Sustainable rice cultivation: a protocol of submergence
Preliminary results of an experiment carried out in Lomellina indicate that an increase in dry periods leads to a reduction in methane emissions and decreased arsenic uptake by the rice grain
Italian rice farming has always had a relevant environmental impact, mainly because of the peculiar water management, which is typical of it. The submerged cultivation, with the consequent condition of soil anaerobiosis, involves the emission of significant quantities of methane which, being a powerful greenhouse gas, contributes to the increase of global warming. Recent studies conducted in Italy, specifically in Lomellina, have shown that methane emissions are responsible for 40-55% of the overall impact of rice cultivation in terms of global warming (carbon footprint). These emissions can be reduced through an increase in dryings or irrigation solutions, limiting the presence of a permanent water layer in the field so as to improve the diffusion of oxygen in the soil.
Submergence also increases the bioavailability of heavy metals: among the most dangerous there are arsenic (As) and cadmium (Cd). Usually, soil submergence reduces the Cd content in different parts of the plant, whereas the increase of anaerobic conditions in rice fields leads to a higher As uptake. On the contrary, the increasing of dryness may lead to a worsening of the quality as far as Cd content is concerned. As with methane, careful water management can influence the uptake of As and Cd by rice plants.
In this regard, the PSR project "BESTsomRICE", funded by GAL Lomellina, is designed to develop a protocol for managing submergence in the paddy field to reduce the environmental impact of paddy production.
Experimental scheme and life cycle analysis
The analysis was carried out through field tests and applying the Life Cycle Assessment (LCA) methodology. The LCA, established by two ISO standards, is the most widely adopted assessment approach for evaluating the environmental impact of a product or process: LCA enables the conversion of the number of production factors consumed and emissions into the environment into a limited number of environmental impact indicators, such as the carbon footprint.
Thanks to the LCA, the environmental impact of the cultivation of two varieties of rice (Carnaroli and Caravaggio) has been evaluated, considering two different management of submergence, the traditional one (control) and an alternative one, characterized by an additional drying introduced during the stem elongation, but before the booting. Studies carried out previously have, in fact, shown how this time is the best to decrease the absorption of As, while limiting the increase of Cd.
During the agricultural season of 2020, preliminary trials were carried out in three different farms in Lomellina. Each variety was cultivated in two adjacent plots with the same cultivation technique, thus changing only the water regulation. In the first farm, the Carnaroli variety was analyzed; in the second, Carnaroli and Caravaggio were in the third only Caravaggio.
For the LCA study, 1 ton of paddy rice at a commercial humidity of 14% was chosen as the functional unite to quantify the environmental impact. The production cycle phases considered were from tillage to drying of the paddy rice.
Through farm visits and field trials, appropriate information about the cultivation technique and yields of individual plots were collected, whereas methane emissions and emissions of nitrogen and phosphate compounds mainly related to fertilization were estimated. Precisely, methane emissions were estimated following the methodology suggested by the IPCC, the UN agency dealing with climate change, considering the quantity and type of organic fertilizers and straw buried, the number of dryings, and the duration of submergence.
In 3 cases out of 4, the yields are not significantly influenced by applying the alternative protocol. Small increases in yield were observed in farm 1 for Carnaroli and farm 3 for Caravaggio (+1.3% and +3.1%, respectively), while a decrease of 4.9% was observed in farm 2 for Caravaggio. The only exception occurred in farm 2, where Carnaroli showed an 18.8% decrease in yield in the treatment compared to the control, mainly due to the presence of a large number of weeds.
Both for the traditional cultivation and the one with the additional dry, the Caravaggio variety cultivated in farm 3 showed the best results. The only exception was the impact related to the emissions resulting from the use of fertilizers; in fact, it is in this farm that the highest quantities of nitrogenous fertilizers were distributed.
A comparison of production and environmental performances
The carbon footprint ranges from 743 kg CO2 eq at best to 1659 at worst. It is impossible to distinguish which variety has the most negligible impact, but it is clear that the addition of dryness allows a significant reduction in climate change.
Variations in the other environmental effects depend mainly on the yield. It should be remembered that methane causes impacts only for carbon footprint and smog formation (POF). For Carnaroli in farm 2, the significant drop in production causes a non-negligible increase in impact for OD (+13%), HT-noc (+16%), HT-c (+13%), POF (+6%), FE (+28%), FEx (16%) and MFRD (+16%). However, it is worth noticing that in Carnaroli in farm 1 and Caravaggio in farm 3, the solution with an additional dryer allows reducing the carbon footprint and all the other environmental impacts.
Arsenic and cadmium content
In the dehulled grain, reductions in arsenic content were measured as 31% and 14% for Carnaroli and 52% and 16% respectively for Caravaggio. In 3 cases out of 4, cadmium increased (with variations from 66% to 97%). Still, the absolute terms remained very low compared with the limit currently imposed by law (0.20 mg/kg) and the more restrictive future limit of 0.15 mg/kg.
Although the results are preliminary, they show that adding an additional dryer during the raising phase of the crop is an effective strategy to mitigate the carbon footprint of rice cultivation without compromising quantitatively and qualitatively the production. Moreover, the alternative management of submergence allows decreasing the As content in paddy rice while keeping the Cd content well below the legal limits.
Future trials foresaw by the PSR project "BESTsomRICE" will hopefully allow us to confirm the promising indications obtained so far. The fine-tuning of cultivation practices aimed at reducing the carbon footprint is constantly evolving and must consider that, sometimes, promising techniques such as minimum tillage can be limited by regulatory choices such as the one that has banned the use of certain herbicides, because they are particularly harmful to other aspects (e.g., protection of biodiversity).