Earthworms and arthropods abundance in an alley-cropping system with poplar and the effect on yield of common wheat and maize intercrops

Introduction A comprehensive understanding of the interactions influencing crop and tree growth is crucial for implementing high-productivity alley-cropping systems. While aboveground interactions, such as microclimatic variations related to shade intensity and windbreak, have been extensively studied for their potential to enhance climate resilience, research on belowground tree-crop interactions, mainly focusing on water relations, is limited1. Soil biological fertility, particularly the abundance and diversity of pedofauna, plays vital roles in nutrient cycling, soil structure, and biodiversity2,3, influencing the long-term sustainability of agroecosystems4,5. However, this aspect remains relatively unexplored. Objectives This study investigates the impact of an alley-cropping system with poplar trees (i) on earthworms (e) and arthropods (ar) biodiversity through Soil Biological Quality Index QBS-e and QBS-ar; (ii) on soil humidity and (iii) on the yield of common wheat and maize intercrops. Methodology The trial took place in 2022-23 at the “Sasse Rami” pilot farm of Veneto Agricoltura (VA) in Ceregnano (Rovigo, NE Italy). It involved an alley-cropping system and a specialized plantation with five-year-old poplar trees. Common wheat (sown on 21/11/22, harvested on 30/06/2023) and maize (sown on 28/04/23, harvested on 12/09/2023) were cultivated in the inter-rows of N-S oriented rows of poplar trees spaced 40m apart and planted along drainage ditches, with 6 meters between trees along the row. The specialized poplar plantation with a 6×6 m design was used as a control (Figure 1). The QBS-e index values were carried out using the earthworms sampling protocol proposed in Paoletti et al.7 with hand-sorting of 30x30x25 cm monolith without using attractive solution. Specimens collected were classified by ecological-functional categories8 and preserved for taxonomic identification and QBS-e calculation8. Sampling occurred twice in 2023, in June and September. For both crops, the soil was sampled along three transects (n=3) perpendicular to poplar rows at distances of +6m and +12m on both east and west sides and in the center of the alley (+20m, Controls). For poplars, earthworms were sampled along the tree row in the alley-cropping system and the poplar plantation. QBS-ar index was determined by collecting soil samples (1 dm^3) later subjected to mesofauna extraction (Berlese-Tullgren method) and microarthropod identification4. Soil organisms are classified into biological forms based on their morphological adaptation to soil environments. The degree of adaptation depends on specific morphological traits, with each form associated with an Eco-Morphological Index (EMI) score ranging from 1 to 204. The final QBS-ar score was derived by summing EMI scores. Sampling occurred in June (wheat and poplars) and September (corn and poplars) at the same QBS-e sampling plots. At crop harvest, wheat and maize were sampled on a 1-m^2 area for each position and threshed to determine grain yield and quality (NIRS technology). Statistical significant differences were detected by R studio (Tukey’s HSD test, P≤0.05). Results In wheat and maize, the QBS-e index generally decreased towards the tree row, though not significantly. In June, the highest QBS-e values were at the alley center (346 in wheat, 439 in maize), except for +12m East in wheat, with the lowest at +6m West (Figure 2). In September, variations were slightly lower than in June, with greater QBS-e decreases in the wheat field. For poplars, a lower QBS-e was observed along tree rows in the alley-cropping system compared to the poplar plantation, though not significantly. In wheat and maize, QBS-ar showed a non-significant increase in the interaction zone with poplars, at +6m in wheat and +12m in maize (west side). For poplars, QBS-ar was significantly higher in the alley-cropping system than in the control plantation (+56 in June, +80 in September, Figure 2). Additionally, the QBS-ar value in the row grass exceeded that in the cultivated field. Wheat yield remained unaffected by tree proximity, with consistent values at all distances, approximately 7 tons per hectare, and a slight increase at +6m East (+10% vs. C; p≥0.05). Maize yields were similar at +12m East and C (approximately 14.7 t/ha), but a significant reduction was observed at +6m (-35% vs. C). Conclusion Earthworms generally decreased while arthropod abundance increased near the tree rows in a silvoarable system with widely-spaced poplar rows and trees at half commercial life span. Poplar rows covered with grass showed the highest QBS-ar values, indicating that diverse habitats enhance biodiversity protection. A positive correlation between earthworms and soil water content was observed in the alley-cropping system, peaking at the center of the inter-row. The positive correlation between crop productivity, particularly in maize, and earthworm density suggests that QBS-e could serve as an informative index for crop yield. Despite higher soil water content, tree rows exhibited lower QBS-e than the arable field, indicating a potential inhibitory effect of poplar on earthworms. However, further research is required to corroborate these preliminary results and possibly clear the role of these agroforestry features (organisms and soil structures, crops, water management, tree species, and ditches).