Ridge gourd (Luffa acutangula (L.) Roxb.) or Luffa is a multi-harvest vegetable crop grown in the South Asian region. It is commonly called “Torai” in rural areas of India.
1. Introduction
The rate of domestic waste discharges has recently grown dramatically as a result of an uncontrolled increase in the human population
[1]. Sewage wastewater is amongst the top contributor of domestic wastes, which has become a difficult to manage problem globally
[2]. It is estimated that nearly 359 billion cubic meters of wastewater are generated globally, with only 52% being sufficiently or partially treated while the remaining is disposed of without any treatment
[3]. Sewage sludge (SS) is a solid or semi-solid substance generated from the different steps of sewage treatment plants (STPs). SS could create several environmental issues if not properly disposed of
[4,5][4][5]. Generally, the management of SS occurs at different stages such as production (minimization), collection, treatment, and reuse
[6]. Being rich in various organic and nutrient substances, SS is widely utilized for secondary purposes such as biogas, biochar, syngas, biofertilizer production, composting, building materials, among many others
[7,8][7][8].
During the recent decades, the increasing use of SS in agriculture has appeared as one of the most effective methods of its management due to the presence of abundant nutrients
[9]. Predominantly, nitrogen, phosphorus, potassium, and other microelements are the major constituents of SS that are useful for soil amendment
[10]. Markowicz et al.
[11] reported that SS mixing up to 15 t/ha was the most efficient composition for the reclamation of nutrient-deficient soil. SS application on lands helps in the mixing of various organic and inorganic nutrients, which further leads to improved crop yields
[12,13][12][13]. However, the excessive mixing of SS, having a high nutrient load, may also damage soil health; thereby, affecting plant growth and reducing crop yields
[14]. Moreover, the nutrients may contain metal elements that have relatively high density, which has been regarded as one of the major problems regarding SS utilization due to their toxicity at higher levels
[15]. Therefore, it is necessary to monitor the migration of such elements from SS-treated soils to plant parts for human health benefits.
Ridge gourd (
Luffa acutangula (L.) Roxb.) or Luffa is a multi-harvest vegetable crop grown in the South Asian region. It is commonly called “
Torai” in rural areas of India. Non-ripened fruits of Ridge gourd having tender dark green color are cooked and eaten due to their high nutritional values
[16]. The ripened fruits of Ridge gourd are widely used for various purposes, such as cleaning sponges and mattresses
[17]. Being a multi-harvest crop, the biggest problem associated with Ridge gourd cultivation is the excessive required quantity of fertilizers, which makes it less profitable for the farmers
[18]. Consequently, the yield and nutritional quality of Ridge gourd start decreasing if the required dose of fertilizers is not supplied timely and adequately
[19]. Also, the repeated use of chemical fertilizers makes the soil unhealthy and harmful for microbial communities
[20]. Therefore, SS application for Ridge gourd cultivation could be a potential technique to minimize chemical fertilizer input and improve soil health.
Plant growth-promoting rhizobia (PGPR) is a group of microorganisms that have distinct capabilities to assist the plant root systems in terms of efficient survival and nutrient deliverability
[21]. Commercial-scale PGPRs commonly known as biofertilizers have shown substantial improvements in plant growth and crop yields, making agriculture more profitable
[22,23][22][23]. A study has found that supplementation of PGPR was helpful for increased nutrient extraction by mustard green (
Brassica juncea). Konkolewska et al. found that supplementation of PGPR was helpful for increased nutrient extraction by mustard green (
Brassica juncea)
[24]. Ke et al. also showed improved nutrient uptake by perennial ryegrass
[25]. Moreover, Ipek et al. reported an increased yield and nutritional profile of strawberries under PGPR treatments
[26]. Considering the role of PGPR in plant-nutrient delivery, it can be potentially used for Ridge gourd cultivation along with SS application for improved nutrient utilization. However, some of the toxic metal elements may also accumulate in edible parts of Ridge gourd, which might not be suitable for human consumption due to health concerns. Hence, biomonitoring of these metal elements needs proper attention, along with their potential health hazard.
2. Impact of SS on Arable Soil Properties
Table 1 provides the physicochemical, nutrient, and metal elements properties of AS, SS, and different treatment groups of SS such as 5, and 10%. The initial basic properties of AS and SS showed that SS was highly enriched with various organic and inorganic nutrients including metal elements. The pH (8.30 ± 0.05), EC (7.21 ± 0.12 dS/m), OM (23.90 ± 3.14%), NO
3-N (31.07 ± 1.380 g/kg), and PO
4-P (14.22 ± 1.10 g/kg) of SS was significantly (
p < 0.05 higher as compared to AS which had pH of 7.28 ± 0.03 units, EC of 2.10 ± 0.08 dS/m, OM of 1.25 ± 0.03%, NO
3-N of 1.60 ± 0.10 g/kg, and PO
4-P of 1.25 ± 0.06 g/kg, respectively. The soil properties were also significantly (
p < 0.05) changed after the SS mixing in both 5 and 10% treatment groups. Contrarily, the availability of all nutrients in AS was slowly increased with an increase in the SS mixing rate which signifies the usability of SS as an efficient nutrient supplement. PCA results showed that SS dose had a significant effect on increased nutrient availability (
Table 2). The data was statistically transformed on two extracted components namely PC 1 and PC2 having eigenvalues of 3.12 and 0.84, while variances of 78.11 and 21.04%, respectively. The axial length of SS treatment in
Figure 2a indicated the positive influence of the AS nutrient properties. Moreover, the analysis of soil physicochemical properties using the Euclidean distance-based neighboring cluster method revealed that metal elements possess more positive interaction (>0.5) with SS dose as compared to other soil nutrients (<0.5) parameters. The heatmap plot shown in
Figure 2b indicates that Zn and Fe were closely related to each other in terms of their available concentration in the soil (mg/kg). Similarly, Mn showed close relation to Cu, while Cd with Cr. Overall, the amendment of SS on AS was helpful to increase its nutrient profile which can be a useful and low-cost fertilization alternative for Ridge gourd crop cultivation in the selected Indian region. Sewage sludge has been widely accepted for soil fertilization at the global level. Markowicz et al.
[11] reported that a 15 t/ha rate of SS application was helpful to rehabilitate the degraded soils in Silesian Upland, Southern Poland. Marotrao et al.
[33][27] also used SS (15 to 45 t/ha) for enhancing the soil mineral properties under the rice-wheat cultivation system. Another study by Marotrao et al. also used SS (15 to 45 t/ha) for enhancing the soil mineral properties under the rice-wheat cultivation They found that the soil metal elements under higher SS doses did not exceed the safe limits except for Cd. Thus, SS obtained from STP Jagjeetpur was rich in various nutrient parameters and can be recommended for AS fertilization, provided prior analysis of other toxic elements.
Figure 2. (a) PCA biplot and (b) clustered heatmap for the interaction of sewage sludge (SS) on soil nutrient availability.
Table 1. Soil properties (mean ± standard deviation, n = 3) used for Ridge gourd (L. acutangula) cultivation under different sewage sludge and plant growth-promoting rhizobia (PGPR) treatments.
Parameter |
Arable Soil |
Sewage Sludge |
Treatments |
|
5% SS |
10% SS |
pH |
7.28 ± 0.03 |
8.30 ± 0.05 * |
7.35 ± 0.05 * |
7.68 ± 0.02 * |
EC (dS/m) |
2.10 ± 0.08 |
7.21 ± 0.12 * |
2.70 ± 0.20 * |