Does association with N-fixing nurse trees improve carbon sequestration in walnut plantations? Case-study on a reclaimed opencast mine in Italy

Mining activities have a major impact on the environment, depressing several ecosystem services. Afforestation is an efficient approach to recovering abandoned mining areas. This paper aims to assess the suitability of common walnut (Juglans regia L.) for planting on mine spoil banks for wood production, hydrogeological protection, and climate change mitigation. The paper also examines if associating walnut with a nitrogen-fixing species, such as Italian alder (Alnus cordata (Loisel.) Duby) or black locust (Robinia pseudoacacia L.), is advantageous compared to monoculture for the mentioned objectives. For this purpose, contiguous stands of pure walnut and walnut intercropped with one of the two nitrogen-fixing species were studied. The stands had been growing for 34 years on the spoil banks of a lignite mine in central Italy that was abandoned in the 1980s. Overall, walnut in monoculture showed good performances on the infertile substrate, in terms of both growth and stored carbon. On average, in fact, walnut showed a mean height of 16.2 m and a woody volume of 70.1 m3/ha, and had stored 16.5 Mg C/ha between above and below ground. However, walnut significantly benefited from the association with Italian alder, which stimulated the early growth of walnut (up to 15 years) and improved the commercial quality of woody assortments. At the current age, walnut showed significant differences in diameter at breast height (DBH, 44.6%) and total height (H, 17.9%) between the stand where it was originally interspersed with 75% Italian alder and the stand where it grew in monoculture. Alder did not improve soil carbon sequestration, while black locust showed a greater soil improving capacity than alder when associated with walnut, both in terms of organic matter enrichment and bulk density decrease. However, black locust impressive growth rate and strength in chasing suckers induced high walnut mortality. Our results demonstrated that Italian alder is to be preferred to black locust as a nitrogen-fixing species to be associated with walnut, at least in the pedoclimatic and environmental conditions we dealt with.