4.1. Soil moisture dynamics
Tephrosia biomass consistently maintained soil moisture levels, particularly during dry months and quantity of biomass retained was directly proportional to the amount of moisture retention. Beneficial effects of Tephrosia candida in retaining soil moisture in bael orchard have also been reported by Das et al (2021). Hernández-Aranda et al. (2021) found that the utilization of similar mulching techniques had a twofold effect in retaining moisture and preventing light penetration, effectively suppressing weeds and promoting microbial growth, along with enhancing soil structure.
4.2. Soil electrical conductivity and soil nutrients
The study clearly indicated improvement in the soil chemical properties with mulching of Tephrosia biomass. The increment in soil electrical conductivity (EC) can be attributed to increased soil moisture (Grisso et al., 2005), nitrate nitrogen, or mineralizable nitrogen (Ju et al., 2007). According to Das et al. (2021) the Tephrosia biomass contains 2.94% nitrogen, 0.16% phosphorus, and 2.94% potassium. Accordingly the total quantity of nutrients per m2 in the mulching material under the treatments 3.0 Kg/m2, 2.0 Kg/m2 and 1.0 Kg/m2 can be calculated to be 88.2 g, 58.8 g, 29.4 g of nitrogen, 4.8 g, 3.2 g, and 1.6 g of phosphorus, and 31.8 g, 21.2 g, and 10.6 g of potassium, respectively. Hence, addition of nutrients in the plant basin through the mulching material was attributed to increase in the content of different nutrients in the plant basin. Apart from that organic mulch application reduces phosphorus movement and fixation, as organic acids released during decomposition minimize phosphorus fixation with aluminium (Mensah, 2015). Higher potassium content in the study may result from reduced leaching, increased soil moisture, electrical conductivity, and organic acid release during biomass decomposition, and easy movement potassium from plant tissues to soil. Organic acids released during decomposition form chelates with micronutrients, increasing their availability in soil and to plants. These chelates are more stable than the free micronutrients and, therefore, are more easily absorbed by the plants (Mackowiak et al., 2001).
4.3. Soil organic carbon and its fractions
Soil with high organic matter content promotes microbial proliferation, nutrient availability and soil structure (Liddle et al., 2020). Intercultural operations have been shown to reduce the soil carbon content by 30–50% (Six et al., 2002; Dinakaran et al., 2014). Application of Tephrosia in the guava orchard helped to improve soil organic carbon and its fractions. The higher Ctot and Csoc in the surface soil might be due the addition of biomass directly in to the surface soil and lack of biological nutrient recycling in the deeper layers (Naik et al., 2017), and this might contribute towards long term carbon sequestration.
Tephrosia treatments improved organic fractions, including highly labile, labile, and non-labile fractions, promoting nutrient cycling and crop productivity. Greater labile pool of organic carbon in the present study may be due to the higher Cmic, DHA, and FDA activity supplied readily decomposable carbon. Tephrosia litter had higher ash, polyphenol, and lignin can lead to organic carbon resistance and addition to the recalcitrant pool of soil organic carbon (Mafongoya et al., 2003). Therefore, increase in non-labile organic carbon in the Tephrosia treated soil may be due to the quality and quantity of leaf litter. Additionally, our present study also showed a 40% of carbon content in Tephrosia candida leaf litter which will apparently contribute towards addition of 1.2 kg carbon per m2. Therefore, the direct application of Tephrosia as mulch might also have contributed towards the increment in soil organic carbon.
4.4. Soil biological properties like microbial biomass carbon dehydrogenase activity and Soil fluorescein diacetate activity (FDA)
Cmic generally accounts for 1–5% of Ctot (Mandal 2005; Naik et al., 2017) and microbial adaptation or community changes may increase carbon use efficiency (Allison et al., 2010). Tephrosia mulching increased Cmic due to the supply of oxidizable or mineralizable carbon, nutrients as substrate and created favourable microclimate for the growth and reproduction by maintaining favourable rhizospheric microclimate, while the lower value in the control might be due the unfavourable rhizospehric microclimate, leaching of available nutrients and lower biomass accelerates the carbon loss (Allison et al., 2010).
DHA measurement shows microbial oxidation (Nannipieri et al., 2002; Patra et al., 2006), and high dehydrogenase activity is a valid soil microbial activity metric affected by temperature, microbiota, moisture, vegetation, and aeration (Klein et al., 1971). Mulching with Tephrosia biomass reliably improved DHA over a 6-month period. Long-term effects are associated with increased oxidizable organic carbon, substrate availability (including soil nutrients and rhizospheric moisture), and soil temperature (Rastin et al., 1988). This can explain the peak in the activity of dehydrogenase during the month of April 2022 in the present investigation.
Similarly, mulching with Tephrosia biomass at 3.0 kg/m2 enhanced FDA activity. Singh et al. (2018) and Kumawat et al. (2017) showed similar increases in soil FDA activity in the context of residue management. FDA activity in April 2022 was 179% greater than the control, whereas it was 149% higher in January. A rapid rise in FDA activity was found with mulch decomposition, similar to Tang et al. (2004), who reported a 60–120% increase in FDA activity from mulched treatments to controls. The monthly fluctuation in enzyme activity is affected by soil temperature, microbiota, moisture, vegetation, and aeration (Rastin et al., 1988). The increased FDA activity in our study implies greater microbiological activity in the mulched soil, improved biological stability of the mulched materials and improved plant nutrient availability. The decline in dehydrogenase and fluorescein diacetate (FDA) activities during the initial months upto January 2022 mainly due to reduced microbial decomposition of the newly applied mulch, because of the higher carbon-to-nitrogen (C:N) ratio, combined with lower temperatures, which collectively hinder microbial decomposition processes (Park and Kremer, 2009).
4.5. Guava leaf nutrient content
Mulching prevents root shrinkage, maintains optimal soil microclimates and soil temperature there by improves the absorption of N, P. K and micronutrients (Kozlowski, 1972; Russell, 1975; Dantas et al., 2007; Dutta, 2009). The increase in leaf nutrient content can be attributed to synergistic effect of applied nutrients (Tiwari et al., 2018) which might contributed towards more absorption and translocation. As the biomass mulching enriches the soil humus, which was the source of easily available nutrients for plant absorption (Bucki and Siwek, 2019).
4.6. Phenological behaviour of guava
The phenological behavior of guava have been reported to be affected by climate conditions such as temperature (Normand and Robert Habib, 2001; Padilla-Ramirez et al., 2012), rainfall (Das et al., 2018), and the initial nutrient content of the soil (Normand and Robert Habib, 2001). The hastening of bud swelling by 29.17 days and a longer duration of bud swelling and appearance of the flower buds with respect to 3.0 kg/m2 of biomass mulching compared to non-mulching might be due to the increase in nutrient in the leaf and soil. The duration of ‘leaves completely developed stage’ was reduced by 0.5 days with mulching @ 3.0 kg/m2 compared to no mulching which can be attributed to the influence of degree-day accumulation and lower temperatures during vegetative flushing.
4.7. Guava plant growth, fruit and leaf parameters
Increased guava plant growth and fruit yield with biomass mulching have been reported by several researchers (Das et al., 2010; Bhattacharjee, 2020; Nandhini et al., 2021). The improvements in guava plant growth parameters with Tephrosia biomass mulching can be attributed to the improvement in soil fertility, as evidenced by increased soil nutrient and soil moisture regime. Das et al., (2021) also reported a better trunk diameter under Tephrosia treatments under bael orchard. The substantial enhancements in plant growth parameters during the second year of the study can be attributed to the cumulative effects of previous years' biomass mulching. There was in increment in fruit weight and yield of guava, similar study was also reported by Shukla et al., (2009) by the use of organic fertilizers and bio fertilizers. The increase in photosynthesis resulting from increase in number of leaves and total chlorophyll content might have contributed towards the increase in fruit weight and fruit yield. The higher leaf number recorded in the present study can be attributed to the increase in macro and micronutrient content in the soil due to biomass mulching.
Leaf anthocyanin pigmentation, an indicator of low temperature stress (Hao et al., 2009). This phenomenon is commonly observed across various guava genotypes. Interestingly, present study revealed that mulching played a significant role in reduction in leaf anthocyanin content which might have contributed towards shortening of the duration of the winter bud stage in our phenological study.