Arsenic in Rice Soils and Potential Agronomic Mitigation Strategies to Reduce Arsenic Bioavailability： A Review

Soils used for rice （Oryza sativa L.） cultivation in some areas contain high concentrations of arsenic （As）due to irrigation with groundwater containing As and intensive use of agrochemicals or industrial residues containing As. To restrict rice uptake of As in these soils, approaches to reduce As input and bioavailability must be considered. One approach to reduce As input into rice soils or uptake by rice is cultivating rice under aerobic, intermittent flooding, or alternate wetting and drying （AWD） conditions, rather than in submerged soils, or use of irrigation water low in As. For reducing As bioavailability in soil, aerobic or AWD rice culture and application of biochar, sulfur （S）, and/or rice polish to soil are promising. Moreover, use of As-hyperaecumulating plant species （e.g., Pteris vittata L.） in rotation or combinations with favourable plant species （e.g., Azolla, Chlorella, or Nannochloropsis species） can also be promoted, in addition to using rice cultivars that are tolerant to As. Though applications of high doses of phosphorus （P）, iron （Fe）, and silicon （Si） fertilizers have shown promise in many instances, these methods have to be practiced carefully, because negative effects have also been reported, although such incidents are rare. Major factors affecting As speciation and bioavailability in soil are chemical properties such as redox status, pH, and Fe, P, Si, and S concentrations, physical properties such as texture and organic matter, and biological properties such as methylation activity by soil microorganisms. However, as many of these factors interact, long-term examination under field conditions is needed before measures are recommended for and implemented in farmers＇ fields.

Nutrient resorption from senescing leaves of epiphytes,hemiparasites and their hosts in tropical forests of Sri Lanka

Aims Epiphytes and hemiparasites do not have direct access to soil nutri-ents.Epiphytes acquire nutrients through symbiosis,foliar leachates and throughfall,whilst hemiparasites have specialized structures(haustoria)to acquire nutrients from their host.Irrespective of the green leaf nutrient concentrations of epiphytes,hemiparasites and their hosts,nutrient-resorption efficiency and proficiency are expected to be the greater in epiphytes than in their hosts and in hemiparasites.These hypotheses were tested.Methods Green and senescing leaves of epiphytes(n=23),hemiparasites(n=9)and their hosts were collected,and leaf area,leaf dry weight,nutrient(nitrogen-N,phosphorus-P,potassium-K,calcium-Ca)and sodium(Na)concentrations were measured,and resorption effi-ciency(%)and proficiency were calculated.Important Findings Concentrations of N,P,K,Ca and Na in green leaves of epiphytes and hemiparasites were either similar or greater than those of their hosts,except for lower green leaf N concentration in hemiparasites.Epiphytes resorbed N,P,K and Na,while their hosts resorbed only N,P and K.Hemiparasites and their hosts resorbed P,K and Na while N was not resorbed.Overall,resorption efficiency was great-est in epiphytes>hemiparasites=hosts,while the resorption pro-ficiencies were similar or greater for epiphytes and hemiparasites compared with their hosts,except for N in hemiparasites.Relatively high nutrient concentrations in epiphytes were associated with greater resorption efficiency.Understanding contrasting nutrient concentrations in epiphytes,hemiparasites and their hosts will be important in recognising their contribution to ecosystem nutrient cycling.