3.1. Change in physicochemical properties
The initial pH in all the composts treated soil was acidic and higher than control (Table 1), with 5.45, 6.52, 6.62 and 6.92 in C, LP, VW and CF composts treatments respectively. During the treatment period it was observed that in all the three types of composts amendment there was no significant variation among the sampling days (Fig. 1), although showing a rise till 120 days and a gradual declining pattern. Maximum increase was recorded in LP with 22% and minimum in CF with 10% (p<0.01) at 270 days (Table 2) with nearly neutral pH of 7.1 in LP and a minimum in CF compost treatment with 6.4(P<0.001).Compost that have a near-neutral or slightly alkaline pH with a high buffering capacity usually elevate pH of acid soils (Shiralipour et al. 1992).Lee et al. (2019) also reported a slight increase in soil pH(7.31 to 7.81) due to treatment of food waste compost at application rates of 10 to 60 Mg/ha compared with control(7.16). Increment in soil pH after compost application was due to decomplexation of metal cations and mineralization of organic N (Amlinger et al. 2007) causing release of K+, Ca2+, Mg2+and OH-during the degradation of organic products in soil (Erana et al. 2019).
During the amendment period soil temperature in composts treated soils showed a higher level till 240days than control with a maximum range of 28.3 to 45.5oC in CF and minimum of 28.0 to 38.5oC in VW compost treated soil (Fig. 2). Soil temperature showed significant variation among the sampling days (P<0.001)and within the different treatments (P<0.001). At 30 days the increase in temperature was maximum in all the three treatments compared to control with 52, 24 and 20% increase in CF, LP and VW respectively (P<0.01). However, a gradual declining pattern was shown in all composts treatments. At 270 days soil temperature was lower than control in LP and almost similar with control in VW but in CF there was still a 2% higher rate than control. Deguchi et al. (2009) have stated, the main factors contributing to increase in soil temperature due to compost application was the decrease in amount of water drawn from the deeper layers which decreased evaporation. Reduction in evaporation results from reduction in bulk density of soil. Amlinger et al. (2007) have cited more absorbing capacity of solar radiation by the dark colored decomposed organic matter and humic substances contained in the compost. The increase in soil temperature was very high although a gradual decline was observed. The reason can be attributed to the high rate of compost application in the present study. A step to reduce the soil temperature can be, allowing the composts to stabilize for at least 30 days before application in field.
OC was higher in all the composts treated soil compared to control in the initial quality (Table 1). Significant variation of OC was observed among the sampling days (P<0.001)and between the different composts treatments (P<0.001). Maximum range of 3.0 to 4.58% was recorded in CF compost treated soil and minimum in VW with 2.1 to 3.5%during the 270 days period. There was a gradual increase in the initial days, however a declining pattern was recorded till the end of the experiment (Fig. 3).Maximum increase of OC was observed in CF compost treated soil with 106% followed by LP with 64% and 24% in VW at 90 days (P<0.01). At the end of 180 days it was more or less similar with control in VW and LP composts treated soil. However, CF compost treatment showed increase of 2% at 270 days. Brown and Cotton (2011) reported an increase of OC in agricultural soils in California by 3 times due to treatment of compost in comparison to control soils. Riwandi et al. (2015) also reported an increase of OC at 2.76% due to application of compost. Total C increased upto 1.86gkg-1 from 0.76gkg-1 indicating a rise of 145% and it was positively correlated with microbial community composition(Lee et al. 2019). Application of compost leads to increase in soil organic matter thereby increased OC and providing more void space in soil. The organic matter has a lower density than the mineral fraction of soil which is also refer to as fluff effect (Kranz et al. 2020).
There was a significant variation of TN among the treatments (P<0.001) and within the sampling days (P<0.01). Compared between the three types of composts treatments initial TN was very high in CF with 2.98gkg-1 as well as throughout 270 days treatment. In CF the TN increased rapidly upto 60 days recording upto 6.36gkg-1 providing a maximum range of 2.7 to 6.36gkg-1. Minimum range was recorded in LP with 1.15 to 1.89 gkg-1. In the composts treated soils a comparatively higher level of TN upto 90 days with 484, 100 and 46% rise in CF, VW and LP (P<0.01) was recorded respectively. However, in LP and VW composts treatments a drastic decline started from 120 days which gradually equals with control at 270 days. However, at 270 days there was still 48% increase of TN in the CF compost treated soil. The high release rate of TN in CF compost can be attributed to the low initial C/N ratio. Similar observations on increase in TN due to treatment of food waste compost was reported by Kelly et al. (2020) with 11%. They also observed that compost derived from food waste provided more N benefits than cow manure-derived composts and have greater nutrient value.
Initial AP was higher in the composts treated soils compared to control (Table 1). Significant variation of AP was observed among the sampling days (P<0.001) and within the treatments (P<0.001).A gradual increase in AP upto 60 days in all composts treatments was recorded (Fig. 5). Maximum was recorded in CF with a range of 43.42 to 121.03mgg-1 throughout the experiment. In VW it varied from 43.73 to 102.42mgg-1 and in LP it varied from 32.0to 108.3 mgg-1. At 90 days a maximum increase of 787% in CF, 541% in LP and 532% in VW (P<0.001) was recorded followed by a gradual declining pattern. At 270 days there was still a very high increase rate with 551, 350 and 269% in CF, VW and LP treatments respectively. Kelly et al.(2020) reported a similar increase of 127% in AP due to treatment of food waste compost in soil. Manirakiza and Seker (2020) was reported an increase of AP by 48.7mgkg-1in soil due to application of compost.
Initial AK was maximum in VW with 320mgkg-1and minimum in CF with 80mgkg-1. Significant variation of AK was also observed among the sampling days (P<0.03) and within the treatments (P<0.001). Maximum AK was recorded in VW with a range of 230.2 to 648.42mgkg-1 and minimum in CF with a range of 103.3 to 256.0mgkg-1throughout the experiment (Fig. 6). Maximum increase of 1383% in VW and minimum of 207% in CF was recorded at 90 days. There was a gradual decline in all treatments however similar to AP at the end of 270 days the level of AK was still high with 703, 503 and 170% increase rate in LP, VW and CF treatments respectively (P<0.05). Manirakiza and Seker (2020) reported an increase of AK by 397 mgkg-1with 2% application rate of compost due to high presence of potassium in compost which was later released into soil through microbial degradation. Akoijam et al. (2017) also reported a significantly higher level of AK in soil due to treatment of flower waste, fish waste and sugarcane bagasse.
3.2. Correlation among the soil characteristics
In LP compost treated soil OC showed significant positive correlation with AP (r=0.83;P<0.05) and AK (r=0.74;P<0.05) (Table 3). Significant positive correlation was also recorded between AP and AK in LP compost treatment(r=0.90;P<0.01). In VW compost treatment significant positive correlation was recorded between AP and AK (r=0.80;P<0.05) only. In CF compost treatment significant positive correlation was recorded between soil temperature and OC (r=0.92;P<0.01); soil temperature and TN (r=0.80; P<0.05) and TN with AP (r=0.83; P<0.05). In control only AP was significantly correlated with AK(r=0.82; P<0.05).The significant correlation of AP and AK in LP, VW and C showed that a similar trend existed in availability and declining pattern of AP and AK in soil. The role of OC and TN in regulating soil temperature in CF compost treatment was prominent from the correlation studies. Since OC and TN was maximum in CF the soil temperature was more regulated.