NOTATIONS
4.4 PHASE IV - COMPOST APPLICATION IN SOIL
4.4.5 Discussion
The composts used in this research work showed enhanced mineral composition and nutrients characteristics compared to other composts prepared from different wastes such as sewage sludge, municipal solid, agro-industrial wastes which have been used so far as organic fertilizers (Paredes et al., 2005; Fernández-Hernández et al., 2014).
Soil Organic matter content is an essential element in the soil that supplies necessary plant nutrients, aid in reducing soil erosion, and improves soil aggregation as well as water holding capacity (Ryals et al., 2014). SOM is the primary source that provides carbon as energy to the soil microbes which regulates nutrients (Cooperband, 2002). The poor SOM content in both soils is mainly due to the heavy rainfall that occurs in the study area (Guwahati, Assam). The recurrent flood washes away the SOM from the topsoil surface. But the application of organic compost shown an immediate positive effect on both soils, i.e., SOM content had shown the significant increase with increasing application rates. However, when 120 days of study is considered the SOM content was found decreasing in all the treatments compared to control SOM content. This fact might be due to the mineralization process that has been occurred in the soil by the microbial biomass (Marschner and Kalbitz, 2003; Kemmitt et al., 2008). The SOM content can also decline because of erosion and repeated cultivation (Wolka and Melaku, 2015).
Other researchers observed similar patterns for SOM during various compost amendments in sandy and different soils (Navas et al., 1998; Baiano and Morra, 2017;
Goswami et al., 2017; Weber et al., 2007).
So, far many researchers have explained the fact of the increase in the SOC after the application of biochar (Lehmann et al., 2011; Kuppusamy et al., 2016). Biochar is a carbon-rich material. Hence, in the present study, the combination of H. verticillata and biochar significantly increased the SOC content with the increasing application rates in both soils. However, SOC was seen to be reducing with time in all the treatments. The carbon reduction from the substrates varies between 10-70% that depends on the soil micro-flora and the synthesized microbial cells.
Even after 120 days of the compost application, the SOC contents were observed as higher compared to the control pots of both soils, i.e., LS0 and AS0, respectively.
The pH values of the raw soil (LS and AS) used in this study were observed to be acidic. This study followed the similar pattern for the soil pH as observed in other studies on sandy soils (Fowles, 2007; Bass et al., 2016). However, it was contradictory to the study carried out by Abujabhah et al. (2016) on the temperate agricultural soil.
The effect of compost application prepared from biochar on soil pH is reliant on the initial pH of the raw materials and the biochar itself, which is dependent on the type of the feedstocks and pyrolysis conditions used for biochar production (Lehmann et al., 2011; Abujabhah et al. 2016). As the biochar utilized in this research work had a slightly basic pH of 8, therefore, it was apparent to exhibit increase in the pH due to its higher buffering capacity. The decrease in organic matter may also the responsible for the rise in the soil pH due to the microbial activity. This pH pattern is following the other studies of compost application conducted on various soils (Jiang et al., 2006; Wolka and Melaku, 2015). According Jiang et al. (2016), the higher pH may increase the microbial community and can able to change its composition.
The LS soil exhibited lower nutritional (TKN and AP) values compared to that of AS.
As anticipated, application of the compost enriched with various nutrients shown immediate positive effect during treatment of both soils. Multiple researchers observed the similar patterns for TKN and AP after compost application (Lehmann et al., 2011;
Abujabhah et al. 2016; Baiano and Morra, 2017; Goswami et al., 2017). The TKN and AP were seen to be reducing with time in all the treatments. According to Wolka and Melaku (2015), the decline in nutrients might be due to leaching or nitrification process.
The reduction also depends on the soil micro-flora and the synthesized microbial cells.
Even after 120 days of the compost application, the TKN and AP contents were depicted higher compared to the control pots of both soils, i.e., LS0 and AS0, respectively. A fascinating observation was represented in this study that despite higher compost application rate in AS soil, i.e., AS30 showed lesser TKN and AP content compared to AS20. This fact is due to the more significant reduction in porosity, which caused substantial leaching of the nutrients. However, in the case of LS30, the higher amount of TKN and AP concentration was observed.
The impact of compost application on soil physicochemical properties may be measured concerning advantageous effects on soil sorption capacity, i.e., WHC and CEC. Various researchers observed the similar results for CEC and WHC after compost application (Gallardo-Lara and Nogales, 1987; Leifeld et al., 2002; Weber et al., 2007), however, regardless amounts of compost, amendments did not affect the soil reaction.
The compost prepared using biochar had shown significant changes in the soil porosity that aid in improving its sorption or hydraulic properties. Curtis and Claassen (2005) reported a 2-fold increase in WHC after compost application at the rate of 24%. In the present study, WHC and CEC both increased with the increasing rate of application.
This WHC pattern is following the other studies of compost application conducted on various soils (Aggelides and Londra, 2000). However, the study by Mamo et al. (2000) reported no significant effect on WHC after compost application.
Biochar is also known for its higher affinity for CEC and WHC. In the present study, also due to an addition of Biochar the CEC and WHC observed to be improved. Such increasing pattern of WHC in sandy soils was also reported (Abel et al., 2013). But according to Xu et al. (2012), improvements in soil WHC by Biochar additions are mainly restricted to coarse-textured soils. Previous studies suggest the improvement in physical properties of soil after compost application (Celik et al., 2004; Głąb, 2014). The significant impact on physical properties of the soil was observed in this study after compost application. According to Głąb et al. (2018), these changes in BD and TP are primarily due to an addition of less dense material (compost) with the soil. Similar patterns were also observed on fine and coarse-textured soils in the earlier studies (Celik et al., 2004; Głąb, 2014; Głąb et al., 2016). A review study also presents a healthy relationship between the compost application and its impact on physical properties (Hargreaves et al., 2008). The higher the compost application rates, the lower the BD and higher the TP. This variation pattern in BD and TP is following the other studies of compost application conducted on various soils (Aggelides and Londra, 2000; Pagliai et al., 2004). Variations in BD were suggested in the differential porosity of the soil. Thus compost application increased pore volume as compared to control. A similar effect was observed in the study conducted by Larney and Angers (2012) who noted that soil microporosity and macroporosity increased with the compost or livestock application.
Moreover, an addition of the biochar during composting significantly aid in improving the physical health of the soil in relation to the control study after compost application in the LS and AS.
According to Tejada and Gonzalez (2008) and Jien and Wang (2013), the biochar amendment in soil contributes to altering soil aggregate sizes, which in turn decreases the bulk density of the soil. Even after 120 days of the compost application, the BD and TP contents were depicted lower compared to the control pots of both soils, i.e., LS0 and AS0, respectively.
Table 4.17. Results from a two-factor ANOVA testing for the effects of days and compost rates on two different soil parameters
Factors df Parametersa
SOM SOC pH TKN
F p F p F p F p
Treatments 7 26 < .05 47.7 < .05 514.2 < .05 66.6 < .05 Days 6 16 < .05 16.1 < .05 11.4 < .05 15.9 < .05
a SOM: Soil organic matter; SOC: Soil organic carbon; TKN: Total Kjeldahl Nitrogen;
Factors df Parametersa
AP CEC WHC BD TP
F p F p F p F p F p
Treatments 7 78.4 < .05 374 < .05 69.6 < .05 64.8 < .05 20.1 < .05 Days 6 10.0 < .05 14.3 < .05 15.5 < .05 8.2 < .05 11.4 < .05
AP: Available Phosphorus; CEC: Cation-exchange capacity; WHC: Water Holding capacity; BD: Bulk density; TP: Total Porosity