Plant-soil feedbacks and invasive spread

Plant invaders have been suggested to change soil microbial communities and biogeochemical cycling in ways that can feedback to benefit themselves. In this paper, we ask when do these feedbacks influence the spread of exotic plants. Because answering this question is empirically challenging, we show how ecological theory on 'pushed' and 'pulled' invasions can be used to examine the problem. We incorporate soil feedbacks into annual plant invasion models, derive the conditions under which such feedbacks affect spread, and support our approach with simulations. We show that in homogeneous landscapes, strong positive feedbacks can influence spreading velocity for annual invaders, but that empirically documented feedbacks are not strong enough to do so. Moreover, to influence spread, invaders must modify the soil environment over a spatial scale larger than is biologically realistic. Though unimportant for annual invader spread in our models, feedbacks do affect invader density and potential impact. We discuss how future research might consider the way landscape structure, dispersal patterns, and the time scales over which plant–soil feedbacks develop regulate the effects of such feedbacks on invader spread.     

Increased ecological amplitude through heterosis following wide outcrossing in Banksia ilicifolia R.Br. (Proteaceae)

To assess whether wide outcrossing (over 30 km) in the naturally fragmented Banksia ilicifolia R.Br. increases the ecological amplitude of offspring, we performed a comparative greenhouse growth study involving seedlings of three hand-pollinated progeny classes (self, local outcross, wide outcross) and a range of substrates and stress conditions. Outcrossed seedlings outperformed selfed seedlings, with the magnitude of inbreeding depression as high as 62% for seed germination and 37% for leaf area. Wide outcrossed seedlings outperformed local outcrossed seedlings, especially in non-native soils, facilitated in part by an improved capacity to overcome soil constraints through greater root carboxylate exudation. Soil type significantly affected seedling growth, and waterlogging and water deficit decreased growth, production of cluster roots, root exudation and total plant P uptake. Our results suggest that the interaction of narrow ecological amplitude and the genetic consequences of small fragmented populations may in part explain the narrow range of local endemics, but that wide outcrossing may provide opportunities for increased genetic variation, increased ecological amplitude and range expansion.     

Distribution of Carboxylates and Acid Phosphatase and Depletion of Different Phosphorus Fractions in the Rhizosphere of a Cereal and Three Grain Legumes

This study investigates the distribution of carboxylates and acid phosphatases as well as the depletion of different phosphorus (P) fractions in the rhizosphere of three legume crop species and a cereal, grown in a soil with two different levels of residual P. White lupin (Lupinus albus L.), field pea (Pisum sativum L.), faba bean (Vicia faba L.) and spring wheat (Triticum aestivum L.) were grown in small sand-filled PVC tubes to create a dense root mat against a 38-μm mesh nylon cloth at the bottom, where it was in contact with the soil of interest contained in another tube. The soil had either not been fertilised (P0) or fertilised with 15 (P15) kg P ha⁻¹ in previous years. The mesh size did not allow roots to grow into the soil, but penetration of root hairs and diffusion of nutrients and root exudates was possible, and a rhizosphere was established. At harvest, thin (1 mm) slices of this rhizosphere soil were cut, down to a 10-mm distance from the mesh surface. The rhizosphere of white lupin, particularly in the P0 treatment, contained citrate, mostly in the first 3 mm, with concentrations decreasing with distance from the root. Acid phosphatase activity was enhanced in the rhizosphere of all species, as compared with bulk soil, up to a distance of 4 mm. Phosphatase activity was highest in the rhizosphere of white lupin, followed by faba bean, field pea and wheat. Both citrate concentrations and phosphatase activities were higher in P0 compared with P15. The depletion of both inorganic (P_i) and organic (P_o) phosphorus fractions was greatest at the root surface, and decreased gradually with distance from the root. The soil P fractions that were most depleted as a result of root activity were the bicarbonate-extractable (0.5 M) and sodium hydroxide-extractable (0.1 M) pools, irrespective of plant species. This study suggests that differences among the studied species in use of different P pools and in the width of the rhizosphere are relatively small.     

Differences in investment and functioning of cluster roots account for different distributions of Banksia attenuata and B. sessilis,with contrasting life history

Aims

Banksia attenuata is a resprouting species growing in deep sand, while B. sessilis is a fire-killed species occurring in shallow sand over laterite or limestone. We aimed to discover the ecophysiological basis for their different distributions by exploring their investment in deep non-cluster roots and shallow cluster roots, and their cluster-root functioning.

Methods

Deep-pot (1 m), shallow-pot (400 mm), hydroponic experiments and phosphorus (P)-extraction experiment were carried out. Biomass allocation, cluster-root exudation, plant P and leaf manganese (Mn) concentrations were measured.

Results

Banksia attenuata allocated more biomass to deep roots and less biomass to cluster roots than B. sessilis did in deep pots. The two Banksias released similar carboxylates in all experiments, with similar carboxylate-exudation rates in hydroponics. The carboxylate amount per unit cluster root of B. sessilis grown in shallow pots was greater than that of B. attenuata, and B, sessilis acquired more P than B. attenuata did in limestone substrate.

Conclusions

Greater investment in deep roots for water uptake accounts for the presence of B. attenuata in deep sand, and vice versa for the absence of B. sessilis. A larger investment in cluster roots, which released greater amounts of carboxylates, likely accounts for B. sessilis occurring over limestone. Trade-offs in investment and cluster-root functioning support the species’ distribution patterns and life histories. Leaf Mn concentration was a good proxy for the plant capacity to acquire P.

     

Changes in soil phosphorus fractions following sole cropped and intercropped maize and faba bean grown on calcareous soil

Aims

This study aimed to investigate the effects of coexistence with faba bean, a phosphorus (P)-efficient crop, on soil-accumulated P use by a maize/faba bean intercropping system on dynamic changes in soil P pool.

Methods

Maize and faba bean were grown in P-accumulated soil as either sole cropping or intercropping. After one year (Stage I) or four years (Stage II) of no P application, soil samples were collected respectively and analyzed for soil P pools using sequential fractionation. Aboveground biomass and P content were annually measured from 2013 to 2016 to assess the annual P balance.

Results

The intercropped maize/faba bean system showed a P-uptake advantage, with a Land Equivalent Ratio (LER) ranging from 1.2 to 1.5. The average shoot P content over the four years in intercropped maize and faba bean was significantly greater than that of the corresponding sole crops by 29% and 30%, respectively. Over the three-year P depletion period, the three cropping systems primarily depleted the 1 M HCl-P_i fraction, followed by sole maize, which depleted the NaOH-P_i and concentrated HCl-P_o fractions. Sole faba bean depleted the alkali-soluble P_o fraction (extracted by NaHCO₃ and NaOH), and the intercropped maize/faba bean system depleted the conc. HCl-P_o fraction, which was similar to the effect of sole maize.

Conclusions

Both sole crops and intercrops mainly depleted 1 M HCl-P_i, but differed in P_o depletion. Sole maize and maize/faba bean intercropping depleted the sparingly labile P_o fraction, while sole faba bean depleted the labile and moderately labile P_o fractions.

     

Correction to: Strong phosphorus (P)-zinc (Zn) interactions in a calcareous soil-alfalfa system suggest that rational P fertilization should be considered for Zn biofortification on Zn-deficient soils and phytoremediation of Zn-contaminated soils

Plant and Soil - A Correction to this paper has been published: https://doi.org/10.1007/s11104-021-04966-1     

A long-term experimental test of the dynamic equilibrium model of species diversity

The dynamic equilibrium model of species diversity predicts that ecosystem productivity interacts with disturbance to determine how many species coexist. However, a robust test of this model requires manipulations of productivity and disturbance over a sufficient timescale to allow competitive exclusion, and such long-term experimental tests of this hypothesis are rare. Here we use long-term (27 years), large-scale (8 × 50-m plots), factorial manipulations of soil resource availability and sheep grazing intensity (disturbance) in grasslands to test the dynamic equilibrium model. As predicted by the model, increased productivity not only reduced plant species richness, but also moderated the effects of grazing intensity, shifting them from negative to neutral with increasing productivity. Reductions in species richness with productivity were associated with dominance by faster growing (i.e. high specific leaf area) and taller plants. Conversely, grazing favoured shorter plants and this effect became stronger with greater productivity, consistent with the view that grazing can lead to weaker asymmetric competition for light. Our study shows that the dynamic equilibrium model can help to explain changes in plant species richness following long-term increases in soil resource availability and grazing pressure, two fundamental drivers of change in grasslands worldwide.     

Downregulation of net phosphorus-uptake capacity is inversely related to leaf phosphorus-resorption proficiency in four species from a phosphorus-impoverished environment

Background and Aims

Previous research has suggested a trade-off between the capacity of plants to downregulate their phosphorus (P) uptake capacity and their efficiency of P resorption from senescent leaves in species from P-impoverished environments.

Methods

To investigate this further, four Australian native species (Banksia attenuata, B. menziesii, Acacia truncata and A. xanthina) were grown in a greenhouse in nutrient solutions at a range of P concentrations [P]. Acacia plants received between 0 and 500 µm P; Banksia plants received between 0 and 10 µm P, to avoid major P-toxicity symptoms in these highly P-sensitive species.

Key Results

For both Acacia species, the net P-uptake rates measured at 10 µm P decreased steadily with increasing P supply during growth. In contrast, in B. attenuata, the net rate of P uptake from a solution with 10 µm P increased linearly with increasing P supply during growth. The P-uptake rate of B. menziesii showed no significant response to P supply in the growing medium. Leaf [P] of the four species supported this finding, with A. truncata and A. xanthina showing an increase up to a saturation value of 19 and 21 mg P g⁻¹ leaf dry mass, respectively (at 500 µm P), whereas B. attenuata and B. menziesii both exhibited a linear increase in leaf [P], reaching 10 and 13 mg P g⁻¹ leaf dry mass, respectively, without approaching a saturation point. The Banksia plants grown at 10 µm P showed mild symptoms of P toxicity, i.e. yellow spots on some leaves and drying and curling of the tips of the leaves. Leaf P-resorption efficiency was 69 % (B. attenuata), 73 % (B. menziesii), 34 % (A. truncata) and 36 % (A. xanthina). The P-resorption proficiency values were 0·08 mg P g⁻¹ leaf dry mass (B. attenuata and B. menziesii), 0·32 mg P g⁻¹ leaf dry mass (A. truncata) and 0·36 mg P g⁻¹ leaf dry mass (A. xanthina). Combining the present results with additional information on P-remobilization efficiency and the capacity to downregulate P-uptake capacity for two other Australian woody species, we found a strong negative correlation between these traits.

Conclusions

It is concluded that species that are adapted to extremely P-impoverished soils, such as many south-western Australian Proteaceae species, have developed extremely high P-resorption efficiencies, but lost their capacity to downregulate their P-uptake mechanisms. The results support the hypothesis that the ability to resorb P from senescing leaves is inversely related to the capacity to downregulate net P uptake, possibly because constitutive synthesis of P transporters is a prerequisite for proficient P remobilization from senescing tissues.