Nematodes as biological indicators of urban heat island effects in the Chongqing area urban green spaces

Abstract

The urban heat island effect (UHIE) is defined as the tendency of urban regions to have higher temperatures than suburbs in their immediate vicinity. A better understanding of UHIE and its biological indicators facilitates the prediction of ecological changes, which is particularly important due to ongoing global warming. To determine whether soil nematodes may serve as biological indicators of UHIE, we compared soil nematode communities under three urban green spaces, namely, urban cold islands (CI), urban heat island transition regions (HIT), and urban heat islands (HI). Distinct soil nematodes characteristics were found in the 3 urban green spaces. In addition, seasonal variations in community composition were identified. Among the 30 nematode genera identified, Eucephalobus was the most dominant in CI and HIT, while Monhystera was the most dominant in HI. The mean nematode abundance significantly differed (p ≤ 0.01) between CI (322 ± 24 individuals/100 g dry soil), HIT (112 ± 9 individuals/100 g dry soil), and HI (107 ± 10 individuals/100 g dry soil). We found that UHIE had detrimental effects on the nematode community structure (i.e., abundance and Margalef, Shannon-Wiener, and Simpson diversity indices) and ecological function [Free-living Nematode Maturity Index (MI), Nematode Channel Ratio (NCR), Enrichment Index (EI), and Structure Index (SI)]. These detrimental effects were reflected in nematode community structure indices, which were all highest in CI and lowest in HI, as well as in MI and SI, which were lower in HI than in CI. Furthermore, nematodes found in HI were shorter (L = 502.15 μm), thinner (W = 38.42 μm), and exhibiting higher length-to-width ratios (L:W) (15.83) compared to nematodes found in CI (L = 986.13 μm, W = 83.58 μm, L:W: 12.24) and HIT (L = 786.80 μm, W = 58.88 μm, L:W: 14.82). Nematode biomass spectra (NBS) shifted towards smaller biomass classes in HI compared to CI, while nematodes of the HIT showed intermediary spectra, further confirming the detrimental effects of UHIE on the soil nematode community. Regarding the physicochemical properties of the 3 green spaces, CI was found to have the highest soil pH, moisture, organic carbon content, total nitrogen content, and available potassium content, while HI had the lowest. Soil physicochemical characteristics were inversely correlated with soil temperature, except for available soil phosphorus, which was positively corrected with temperature. We also found that soil characteristics were significantly correlated with nematode community structure and ecological function, suggesting that the effects of UHIE on soil nematodes could have been indirectly affected by changes in soil physicochemical characteristics.