Impact of seasonal variations on physico-chemical and exchangeable cations of forest soil in Doon Valley, Uttarakhand, India

ABSTRACT The aim was to investigate the role of seasonal changes of physico-chemical parameters in forest soil of Doon Valley of Uttarakhand, India. The samples were collected from four different environmental conditions viz. urbanized area, industrial area, urbanization with dense forest area, and wetland area during summer, monsoon, and winter seasons from two different depths of 0–15 cm and 15–30 cm, respectively. The soil parameters were analyzed such as pH, total nitrogen, total phosphorous, total hydrogen, total carbon, organic carbon, inorganic carbon, exchangeable cations such as sodium, potassium, calcium, and magnesium. The result revealed that site-I and IV of both depths were found to be of good quality due to less human interference, low urbanization effect, and covered by the forest area whereas site-II and III were confirmed of poor quality. Pearson’s correlation coefficient analysis suggest soils at both depths has following relation: total carbon and nitrogen (r = 0.90), magnesium and potassium (r = 0.74), Ca with N, P, C, OC, Na, K, Mg, organic carbon and total phosphorous (r = 0.85), organic carbon and nitrogen (r = 0.91). Hence, the soil fertility is good and very important for plant growth and development in agriculture process for sustainable development.


Introduction
Soil supports all the terrestrial ecosystems that cycle a major portion of the atmospheric as well as terrestrial carbon and is a sink and source of carbon dioxide emission. Soil serves as a key part among all of the terrestrial systems, providing dwelling place for plants, animals, and micro-organisms (Deyn & Van der Putten, 2005). It is considered as an active and significant part associated with global carbon emission and sequestration as it contains more inorganic and organic carbon than the atmosphere-biosphere (Post & Mann, 1990). Rapid urbanization and industrialization with excess usage of chemicals have been associated with deterioration of soil quality. It acts as a loose covering of mineral particles finely covering the earth's surface (Birkeland, 1999) and a biological medium for plant growth (Ghorade, 2013); in addition host enormous biodiversity in the terms of species abundance and functionality (Adedokun & Ataga, 2007;Adenipekun, 2008;Onuh et al., 2008). Soil is developed as a result of pedogenic processes . The soil texture depends upon the percentage of silt, clay, and sand particles in the soil (Gaikawad et al., 1974). The lifesupporting property of soil can be attributed to the five main processes including pollutant detoxification, biomass productivity, cycling of C, N, P, S, and H 2 O and carbon sink (Blakeslee, 2010;Hansen et al., 2008;Lal, 2004). Constructive forces produce new chemical compounds, including both mineral and organic and provide new distribution characteristics, chemical compositions, and structural properties. These factors affect the growth of the plant in the soil (Anu et al., 2010). Singh et al. (2012) investigated plant community characteristics and soil status in forested state. Kumar et al. (2013) highlighted the role of forest in soil of different land use system of North-east, India. Rawat et al. (2019) investigated the role of topography on soil quality of Jharkhand, India. Yinga et al. (2020) have evaluated the effect of various existing hill landuse practices in Mizoram, India, on soil quality. Rawat et al. (2020) has studied the nickel distribution in soil in different agro-climatic zone.
Soil fertility is generally connected with soil superiority and soil conditions (Ajgaonkar, 2017). Soil properties of terrestrial ecosystems rely on an assortment of abiotic and biotic components that fluctuate both spatially and seasonally (Dar Zubair et al., 2018;Peverill et al., 1999). Forest soil plays a crucial role in determining the sustainable efficiency of forest ecologies (Dar Zubair et al., 2018). The present study focuses on physic-chemical parameters evaluation i.e., pH, total nitrogen, total phosphorous, total hydrogen, total carbon, organic carbon, inorganic carbon and exchangeable cations like sodium, potassium, calcium and magnesium and seasonal variability in forested soil.

Study area
The Doon valley is a wide bouldry valley of Uttarakhand, India, which lies between Shivalik hills and Lower Himalayas (latitudes 29°55ʹN and 30°30ʹN and longitudes 77°35ʹE and 78°24ʹE) and covers an area of about 150 km 2 ; approximately 20 km wide and 80 km long. The average annual temperature of Dehradun was 29.2°C. In addition Dehradun received an average annual rainfall of 2456.1 mm. According to 2011 Census of India, Dehradun district had a population of 1,696,694 and population density of 550 per/km 2 .
The physico-chemical parameters and exchangeable cations of soil had determined at four study sites represent different environmental conditions due to urbanization at Clock Tower (Site-II), industrial area with rapid urbanization at Selaqui (Site-III), FRI campus (Site-I) has some urbanization with forest areas and Karwapani (Site-IV) a wetland area with some traditional villages.

New forest area (FRI campus)
New forest estate is located north of Dehradun-Chakrata Road, approximately 6 km from main Dehradun town surrounding the clock tower. The area is situated between 30°20ʹ05.4" N Latitude and 77°59ʹ15.3" E longitude and 640.08 m above mean sea level (amsl).

Clock tower (Urban area)
The clock tower is the main part of Dehradun city and is encircled by the city's commercial centres such as "Paltan bazar" with its old traditional shops, cinema halls etc. The area is situated between latitude 30° 19ʹ32.4" N and longitude 78°02ʹ34.4" E and is located at an elevation of 640 m (amsl).

Selaqui (Industrial area)
Selaqui is located west to the main township of Dehradun along the Chakrata road and is approximately 19 km from the clock tower. The area is situated between latitude 30°21ʹ40"N and longitude 77° 50ʹ44.8"E at an altitude of 635 m (amsl). Industrialization is taking place very rapidly in the region.

Karwapani (Rural area)
There are a number of wetlands in the Doon valley, the most common are freshwater swamps include Karwapani swamp is situated near Manak Sidh temple nearby Shimla road and is about 15 Km north of Dehradun occupying an area of about 3-4 m 2 . It lies in the latitude 30°2" to 30° 26" (N), and Longitude 77° 52" to 78°-19" (E) (Subba et al., 2014).

Sample collection and analysis
Soil samples were collected from four sites namely site-I, site-II, site-III, and site-IV, respectively, from two different depths i.e., 0-15 cm and 15-30 cm. The samples were air dried and sieved with 2 mm sieve for further analysis. Physico-chemical characteristics of soil along with exchangeable cations were estimated and followed standard method of Jackson (1967), Piper (1950, Olsen and Dean (1965), and Walkley and Black (1934), and Vogel (1961), respectively.

Statistical analysis
The data obtained from soil analysis were further analyzed statistically using GenSTAT software. The significance tests were performed among sites, seasons, and among sites and seasons together at 0.05 levels using ANOVA test. ANOVA help to identify the variability among the group and between the groups (Gautam et al., 2021). Correlation analysis was performed which help in identifying the relationship between the parameters using SPSS software.

Physical and chemical property of soil
The pH of soil at all sites is acidic in nature. The pH of soil samples in 0-15 cm depth was recorded higher (6.56) at site-II and comparatively lower pH (6.44) at site-IV. However, soil samples of 15-30 cm depth, the higher pH (6.69) was observed at site-III and lower pH (6.59) at site-II ( Figure 1). The pH was recorded in the order of winter>summer>monsoon in surface (0-15 cm) soil and summer >winter>monsoon in subsurface (15-30 cm) soil.
Soil pH is a good indicator for possible nutrient status of the land. Soil having pH < 5.6 is usually considered to be acidic in nature, pH from 5.6 to 6.0 ranges is moderately acidic while < 5.5 are strongly acidic in nature (ICAR, 2005). Similarly, soil pH is the most important factor that affects the heavy metal solubility and their availability to the plants (Reddy & Patrick, 1977;Zeng et al., 2011;Zhao et al., 2010). The natural forest has lower pH level as compared to the grasslands because the organic matter present in the form of plant litter, compost, and manure decreases the soil pH through the decomposition processes (Brady & Weil, 2002). Robertson and Vitousek (1981) and Adams and Sidle (1987) also recorded low pH level in the soils of undisturbed natural forest compared to disturbed forest soils. It was reported that for balanced nutrient supply, and forest soil should be slightly acidic in nature (Leskiw, 1998). Semwal et al. (2009) reported that the pH levels were very acidic in the undisturbed forest when compared with the disturbed forest. Elevated levels of humus in forest soils are attributed for the low pH levels (Dimri et al., 1987).
Total nitrogen in soil samples revealed that the higher value of total nitrogen in 0-15 cm depth was at site-IV (0.18%) and lower was at site-III (0.13%), whereas, in case of 15-30 cm depth, the highest nitrogen (0.17%) was recorded in both site-I and site-IV and lowest was at site-III (0.10%; Figure 2).
Total nitrogen percentage was recorded maximum during winter followed by summer and monsoon in surface soil and similar pattern of distribution was recorded in sub-surface soil. Nitrogen is an important nutrient that affects both the soil fertility and plant strata. Evidence show that the increased biological nitrogen fixation as well as the increased rate of mineralization occur during the rainy season, which results in increased levels of nitrogen during this season (Bergeron et al., 2002). Elevated levels of total nitrogen in the soil during rainy season indicates blue green algae fixation, input from rain water and higher release rate of mineral nitrogen by means of microbial decomposition (Birch, 1958;Choudhri & Sharma, 1975). Singh and Singh (2006) reported that plant uptake of nutrients during dry periods is significantly reduced and the process of N-mineralization and nitrification are either immobilized in the microbial biomass or are accumulated in the soil in the form of inorganic nitrogen.
Total phosphorous in soil samples recorded higher value (0.04%) at site-IV in depth of 0-15 cm and site-III recorded comparatively lower value (0.02%) in the same depth. Meanwhile, in 15-30 cm depth, the higher value (0.03%) recorded at site-I and site-IV both and lower (0.02%) recorded at site-III ( Figure 3).
In during all three season, almost equal pattern was recorded in surface and sub-surface soil and maximum was recorded in winter followed by summer and monsoon season. Phosphorus being a part of each plant cell every plant activity including the growth, respiration and reproduction depends upon the phosphorus content (Rai et al., 2011). The carbonphosphorus and nitrogen-phosphorus ratio depends upon the parent material which in turn is decided by extent of weathering and by other means (Paul & Clark, 1996). The rates of weathering also affect the availability of phosphorus to the plants. Phosphorus in turn affects the input level of the plant residue (Brown et al., 1994). The amount of phosphorous in the soil at a place determines its characteristic for allowing the growth of the specific type of plants and hence determine the type of vegetation in that area. Phosphorus is essential in cell division and energy transformation and also metabolic processes in plants (Rai et al., 2011). Total hydrogen percentage compared in soil samples and the highest value was recorded at site-I (0.74%) and lower was at site-III (0.36%) in the depth of 0-15 cm, where as in 15-30 cm depth, the highest value was recorded at site-I (0.80%) and lowest was recorded at site-III (0.39%) as shown in Figure 4.
The total hydrogen was found maximum during summer followed by winter and monsoon in surface soils and sub-surface soil, respectively.
Total carbon of soils were also analyzed and the highest value was recorded at site-IV (2.370%) while the lowest at site-III (0.980%) in 0-15 cm depth ( Figure 5). However, in 15-30 cm depth, the highest value was recorded at site-IV (1.946%) while the lowest value was recorded at site-III (0.975%).
Total carbon was recorded maximum in winter in surface soils while; minimum was recorded in monsoon of sub-surface soils. Similarly, percentage organic  carbon determined in the soil samples and it was found highest in 0-15 cm depth, was recorded at site-IV (2.368%) while the lowest at site-III (0.997%); while in 15-30 cm of soil depth also organic carbon was the highest (1.943%) at site-IV and lowest (0.972%) at site-III ( Figure 6). The percentage organic carbon in surface and subsurface soils was found maximum in winter and minimum in monsoon. Sevgi and Tecimen (2008) reported that due to the production and return of high amount of litter in natural forest, the organic carbon content is also high. The nutrients release after litter decomposition occurs naturally in the internal biogeochemical cycle of an ecosystem (Yinga et al., 2020). The decomposers recycle a huge amount of carbon that remains bounded in the plant or tree into the atmosphere. The results also revealed that the soil in the natural forest had the maximum organic carbon content in all the seasons while the minimum was recorded under the grassland in all the seasons. This may be due to the fact that forests have greater canopies and they provide litter in greater quantity (Yinga et al., 2020), hence accumulating higher carbon content compared to the grasslands. Approximately, 40% of the total stock of SOC in the global soils resides in the forest ecosystem . However, inorganic carbon in soil was recorded highest in soil of site-II (0.0032%) followed by site-III (0.0031%) and lowest was at site-IV (0.0020%) in depth of 0-15 cm ( Figure 7). However, in 15-30 cm depths, the highest inorganic carbon was recorded at site-III (0.0031%) and lowest at site-IV (0.0020%).
The inorganic carbon was observed in the order of winter>summer>monsoon in both surface and subsurface soils, respectively.

Exchangeable cation and anions
Exchangeable sodium in soil sample was assessed and the highest concentration of exchangeable Na was recorded at site-IV (23.94 mg/kg) and lowest concentration at site-II (14.56 mg/kg) in 0-15 cm depth. While in 15-30 cm depth also, the highest concentration of Na was recorded at site-IV (19.61 mg/kg) and lowest concentration (12.44 mg/kg) was recorded at site-II. It is observed that Na in both the depths was higher in concentration at site IV and lower at site II of all four study sites (Figure 8). The Na concentration maximum during winter followed by monsoon and summer in surface soil while in sub-surface soil, minimum was in monsoon. An increase in exchangeable sodium percentage would bring about dispersion of the soil macro-colloid and highly dispersed soil is characterized by defluctuation and crusting (Foth, 1984). High level of exchangeable sodium affects the permeability of soil and may also be toxic to the sensitive plants.
Exchangeable K in soil was depicted and the highest concentration of exchangeable potassium was recorded site-IV (94.06 mg/kg) and lowest concentration in soil samples of site-II (69.67 mg/kg) in 0-15 cm depth. The same trend was observed in the depth of 15-30 cm as the highest concentration in this depth also recorded at site-IV (79.11 mg/kg) and lowest concentration at site-II (63.00 mg/kg). K has also shown highest concentration at site IV and lowest at site II just like Na, in both depths of soil (Figure 9).
The K content was observed in the order of win-ter>monsoon>summer in surface soil and monsoon>-winter>summer in sub-surface soils. The decrease of K is associated with leaching and drainage processes which leads to the destruction of vegetation (Basumatary & Bordoloi, 1992). Potassium performs extremely important roles in different process such as regulating both transpiration and respiration, influencing the enzymatic action, synthesis of proteins and carbohydrates etc. (Brady, 1996). A positive correlation between the organic matter content and available potassium indicating that with the increase in organic matter there is a corresponding increase in the accumulation of available potassium in the soil reported by Chauhan (2001).
Exchangeable calcium shown highest concentration in soils of site-IV as 80.98 mg/kg and 71.36 mg/kg, respectively at both the depths i.e., 0-15 cm and 15-30 cm. The lowest concentration of exchangeable calcium was recorded as 56.04 mg/kg in the soils at depth of 0-15 cm at site-III and 55.26 mg/kg in the depth of 15-30 cm of site-III again can be seen in Figure 10. Hence, site IV had the highest and site III had the lowest concentration of Ca in both depths of soil.
The Ca concentration was maximum during winter followed by summer and monsoon in surface soil and winter>monsoon>summer in sub-surface soils.
Exchangeable magnesium in soil sample was determined and the highest concentration was recorded at site-IV (44.73 mg/kg) and lowest concentration at site-III (21.91 mg/kg) in depth of 0-15 cm. In case of 15- 30 cm depth, again the highest concentration was recorded at site-IV (44.79 mg/kg) but the lowest concentration was recorded in soil samples of site-III (19.54 mg/kg). Again site IV has shown highest concentration of Mg in its soil samples as shown in Figure 11. The Mg content was maximum during summer followed by winter and monsoon both in surface subsurface soil, respectively. The mobility of calcium and magnesium in soil is relatively restricted because of their higher ionic charge and thus are bound more to the exchangeable sites than ions of lower ionic charge. Exchangeable calcium and magnesium are the secondary nutrients which are required in comparatively smaller yet in appreciable quantities (Tukura et al., 2013). Calcium deficiency is very uncommon if the soil pH is adequate (Snober et al., 2011).

Analysis of variance
ANOVA was used to see the correlation among sites, among seasons and sites in 0-15 cm depth as can be seen in Table 1. The results revealed that there was non-significant difference among sites but nonsignificant difference in seasons and in between sites and seasons.  The ANOVA in depth of 15-30 cm also reveals the same i.e., there was non-significant difference among sites and seasons but significant difference in between sites and seasons ( Table 2). The ANOVA reveals that there was non-significant difference among sites, season and in between sites and seasons in both 0-15 cm and 15-30 cm depths as shown in Tables 4 and 5, respectively.

Pearson's correlation coefficient matrix
Pearson's correlation coefficient matrix worked out for various physico-chemical attributes of soil with exchangeable cations in both the depths at all four study sites. A correlation analysis presents an effective approach to understand the relationships between multiple different variables and thus have been useful to identify the influencing factors and the sources of chemical components. Pearson's correlation coefficient analysis is used to measure the degree of correlation between the exchangeable cations and physicochemical parameters of soils in the selected sites of study areas. Table 3 shows this correlation coefficient matrix for 0-15 cm depth of soil along with their level of significance at 0.05 or at 0.01. As it is clear from the table there are some pairs of soil attributes at 0-15 cm depth showed positive correlation between them; as nitrogen and phosphorous are positively correlated with each other at 0.01 significance level with correlation coefficient r = 0.74.
Similarly, total carbon and phosphorous (r = 0.85), total carbon and nitrogen (r = 0.90), organic carbon and total phosphorous (r = 0.85), organic carbon and nitrogen (r = 0.91) have shown positive relation because of good indicator of soil fertility and it is very important for plant growth and development, organic carbon and total carbon (r = 0.99), sodium and nitrogen (r = 0.53), sodium and total carbon, potassium and phosphorous (r = 0.71), potassium and nitrogen (r = 0.70), potassium and total carbon (r = 0.65), potassium and organic carbon (r = 67), magnesium and phosphorus (r = 0.62), magnesium and nitrogen (r = 0.69), magnesium and total carbon (r = 0.70), magnesium and organic carbon (r = 0.71), magnesium and sodium (r = 0.70), magnesium and potassium (r = 0.74), calcium with phosphorous, Ca with N, P, C, organic carbon, Na, K, Mg has positive correlation at 0.01 level.
Similarly, the correlation between the soil attributes including heavy metals in soil of 15-30 cm has been developed and depicted in Table 4 which shows both positive and negative correlations. There was positive correlation between nitrogen and phosphorous (r = 0.51), total carbon and nitrogen are positively correlated (r = 0.66) at 0.01 level of significance.

Cation exchange capacity (CEC) and the percent base saturation (%)
In the case of deep soils having low cationic concentrations, they may have either moderate or high total quantities of base cations. The level of exchangeable cations is affected by various factors such as depth to the underlying bedrock and slope position as well as an additional split by the bedrock formation. In the present study only surface soil has been studied up to 30 cm depth, hence the concentration of Mg in all cases is low as shown in Table 5.
The cation exchange capacity describes the holding capacity of a particular soil for positively elements (cations). It also describes the capacity for a soil to exchange cations for another. Higher the clay content,  higher the cation exchange capacity, because clay particles have the maximum surface area per unit volume of soil and therefore can hold most cations (Yinga et al., 2020). The maximum percentage contains calcium (44.16%) at site II and magnesium (40.67%) at site III, however in soil maximum amount of free calcium and magnesium, respectively present in soil solution.

Conclusion
The large-scale chemical analysis of the soil samples can be used as a database for identifying the potential micronutrient deficiency. The natural forests have the lower pH values which are attributed towards their undisturbed nature along with high organic content in the form of litter over the soil including humus, microorganisms, etc. pH value of both depth shows that the soil is acidic in nature. Total nitrogen at site-I and site-IV is higher in both the depths as compared to site-II and III. Nitrogen in soil is an indicator of good soil. Similarly total Carbon and Organic Carbon values are also indicator of good/fertile soil. Total Carbon and Organic Carbon both are more in soils of site I and sit-IV. Exchangeable cations i.e., Na, Ca, and Mg, are also higher at site-IV then at site-I in concentration as compared to site-II and site-III. Overall, site-I and IV of study sites in both depths were found good quality while at site-II and site-III was in bad quality due to human pressure or anthropogenic activities. The low levels of the Ca and Mg may be linked with the increased binding of Ca and Mg in soil to macrocolloid lead to the reduction in the availability in the soil solution. Level of Na is related to the problem of the soil dispersion. The main reason behind the decrease in K level is both leaching and drainage, which subsequently results in the destruction of vegetation. This work is useful to the soil resource managers and agriculturalists.