4. Discussion
Growth and productivity of plants in ravines are adversely affected by nutrient and moisture deficits, soil compaction, and high summer temperatures as well as by biotic stressors. Consequently, vegetation in ravine lands experiences multiple stresses. Despite these challenges, there is a lack of extensive research focusing potential of resilient perennial fruit crops and sustainable land management techniques to enhance productivity and fruit quality in degraded ravines. Few fruit crops identified for marginal lands near ravines; however, they have not specifically addressed the unique challenges faced by ravine slopes in rain-fed environments. The present results showed significant effect of SMSTs on the tree growth and fruit yield of rain-fed sapota cultivated in degraded ravine lands. The incremental changes in growth parameters such as tree height, canopy spread and volume, collar diameter under SWCMs viz., terraces and trenches shows positive influence of SWCMs in or improving the various physical, chemical and biological properties of soil (Dagnachew et al., 2020; Moradi et al., 2014; Ping et al., 2012; Zhang et al., 2023; Yue et al., 2022; Mahajan et al., 2021). The two crucial factors for the growth of any plant in general and more specific under rain-fed cultivation are water and nutrients. SMSTs play instrumental role in enhancing availability of essential soil nutrients and moisture that results in the better photosynthesis, translocation of photosynthates and other growth promoting physico-biochemical processes that lead to higher plant growth. On the other hand, slope conditions are deprived of these two precious resources from the soil root zone (Panigrahi et al., 2009 & 2017; Hishe et al., 2017). The increased availability of soil nutrient and moisture under field might be owing to minimized runoff and soil loss, higher SOC under SMSTs. Lower runoff generation might have enhanced deeper water infiltration and prolonged soil moisture availability in post-rainy season (Panigrahi et al., 2009; Tsui et al., 2004; Negi et al., 2012). These findings signifies the SWCMs induced greater nutrient and water accumulation that have positively influenced the plant growth parameters such as tree height, canopy spread and stem diameter in harsh climates and degraded lands. The improvement in plant photosynthesis rate under sufficient water situations has already been reported in crops (Vu and Yelenosky, 1988; Mpelasoka et al., 2001). Likewise, higher plant growth was achieved in normal irrigation compared to deficit irrigation (Mpelasoka et al., 2001; Hajlaoui et al., 2022). In the present experiment also SMSTs enhanced soil moisture, SOC, availability N, P, K etc. under degraded ravines. Therefore, higher vegetative growth of sapota plants under SMSTs (Fig. 2a) in this experiment can be attributed to higher availability of nutrients and moisture through reduction in runoff, which are indispensable for plant growth and development.
Enhanced yield and RWP of sapota under degraded ravine lands through SMSTs (SCCBT, followed by SBT, SSTS) can also be attributed towards optimum water and nutrients availability in the soil. This might have improved physiological activities of plants e.g. greater photosynthesis and higher availability, translocation of photosynthates to fruits (Zhang et al., 2023). As reported earlier, the decrease in photosynthetic capacity of leaves exposed to water deficit during water stress may results in the reduction of assimilates offered for fruit growth and a stronger contest for assimilates between sink organs (Moutinho-Pereira et al., 2004 and Hajlaouiet al., 2022). Another reason could be improvement of micro-climatic conditions for fruit setting and fruit development events including pollination and fertilization activities resulted in to better fruit set, higher fruit number and fruit growth. In the present study, as discussed above these SMSTs have improved soil properties made soil conditions more favorable for plant growth and development on ravines. Further, SMSTs induced enhancement of soil moisture could be one of the crucial points for higher fruit setting and fruit growth in rain-fed fruit cultivation (Huang et al., 2014). Under rain-fed cultivation, fruit crops face water scarcity in different growth phases; resulting in the reduction of plants growth, mortality of young plants, hampered fruit set and fruit quality. Thus, the utilization of SMSTs can mitigate the adverse effects of water stress on plant growth during various stages of the growth cycle in rain-fed conditions by increasing soil moisture. The crucial role of water in rain-fed cultivation can be justified by its functionality in enhancement of number of fruits, fruit weight and size, and yield (Panighrahi et al., 2017; Wang et al., 2019; Mpelasoka et al., 2001; Hajlaoui et al., 2022).
Present study observed positive influence of SMSTs (SCCBT and SBT) on fruit quality parameters like fruit and pulp weight, and fruit diameter. These three parameters are one of the decisive to make good fruit quality, appealing and market value. Higher partitioning of photosynthates towards fruits and availability of soil moisture produced fruits with higher weight; on the contrary, moisture deficiency reduces the fruit weight (Wang et al., 2019). Further, water stress (drought) can lead to insufficiency of water necessary for enlargement of cells; consequently reduces the fruit size (Yildirim et al., 2015). Current study also showed that the SMSTs treatments (SCCBT and SBT) produced fruits with higher moisture which lead to reduction in TSS and total sugars; this might be due to the higher moisture dilution effect in the fruits (Panighrahi et al., 2017; Davis & Albrigo, 1994). Whereas, reduced fruit moisture, higher TSS, total sugars, reducing and non-reducing sugars were observed in treatment without any SMSTs i.e. SS. Water stress conditions enhances the TSS, soluble sugars, and reduces TA in fruits. This may displays improved conversion of acids to sugars in dehydrated juice sacs that is essential to sustain the solute potential of fruit cells under modest moisture stress situations (Huang et al., 2000; Navvaro, 2010). This also indicates the adaptation in solute pathway stimulated by the plants in the degraded ravines under moisture deficit. Many times light moisture deficit is recommended to enhance the fruit quality (Mpelasoka et al., 2001; Wang et al., 2019). Further, SMSTs enhanced vitamin C content in the sapota fruits compared to control. Higher vitamin C under SMSTs has been earlier been observed by Liu et al. (2012) and Panighrahi et al. (2017). It has been reported that soil moisture regime is negatively and positively correlated with vitamins and polyphenols, respectively in wolfberry (Wang et al., 2020). Results of present study confirms the observation with respect to organic acids, phenols however contradict with the vitamin C. We have reported increase in vitamin C content under SCCBT and lowest under SS on par with SST. Whereas, total phenol content in the fruits observed more in control plot which received no SMST followed by SCCBT and lowest when Sapota grown alone with bench terrace and trenches. Present result revealed that the moisture deficit conditions improved the phenolic compounds. This rise in phenolic concentrations can be attributed to the activation of metabolic responses triggered by the moisture stress, which aims to adapt to the unfavorable conditions (Buendia et al., 2008; Guizani et al., 2022).
Present investigation showed that natural slopes produced highest runoff, whereas terrace reduced run-off and the latter may be due to the tillage activities and rainfall interception by both trees and crops. Earlier studies have also reported that the steep slopes, reduced or no tillage, poor organic matter, bare soils, and high rainfall intensity etc., are the major factor responsible for generating the higher runoff and consequently higher erosion risks in similar kinds of lands (Barrena-Gonz´alez et al., 2020; Ben-Salem et al., 2018; García-Díaz et al., 2016; Salom´e et al., 2016; Baiamonte et al., 2019). Therefore, reduced slope, tillage operation and rainfall interception by tree canopy cumulatively reduces runoff in the bench terrace (SCCBT and SBT) compare to control (Fang et al., 2021; Keesstra et al., 2016). Further, in terrace based SMSTs, the reduction in water velocity and subsequent its more retention over surface leads to the greater water infiltration into the soil that consequently reduced runoff generation (Zhnag et al., 2014). Though SCCBT produced lowest runoff but caused highest soil loss which could be due to the application of tillage operations during cowpea and castor cultivation (Fig. 2c). On the other hand, the highest runoff and moderate soil loss in SS was resulted from the generation of greater runoff water velocity on the undisturbed slope (Bagagioloet al., 2018; Keesstra et al., 2016). Since the development of agriculture, tillage is the prime provoker of the soil erosion (Brevik and Hartemink, 2010; Nie et al., 2016). Tillage makes the soils bare, loose and consequently makes it prone to higher soil erosion. Tillage is typically regarded as the major factor responsible for soil erosion in rain-fed agriculture (Kurothe et al. 2014) and is accountable for the acceleration of sediment fluxes on all continents (Van Oost et al., 2009). Despite this, tillage is considered a crucial factor in agricultural production (Singh et al., 2014) because it alters soil characteristics such as SOC (Hassan et al., 2014), biota habitat (Balota et al., 2014), and chemical properties (Laudicina et al., 2015; Zornoza et al., 2015). Therefore, findings showed that the despite the higher soil loss recorded under SCCBT treatment, it also contributed greater fruit yield as well as quality.
The present investigation revealed the improvement of availability major nutrients under SMSTs measures, compare to natural slopes of degraded ravine lands. These measures influenced improvement in status of soil moisture, SOC, soil nutrients like N2O, P2O5, and K2O could be attributed to the erosion reduction under SMSTs. The higher SOC in the SCCBT, SBT, and SST treatments could be the result of accumulation and retention of organic matter in the soil. Conversely, lower levels of SOC in SS may be due to the loss of decaying plant parts from the surface soil through soil erosion because of inadequate physical barriers. Improvement in SOC through different SMSTs has been reported previously in different studies (Gu et al., 2017). Eroded soils lose a large amount of surface SOC because of its preferential removal by the erosion process and relatively low density of SOC (Lal, 2003). Further, Mahajan et al. (2021) reports that continuous contour trenches and vegetative barrier improved SOC (1.41% to 2.02%), SOC stock (44.9-57.8 Mg C ha-1), rate of SOC sequestration (1.5 Mg C ha-1 yr-1), soil microbial biomass carbon (33.9 mg kg-1), and had lowest metabolic quotient (0.52 mg CO2-C g-1 h-1), compared to the control, which indicates improved soil biological activity and reduced environmental stress on the soil microbes. Several studies report the similar findings of loss of SOC under accelerated soil erosion process (Rhoton and Tyler, 1990; Kimble et al., 2001; Zhang et al., 2023). Further present study revealed that SMSTs improves properties of soils one or other way. Likewise, Dagnachew et al. (2020) reported that farmlands with SMSTs had significantly improved soil physical properties such as silt and clay fractions, and volumetric soil water content and chemical properties like pH, SOC, TN, C:N ratio, and available phosphorus, compared with farmlands without SWC measures. Numerous studies, including those by Hailu et al. (2012), Ademe et al. (2017), Abay et al. (2016), Stahr (2010), and Zhang et al. (2023), have reported that farmlands implementing SMSTs and lower slope gradients tend to demonstrate considerably greater levels of SOC, total nitrogen etc. when compared to non-conserved lands and having higher slope gradients. Similar to our study, the higher availability of P2O5 and K2O in case of SMSTs and lower slopes were also observed by the previous researchers (Dagnachew et al., 2020; Mengistu et al. 2016; Ademe et al., 2017). Therefore, higher accumulation of SOC and soil nutrients like available N2O, P2O5, and K2O can be attributed towards positive effects on SMSTs on sites.