Si and Water Management Drives Changes in Fe and Mn Pools that Affect As Cycling and Uptake in Rice

Abstract

Arsenic availability to rice is tied to biogeochemical cycling of Fe and Mn in rice soils. Two strategies to minimize As uptake by rice—increasing Si and decreasing water—affect soil Fe and Mn pools. We synthesized data from several soil-based experiments with four rice cultivars across pot and field trials with manipulations of Si, water, or both. Increasing Si alters the mineral composition of Fe plaque more than decreasing water, with the former promoting relatively more ferrihydrite and less lepidocrocite. Nonflooded conditions decrease lepidocrocite but slightly increase goethite compared to flooded rice. Plaque As, which was a mixture of arsenite (15–40%) and arsenate (60–85%), was correlated positively with ferrihydrite and negatively with lepidocrocite and goethite. Plaque As was also positively correlated with F1 and F2 soil As, and F2 was correlated positively with porewater As, total grain As, and grain organic As (oAs). Grain inorganic As (iAs) was negatively correlated with oxalate-extractable Fe and Mn. Our data and multiple linear regression models suggest that under flooded conditions iAs is released by poorly crystalline Fe oxides to porewater mainly as iAs(III), which can either be taken up by the plant, adsorbed to Fe plaque, oxidized to iAs(V) or methylated to oAs. Increasing Si can promote more desorption of iAs(III) and promote more poorly-ordered phases in plaque and in bulk soil. The ultimate effectiveness of a Si amendment to decrease As uptake by rice depends upon it being able to increase exogenous Si relative to As in porewater after competitive adsorption/desorption processes. Our data further suggest that poorly crystalline Fe and Mn soil pools can retain inorganic As and decrease plant uptake, but these pools in bulk soil and plaque control grain organic As.

Carryover effects of silicon‐rich amendments in rice paddies

“…The results of our study partially supported our hypothesis, showing that the three Si‐rich amendments tested possessed a similar carryover of Si, resulting in straw Si 55% higher than the Control (Figure 3), elevated CaCl 2 –extractable soil Si (Table 1), elevated porewater Si during reproductive growth (Figure 1c), and higher concentrations of Si in the root iron plaque leading to increased ferrihydrite in the plaque (Figure 7) (Seyfferth et al., 2019a). The Si‐rich amendments continued to increase CaCl 2 extractable Si between harvests in Year 1 and 2 by 6–22% compared with the 7% decrease in the Control treatment, demonstrating increased release and availability of Si from Si‐rich amendments (Table 1).…”

Silicon-rich soil amendments impact microbial community composition and the composition of arsM bearing microbes

“…We sequenced the 16S rRNA and arsM genes from the rhizosphere soils collected at grain ripening (88 DPT) of 12 rice paddy mesocosms amended with husk, char, silicate, or unamended control and paired these data with geochemical measurements from the rice paddies that were previously described (Seyfferth et al 2019b;Limmer and Seyfferth 2020). We obtained 36,829 raw arsM sequences (with at least 5 circular consensus reads) from PacBio sequencing, of which 24,627 (67%) remained after quality ltering.…”

“…Because porewater between 55-89 DPT was most predictive of mature grain As concentrations (Limmer and Seyfferth 2020), the median porewater or median methane ux value in this range of sampling was used for analysis with microbial data. Following the 2016 growing season, ve soil cores from 0-10 cm depth were collected in diagonal pattern across each rice paddy and composited for analysis of solid-phase As pools by As sequential extraction, as described in detail by (Seyfferth et al 2019b). The extractants were 0.05 M ammonium sulfate for fraction F1 (non-speci cally sorbed As), 0.05 M ammonium phosphate for fraction F2 (speci cally sorbed As), 0.2 M acid ammonium oxalate for fraction F3 (As associated with amorphous Fe and Al oxides), 0.2 M ammonium oxalate and ascorbic acid for fraction F4 (As associated with crystalline Fe/Al oxides) and nitric acid and hydrogen peroxide for fraction F5 (residual As) (Motuzova et al 2006).…”

Socio-Technical Changes for Sustainable Rice Production: Rice Husk Amendment, Conservation Irrigation, and System Changes

“…Huskderived Si has been shown to beneficially compete against As in its assimilation into the plant and can combat the As-induced yield loss in flooded rice (Teasley et al, 2017;Limmer et al, 2018b). Husk addition increases the proportion of ferrihydrite in root plaques, which may contribute to As retention in the rhizosphere and less As uptake (Seyfferth et al, 2019b). Moreover, soil incorporation of husks can decrease toxic iAs in rice grain by 25-50% without affecting grain Cd in flooded and nonflooded soils (Seyfferth et al, 2016(Seyfferth et al, , 2019aTeasley et al, 2017).…”