Changes in abundance, diversity and metabolic footprint of soil nematode community along an elevation gradient under temperate vegetation cover in Banihal-Pass of Pir-Panjal mountain range


 Despite enormous diversity, abundance and their role in ecosystem processes, little is known about how community structures of soil-inhabiting nematodes differ across elevation gradient. For this, soil nematode communities were investigated along an elevation gradient of 1000 to 2500 m asl across a temperate vegetation in Banihal-Pass of Pir-Panjal mountain range. We aimed to determine how the elevation gradient affect the nematode community structure, diversity and contribution to belowground carbon assimilation in the form of metabolic footprint. Our results showed that total nematode abundance and the abundance of different trophic groups (fungivores, herbivores and omnivores) declined with the increase of elevation. Shannon index, generic richness and evenness index indicated that nematode communities were more diverse at lower elevations and declined significantly with increase in elevation. Nematode community showed a pattern of decline in overall metabolic footprint with the increase of elevation. Nematode abundances and diversity proved to be more sensitive to elevation induced changes as more abundant and diverse nematode assemblage are supported at lower elevations. Overall it appears nematode abundance, diversity and contribution to belowground carbon cycling is stronger at lower elevations and gradually keep declining towards higher elevations under temperate vegetation cover in Banihal-pass of Pir-Panjal mountain range.


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Soils harbour some of the most diverse microbial community on earth, provide shelter to 25% world-50 wide described species and thus considered as a crucial biodiversity reservoir 1-6 . Soil organism's 51 distribution patterns play critical roles in determining the above and belowground primary production and 52 its composition 7,8 . Elevation and its related factors significantly affect the soil abiotic characteristics and 53 the density/biomass patterns of nematode communities, thus alter the microbial functions of soil 54 ecosystem 9-12 55 All chemical and physical changes in the soil are rapidly reflected through changes in richness and 56 composition of nematode species 13 . In addition to diversity and functional indices which are useful 57 descriptive tools for assessment of food web and ecosystem condition, various metabolic footprint 58 indices 14 have been developed to estimate contribution of nematodes to various ecosystem services and 59 functions. Among the landscape properties, altitude rather than vegetation cover is found to have vital 60 effect on nematode community 15 , because altitudinal climatic conditions strongly constrain the 61 availability and turnover of basal resources and can be viewed as nature's own field experiments 16 .

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Various elevation patterns of diversity have been studied across a wide range of taxonomic groups in 63 aboveground organisms, including trees, mammals, birds, insects and amphibians 17 . However, elevational 64 diversity patterns of belowground organisms, especially those of soil invertebrates which often represent 65 decomposer subsystems, have remained understudied 18,4 . 66 In the present study, we tried to unravel the distribution and contribution of soil dwelling nematodes to 67 belowground soil food web from 1000 to 2500 m asl elevation gradient along a pristine temperate forest 68 vegetation cover at Banihal-Pass of Pir-Panjal mountain range which lies in the Western Himalayan 69 region. We assessed total and trophic abundance, biomass, alpha diversity, and metabolic footprint pattern 70 of soil nematodes along the elevation gradient. Since that abiotic factors shape species distributions, 71 fostering fewer species and abundance at high elevations due to harsh climatic conditions compared to 72 low elevations. Keeping this in consideration, we hypothesized: (1) a decline in nematode abundance 73 and diversity with increasing elevation and (2) metabolic footprint of nematodes will be suitable 74 indicators of elevation change.   82 The total nematode abundance, biomass and diversity were correlated and tested with simple linear 83 regression with elevation (Table 1). From 45 soil samples 30955 nematodes were assigned to 49 different 84 genera (Table 2) in five different trophic groups (bacterivore-12, fungivore-12, omnivore-4, predatory-8 85 and herbivore-13). Total nematode abundance ( Fig    In line with our first hypothesis here we observed that total soil nematode abundance and diversity exhibit 111 a significant decline with elevational gradient which is consistent with the results of some earlier studies 112 on nematodes 19, 20 and with other soil fauna including springtails and mites 21 , and other 113 microarthropods 22,23 . In contrast to this, various patterns are found to occur in nematode community along 114 the elevation, at mid-elevation maximum diversity and species richness have been documented 24,25 , in 115 alpine grasslands abundance and diversity increased with elevation 26 . However similar studies on 116 nematodes along gradient did not find any definite pattern 27,28 . As mentioned earlier mountains in Pir-

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Panjal range tend to have greater climatic adversities that alter soil conditions, and it is possible that due 118 these altered soil conditions explains the survival of sparse and coexistence of fewer nematode species in 119 the higher elevations. Several environmental gradients are held responsible for shaping soil nematode 120 communities .Among the environmental conditions that shift strongly with elevation is temperature, 121 which gradually decreased with elevation ( Fig. S1) and proved detrimental for nematode community like 122 other soil organisms. Nematodes are found to be very sensitive to temperature changes with optimal of 123 20-25°C for the survival and propagation, any change in temperature from optimum value leads to 124 significant inhibition in nematode population 29,30,31 . In our study, temperature was below optimum value 125 possibly affecting soil nematode abundance and diversity as conditions were much harsher at higher pressure from predators 38 . Elevation has no effect on bacterial feeding nematodes in our study, which may 141 be due to basal characteristics bacterial feeders in food web that are mostly dominated by basal fauna.

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However another studies 26,29 reported a definite pattern of increase in bacterivore nematode from lower to 143 higher elevations. The non-significant decline of predatory nematodes could be due to their key position 144 in the soil food web, where they can switch on to different prey based on availability.

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Similar results were reported by Kergunteuil et al. 26 .

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The metabolic footprint provides information on magnitude of carbon and energy flow in soil food 147 webs 14 . The nematode metabolic footprint consists of composite metabolic footprint, enrichment footprint 148 and structure footprint which are the representatives of whole nematode community 14 . In our study, we 149 found that the carbon assimilation in soil food web from autotrophic organisms decreased with elevation  Overall, this decrease in carbon assimilation of whole nematode community could be attributed to 158 retention of more carbon in tree biomass than in soil of these temperate forests at higher elevation, 159 although productivity is high 42 . However, decline in the efficiency of carbon to enter the soil faunal food 160 web through nematodes with increasing altitude may be due to decrease in the availability of amplifiable 161 prey for nematodes. Thus for the mitigation of elevated atmospheric carbon, high altitude forest soils 162 cannot be relied. Additional expansion of lower altitude forests in these regions need to be considered for 163 mitigating and stabilising the elevated atmospheric carbon in soil food webs. Accordingly our second 164 hypothesis that nematode metabolic footprint are the suitable indicators of elevation change suggests 165 whether carbon is partitioned in the soil food web or not.  Cobb's method 44 . Each soil sample was put into a 1 L beaker and mixed with tap water. The water 205 suspension was stirred and decanted into another 1 L beaker through 2 mm mesh sieve to remove stones 206 and large debris. The water suspension was mixed further and then decanted through 53 μm mesh sieve.

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The material left on the sieve was collected in 250 ml beaker and further extraction was carried out by 208 Baerman's funnel method. Nematodes were removed for 2 days, stored at 4 ºC, fixed in TAF and counted 209 using inverted microscope (Olympus SZX10). Additionally, 200 individuals per sample were identified to 210 generic level using compound microscope (Olympus BX41), with the aid of various texts 45,46,47,48,49 . The 211 genera identified were also assigned to different functional guilds 50,40 .

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The authors declare that they do not have any competing interests.        (coe cient of determination), p (signi cance) of linear regression and regression line are displayed only for signi cant relationships.

Figure 4
Relationship between elevation and total nematode diversity (A), generic richness (B) and evenness (C).
Equation for Y, R2 (coe cient of determination), p (signi cance) of linear regression and regression line are displayed only for signi cant relationships.

Figure 5
Relationship between elevation and composite metabolic footprint (A), enrichment footprint (B) and structure footprint (C). Equation for Y, R2 (correlation coe cient), p (signi cance) of linear regression and regression line are displayed.

Supplementary Files
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