Irrigation as an Effective Way to Increase Potato Yields in Northern China: A Meta-Analysis

Abstract

A meta-analysis was conducted with the aim of exploring the influence of irrigation on potato yield, evapotranspiration (ET), and water-use efficiency (WUE) in northern China, considering factors such as irrigation methods, growing region, irrigation water-use efficiency (IWUE), irrigation frequency, soil types, and nitrogen (N) fertilizer rate. Overall, irrigation significantly increased potato yield and ET by an average of 45 and 54% compared to non-irrigation, respectively, but did not significantly increase the WUE. The increase in potato yield under irrigation is the most evident in aeolian sandy soil in northeast China and northwest China. Drip irrigation demonstrated the highest positive impact on both yield and WUE. Optimal yields were achieved with an irrigation amount ranging from 100 to 200 mm, while the highest WUE was observed with an irrigation amount of 30–50 mm. When the amount of irrigation exceeded 100 mm, the irrigation significantly resulted in decreased WUE compared to non-irrigation. The relative increase in yield per unit of irrigation amount and IWUE gradually decreased and eventually stabilized when the irrigation amount exceeded 100 mm. Therefore, the yield and WUE perform best when the irrigation amount is below 100 mm and the irrigation frequency is less than three times for sprinkling and flood irrigation methods. The greatest increases in yield and WUE under irrigation were found under a moderate N rate (150–250 kg N ha⁻¹). Additionally, the relative increase in yield per unit of irrigation amount decreases gradually as the organic matter content increases. These findings suggest that the optimal benefits from irrigation might be realized when the irrigation amount is below 100 mm, with a moderate nitrogen fertilizer application and an irrigation frequency of three times. However, it is essential to consider local environmental factors such as the growing region, soil types, and organic carbon content.

1. Introduction

Water scarcity and frequent droughts present major hurdles for sustainable agricultural growth, especially in arid regions. Growing demands for water in cities and industries put further pressure on agricultural water supplies. Wasteful irrigation practices exacerbate the issue, depleting regional water resources even faster. As water availability shrinks, areas facing water scarcity tend to use the limited water more efficiently. Prioritizing efficient irrigation methods is crucial for maximizing economic yields and safeguarding water resources for the future [1,2,3,4]. The agricultural sector faces a crucial challenge: boosting food production while minimizing water use. This necessitates improving crop WUE (expressed by dividing yield by evapotranspiration), the ratio of yield to evapotranspiration. Optimizing water use for maximum benefits with minimal irrigation is the driving force behind ongoing research efforts. China’s recent construction of water conservancy infrastructure, including inter-regional water transfer projects, demonstrates its commitment to solving water scarcity by redistributing resources from wetter regions to drier regions. To combat water scarcity in arid regions, especially in potato farming, advanced irrigation methods offer a crucial solution. These methods prioritize high water efficiency and can substantially improve overall IWUE (expressed by dividing yield by irrigation amount). However, implementing these new technologies requires addressing the research gap in water-saving techniques currently plaguing many such regions, where traditional, wasteful irrigation practices remain common [5].

Potatoes play a crucial role in feeding the world, ranking fourth among the most-consumed food crops after maize, wheat, and rice [6,7,8]. China, has been a leading potato producer since 1993, contributing roughly 27% of the global planting area and 24% of the total yield [8,9]. This starchy staple has played a crucial role in feeding China’s growing population, especially since the 2015 policy promoting its consumption [10]. However, despite this growth, China’s average potato yield (18 tons/ha) falls behind other countries (30–70 tons/ha) and suggests untapped potential for resource efficiency improvements [11,12,13,14,15,16]. China’s diverse potato production spans four distinct agroecological regions: the dominant north single (NS) planting region (49%), the central double (CD) planting region (39%), the south winter (SW) planting region (7%), and the southwest mixed (SWM) planting region (5%) [7,8,17]. However, potato yield in these regions depends heavily on soil, climate, and water availability. Potatoes typically need 400–800 mm of water during their growth season [5], making them vulnerable to water deficits. Short-term droughts, especially in northern China’s semiarid regions, can significantly reduce yields if their water demands are not met. Given this challenge, irrigation emerges as a crucial technology for boosting potato production, particularly in arid and semiarid areas. Prior research on potato irrigation has primarily focused on identifying the optimal timing [1,16] and methods [1,18,19,20,21,22,23,24,25] for water application. However, further research is needed to address water efficiency and resource utilization across these diverse regions with their unique water challenges.

While acknowledging the significance of irrigation in arid regions [18,19], research reveals the complexity of maximizing potato yield. Optimal timing and water productivity appear to vary greatly, with conflicting evidence between tuberization and vegetative stages [20,21]. Additionally, local precipitation patterns significantly influence yield response to limited irrigation [19]. While advanced methods like sprinklers and drip irrigation demonstrate promising gains in yield and water efficiency compared to traditional approaches [1,26,27], a critical question remains unanswered: what is the optimal national or regional strategy for maximizing potato productivity, ET, WUE, and IWUE under different irrigation methods? Comprehensive research to address this gap is urgently needed. To uncover the key factors influencing potato growth in northern China’s diverse landscapes, this research uses a robust statistical method called meta-analysis, which combines findings from various field experiments. The study aims to bridge the knowledge gap by (1) analyzing how water application, the chosen irrigation method, region, soil type, and nitrogen fertilizer all impact potato yield, water-use efficiency (ET), and (WUE and IWUE); and (2) studying how the amount and frequency of irrigation affect these crucial metrics. We hypothesize that irrigation will enhance yield while maintaining WUE compared to dryland cultivation (non-irrigated conditions).

2. Materials and Methods

2.1. Data Search and Collection

A comprehensive literature review was carried out utilizing keywords such as potato, irrigation, productivity and WUE. Our search was limited to publications before July 2022 and we utilized databases like Google Scholar, Springer, ISI-Web of Science for English language data, while Chinese language data were acquired from Baidu Scholar, and China National Knowledge Infrastructure (CNKI). To ensure an unbiased selection, we included only field studies with irrigation and control groups and readily available data for key variables (means, standard deviations/errors, and sample sizes). This rigorous process yielded 356 observations from 19 research papers, forming the foundation of our study (Figure 1). To account for the diverse geography, climate, and cropping systems in northern China’s potato cultivation, we divided the study area into three distinct regions: (1) Northeast, (2) Northwest, and (3) North-central China. We further categorized nitrogen fertilizer application as low (<150 kg N ha⁻¹), medium (150–250 kg N ha⁻¹), and high (>250 kg N ha⁻¹). Additionally, reflecting the region’s diverse soil types, we classified them as aeolian sandy soil, chestnut soil, sierozem, chernozem, meadow soil, heilu soil, and huangmian soil [28]. Soil organic carbon content was categorized into six ranges as follows: <0.5, 0.5–1.0, 1.0–1.5, 1.5–2.0, 2.0–2.5, and >2.5(%). Finally, we distinguished four irrigation methods: flood, furrow, sprinkler, and drip irrigation.

2.2. Statistical Analysis

Irrigation water-use efficiency (IWUE) was calculated as:

$$\mathrm{I}\mathrm{W}\mathrm{U}\mathrm{E}=\frac{\mathrm{Y}\mathrm{t}}{I}$$

(1)

where Yt represents the potato yield achieved when irrigation is applied, while I represents the amount of water under irrigation treatment.

Relative yield increase (RYi) quantifies the increase in potato yield due to irrigation by comparing yields under irrigation treatment (Yt) and yields under non-irrigation treatment (Yck) that could be calculated from the obtained data. Relative yield increase and relative yield increase per unit of the irrigation amount were calculated as:

$$\mathrm{R}\mathrm{Y}\mathrm{i}=\left(\mathrm{Y}\mathrm{t}-\mathrm{T}\mathrm{c}\mathrm{k}\right)$$

(2)

$$\mathrm{R}\mathrm{Y}\mathrm{e}=\frac{(\mathrm{Y}\mathrm{t}-\mathrm{T}\mathrm{c}\mathrm{k})}{I}$$

(3)

where RYi is the relative yield increase, RYe is the relative yield increase per unit of irrigation amount (kg mm⁻¹ ha⁻¹) and Yck is the potato yield under non-irrigation treatment.

To synthesize and evaluate the impacts of irrigation on potato productivity and WUE in northern China, we conducted a meta-analysis. Borrowing from established methodology [7,29,30,31], we expressed the influence of irrigation on these variables as the natural logarithm of the response ratio (lnR)—a statistical measure of effect size.

$$\mathrm{l}\mathrm{n}R=\ln \left(\frac{X_T}{X_C}\right)=\mathrm{l}\mathrm{n}{X}_T-\mathrm{l}\mathrm{n}{X}_C$$

(4)

where X_T is the average of the potato yield, ET, or WUE in the treatment and Xc is the average of the potato yield, WUE, or ET in the control group. If the lnR > 0, lnR < 0, or lnR = 0 for the tested variable, it indicates that the treatment had a positive, negative, or neutral effect on crop yield, ET and WUE, respectively.

The variance (V) of each lnR for each study was calculated as follows:

$$V=\frac{{S_T}^2}{N_T {X_T}^2}+\frac{{S_C}^2}{{N_C X_C}^2}$$

(5)

where S_T and S_C are the treatment and control group’s standard deviations, respectively, and N_T and N_C represent the number of replicates in the treatment and control group, respectively.

The weighting factor (W_i) and weighted response ratio (lnR₊) were calculated as follows [31,32]:

$$\mathrm{l}\mathrm{n}{R}_{+}=\frac{{\sum }_{i=1}^k{{ W}_i}^{\prime } {\mathrm{l}\mathrm{n}R}_i}{{\sum }_{i=1}^k{{ W}_i}^{\prime }}$$

(6)

$${w_i}^{\prime }=\frac{1}{V_i}$$

(7)

where the weighting factor W_i′ is the reciprocal of the total variance of lnR_i.

We first checked if the data followed a normal distribution using the Kolmogorov–Smirnov test. If not, we relied on alternative parameter estimation methods for analysis. We then assessed the influence of supplemental irrigation and various irrigation methods on each variable via a random model with 9999 iterations, performed using the MetaWin 2.1 software (Systat Software, Inc., San Jose, CA, USA). We considered a treatment to have a significant effect (increase or decrease) compared to the control group (p < 0.05) if its 95% confidence interval did not overlap with zero. Similarly, we determined significant differences between means for categorical variables if their 95% bootstrapping confidence intervals (Cis) did not overlap [7,17,31].

3. Results

3.1. The Comprehensive Impact of Potato Yield, ET, and WUE on Irrigation in Northern China

On average, the yields were 17.6 and 25.3 Mg ha⁻¹ for the irrigation and non-irrigation controls, respectively. The corresponding ET values were 258 and 430 mm under irrigation and non-irrigated conditions, while the WUE was 99.2 and 65.3 kg mm⁻¹ ha⁻¹, respectively. Irrigation resulted in a significant increase in yield and ET by 44.6% and 54.3%, respectively, compared to non-irrigation. However, there was no significant increase in WUE with irrigation, and the WUE under irrigation exhibited a range from −9.2% to 16.4% compared to non-irrigation (Figure 2).

3.2. Response of Potato Yield and WUE to Irrigation as Affected by Irrigation Methods

The effect of irrigation on potato yield and WUE varied with irrigation methods (Figure 3). In comparison to the non-irrigated control, the positive influence of irrigation on potato yield was the most pronounced or drip irrigation (44.2%), followed by sprinkler irrigation (34.7%), and was the least significant for flood irrigation (30.6%). Sprinkler irrigation and flood irrigation led to significant increases in ET of 170.0% and 114.9%, respectively, compared to non-irrigation. However, overall, ET increased by 11.3% (7.6–16.7%) under irrigation compared to the non-irrigated control. Compared to non-irrigation, sprinkler irrigation, and flood irrigation significantly decreased WUE by 24.3% and 25.4%, respectively, but drip irrigation significantly increased WUE by 4.78% (Figure 3).

3.3. Response of Potato Yield and WUE to Irrigation as Affected by Growing Region, Soil Types and Organic Carbon Content

The influence of irrigation on potato yield and WUE in northern China exhibited unpredictability across different growing regions (Figure 4). The most notable increase in yield due to irrigation was observed in Northeast China and Northwest China, showing a substantial 71.6% improvement, followed by a 27.6% increase in North-central China. In North-central China, irrigation led to a significant enhancement in the WUE of potatoes compared to the non-irrigated control. Additionally, no significant difference in WUE was observed between the irrigation and non-irrigated control in Northwest China. In this region, irrigation resulted in variations in ET ranging from 15.3% to 39.7%, and WUE ranging from −2.4% to 32.1% when compared to the non-irrigated control (Figure 4).

Moreover, the impact of irrigation on potato yield, ET, and WUE in the northern region of China demonstrated variations based on soil types (Figure 5). In general, the changes in yield align with the trend of ET alteration, where a greater increase in ET corresponds to a more significant increase in yield. Within similar ET ranges, irrigation led to a significant improvement in the WUE in chestnut soil and sierozem. However, the WUE was slightly reduced by 14.2% (−34.9–18.5%) in huangmian soil compared to the control without irrigation (Figure 5). The positive impact of irrigation on potato yield varied with the organic carbon content (Figure 6). Regression analysis shows that the relative yield increase per unit of irrigation amount gradually decreases with the increase in organic carbon content.

3.4. Response of Potato Yield and WUE to Irrigation as Affected by N Fertilizer Rate and Irrigation Amount

In northern China, the impacts of potato yield, ET, and WUE on irrigation were distinct depending on the amount of nitrogen fertilizer applied. The highest yield and WUE increase were observed when less than 250 kg N ha⁻¹ was applied (Figure 7). A high N rate (>250 kg N ha⁻¹) reduced the beneficial effect of irrigation on the yield and WUE. On average, the greatest increase in yield was found for 150–250 kg N ha⁻¹ (43.5%), followed by 0–150 (37.9%) and >250 kg N ha⁻¹ (21.6%). The WUE under irrigation was significantly increased by 17.8, 18.6 and 10.9% for low (<150 kg N ha⁻¹), medium (150–250 kg N ha⁻¹) and high (>250 kg N ha⁻¹) N rates, respectively, when compared to the non-irrigated control.

The irrigation amounts significantly affected the yield, ET, and WUE of the potato (Figure 8). As the irrigation amount increases, the change in yield of irrigation treatment first increases and then decreases. The relative yield reached its maximum value within the irrigation range of 100–200 mm. The ET value gradually increases as the irrigation amount increases. When the irrigation amount increased within the range of 30–300 mm, the ET of the irrigation treatment significantly increased from 11.9% to 88.2%. When the irrigation amount exceeded 400 mm, the ET of irrigation significantly increased by 184%. The WUE change reached its maximum value within the irrigation range of 30–50 mm. When the irrigation amount was between 50 and 100 mm, the WUE of irrigation no longer significantly increased. Conversely, when the irrigation amount exceeded 100 mm, irrigation significantly reduced the WUE by 26.5–49.5%.

The associations between irrigation amount and the relative increase in yield, relative increase in yield per unit of irrigation amount, and irrigation water-use efficiency are depicted in Figure 9. Consistent with the findings in Figure 8, the increase in yield ceased to be significant when the irrigation amount surpassed 200 mm. The correlation between irrigation amount and both the relative increase in yield per unit and irrigation water-use efficiency can be characterized by an exponential function. Within the irrigation range of 0–100 mm, there was a steep decline in the relative increase in yield per unit of irrigation amount and irrigation water-use efficiency as the irrigation amount increased. However, when the irrigation amount exceeded 100 mm, the reduction in the relative increase in yield per unit of irrigation amount and irrigation water-use efficiency gradually approached a plateau, reaching a stable value.

3.5. Response of Potato Yield and WUE to Irrigation as Affected by Irrigation Frequency

In northern China, the impacts of potato yield, ET, and WUE on irrigation were distinct based on irrigation frequency (Figure 10). Irrespective of the irrigation amount, the positive impacts of irrigation on potato yield initially increased and then decreased with the rise in irrigation frequency. The maximum yield effect was achieved when the irrigation frequency was 3–4 times. However, with the increased irrigation frequency, potato ET also increased under the irrigation frequency of 3–4 times, leading to a decline in WUE.

For a precise examination of the influence of irrigation frequency on yield, we conducted a reanalysis using data with the same irrigation amount but varying irrigation times. The findings revealed that when the irrigation amount remained below 50 mm, an increase in irrigation frequency led to a gradual increase in yield. Conversely, the WUE exhibited the opposite trend, with no significant increase even when the irrigation frequency reached three times (Figure 11). With an irrigation amount in the range of 50–100 mm, the positive impact of irrigation on yield initially increased and then decreased, reaching its highest proportion of increase when the irrigation frequency was three times. Similarly, with an irrigation frequency of three times, the WUE gradually decreased (Figure 11).

4. Discussion

4.1. Overall Response of Potato Yield and WUE to Irrigation

Irrigation has been shown to have a substantial positive impact on potato yield [16]. In this meta-analysis, the initial hypothesis was that additional irrigation would enhance the potato yield in northern China without compromising WUE. In line with previous studies [33,34,35,36], our findings indicate that irrigation indeed increased both potato yield and ET without adversely affecting WUE. However, it is noteworthy that while irrigation led to an increase in potato yield, there is a potential risk of diminishing WUE due to a significant rise in evapotranspiration, particularly when the irrigation amount is high [37,38].

4.2. Effect of Irrigation Methods and Growing Region on Potato Yield and WUE under Irrigation

Conventional irrigation methods such as flood or furrow irrigation, while low in input costs, suffer from significant water loss, leading to diminished water utilization efficiency. On the other hand, sprinkler irrigation stands out for its high-water utilization efficiency. However, it is worth noting that traditional methods like flood and furrow irrigation can result in bacterial damage to potato stems and leaves due to wetting [39]. In contrast, drip irrigation not only conserves water resources but also saves time and labor, thereby enhancing fertilizer utilization efficiency, potato yield, and WUE [40]. Nevertheless, there is insufficient extensive research on the regional-level effects of these three irrigation methods on both potato yield and WUE. In this investigation, it was found that flood irrigation and sprinkler irrigation resulted in the least variation in yield, but both contributed to a reduction in the WUE of potatoes. Conversely, drip irrigation demonstrated the most significant increase in yield and the least alteration in ET, ultimately improving the WUE of potatoes. This suggests that drip irrigation showcases notable potential for water conservation, aligning with findings from previous studies [23,39,40,41].

Supplemental irrigation had a limited positive effect on potato yield in North-central China, mainly attributed to the region’s unsuitability for potato cultivation, primarily due to unfavorable summer temperatures [7]. In Northwest China, where there is a relatively high level of irrigation, the maximum ET was obtained. Consequently, there exists a potential for negative fluctuations in the WUE values. This underscores the pressing need to optimize the irrigation amount for potatoes in this region, emphasizing the importance of a comprehensive approach to simultaneously enhance both yield and WUE.

4.3. Effect of Irrigation Amount and Irrigation Frequency on Potato Yield and WUE under Irrigation

Efficient irrigation management plays a pivotal role in agricultural operations, aiming to achieve cost-effective yields, optimize crop WUE, and safeguard environmental quality by decreasing water wastage through runoff and deep drainage [42]. Both the irrigation amount and frequency exert significant impacts on crop yield and WUE [24,43,44,45]. Appropriate irrigation amounts have the potential to substantially improve the number of tubers per plant, consequently enhancing potato yield [38]. Within our research, the relative yield increase ceases to grow beyond an irrigation amount of 200 mm (Figure 8A and Figure 9A). Conversely, WUE experiences a significant decrease once the irrigation amount surpasses 100 mm. Additionally, the relative yield increase per unit of irrigation amount and IWUE show a tendency to plateau after exceeding 100 mm of irrigation (Figure 9B,C). Therefore, considering these three indicators collectively, it is advisable that the suitable irrigation amount should not exceed 100 mm in northern China.

The potato yield and WUE did not show a continuous increase as the irrigation frequency increased [46]. Generally, irrigating 3–4 times significantly enhances potato yield without compromising the WUE. Under an identical irrigation amount, the synergistic improvement of yield and WUE can be achieved with a frequency of two times. However, if the irrigation frequency surpasses three times, there is a potential risk of diminishing the WUE. These findings can guide the determination of irrigation frequency, offering evidence for optimizing yield and WUE while minimizing water replenishment.

4.4. Response of Potato Yield and WUE to Irrigation as Affected by Soil Types, Organic Carbon Content, and N Fertilizer Rate

The yield increase effect of irrigation depends on soil type. In aeolian sandy soil, irrigation yields have the most substantial positive effects on both yield and WUE. This is primarily attributed to the frequent droughts, poor soil water retention capacity, and high soil evaporation in these areas. Irrigation replenishes soil moisture, increases nutrients, and stimulates crop growth. Conversely, there is a significant risk of diminishing WUE when irrigating in meadow soil and huangmian soil. The organic carbon content in soil serves as a crucial indicator of soil fertility, generally reflecting high soil fertility and a robust nutrient activation capacity [47,48]. The decrease in the relative yield increase per unit of irrigation amount corresponds to an increase in the organic soil carbon content, suggesting that higher soil fertility leads to a lower yield increase but enhanced irrigation efficiency. This tendency is likely due to soils with low organic carbon content, such as sandy soil and loess, being more susceptible to water and wind erosion. These findings underscore the significance of enriching soil carbon to enhance both crop yield and water-use efficiency [24,44,45,46].

Nitrogen is the most important essential element in agricultural production [49]. Effective nitrogen fertilizer management is vital for crop production, where appropriate nitrogen application can lead to optimal yields and WUE [50,51]. The nitrogen fertilizer rate significantly influences the positive impact of irrigation on both potato yield and WUE. Optimal performance in yields and WUE is observed at low and medium nitrogen rates, likely due to the restricted growth of potatoes and its impact on yield effects under an insufficient nitrogen supply. Conversely, a high nitrogen rate increases ET due to an augmented leaf area [50], with the increase in ET surpassing the increase in yield under a high nitrogen supply, consequently reducing the WUE [7,50,51].

5. Conclusions

This meta-analysis study was based on field experiments on various soils and growing environments in northern China, underscoring the significant potential of irrigation to enhance both potato yield and WUE. However, it was revealed that the response of potatoes to irrigation depends on variables such as the amount and frequency of irrigation, the method employed, nitrogen application rate, soil type, and the growing region. Our study suggested that irrigation could be particularly advantageous for improving potato yield and WUE in aeolian sandy soil with drip irrigation and a moderate nitrogen application rate (150–250 kg N ha⁻¹), especially when the irrigation amount is limited to no more than 100 mm. These findings emphasize the prospect of irrigation in achieving high yields and efficient potato production in semiarid regions.