Crucially, fast initial fern growth in the medium-textured soil, likely due to higher nutrient content and/or lower arsenic phytoavailability, led to a decrease in effluent flow and therefore arsenic leaching, as transpiration exceeded water application. In this soil, the fern required less energy to acquire nutrients, released less arsenic from soil via nutrient scavenging, and produced greater biomass such that arsenic concentrations in biomass were lower. The increase in mass of arsenic accumulated from 11 to 21 weeks, coupled with lower final biomass of mature and young compared to senescent fronds, shows that arsenic accumulation continued even as growth slowed. This arsenic accumulation could be due to increased nutrient scavenging associated with drought stress.The increase in arsenic concentrations in leachate in the presence of ferns growing in the medium-textured soil reveals the importance of rhizosphere processes to arsenic release for uptake and leaching. We found that arsenic depletion from the medium-textured soil was the greatest in surface soil where P. vittata roots are primarily located . Moreover, in both soils we calculated that arsenic concentrations in rhizosphere pore water must be greater than those in bulk soil pore water,square planter pot because assuming arsenic concentrations to be the same in rhizosphere and bulk pore water indicated a discrepancy between arsenic intake through transpiration flux, and fern arse nic content. Processes other than mass flow of soluble arsenic from bulk soil to roots must be important for arsenic uptake .
If nutrients availability in soil is lower than P. vittata demand, P. vittata could employ nutrient-scavenging processes that release iron, phosphorus, and therefore arsenic from soil into pore water in the rhizosphere , increasing pore water arsenic concentrations locally to potentially very high concentrations. We suggest that the majority of the arsenic taken up into P. vittata was mobilized directly in the rhizosphere, similarly to others who found greater desorption of cadmium in cadmium hyper accumulator rhizospheric compared to bulk soils . We hypothesize that higher diffusivity due to greater connected pore space in the medium-textured soil could lead to transport of the arsenic re leased in the rhizosphere to the bulk soil, where it is then available for leaching. Similarly, rhizosphere DOC could be transported to the bulk soil and promote release of arsenic. However, in the coarse-textured soil characterized by lower porosity, larger pores, lower saturated fraction, and pre dominantly advective flow, arsenic and DOC released in the rhizosphere did not contribute to bulk leachate arsenic concentrations and, conversely, arsenic in the bulk pore water was not as accessible to the plants.We suggest rhizosphere arsenic mobilization is a byproduct of nutrient scavenging processes, particularly iron-scavenging in the medium-textured soil, where we found higher iron concentrations in ferns and in root zone pore water. Specifically, arsenic release from soil could be coupled to phosphorus and iron release from soil iron oxide minerals . Release processes could include ion exchange, ligand-enhanced dissolution, and reductive dissolution , likely tied to release of root exudates from P. vittata roots . We found primarily oxidized arsenic in our well-drained rhizosphere soil, suggesting processes including ion exchange and ligand-enhanced dissolution, likely coupled to rhizosphere DOC, are more important than reductive dissolution, similarly to in the Pine rhizosphere . Alternately, the predominance of oxidized species could indicate P. vittata preferentially took up reduced species, leaving oxidized species behind. We found evidence of reductive processes in the rhizosphere, with up to 41% of the arsenic present as arsenic in rhizosphere soil, up to 100% of the arsenic present as arsenic on and within roots, and iron phases in rhizosphere soil, suggesting reduced arsenic and iron could play a secondary role in arsenic release and uptake.
A high fraction of surficial arsenic could indicate transport of arsenic toward the root and accumulation in the rhizoplane, with slower uptake of arsenic enriching arsenic relative to arsenic on the root surface. The presence of arsenic on the root surface could also indicate efflux of arsenic from roots, which has been proposed to be a secondary tolerance mechanism in P. vittata and other plants under arsenic stress . In bulk pore water, bulk soils, and soil aggregates, the predominance of arsenic indicates arsenic can leach under oxic conditions. Arsenic availability for leaching, whether due to soil characteristics or influence of plant growth, is not dependent on reducing conditions. Indeed, arsenic mobility in soil increases at the circumneutral to alkaline pore water pH we observed . Arsenic mobilized as arsenic could be oxidized, perhaps coupled to reduction of the moderately-available soil manganese. Leaching of root derived dissolved organic carbon could also increase arsenic release from bulk soil for leaching.Rhizosphere nutrient acquisition processes have a specific significance in the case of hyper accumulators. Infertile soils could characterize the hyper accumulator ecological niche , such that P. vittata employs scavenging techniques and associates with indigenous AMF to acquire necessary phosphorus and other nutrients. We found Glomus spp. including F. mosseae were present across all treatments whether due to colonization by indigenous mycorrhiza or due to inoculation. In the very low nutrient coarse-textured soil, we hypothesize that extensive use of these scavenging processes cost metabolic energy, locally in creased already high arsenic availability, led to high uptake of arsenic and consequently even more energy expenditure to sequester this arsenic, and ultimately resulted in low biomass containing arsenic at high concentrations. The lack of effect of supplemental phosphorus in the coarse textured soil suggests it is a balance of phosphorus and other nutrients which are required to meet P. vittata nutritional needs. In contrast, in the medium-textured soil, we hypothesize the ferns used less energy to acquire nutrients.
Iron scavenging here was successful, apparently meeting fern nutrient needs and therefore limiting “byproduct” arse nicreleased from soil. Hence, P. vittata growing in the medium-textured soil experienced lower metabolic costs and consequently higher biomass until drought stress limited biomass production. In keeping with evolution under phosphorus starvation conditions , our results suggest P. vittata is less tolerant to extractable phosphorus concentrations greater than that of the medium-textured soil . Fronds of P. vittata growing in its native habitat in China were only 0.08% phosphorus, and ferns including P. vittata had the lowest phosphorus content of any flora group in China . We found phosphorus application delayed fern growth in both medium- and coarse-textured soils,hydroponic nft channel as has been shown for tropical forest ferns , leading to smaller senescent fronds containing lower amounts of arsenic.Our findings suggest that P. vittata is a good choice for remediation at the mesoscale, because arsenic uptake in ferns exceeded cumulative loss by leaching by an order of magnitude, and transpiration limited leaching compared to the absence of ferns. Decreased effluent volumes and cumulative arsenic leaching in both soils in the presence of ferns confirms the critical role transpiration plays in limiting water percolation and leaching of soluble, plant available constituents . The leaching to uptake ratio measured in this mesocosm system is not directly scalable to field conditions. We demonstrate that arsenic leaching during phytoextraction depends on soil characteristics, fern growth, and water input/evapotranspiration ratios, and therefore must be measured at the field scale. The constant water application required in our column study design could have increased leaching of arsenic relative to field applications. On the other hand, our experimental design could have limited plant growth and therefore nutrient scavenging activities, which we showed can increase arsenic release from soil. Larger biomass under field conditions could increase the influence of the nutrient scavenging geo chemical processes observed here and lead to increases in arsenic mobilization for both uptake and potential leaching, explaining the excess loss of arsenic from soil observed under field conditions . Counter intuitively, because we showed that P. vittata continued to phytoextract arsenic under drought conditions from the medium-textured soil to effectively limit arsenic leaching, phytoextraction could be best suited for dry soils with lower arsenic availability. Here, even though frond arsenic concentrations were an order of magnitude greater in coarse-textured soil ferns, mass of accumulated arsenic in coarse-textured soil ferns was only 1.2 to 2.4 times that of medium-textured soil ferns, while leached arsenic was also greater in coarse-textured soil, due to the lower biomass and lower transpiration. Alternatively, phytostabilization with species with high transpiration rates but lower iron demand could limit biotic and abiotic arsenic leaching.Such substrates typically have an inorganic and organic component . The organic component provides high porosity, low bulk density, and nutrient retention , which makes Sphagnum peat moss a strongly suitable option with widespread use .
However, increasing expense and competing uses for peat , impacts of its harvest on wetland ecosystems , including loss of peat bogs as a key global C sink , and its perception as unsustainable have spurred recent investigations of substitutes for peat in soil-free substrates, including biomass waste products such as compost and sawdust . Biochar has been recently proposed as a strong candidate to substitute for peat because of its high porosity, low density and high cation-exchange capacity. Biochar is a carbon -rich material produced by pyrolysis of biomass and has been a major subject of study as a soil amendment in the last decade . In addition to providing high nutrient and water retention, replacing peat with BC could offset or reverse the C footprint of soil-free substrates into a net C sink . Evidence to-date suggests neutral or positive effects of BC use in substrates on nutrient availability and plant growth , though many studies examine additions of BC to peat-based substrates, rather than replacing a substrate component such as peat .Evaluating effects of high BC substitution rates on substrate properties and plant growth is necessary to understand the extent to which BC can replace peat. At low amendment or substitution rates, BC has been found to maintain or improve plant growth as a result of increased nutrient availability , reduced nutrient and water loss , and amelioration of peat acidity , though these effects may be BC-specific due to feed stock and pyrolysis influences on BC properties . However, at high substitution rates, substrate properties conducive to plant growth may be compromised. In particular, the high pH of many BCs could result in BC-substituted substrates with pH values unfavorable to plant growth. For example, pelleted wood BC substitution for peat required adjustment of pH due to the liming effect of the BC . The neutral to alkaline pH of BCs and their liming potential means that BC substitution for peat can increase pH beyond optimum for plant growth in potting media . Explicit eva luation of BC effects on substrate pH and plant performance provides a basis to improve design of BC-based substrates and inform trade-offs in this application of BC . The objective of this study was to determine the effects of BC substitution for peat and substrate pH on greenhouse production, using marigold as a model crop. In the United States, the wholesale value of marigolds plants was 30.3 million USD in 2015 . Softwood BC was substituted for peat in a typical 70:30 peat:perlite mixture at 10%v increments. Since many BCs are alkaline and will increase pH of substrates in proportion to the degree of substitution, the effect of adjusting pH of substrates to typical soil free substrate values was also evaluated. Marigold germination and growth were measured over 9 weeks. We hypothesized that under greenhouse conditions , marigold germination and growth would be more sensitive to BC substitution at higher rates and that this would be due to elevated substrate pH. Additionally, we hypothesized that pH adjustment of BC substrates would increase the extent to which this softwood BC could be substituted for peat without compromising plant growth.Marigold var. ‘Crackerjack’ seeds were sown directly in 0.7 L of substrate pre-fertigated to 100% WHC using 0.5% Hoagland solution in 1.2 L polypropylene pots in a greenhouse at the UC Davis Plant Growth Facility. Pots were arranged 18 cm apart in a completely randomized block design with four replicates per substrate-pH treatment . Pots were drip fertigated with 0.5% Hoagland solution at 66 mL d−1 for weeks 1–6 and 99 mL d−1 for weeks 7–9.