Investigation of Frequency and Biodiversity of Soil Macrofauna in Irrigated and Rainfed Wheat (Triticum aestivum L.) Fields, Ilam Province (Iran)

IntroductionSoil is the richest and most diverse living community in any ecosystem. Soil organisms are responsible for the change and transformation of organic matter and the deformation and transport of nutrients such as nitrogen and sulfur. Therefore, they are an integral part of soil quality. In agricultural systems, useful macrofauna includes species that are involved in increasing crop yield and the ecological stability of the system. The objective of this study was the effect of agricultural systems (irrigated and rainfed) on the abundance and diversity indices of soil macrofauna in wheat fields of Ilam province, west of Iran. Materials and MethodsIn this study, the abundance and biodiversity of the soil macrofauna arthropods in wheat fields of Eyvan (Ilam province) was investigated. Five samplings were performed during irrigated and rainfed wheat growth in the year 2019. For this purpose, five sampling units (replicates) were selected for ten fields. In each sampling unit, soil macrofauna was collected using pitfall traps and then counted. Diversity was calculated by Shannon-Wiener and Simpson indices, evenness by Pielou evenness index, and species richness with Margalef index. PAST software was used to calculate biodiversity indices. Results and discussionFrom 3753 macrofauna samples collected, 11 families belonging to different orders of insects and spiders were identified: (1) Diptera including Syrphidae and Muscidae, (2) Hymenoptera, i.e., Formicidae, (3) Coleoptera, i.e., Carabidae, Staphylinidae, Silphidae, Scarabaeidae and Cicindelidae, (4) Hemiptera i.e., Pyrrhocoridae, (5) insects larval from different families, and (6) spiders. The tiger beetles had the highest abundance in both irrigated wheat (25.48%) and rainfed (20.57%) fields. The T-test results showed that the agricultural environment change from irrigated to rainfed fields did not increase or decrease the number of families but increased the average total frequency (from 2100 in rainfed farms to 1653 in irrigated farms). Changing the agricultural environment from irrigated to rainfed caused an increase in the Shannon index, Simpson and Margalof; also, this change of agricultural environment from irrigated to rainfed has caused a decrease in the value of the Pielou index. The results showed that the Shannon-Wiener index and the Margalef richness in the rainfed wheat fields were significantly higher than in the irrigated fields (P≤0.05), however, the Simpson diversity in rainfed fields was higher than in irrigated wheat fields, but this difference was not significant (P≥0.05). Also, the Pielou evenness in the irrigated fields was higher than in rainfed, but this difference was insignificant (P≥0.05). The results also showed that the highest and lowest number of arthropods trapped in pitfall traps in rainfed fields were related to tiger beetles (21.4 ± 4.87) and red bugs (1.68 ± 0.18), respectively, and in irrigated fields, it was related to tiger beetles (13.60 ± 2.82) and dung beetles (2.16 ± 0.16). In general, the results showed that the frequency and diversity of soil macrofauna in the rainfed wheat fields were higher than in irrigated wheat fields. Conclusion The diversity of soil macrofauna arthropods in rainfed wheat fields was better than in irrigated wheat. Less use of chemical pesticides and agricultural machinery in rainfed wheat fields improves soil physical properties as well as provides suitable habitat for soil macrofauna arthropods, which increases diversity. Useful soil macrofauna is very sensitive to environmental changes, so as the environment is destroyed, the diversity and number of these macrofauna decrease. In general, according to the obtained results, it is necessary to avoid the excessive use of chemicals in irrigated wheat fields and to provide conditions for improving the biodiversity of soil macrofauna. These conditions can be created by reducing the use of chemicals on farms, but if you cannot for any reason, avoid the use of chemicals on farms, use other methods such as fallow, crop rotation, and also the cultivation of legumes in the agricultural program of the region provided these conditions.