Plant Soil DOI 10.1007/s11104-015-2682-3
REGULAR ARTICLE
Biogeochemical impacts of silicon-rich rice residue incorporation into flooded soils: Implications for rice nutrition and cycling of arsenic Evanise S. Penido & Alexa J. Bennett & Thomas E. Hanson & Angelia L. Seyfferth
Received: 17 July 2015 / Accepted: 17 September 2015 # Springer International Publishing Switzerland 2015
Abstract Aim Soil incorporation of Si-rich rice residues may aid smallholder rice farmers in improving crop yields and may affect As uptake. Here, the biogeochemical impacts of rice residue incorporation into flooded soil without plants were evaluated. Methods Various particle sizes of fresh rice straw (FS), fresh rice husk (FH), rice straw ash (RSA) and rice husk ash (RHA) residues were incorporated into soil (1 % w/w) in a flooded pot experiment. Pore-water chemistry was monitored weekly and dissolved CH4 concentrations and genes specific for methanotrophy and methanogenesis in DNA extracts of soil were evaluated. Results FH-amended soil had the highest level of dissolved Si, followed by FS and then ash (RHA or RSA). Particle size had little effect on the dissolved Si concentration for any residue tested. No amendments had any substantial effect on pore-water pH. FS-amended soil had much higher As, Fe, and CH4 concentrations in
pore-water compared to ash and FH, and its extracted DNA also had higher amplifications of genes indicative of methanogenesis. Conclusion FH, RHA, or RSA are attractive amendments for smallholder rice farmers to increase plantavailable Si without exacerbating CH4 emissions, which may improve rice nutrition through the beneficial impacts of Si on combating biotic and abiotic stress including decreased arsenic uptake. Keywords Sustainable farming . Methane emissions . Methanogenesis . Biocycling . Abiotic and biotic stress Abbreviations FH Fresh husk FS Fresh straw RHA Rice husk ash RSA Rice straw ash
Responsible Editor: Robert Reid. E. S. Penido : A. L. Seyfferth (*) Department of Plant and Soil Sciences, University of Delaware, Newark, DE, USA e-mail:
[email protected] A. J. Bennett : T. E. Hanson School of Marine Science and Policy, University of Delaware, Newark, DE, USA Present Address: E. S. Penido Department of Soil Science, Federal University of Lavras, Lavras, MG, Brazil
Introduction Rice is an important staple food for over half of the global population, but its yield is affected by biotic and abiotic stress that will worsen with climate change (Challinor et al. 2014). Direct climate change-induced abiotic stressors such as increased temperature and rising atmospheric CO2 clearly affect plant physiology and thus rice yields (Arai-Sanoh et al. 2010; Lin et al. 2010; Myers et al. 2014; Welch et al. 2010; Ziska et al. 1996), but the indirect effects of climate change are less well
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understood. These indirect or secondary effects include more release and uptake of toxic metal(loid)s such as As due to higher transpirational water flux, microbial activity, and elemental cycling in paddy fields (Guo et al. 2011) if dry seasons get drier, and increased susceptibility to infection by devastating fungal pathogens such as Magnaporthe oryzae (Kobayashi et al. 2006), causal agent of rice blast if wet seasons get wetter. Both As uptake and rice blast currently affect rice quality and yield (Abedin et al. 2002; Bevitori and Ghini 2014; Hossain et al. 2008; Hossain et al. 2009; Panaullah et al. 2009; Wilson and Talbot 2009), which is only expected to worsen as the climate shifts. Increasing dissolved and plant-available Si, a nontoxic and beneficial element for rice, has positive impacts on rice yield and quality by protecting against abiotic and biotic stressors including As uptake/ toxicity (Bogdan and Schenk 2008; Li et al. 2009; Seyfferth and Fendorf 2012) and M. oryzae infection (Deren et al. 1994; Rodrigues et al. 2004; Rodrigues et al. 2001; Seebold et al. 2001). While silicon is not considered a plant-essential element, its beneficial effects on monocots, including rice, are well-documented, and Si comprises up to 10 % of the dry matter in rice straw and husk—much higher than other mineral nutrients (Epstein 1994, 1999). With respect to As, dissolved silicon (silicic acid) and arsenite, the dominant dissolved species in paddy soil solutions, are chemical analogs at circumneutral pH and are taken up by rice roots through the same transport mechanism (Ma et al. 2008). Therefore, increasing dissolved Si leads to competitive inhibition of As uptake (Seyfferth and Fendorf 2012). For M. oryzae, increased Si deposition in epidermal tissues of rice leaves renders them more resistant to fungal penetration (Seebold et al. 2001). Prior work demonstrating the benefits of Si on rice were conducted with synthetic silica fertilizer (e.g., calcium silicate or silica gel), which is prohibitively expensive for smallholder farmers in developing countries who rely on stable food production for economic and caloric security. Moreover, since many soils in subtropical and tropical ricegrowing regions are well-weathered and Si depleted, finding sustainable approaches to improve Si availability is of utmost importance (Savant et al. 1997a). The soil-incorporation of rice plant residues (e.g., rice husk and straw) as a source of Si could help rice to defend itself against biotic and abiotic stress in a sustainable way. The effects of rice straw incorporation
have been studied with respect to CH4 emissions (Adachi et al. 1996; Bhattacharyya et al. 2012; Cai et al. 2001; Chidthaisong and Watanabe 1997; Dominguez-Escriba and Porcar 2010; Espiritu et al. 1997; Ma and Takahashi 1991; Seyfferth et al. 2013), but very little work has been conducted on the impacts of soil incorporation of other Si-rich rice residues including rice husk and the ash of both rice straw and husk. These residues are continually available to smallholder farmers in developing countries and could be incorporated into paddy soils with minimal change and cost to current practices, thus providing a sustainable approach to improve Si nutrition in flooded rice paddies. However, the biogeochemical impact of incorporating these residues into flooded soils is unknown. The purpose of this study was to evaluate the impacts of Si-rich rice residue incorporation to flooded soil on the release of Si, As, P, and Fe to soil solution, and on the production of CH4 in the sediments. We monitored pore-water pH, redox potential, dissolved CH4 concentrations, and availability of inorganic plant nutrients (Si, Fe, P) and As over time in flooded pots amended with 1 % w/w residue, and evaluated the abundance of genes diagnostic for methanogenesis or methanotrophy as a function of depth in each pot. Our findings are discussed in the context of global rice production and health.
Materials and methods Rice residue collection and preparation Fresh rice straw (FS), fresh rice husk (FH), rice straw ash (RSA) and rice husk ash (RHA) were obtained from Cambodia in 2011 and 2012. Rice straw was obtained from 23 sites among five provinces; see Seyfferth et al. (2014) for details. Briefly, at least three handfuls of rice straw was obtained from at least three locations in rice paddy fields or, just after harvest, from straw piles on home-owners’ farms and was pooled into a composite sample. Straw was rinsed and oven-dried at 65 °C prior to use. Straw was cut into pieces of approximately 1-cm length (FS 1-cm) and a portion of the cut straw was ground into a powder using a Wiley Mill (FS powder). Powdered straw was combusted in a muffle furnace at 450 °C at the University of Delaware to create RSA. Fresh rice husk (FH) and rice husk ash (RHA) were obtained from a rice processing facility in Cambodia, Battambang Province in 2012, where FH is combusted
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as a fuel source to drive the milling machinery leaving behind an ash residue (RHA). FH was used either whole (FH-whole) or powdered (FH powder) using a Wiley Mill. Soil collection and preparation Soil from the upper 30 cm of the profile, after removal of the organic horizon (sod) was collected from the University of Delaware Newark Farm in October 2012. The field was selected because of its similar characteristics to typical Ultisols/Acrisols found in regions of Southeast Asia, representing a weathered soil in humid regions that has undergone desilication. The soil is classified as a fine-loamy, mixed, semiactive, mesic Typic Hapludults according to the US Soil Taxonomic description, or an Acrisols according to the Food and Agriculture Organization classification system. Approximately six shallow soil pits were hand dug and the soil was placed into large plastic bags. The soil was field moist when collected and bags were left open to air dry in the laboratory air prior to use. Soil was combined into a composite sample using large plastic tubs with special care to preserve soil structure as much as possible. Subsamples of the composite soil were obtained and utilized for soil characterization and the remainder was utilized in pot experiments. Soil and rice residue characterization Total elemental analysis of Si, As, P, and Fe in the FS, FH and RHA rice residues and UD Farm soil was conducted using X-ray fluorescence spectrometry (Spectro XEPOS). Residues and soil were ground into a fine powder by filling a plastic jar with sample and five methacrylate beads and shaking with a high-energy ball mill to pulverize the sample. The fine powders were packed into sample cups and introduced into the instrument autosampler tray along with NIST certified reference material. Absolute error of the Si, As, P, and Fe measurements were at most 0.03 %, 0.1 μg g −1 , 0.001μg g−1, and 2μg g−1, respectively. Analysis of a NIST certified reference soil material (San Joaquin soil SRM 2709) gave Si, As, P, and Fe recoveries of 105, 95, 102, and 109 %, respectively, and plant material (Peach leaves, SRM 1547) gave P and Fe recoveries of 101 and 96 %, respectively; no value is indicated for Si and As was below detection in the Peach leaves.
Soil was further characterized by first sieving a subsample of composite soil to the 2-mm fraction and utilizing that subsample for various analyses per standard methods (Sparks 1996). Soil was analyzed for pH using a 1:1 soil:water slurry. BaCl2-extractable Ca, K, Mg, Mn, Na, P, S, and Si were determined by extracting 2.0 g of
