substrate-induced respiration (SIR), denitrification and nitrification enzyme activities (DEA and NEA, respectively), root procyanidin concentrations, and denitrification inhibition by procyanidins were measured on five sites under P. aquilinum and E. cinerea stands. NEA and nitrate levels were higher, and ammonium levels and SIR lower, for P. aquilinum in the most acid soils. Procyanidins from both species induced the same level of denitrification inhibition, soil nitrate being correlated with root procyanidin concentration for both species. Soil nitrate correlated with NEA only for P. aquilinum. Our results show that both species increased procyanidin production in the most acid soils, thereby reducing denitrification and decreasing nitrate loss, this process being more efficient for E. cinerea. However, P. aquilinum additionally increased nitrification, and this double control on nitrification and denitrification was very efficient to increase soil nitrate availability in the most acid soils. This may participate to the success of P. aquilinum invasions in heathlands. This shows that approaches for bracken control in heathlands should better account for belowground processes and, more generally, that biological denitrification inhibition by plants may be a widespread phenomenon influencing soil N dynamics in N-poor environments. DeLuca, T.H., Zewdie, S.A., Zackrisson, O., Healey, J.R. and Jones, D.L., 2013. Bracken fern (Pteridium aquilinum L. Kuhn) promotes an open nitrogen cycle in heathland soils. Plant and Soil, 367(1), pp.521-534. DOI jstor Abstract Background and Aims: In spite of the broad array of studies conducted on the ecology of bracken fern (Pteridium aquilinum (L.) kuhn), there is currently only a limited understanding of how P. aquilinum alters the soil environment in which it succeeds. P. aquilinum is one of the world’s most aggressive invasive species and is known to effectively invade conservation priority habitats such as Calluna vulgaris (L.) heathland. The aim of this study was to evaluate differences in soil properties between intact stands of C. vulgaris and neighboring P. aquilinum to assess how P. aquilinum alters soil N transformations in a manner that might promote its success. Methods: Replicate plots in five independently paired stands of P. aquilinum and C. vulgaris were established on land in which P. aquilinum is actively invading. Soils under the two plant types were evaluated for total N, mineralisable N, net nitrification, nitrifier activity, denitrification enzyme activity, polyphenol N complexing capacity, and resin sorption of inorganic N. Results: Soils under P. aquilinum were consistently higher in NO3 - and NH4 + concentrations compared to C. vulgaris. Extractable organic and inorganic N concentrations for soil under P. aquilinum were respectively 65 %, 77 % and 358 % greater in amino N NH4 +-N and NO3 --N compared to that under C. vulgaris. In-situ net nitrification (NO3 - sorption to ionic resins) was found to be nearly 300 times greater under P. aquilinum than under C. vulgaris. Conclusions: P. aquilinum alters the soil environment as to create an inorganic N-rich environment that is favorable to its growth and development.

Johnson-Maynard, J.L., McDaniel, P.A., Ferguson, D.E. and Falen, A.L., 1997. Chemical and mineralogical conversion of Andisols following invasion by bracken fern. Soil Science Society of America Journal. 61 (2): 549-555., pp.549-555. DOI PDF Researchgate Description Andisols support ≈ 200 000 ha of mid-elevation grand fir (Abies grandis [Dougl. ex D.Don] Lindl.) forests in the Pacific Northwest region that are characterized by little or no natural conifer regeneration following removal of the forest canopy. Previous work suggests that the properties of these Andisols have been altered as a result of the establishment of successional communities dominated by bracken fern (Pteridium aquilinum [L.] Kuhn) in deforested areas. In this study, we compared soil properties in a 30-yr-old bracken fern site (clear-cut in 1965), a natural bracken fern site that is estimated to be centuries old, and an adjacent undisturbed forest in the Clearwater National Forest of northern Idaho. Results indicate that changes in chemical properties have accompanied establishment of successional communities. Mean weighted pH within the ash cap of the 30-yr-old bracken fern site (4.6) is significantly lower than that of the undisturbed forest (5.2). Mean values for AI saturation range from 27% in the undisturbed forest to 52% in the 30-yr-old bracken fern site; organic C is also lower in the undisturbed forest (37 g/kg) than in the 30-yr-old bracken fern site (54 g/kg). The dominant secondary mineralogical component of soils of the undisturbed forest is inorganic, short-range-order AI-Fe minerals, while metal-humus complexes are dominant in the bracken-fern-influenced soils. Data indicate that bracken fern successional communities are responsible for a shift from allophanic to nonallophanic properties in these soils, probably due to increased levels of soil organic C associated with bracken fern and a subsequent increase in formation of AI-humus complexes. Furthermore, such a mineralogical shift may contribute to the observed problems with conifer regeneration. McDaniel, P., Jimenez, J., Johnson-Maynard, J., Ferguson, D. and Falen, A., 2006, July. Decade-scale Conversion to Non-allophanic Andisols with Secondary Succession. In The 18th World Congress of Soil Science. Poster Abstract Successional communities dominated by bracken fern (Pteridium aquilinum) establish on Andisols (Andosols) after removal of forest canopy in northern Idaho, USA. Many of these communities, known as bracken glades, have been created as a result of clear cutting over the past 50 yrs; others, based on radiocarbon dates, have apparently existed for millennia. We sampled Andisols supporting bracken glades ranging in age from 10 yrs to 7,700 yrs BP. Chemical and mineralogical characteristics of these soils were measured and compared to those of adjacent grand fir forest communities. Data show changes in several soil properties following establishment of bracken glades. Quantity and quality of belowground C increase as a result of bracken fern inputs in the form of rhizomes and fine roots (Fig. 1a). As much as 4.9 kg m-2 of belowground biomass was measured in a 40-yr-old bracken glade, more than twice the amount in the adjacent forest. In addition, pH decreases and active forms of Al3+ increase (Fig. 1b). These changes are consistent with a conversion from allophanic to non-allophanic mineralogy. More surprisingly, our data indicate that this conversion is initially very rapid, occurring on a decadal scale. Properties observed within several decades of bracken fern establishment are similar to those of Andisols that have supported bracken glades for millennia.

Grasslands in general
https://www.youtube.com/watch?v=Yy191KVBNP0&t=36s&ab_channel=GeoDiode

Southern African grasslands
Killick, D.J.B., 1963. An account of the plant ecology of the Cathedral Peak area of the Natal Drakensberg. Memoirs of the Botanical Survey of South Africa 34: 1–178. archive.org

Edwards, D., 1967. A plant ecology survey of the Tugela River basin, Natal. Pietermaritzburg: Natal Town and Regional Planning Commission. (also Memoirs of the Botanical Survey of South Africa 36).biodiversitylibrary.org

O’Connor T.G., Bredenkamp G.J., 1997. Grassland. In: Cowling RM, Richardson D (eds) Vegetation of Southern Africa. Cambridge University Press, Cambridge, pp 215–257

Palmer, A.R. and Ainslie, A.M., 2005. Grasslands of South Africa. Grasslands of the World, 34, p.77.