Earthworm population
A total of 347 earthworms were collected, (217 from agricultural sites and 130 from non-agricultural sites). The earthworms belonged to three ecological categories, six species [A. corticis (Kinberg, 1867), A. morrisi, A. robustus, M. posthuma (Vaillant, 1868), P. excavatus (Perrier, 1872), and B. parvus (Eisen, 1874)] and were distributed variably in the study site (Table 1).
Out of these six species, five belonged to family Megascolecidae (M. posthuma, P. excavatus, A. corticis, A. morrisi and A. robustus) and one species belonged to family Lumbricidae (B. parvus). Furthermore M. posthuma, A. corticis, A. morrisi, A. robustus and B. parvus were exotic species and P. excavatus was the only indigenous species. M. posthuma, A. corticis, P. excavatus were distributed throughout the wetland area, whereas A. robustus was restricted to HK1 (present only during winter), B. parvus was restricted to HK2 and A. morrisi was restricted to MV only (Table 1).
A. corticis and P. excavatus were the second and third most abundant species. However, M. posthuma was not recorded from HK1 during winter and P. excavatus was absent from non-agricultural sites during spring and summer. Seasonal variation in the abundance of earthworms (species wise) in the study area is given in Fig. 1.
Heatmap showing the abundance of earthworm species across different sites and seasons.
X-axis represents sites and seasons, y-axis represents species. The color gradient indicates abundance, with red representing the highest values and dark blue the lowest.
We observed maximum number of individuals (171) during monsoon season (112 from agricultural sites and 59 from non-agricultural sites). Lowest number of worms (24) was recorded in the winter season from all the sites collectively (Table 2). A. corticis was present only at the agricultural sites during all the seasons except for monsoon when it was observed at all the sites. During this month, it dominated the population with numbers (59) more than that of M. posthuma (56). P. excavatus was observed at the agricultural sites throughout the year while it was present at all the sites during monsoon (Fig. 1). The earthworm population included highest percentage of M. posthuma (41.78) followed by that of A. corticis (27.66), P. excavatus (18.44), A. morrisi (6.91), B. parvus (4.89) and A. robustus (0.28). Out of the four sites, MV had maximum earthworm biomass (114 worms, 32.85% of total earthworms) while HK1 had lowest biomass (38 worms, 10.95% of total earthworms) throughout the study period.
After the flood (during autumn), we noted a decline in earthworm density across all the sites. Floodwater entered up to 80 m of the study area at MV {60 m of first study zone (whole area) and 20 m of the second study zone} and up to 70 m at KQK {60 m of first study zone (whole area) and 10 m of the second study zone}. Although HK1 and HK2 were less flood-prone, but during the monsoon season we observed that a 10 m wide stretch at HK2 (adjacent to the water body) became submerged under approximately 6 cm water. Water receded by 40 m at MV, by 30 m at KQK and by 5 m at HK2 during autumn. We found that the areas impacted by flood showed reduced earthworm populations in comparison to the unflooded areas. At MV, study zones 1, 2 and 3 contributed 17.64, 36.76 and 45.58% individuals to the earthworm population, respectively, during monsoon. During autumn, however, study zone 1 and 2 collectively contributed 28.57% and study zone 3 contributed 71.42% individuals. Similarly, at KQK also, the first study zone contributed 18.18%, while the second and third study zones contributed 36.3 and 45.45%, to the population of worms, respectively, during monsoon. During autumn, the biomass contribution was 17.39, 30.43 and 52.17%, respectively. The floodwater impacted only 10 m of HK2 site during monsoon, and contribution to the earthworm population was 14.63% by the affected study zone, but 34.14 and 51.21% by the second and third study zones, respectively. During autumn, contribution to biomass was 7.69, 30.76 and 61.53% by the study zones 1, 2 and 3, respectively. Although HK1 was unaffected by floodwater throughout the study period, abundance of worms declined during autumn here also. During monsoon, the study zones 1, 2 and 3 of HK1 contributed 11.11, 38.88 and 50% to the earthworm population, respectively, while during autumn, contribution by these zones was 10, 23.33 and 66.66%, respectively. Furthermore, we noticed a change in species diversity as there was absence of A. corticis, M. posthuma and P. excavatus from MV, HK1 and HK2, respectively, during autumn whereas these were present at the respective sites during the monsoon season. The species composition did not change at KQK even during autumn.
Diversity indices
Table 2depicts the seasonal variation in earthworm diversity indices [(Shannon-Wiener Index (H′), Simpson’s Diversity Index {S(1−D)}, and Species Evenness Index (SEI)] of the study area. The data clearly show that there was a non-significant difference in the values of three indices for the selected sites. The mean value of Shannon-Wiener Index (H′) was highest during monsoon (1.199) followed by autumn (0.925), spring (0.839), summer (0.802) and winter (0.739). Amongst the sites, maximum value of H′ was observed at HK2 during spring, summer and monsoon but it was maximum for KQK during autumn and for HK1 during winter. HK1 had lowest value of H’ during spring, monsoon and autumn but it was lowest for KQK during summer and for HK2 during winter.
The mean value for Simpson’s Diversity Index {S(1−D)}was highest during monsoon (0.685), declined during autumn (0.573) and became minimum during winter (0.498). During spring, the mean value increased to 0.541 again but then declined during summer (0.508). Site wise comparison showed that HK2 had maximum value of S(1−D) during spring, summer and monsoon but it was highest at KQK during autumn and at HK1 during winter. Mean value of Species Evenness Index (SEI) was highest during monsoon (0.958) followed by winter (0.947), spring (0.946), autumn (0.931) and summer (0.911).
Physico-chemical characteristics of soil and earthworm density
Two way ANOVA revealed that pH, moisture, EC, OM, clay, silt and sand in the wetland soil varied significantly during different seasons (p < 0.05), whereas only moisture, OM, clay, sand and EC varied significantly (p < 0.05) amongst the sampling sites during the study period. The interaction between sites and seasons was significant for moisture, OM, and clay. The earthworm density, however, varied significantly (p < 0.001) due to both seasons and sites (Table 3).
Values are presented as Mean ± SD. Superscripts a-c (seasons) and p-r (sites) indicate significant differences (p < 0.05). Significance levels are indicated as *** (p < 0.001), ** (p < 0.01), and * (p < 0.05).
Soil pH was in the basic range during spring, summer, monsoon, and winter but it became slightly acidic during autumn season. Comparison of the data shows that during the period of study, the soil pH was maximum at KQK in winter season while it was minimum at HK1 in autumn season. Out of all the sites, HK1 had minimum moisture (except for winter) while HK2 had maximum moisture (except for summer and monsoon) throughout the study period. Least variation was observed in the moisture content of HK2 during different seasons. Moisture in the soil was found to be maximum during monsoon (highest at MV), however, its lowest level was observed during winter (minimum at MV).
Soil temperature in the selected sites of the study area varied between 10.66 ± 0.57 -34.00 ± 1.00 oC during 2022. Soil of HK2 exhibited both highest (during summer) and lowest (during winter) temperature. Organic matter (OM) in the wetland soil ranged between 2.47 ± 1.31–11.30 ± 1.15% during different seasons. Content of OM was highest at MV during monsoon, whereas HK1 had lowest OM level throughout the year except for winter when it was lowest at KQK. Highest value of EC (289.00 ± 56.32 µS/cm) was recorded at HK1 during autumn while it was minimum at MV during monsoon (176.23 ± 36.90).
Maximum clay content in the soil was observed at MV during monsoon (30.33 ± 1.1%) and it was lowest at HK1 during winter (9.00 ± 1.0%). Further, silt content of the soil at various sites was maximum during winter and minimum during monsoon. Silt content showed maximum variation at KQK as it was highest (28.6 ± 3.5%) as well as lowest (16.00 ± 3.4%) at this site during the study period. Sand content of the wetland soil varied between 48.66 ± 14.7% at KQK during summer and 68.00 ± 3.6% at HK1 during spring. Sand content was found to be highest at HK1 and lowest at KQK. Earthworm abundance in the wetland area was highest (6.00 ± 1.73–22.66 ± 3.05) during monsoon but it was lowest (1.66 ± 0.57–2.66 ± 2.08) during winter. It was highest at KQK during spring, monsoon and winter but at MV during summer and autumn. However, the abundance was lowest at HK1 throughout the year (Table 3).
Soil macronutrients
There was a significant (p < 0.05) seasonal variation in the soil macronutrients at different sites (Table 4). Maximum OC in the wetland area was recorded during monsoon while it was minimum during winter. A comparison of the sites showed that OC was maximum at MV (6.57 ± 0.66%), however, it was lowest at HK1 during all the seasons except for winter when it was lowest at KQK.
Values are presented as Mean ± SD. Superscripts a-c (seasons) and p-r (sites) indicate significant differences (p < 0.05) Significance levels are indicated as *** (p < 0.001), ** (p < 0.01), and * (p < 0.05).
Highest TN, P, and Na in the soil of the study area were observed during monsoon, while these were lowest during winter. Site wise comparison showed that the level of TN and P was lowest at HK1 while Na was lowest at HK2 throughout 2022. Highest content of TN (except for spring and autumn) and P was observed at MV and that of Na at KQK throughout the study period.
Concentration of K in the wetland soil was maximum during monsoon but lowest during summer. It was highest at KQK (22.77 ± 1.87 g/kg), but lowest at HK2 (8.79 ± 0.81).
Heavy metals
Contents of Zn, Fe, Cu, Ni, Pb, Mn, Cr, and Li in the soil showed a significant seasonal variation across the selected sites (p < 0.05) of the wetland during the study period (Table 5).
Values are presented as Mean ± SD. Superscripts a-c (seasons) and p-r (sites) indicate significant differences (p < 0.05).Significance levels are indicated as *** (p < 0.001), ** (p < 0.01), and * (p < 0.05).
Maximum (0.86 ± 0.04 g/kg at HK2) and minimum (0.39 ± 0.15 g/kg at MV) content of Zn was recorded during spring and monsoon, respectively. Contents of Fe (106.29 ± 23.06 g/kg at HK1), Pb (0.13 ± 0.02 g/kg at HK1), and Mn (2.81 ± 0.52 g/kg at KQK) were highest in the soil during the winter season while their respective lowest values were found to be 50.28 ± 26.13 (monsoon), 0.04 ± 0.02 (summer) and 1.08 ± 0.26 (monsoon) g/kg at MV. Maximum (2.81 ± 0.52 g/kg at KQK) and minimum (1.08 ± 0.26 g/kg at MV) content of Mn was observed during winter and monsoon, respectively. Highest contents of Ni (KQK) and Cr (HK1) were observed during monsoon. However, minimum contents of Ni (MV) and Cr (KQK) were recorded during the summer season. Highest content of Cu (0.21 ± 0.03 g/kg at HK1) was recorded during summer, while its minimum concentration (0.06 ± 0.04 g/kg at MV) was recorded during monsoon. On the other hand, Li was maximum (KQK) during spring and minimum (HK2) during summer.
Principal component analysis for interaction between soil parameters and earthworm density
PCA extracted five principal components (PCs), collectively explaining 84.24% of the total variance in physico-chemical properties of the soil and earthworm abundance. The variance due to PC1, PC2, PC3, PC4 and PC5 was 41.09%, 14.74%, 13.76%, 7.67%, and 6.98%, respectively, with corresponding Eigen values of 9.443, 4.283, 2.211, 1.500, and 1.098 (Table 6).
The variance observed in PC1 was attributed to the strong positive loading of moisture, density of earthworms (DE), clay, OC, P and TN and moderate positive loading of temperature, along with a strong negative loading of Cu, EC and Fe, and moderate negative loading of sand, Pb and Mn. The variance exhibited in PC2 appeared to be due to strong positive loading of K and Ni, moderate positive loading of Li, OM and Cr, weak positive loading of temperature and Fe and weak negative loading of silt. Further, the variance observed in PC3 was due to moderate positive loading of silt, Na and Zn and weak negative loading of temperature. The PCA plot (Fig. 2) illustrates the distribution of soil parameters across three principal components. PC1 is primarily linked to P, TN, Fe, EC, DE, Zn, Pb, Mn, Clay, Sand, OC, temperature and moisture, while PC2 is largely linked to Ni, Cr, OM, K and Li. In contrast, PC3 is associated with silt, Na and Zn. On the other hand, PC4 had a positive loading of pH with Eigen value 1.5. The variance of PC5 was attributed to the moderate positive loading of Zn. The spatial arrangement of these parameters highlights their respective roles in soil variability at different sites.
Interaction of physico-chemical characteristics of soil with earthworm density.
OM-organic matter, DE- density of earthworms, OC- organic carbon, EC- electrical conductivity, TN- total nitrogen, K- potassium, P- phosphorus, Na- sodium, Fe- Iron, Ni- nickel, Pb- lead, Mn- manganese, Cu-copper, Li- Lithium, Zn- Zinc and Cr- chromium. (Closely placed parameters behave similarly, farther the location from the center stronger the influence on a PC)
Correlation between physico-chemical parameters of the soil and earthworm density
Pearson’s correlation analysis revealed a strong positive relationship (r ≥ 0.7) of earthworm density with moisture, clay content, OC, P, and TN. A moderate positive relationship (0.7 ≤ r ≥ 0.5) was observed between earthworm density and temperature. Further a weak positive relationship (r ≤ 0.5) of earthworm density was observed with pH, Li and Ni. In contrast, earthworm density showed a strong negative correlation (r≤-0.7) with EC and a moderate negative correlation (-0.7 ≤ r≤-0.5) with sand content(Fig. 3).
Correlation between physico-chemical parameters and earthworm density.
OM-organic matter, OC- organic carbon, EC- electrical conductivity, P- phosphorus, TN- total nitrogen, K- potassium, Li- lithium, Na- sodium, Zn- zinc, Cu- Copper, Fe- iron, Ni- nickel, Pb- lead, Mn- manganese, Cr- chromium and DE- density of earthworms.
A positive correlation was observed between pH and three parameters, namely, silt, OC and OM throughout the study period. On the other hand, temperature, OC and OM showed a positive correlation with the moisture and clay content of soil. However, EC was observed to be positively correlated only with silt and sand contents. We observed that earthworm density was positively correlated with OC, OM, moisture, and clay content, but it was negatively correlated with EC, silt content, and sand content.
