Do alternative irrigation strategies for rice cultivation decrease water footprints at the cost of long-term soil health?

To assess the impact of water-saving irrigation practices (alternate wetting and drying and mid-season drainage) on SOC, a literature search was conducted to gather articles reporting changes in SOC from split plot experiments where either of the two water-saving irrigation practices were compared against continuous flooding irrigation. The literature search was conducted within Web of Science, on 26th November 2018. The following string was used: ((rice OR paddy OR 'oryza sativa' OR 'oryza s*') AND (SOC OR 'soil carbon' OR 'soil organic carbon' OR 'soil c' OR 'organic carbon' OR 'carbon sequestrat*' OR 'C sequestrat*' OR 'soil organic C' OR 'carbon pool' OR 'carbon stock' OR 'carbon storage' OR 'soil organic matter') AND (water OR irrigat* OR 'water management' OR flooded OR anaerobic OR aerobic OR 'alternate wetting and drying' OR AWD OR 'intermittent wetting and drying' OR MSD OR 'midseason drainage' OR 'mid-season drainage')).

The search returned 1680 research articles. These articles were then sifted through a systematic title and abstract screening to exclude articles which were: (1) not related to irrigation of rice that could be equated to either alternate wetting and drying or mid-season drainage; (2) were not field based. Articles were kept in instances where it was unclear whether they met the two exclusion factors detailed above. The title and abstract screening resulted in the identification of 56 articles. These 56 articles were then assessed through reading of their content, including supplementary material, and were retained for analysis if they reported SOC concentrations for each water treatment at the end of at least one season, resulting in seven articles being carried forward into the analysis.

As changes in SOC are affected by emissions of both CH₄ and CO₂ (figure 1), a second search was conducted 5th December 2018 to identify articles reporting emissions of both gases. The search string for this search was: ((rice OR paddy OR 'oryza sativa' OR 'oryza s*') AND ('carbon dioxide' OR methane OR CO₂ OR CH₄ OR minerali* OR respiration) AND (water OR irrigat* OR 'water management' OR flooded OR anaerobic OR aerobic OR 'alternate wetting and drying' OR AWD OR 'intermittent wetting and drying' OR MSD OR 'midseason drainage' OR 'mid-season drainage')), returning 4377 results. As with the first search, articles were then sifted to exclude those that were not related to alternate wetting and drying or mid-season drainage irrigation of rice, and were not field based, identifying 59 articles. These 59 articles were then assessed and were included in the analysis if they reported cumulative CH₄ and CO₂ emissions for each water treatment at the end of at least one season. This assessment resulted in seven articles being retained for analysis.

In both searches, only articles where all other aspects of the treatments (chemical fertilization, organic amendments, residue retention, planting method, tillage, initial soil properties, etc) are the same between control and treatment, and only water management is adjusted were kept.

As we also sought to assess changes in CO_2eq emissions, which are the product of changes to CH₄, CO₂, and N₂O, from the introduction of water-saving irrigation, the second literature search was also used to identify articles reporting cumulative N₂O emissions. Although the search string did not explicitly mention key words for identifying articles reporting N₂O emissions, it was assumed that relevant articles would be captured by the search string (i.e. articles reporting N₂O emissions alongside CH₄ and CO₂ would appear in the search results).

For each identified study, primary data were collected, including initial and final SOC concentrations, BD, cumulative growing season GHG emissions, study period, experimental replication, grain yield and water use. Secondary data of potentially regulating variables were also recorded, including soil texture, pH, total nitrogen, total phosphorus, chemical and organic fertilizer additions, tillage practices, average and annual mean temperature and precipitation, and geographic coordinates. While the intention was to assess changes in SOC stocks and GHG emissions in relation to secondary variables, the limited number of articles found and the inconsistent reporting of the secondary variables in those articles prevented us from assessing their regulating effect on stocks or emissions.

In studies that ran over multiple years or seasons only SOC concentrations after the final harvest within the study were recorded. In these studies, SOC concentrations were collected for the first 20 cm of soil or to the deepest depth that could be obtained if sampling within an individual study was not conducted down to 20 cm.

The effect of alternate wetting and drying has been tested at many different drying thresholds. Here, if more than two drying thresholds were used in any single study, only results from the two least extreme drying levels were used. Where GHG monitoring was conducted over multiple seasons, the average gas flux for the monitoring period was used to avoid over-weighting results from studies which ran over more than one season compared to single season studies. As some studies tested the effect of water-saving irrigation in relation to different quantities of organic additions or chemical fertilizers, if these additions were tested in both continuous flooding and alternate wetting and drying or mid-season drainage, each level of addition was treated as a pair.

To evaluate the compound effect of the GHG emitted from flooded rice fields, i.e. CO₂, CH₄ and N₂O, we calculated CO_2eq emissions as the sum of emissions from the three individual gasses, multiplied by their respective IPCC 100-year emission factors (Myhre et al 2013):

$$\begin{eqnarray*}&&{{\rm{CO}}}_{2{\rm{e}}{\rm{q}}}={{\rm{CO}}}_{2}+\left({{\rm{CH}}}_{4}\times 28\right)+({{\rm{N}}}_{2}{\rm{O}}\times 265).\end{eqnarray*}$$

The direction of the change in C emissions is relevant to our understanding of changes in SOC. Changes in SOC stocks result from the difference between input of organic C and output of CO₂, CH₄, and dissolved organic C (not quantified here due to lack of data). To estimate C emissions on a mass basis (consistent with measured changes in SOC), CO₂ and CH₄ fluxes were converted to C-mass fluxes considering the molar ratios of CO₂ and CH₄.

Differences in soil organic nitrogen (SON) stocks between treatment pairs were estimated from SOC concentrations, bulk density (BD), and C:N ratios provided in the studies for each treatment. SOC stocks were estimated using reported SOC concentrations and estimated BD. When BD was not reported, the average value from all studies (1.3 g cm⁻³) was used. Despite some variability in the organic matter C:N ratio across studies (6.6 to 12.4 g C/g N), changes across treatments were small and inconsistent. Therefore, in the three studies lacking information on SON, we used the average C:N ratio from all reporting studies.

The effect of 30 year (1987–2016) mean annual precipitation (P), potential evapotranspiration (PET), temperature (T) on SOC changes was also explored. Mean annual climatic variables were obtained using the CRU TS 4.01 P, T, and PET gridded data products (Harris et al 2014), and reported site locations.

For each study, response ratios, i.e. the ratio between treatment (alternate wetting and drying or mid-season drainage) and control (continuous flooding), were calculated for SOC concentrations, cumulative seasonal GHG emissions (CH₄, CO₂, N₂O), CO₂ equivalent (CO_2eq), and total C emissions (the sum of C in CH₄ and CO₂ fluxes). The effect size of the water management treatments was then determined through the natural logarithm of the response ratio, R:

$$\begin{eqnarray*}&&{\rm{ln}}\,{R}=\,{\rm{ln}}\left(\displaystyle \frac{{{x}}_{{\rm{t}}{\rm{r}}{\rm{e}}{\rm{a}}{\rm{t}}{\rm{m}}{\rm{e}}{\rm{n}}{\rm{t}}}}{{{x}}_{{\rm{c}}{\rm{o}}{\rm{n}}{\rm{t}}{\rm{r}}{\rm{o}}{\rm{l}}}}\right).\end{eqnarray*}$$

The arithmetic mean and 95% confidence interval were then calculated, where the confidence interval is based on Student's t-distribution due to the small sample size. Results were then back-transformed, taking the exponent of the arithmetic mean and confidence interval, giving the geometric mean and its corresponding confidence interval. This allowed for results to be expressed as percent change, here termed effect size. All studies collected had between three and four replicates, but neither standard deviation nor standard error were consistently reported. In other meta-analyses where standard deviation, standard error or the number of replications were not consistently reported, equal weights were given to each sample set (Johnson and Curtis 2001, Gopalakrishnan et al 2014, Qin et al 2015, Han et al 2016). The same approach was followed here.

Only seven studies were found which reported CO₂ alongside CH₄ emissions in paired rice fields under different water managements. All studies demonstrated a consistent trend of increased CO₂ emissions coupled with decreased CH₄ emissions. Despite the limited number of data points, the change in CH₄ emissions (−52.3%) is consistent with those reported elsewhere (e.g. approximately −50%, in the meta-analysis by Sanchis et al (2012), and −53% by Jiang et al (2019)). This consistency suggests that, while little work has been done on CO₂ emission responses to water-saving irrigation (Yang et al 2017), the mean 44.8% increase in CO₂ emissions found here may be a reasonable estimate also for other sites. However, the mean increase in N₂O emissions in this study is lower than the mean increase of 102%, but within the confidence interval, found by Jiang et al (2019). The difference across studies may be due to the highly episodic nature of N₂O emissions not being captured by the relatively coarse gas measurement designs of the seven studies used within this meta-analysis. Only two of the seven studies conducted gas samping on a daily basis, with the others being between twice per week and every 10 d. Within the five studies that sampled less than once per day, only two intensified the sampling during fertilization or during drying events. Therefore, these studies may have not captured periods of high N₂O emissions.

Water-saving irrigation reduces emissions of CH₄, having a benifical effect on total GHG emissions. However, this benificial effect is partially negated through increased emissions of CO₂ and N₂O. Shifting from continuous flooding to alternate wetting and drying or mid-season drainage reduces water demand and global warming potential. While both N₂O and CO₂ emissions increase and off-set some of the benefits of reduced CH₄ production, water-saving irrigation reduced CO_2eq emissions (figure 3(a)). Nitrous oxide has the greatest warming potential of the three reported gases, but actual emissions was very low and not sufficient to negate the benefit of CH₄ reductions. Conversely, while the increase in actual emissions of CO₂ was much greater than the reduction in CH₄, the greater warming potential of CH₄ resulted in a more marked effect on the CO_2eq emissions than CO₂. Therefore, from a greenhouse warming perspective, switching from continuous flooding to alternate wetting and drying or mid-season drainage potentially reduces the impact of rice production on climate change.

Recent work has criticized the use of CO_2eq in policy setting (Allen et al 2018). Short-lived and long-lived GHGs affect the trajectory to, and recession from, peak warming. The replacement of CH₄ emissions with those of CO₂ and N₂O does not necessarily provide a positive outcome in regards to mitigating warming. Therefore, while CH₄ reductions have often been highlighted as a benefit of water saving irrigation, this conclusion can only be made with full consideration of the conseqences on present and future emissions. As such, care must be taken in stating the positive benefits of CH₄ reductions in regard to climate change mitigation.

Maintaining yields and reducing the global warming potential, while simultaneously reducing water inputs (up to about 30%; Carrijo et al 2017) are all positive consequences of improved water management. The increased emission of CO₂ seen in figure 3(a) was not large enough to negate the positive effect that water-saving irrigation has on reducing CO_2eq emissions, but it signals a departure between the SOC concentrations in continuous flooding and alternate wetting and drying or mid-season drainage fields. As the gaseous C loss is mainly due to CO₂, the soil-to-air C flux under water-saving irrigation was significantly increased (figure 3(b)), suggesting a reduction in SOC concentration compared to continuous flooding. This is further confirmed by the 5.2% decrease in SOC concentration shown in figure 3(b).

Due to limited reporting of changes in BD in the selected studies, it was not possible to calculate changes in SOC stocks. Yet, the observed increases in CO₂ emissions suggest that, regardless of changes in BD, water-saving irrigation can result in either an absolute reduction in SOC stocks (rice fields becoming a C source) or a reduction in C sequestration rates compared to flood irrigation (becoming a weaker C sink). The consequence of this depends on the severity of the drying, the state of the SOC stock, additions of organic and chemical fertilisers, and the C storage potential. As SOC concentrations follow saturation curves, which in turn are dependent on the above stated factors, the carbon loss or gain rate slows as the system approaches a new equilibrium. In all the reviewed studies, alternative irrigation approaches have been adopted for between 1 and 17 years. Despite the short duration of most studies, the mean reduction in SOC under alternate wetting and drying or mid-season drainage is statistically significant, suggesting that even larger SOC losses may occur in the long-term. However, the overall long-term consequence is site specific, making it difficult to anticipate whether long-term trends in SOC will begin to have an effect on yields.

The significant effect of precipitation on effect size may be a consequence of the native saturation capacity of the soils at the study sites. SOC has been shown to be positively and linearly correlation with precipitation (Chang et al 2014). Flooded rice production may have caused larger accumulation of SOC in relatively drier sites, where the background SOC was lower. Thus, the removal of flooding and the establishment of water-saving irrigation practices may have a more marked effect on SOC loss in these locations, where SOC in paddy fields is disproportionally higher than in non-irrigated land.

Flooded rice systems enable the accumulation of SON (Ladha et al 2011), which may be lost through leaching in water-saving irrigation (Tan et al 2013). The approximate calculation of organic N losses (138 kg N/ha per year on average when assuming no increase in BD) highlights the risk of losing large amounts of nutrients that were chemically bound in organic matter when continuous flooding is substituted by water-saving irrigation. In low fertilization systems that rely on N supply from soil organic matter, a part of the lost nutrient stocks could have supported crop growth. Further, where the loss will result in leaching, increased N loads to water bodies may cause eutrophication. However, leaching may also be reduced due to decreased percolation in water saving irrigation compared to continuous flooding (Peng et al 2011).

While potentially being beneficial for reducing the global warming potential of rice production, the impacts of improved water management may extend beyond the other commonly reported benefit of water saving. The introduction of aerobic conditions during the growing period alters soil processes and chemistry. The consequences of these changes need to be fully understood to determine the sustainability of improved water management. Here, we highlight the consequences of water-saving irrigation on SOC and the impact this may have on long-term production. These long-term consequences may not be immediately experienced, but attention should be paid to their potential impact.

While few studies have assessed changes in SOC as a result of alternate wetting and drying or mid-season drainage, much attention has been placed on the capacity of these water management strategies to reduce water consumption without sacrificing yields. The meta-analysis by Carrijo et al (2017) found that mild-alternate wetting and drying increased water productivity (grain yield per unit water) by 24% without significant decrease in yields. The same study also identified that the effect of water management is dependent on soil characteristics and the severity of soil drying. Notably, SOC concentration played a role in limiting yield reductions, where the impact of alternate wetting and drying on yield was lower when SOC concentrations were above 1%. Elsewhere, SOC has also been positively correlated to yield (Pan et al 2009), with paddy rice systems often promoting SOC sequestration (Pan et al 2010). Conversely, cessation of paddy rice production has also been linked to a rapid loss of SOC (Desjardins et al 2006, Chen et al 2017). Our results and this evidence point consistently to water-saving irrigation causing reduction in plant-available nutrients, which may, in the long-term, contribute to yield declines.

To counteract SOC and nutrient losses, management strategies could be adjusted to increase the amount of biomass being returned to the soil. This goal can be achieved via additions of crop residues (Liu et al 2014), organic amendments (Kätterer et al 2011), chemical fertilizers (Geisseler et al 2017, Poeplau et al 2018), and changes to tillage practices (Zhang et al 2014). However, without first recognizing and quantifying the risk of SOC loss, appropriate countermeasures cannot be identified.

Consequences of water-saving irrigation extend beyond soil properties. There are clear short-term benefits to individual farmers from improved water productivity. There are also site-specific benefits, such as reduced lodging (Yamaguchi et al 2017), and community benefits through more equitable water allocation between farms (Lampayan et al 2015). Conversely, there are potentially long-term effects to the ability of soils to maintain yields, due to the slow response of these soils to changes in land management. Moreover, flooded rice fields are multi-functional systems that provide, for example, habitats for fish species—a saleable protein source and pest control (Berg et al 2017). These multi-functional benefits may be lost through increased management of water resources resulting from water-saving irrigation, but these long-term effects—as those for soil fertility—are still poorly characterized.