Deficit irrigation and fertilization strategies to improve soil quality and alfalfa yield in arid and semi-arid areas of northern China

Introduction

Exploitation and utilization of brackish groundwater has become a concern in many countries (Oster, 1994; Mondal, Bhuiyan & Franco, 2001; Jiang et al., 2012). Many agricultural irrigation studies have shown that rational use of brackish groundwater does not result in a reduction of crop yields (Wang et al., 2007; Kang, Chen & Wan, 2010). However, prolonged periods of brackish water irrigation have been found to increase soil salinity, resulting in the destruction of soil structural stability (Lax et al., 1994), in addition to reduced microbial biomass and soil enzyme activity (Rietz & Haynes, 2003), resulting in severe soil degradation (Romic et al., 2008; Wang et al., 2016). In the arid and semi-arid areas of China, natural precipitation cannot meet the needs of crop growth, due to the shortage of fresh water resources on the surface, and therefore farmers mainly rely on the use of brackish groundwater for irrigation purposes (Kang, Chen & Wan, 2010; Jiang et al., 2012). Although brackish water can be used to provide more water for crops, it also introduces large amounts of salt into the soil, resulting in large areas of soil secondary salinization, and as a consequence, many farmlands have been abandoned (Pang et al., 2010; Bouksila et al., 2013). Alfalfa (Medicago sativa L.), as a good legume forage, not only feeds livestock but also improves soil quality (Raiesi, 2007; Yong, 2007), and is widely grown in many countries (Raiesi, 2007; Li & Huang, 2008; Bagavathiannan, Gulden & Acker, 2011). Alfalfa is cultivated over more than 4 ×10⁶ hm² in China, mainly in the arid and semi-arid regions of northern China (Jing et al., 2016); however, large areas of alfalfa in these area need irrigation to maintain normal growth and development. Therefore, it is important to study rational irrigation patterns to increase alfalfa production and prevent land degradation. In recent years, deficit irrigation has been widely used in agricultural production to improve crop yield and water-use efficiency (Shock et al., 2007; Du et al., 2010; Lindenmayer et al., 2011). However, it is not known whether using brackish water for deficit irrigation can improve soil quality and increase alfalfa production, and thus this needs further study.

It is well known that chemical fertilizers can be used to increase soil nutrients and that their application is a major approach for increasing crop yields (Verma & Sharma, 2007; Moharana et al., 2012). However, overapplication of chemical fertilizers has resulted in lower soil organic matter content and decreased soil enzyme activities and microbial populations (Saha et al., 2008), which in turn has caused severe soil degradation. The level of soil enzyme activity reflects the biological activity and biochemical reactions in the soil and is an important index used to evaluate soil health (Li et al., 2010). Soil microorganisms play an important role in the decomposition of organic matter, altering soil enzyme activity and nutrient cycling (Bastida et al., 2008; Saha et al., 2008). Most studies have shown that organic manure is beneficial to soil microbial growth (Hao et al., 2008; Liu et al., 2010) and can improve soil organic carbon content and enzyme activity (Liang et al., 2005; Yang et al., 2008), thereby improving the soil environment. However, compared with chemical fertilizers, the available nutrients in organic manures are low, which is not beneficial for crops to absorb and utilize nutrients in a short time (Marinari et al., 2000; Hao et al., 2008). The use of organic manure alone leads to a decrease in the yield of some crops (Enujeke & Egbuchua, 2013; Xin et al., 2017). However, some studies have shown that a combination of organic manure and chemical fertilizer is a more effective means of fertilization, which can significantly increase soil available nutrient content, soil enzyme activity, and soil microbial biomass carbon and nitrogen (Qiu et al., 2004; Xin et al., 2017), and also modify soil respiration rate (Bastida et al., 2008; Hao et al., 2008), thus increasing the crop yield while maintaining a good soil environment (Moharana et al., 2012). Although many studies have demonstrated the utility of applying organic manure in combination with chemical fertilizers, it is necessary to study how to use deficit irrigation with brackish water under combined fertilization conditions.

In order to improve soil quality in abandoned farmland and increase pasture production, we studied the effects of three fertilization modes and different deficit irrigation rates on soil enzyme activities, organic carbon, water-soluble carbon, total nitrogen, microbial biomass, respiration rate, and alfalfa yield. The purpose is to determine the appropriate fertilization methods and the irrigation volume of brackish groundwater. We believe that an appropriate combination of organic manure and chemical fertilizer with deficit irrigation can improve the soil environment and alfalfa yield in arid and semi-arid areas of northern China.

Materials and Methods

Study site and materials

Field studies were performed in 2013 and 2014 in Yanchi County, NingXia (106°30′–107°47′E, 37°04′–38°10′N) on the Loess Plateau, China. The regional climate is a temperate continental climate. The average annual temperature is 7.7 °C, with extremes ranging from a high of 38.1 °C to a low of −29.6 °C. The annual precipitation rate over last 31 years is 288.6 mm and the average annual potential evaporation is 2,710 mm. The amount of rainfall was 290.4 mm in 2013 and 346.9 mm in 2014. The monthly rainfall amounts during these 2 years and the 31-year monthly averages (1982–2012) are shown in Fig. 1. The study area was farmland that has been abandoned for 11 years and has abundant reserves of brackish groundwater (NaCl: 2.3 g L⁻¹). The soil characteristics of the 0–20 cm soil layer in 2012 were as follows: soil organic carbon, 3.71 g kg⁻¹; total nitrogen (N), 0.28 g kg⁻¹; total phosphorus (P), 0.21 g kg⁻¹; total potassium (K), 15.52 g kg⁻¹; available N, 34.11 mg kg⁻¹; available P, 3.62 mg kg⁻¹; available K, 100.58 mg kg⁻¹; and total salt content, 2.08 g kg⁻¹, with a pH of 8.36. Herdsmen in the area process large amounts of sheep manure into organic fertilizer and the organic fertilizers we used contained 186.5 g kg⁻¹ of organic C, 7.1 g kg⁻¹ of total N, 4.5 g kg⁻¹ of total P, and 13.2 g kg⁻¹ of total K. Chemical fertilizer N was supplied as urea (N 46%), P was supplied as calcium superphosphate (P₂O₅ 20%), and K was supplied as potassium sulphate (K₂O 50%).

Results

Soil microbial biomass carbon and nitrogen (MBC and MBN), the MBC:MBN ratio, soil respiration rate (SR), and metabolic quotient (qCO₂)

In both years of the study, variance analysis revealed highly significant effects of F on MBC, MBN, the MBC:MBN ratio, SR, and qCO₂ (P < 0.01) (Figs. 2 and 3), whereas the effect of I on these indicators was not significant (P > 0.05). In the three fertilization modes, MBC and MBN did not decrease significantly with an increase in irrigation (Fig. 2), but instead increased slightly. This is due to the low salt content of brackish groundwater and the low irrigation amount of deficit irrigation, which means that the microorganisms were less affected by salt stress. Under the same irrigation level, the MBC and MBN of the OM treatment were significantly higher than those of the CF and NF treatments, whereas the MBC:MBN ratio was significantly lower than that in the NF treatment. This shows that the use of organic manure and chemical fertilizers promotes the accumulation of soil microbial biomass, which is conducive to improving the soil environment. Under irrigation, qCO₂ in the OM treatment was significantly lower than that in the CF and NF treatments. The OM treatment significantly increased SR compared with the NF treatment, with or without irrigation. The above results show that a combination of organic manure and chemical fertilizer increased SR and reduced qCO₂, which improved the soil environment more effectively than chemical fertilizer.

Discussion

Application of organic fertilizers can increase soil organic carbon and promote soil microbial metabolism, thereby increasing soil enzyme activity (Liang et al., 2005). Liu et al. (2010) reported that application of organic fertilizer increased phosphorus-dissolving bacteria in the soil, and therefore urease and alkaline phosphatase activities are increased. Previous studies have also shown that using organic fertilizers increases soil dehydrogenase activity compared with the use of chemical fertilizers (Marinari et al., 2000; Yang et al., 2008). This is due to the fact that organic fertilizers add metabolic substrates that can be consumed by microorganisms, and stimulate dehydrogenase activity (Pascual et al., 1998; Liang et al., 2003). Li et al. (2010) reported that the highest catalase, urease, and phosphatase activities were observed in soils in which both organic and inorganic fertilizers have been used. The results of our study are consistent with the findings of the aforementioned studies; that is, we found that under the combination of organic manure and chemical fertilizers (OM) treatment, the activities of urease, alkaline phosphatase, invertase, catalase, and dehydrogenase were significantly higher than those under the no fertilization and chemical fertilizer treatments (Table 1). Correlation analysis showed that the activities of these five enzymes were significantly (P < 0.01) positively correlated with SOC, TN, WSC, MBC, MBN, and SR (Table 4), but negatively correlated with qCO₂ and the MBC:MBN ratio (P < 0.01). This shows that soil enzyme activity is closely related to soil nutrients and microbial biomass.

Soils with high SOC content have a more stable aggregate structure that increases the infiltration and retention of moisture and is less susceptible to erosion and runoff (Watts et al., 2006; Abiven, Menasseri & Chenu, 2009). Ebhin Masto et al. (2006) reported that the combination of organic and chemical fertilizers significantly increased SOC compared with chemical fertilizer alone. Similarly, Bhandari et al. (2002) reported that the combed application of organic and chemical fertilizers will delay the release of nitrogen in soil, resulting in less nitrogen loss and thus higher TN. Combinations of organic fertilizers and chemical fertilizers are more effective measures for increasing soil fertility and improving the soil environment than are applications of single chemical fertilizers (Hopkins & Shiel, 1996; Bharambe & Tomar, 2004). Our results are again consistent with the aforementioned studies, in that the OM treatment significantly increased SOC, TN, and WSC compared with the CF treatment, reducing the SOC:TN ratio (Table 2) and thus improving soil quality better than CF.

Previous studies have shown that a reduction in MBC is often due to a lack of phosphorus, potassium, or other micronutrients in the soil (Moharana et al., 2012). The use of organic fertilizers not only increases phosphorus and potassium in the soil but also introduces a large amount of micronutrients and metabolic substrates, thereby promoting the growth of microorganisms and increasing the MBC (Luo & Sun, 1994; Hao et al., 2008). Organic fertilizers also provide a stable source of organic carbon and nitrogen for the growth of soil microorganisms (Marschner, Kandeler & Marschner, 2003; Mandal et al., 2007), thereby increasing MBC and MBN (Mandal et al., 2007). Our results are consistent with these findings, as we demonstrated that a combination of organic and chemical fertilizers increase SOC, TN, and WSC (Table 2), and thus significantly increased MBC and MBN. The metabolic quotient (qCO₂) is the ratio of SR to MBC, which reflects microbial metabolic activity and is also a valid indicator of soil ecosystem health (Anderson & Domsch, 1993). A lower qCO₂ indicates a higher substrate mass and microbial metabolic efficiency in the soil (Anderson & Domsch, 1990). Previous studies have shown that a lower qCO₂ was observed in soils with organic fertilizer application (Dilly, 2005; Melero et al., 2007; Scheller & Joergensen, 2008). Consistently, we also observed a lower qCO₂ under the OM treatment than under the CF and NF treatments (Fig. 3), indicating that the OM treatment increases substrate quality and microbial metabolic efficiency in soil. We also found that the OM treatment significantly increased SR compared with the NF treatment. This is consistent with the reports of Liang et al. (2005) and Liang et al. (2003), who found that organic fertilizers not only increase the activity of various enzymes but also increase the SR.

Previous studies have shown that the activities of most soil enzymes, such as invertase, polyphenol oxidase, and catalase, are obviously decreased after brackish water irrigation (Guo, 2012), for the following three reasons: (a) Brackish water irrigation increases the amount of salt in the soil, exposing soil microorganisms to osmotic stress, and thereby inhibiting microbial metabolism (Rietz & Haynes, 2003); (b) Salinity in the soil inhibits the dissolution of proteins and destroys the tertiary structure of the proteins, resulting in the denaturation and inactivation of proteases, and thereby affecting enzyme activity (Yuan et al., 2007); (c) Salinity in the soil suppresses the growth of crop root systems, thereby reducing the root enzyme secretion (Ghollarata & Raiesi, 2007). Wang et al. (2009) reported that the use of brackish groundwater irrigation causes an excessive accumulation of salt in the soil and increases in soil pH, thereby significantly reducing soil urease, invertase, and catalase activities. However, alkaline phosphatase activity increased with an increase in brackish water. Similarly, we found that irrigating with 150 mm brackish groundwater per year (I₁₅₀) significantly reduced invertase activity and significantly increased alkaline phosphatase activity compared with no irrigation (Table 1). Singh, Chauhan & Minhas (2009) also found that low concentrations of NaCl (<20 mmol L⁻¹) have a beneficial effect on alkaline phosphatase, and Li et al. (2010) reported that deficit irrigation can increase the activities of soil catalase, urease, and invertase to improve the soil environment. Our study showed that the I₁₅₀ treatment significantly increased soil urease and catalase activities under the OM fertilization mode compared with no irrigation; however, irrigation with brackish groundwater did not significantly increase both enzyme activities without fertilization. This is due to the fact that the combination of organic fertilizer and chemical fertilizer can alleviate the effect of salt stress on soil enzyme activity. In addition, the brackish groundwater in our study area has a low salinity (NaCl: 2.3 g L⁻¹) and the irrigation amount (150 mm) of the I₁₅₀ treatment is also lower than that of the conventional irrigation in the area (300 mm). Therefore, enzyme activities under the OM-I₁₅₀ treatment were less inhibited by salt stress, and indeed, the highest urease, alkaline phosphatase, catalase, and dehydrogenase activities were observed under this treatment.

Previous studies have shown that long-term brackish groundwater irrigation resulted in decreases of 26.8% and 28.0% for organic matter and total nitrogen, respectively (Wang, Zhao & Ouyang, 2010). They also found that brackish groundwater irrigation inhibited soil microbes, reducing MBC and MBN by 24.4% and 42.4%, respectively. Wichern, Wichern & Joergensen (2006) reported that low concentrations of NaCl have a stimulating effect on soil microorganisms, whereas high concentrations of NaCl are not conducive to the growth and reproduction of microorganisms and result in reduced enzyme activity. Our results show that irrigation has insignificant effects on MBC and MBN (Fig. 2). This is due to the lower salt content of the brackish groundwater we used combined with the use of an irrigation volume that is lower than the conventional irrigation practice in the area, and therefore irrigation has less effect on the microorganisms. In addition, we found that MBC and MBN increased slightly with an increasing irrigation volume (Fig. 2), which is similar to the results of Shui et al. (2012), who found that deficit irrigation increased maize yield and soil MBC. It has been reported that excessive irrigation can increase evapotranspiration, wheat dry matter, and leaf area per plant, but it does not significantly increase grain yield and reduces the soil respiration rate (SR) (Kang, Chen & Wan, 2010). Soil respiration is mainly attributable to root respiration and microbial oxidation of organic matter, and is an indicator of soil quality (Rong et al., 2015). Previous research results have revealed that SR improved with increasing irrigation under deficit irrigation, but that overirrigation inhibited SR (Wang, Zhao & Ouyang, 2010). Our results show that deficit irrigation did not significantly increase or decrease SR (Fig. 3), which could be attributable to the combination of lower irrigation volume and brackish groundwater salt stress, resulting in a small change in SR. Triantafilis et al. (2004) showed that irrigation with low-salinity brackish water did not affect crop yields, whereas high concentrations of brackish water were detrimental to crop growth. Talebnejad & Sepaskhah (2015) also reported that irrigating with saltwater below 20 dS m⁻¹ conductivity did not reduce the biomass of quinoa, whereas Zhang (1999) found that saltwater irrigation with less than 3 g L⁻¹ of NaCl promoted the growth of maize seedlings to varying degrees, although more than 3 g L⁻¹ was detrimental to crop growth.

Previously, Singh, Chauhan & Minhas (2009) showed that winter wheat yield increased with brackish water irrigation. In the present study, we showed that irrigation with low saline (NaCl: 2.3 g L⁻¹) brackish groundwater increased alfalfa plant height, LAI, and leaf SPAD values, thereby increasing biomass yield (Table 3). In arid and semi-arid regions, moderate amounts of brackish groundwater provide good water conditions for alfalfa growth. Although brackish groundwater contains some salt, the salt concentration in the soil is lower due to the smaller irrigation volume. Therefore, salt stress had a less inhibitory effect on alfalfa growth. In contrast, under the OM treatment, the use of brackish groundwater for deficit irrigation is more conducive to alfalfa growth. This is due to the significant increase in MBC and MBN under the OM treatment (Fig. 2). This also increased soil enzyme activity and soil carbon and nitrogen contents (Tables 1 and 2), thus providing more nutrients and a good soil environment for alfalfa growth. Finally, we observed the highest alfalfa biomass yield (11.0 t ha⁻¹) under the OM-I₁₅₀ treatment.

Conclusion

The application of organic manure and chemical fertilizers (OM) significantly increased soil urease, alkaline phosphatase, invertase, catalase and dehydrogenase activities, and also increased the SOC, WSC, TN, MBC, MBN and SR, but decreased the SOC:TN ratio and qCO₂, thus improving the soil quality in abandoned farmland. Under the OM treatment, the use of brackish groundwater with low salt concentration increased the activity of urease, alkaline phosphatase, catalase, and dehydrogenase, which is beneficial to alfalfa growth and increases biomass yield. We observed the highest alkaline phosphatase, urease, catalase and dehydrogenase enzyme activities, as well as the highest SOC, TN, WSC, MBC, MBN, and alfalfa biomass yield under the OM-I₁₅₀ treatment. Therefore, the OM-I₁₅₀ treatment could be used as an effective measure to improve the soil quality of abandoned farmland and to make rational use of brackish groundwater in the arid and semi-arid regions of northern China.

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