1a. Objectives (from AD-416):
This study will determine: 1) if rice cultivars differ in grain arsenic accumulation, 2) how do changes in soil chemistry as a result of organic or conventional management practices relate to arsenic accumulation in rice grain, 3) to what extent does milling reduce grain arsenic content as compared to brown rice, and 4) how do different rates and organic fertilizer amendments impact soil chemistry and grain arsenic accumulation.
1b. Approach (from AD-416):
Grain and soil samples from studies conducted previously under organic and conventional management practices will be analyzed for total arsenic content. Soil samples will also be analyzed for pH, organic matter, and mineral content. Treatments include rice varieties, fertilizer amendment products, fertilizer rates, and years. Grain samples will include whole grain brown rice and whole grain milled rice. Grain arsenic and soil chemistry assays will be performed through a commercial lab. On samples that are high for total grain arsenic, speciation analysis will be performed to determined inorganic and organic forms of arsenic. Results will be used to determine to what extent choice of variety, post-harvest processing, and fertilizer amendments can be used to minimize grain arsenic content under organic production systems.
3. Progress Report:
Recent reports have indicated that rice when grown under anaerobic flooded field conditions can accumulate high levels of grain arsenic. It is known that factors such as pH, organic matter, and soil microbial populations can influence how available naturally occurring arsenic in the soil is for plant uptake. To provide the needed nutrients for production of organic crops, effort is made to improve soil health by increasing organic matter and soil microbes. A study was conducted to determine if grain arsenic accumulation differed in rice grown under organic versus conventional management systems. Nineteen rice varieties were evaluated over three years in field trials conducted using organic and conventional production methods. The relative amount of grain arsenic between the two management methods differed in each of the three years. On average the organic fields had higher grain arsenic but this was largely due to much higher levels in the third year. In that year, grain yield under the organic system was also greater than under conventional management. Plant maturity and yield were positively correlated with grain arsenic accumulation under organic management, but not under conventional management. Since irrigation is critically important for weed control under organic management where herbicides are not used, the rice plants are under anaerobic flooded field conditions longer than under conventional management. Anaerobic conditions are known to be conducive for making arsenic available to the rice root system. Thus, it appears that organic management practices that reduce weeds and increase soil fertility may be counter-productive for reducing grain arsenic. However, it was observed that rice varieties differed in the amount of arsenic accumulated in the grain. The variety Sierra had significantly lower grain arsenic than others. This indicates that choice of variety may be one method of minimizing exposure to arsenic in organically produced rice.