EFFECTS OF ALFALFA ON AGGREGATE STABILITY, AGGREGATE PRESERVED-C AND NUTRIENTS IN REGION MOUNTAIN AGRICULTURAL SOILS 1 YEAR AFTER ITS PLANTING Patrizia Guidi 1 * , Livia Vittori Antisari 1 , Boussa T. Marè 2 , Gilmo Vianello 1 , Gloria Falsone 1 1 Alma Mater Studiorum, Università di Bologna – DIPSA, Chimica Agraria e Pedologia, Via Fanin 40, 40127 Bologna, Italy 2 Université de Ouaga 1PJKZ, Unitè de Formation et de Recherche en Sciences de la Vie et de la Terre, Laboratoire de Sols-Matériaux et Environnement, 03, BP 7021 Ouagadougou 03, Burkina Faso Received 8 December 2016; Revised 26 July 2017; Accepted 27 July 2017 ABSTRACT In mountain areas, water erosion plays an important role on soil structure and can strongly affect its functions. Suitable management practices, namely choice of crops, may be able to improve aggregate stability and reduce soil loss by surface runoff. To study the effects of cover crops on aggregation in mountain soils, we investigated a soil planted with wheat (site C) compared with two soils under alfalfa (sites A and B). All investigated soils were Typic Ustorthents. The soil aggregates were isolated by a physical-functional procedure defined on the basis of aggregate resistance against specific breakdown mechanisms such as slaking and water abrasion. One year after alfalfa plantation, its effect on wet aggregate stability and the amount of organic carbon (OC) and nutrients retained by the isolated aggregates were measured. In the less degraded soils (site B), organic matter decomposition was stimulated, and after 1 year, the total OC (TOC) and TOC/N declined. Consequently, a reduction in aggregate stability and ability to preserve elements occurred, as shown by the decrease of wet aggregate stability indexes and total nutrients (e.g. N, P and K) retained by stable aggregates. In more degraded soil (site A), alfalfa generally positively affected the investigated properties. The effect of alfalfa in the studied crop rotation depends on the degree of land degradation due to water erosion. As regards the investigated breakdown mechanisms, our data showed that the aggregate resistance to water abrasion was related to TOC, while the resistance against fast wetting breakdown is correlated to the microbial biomass. Copyright © 2017 John Wiley & Sons, Ltd. key words: soil aggregation; soil organic carbon preservation; nutrients; alfalfa; wheat INTRODUCTION Soil aggregation is an important aspect of agricultural soil quality, and its preservation and improvement are keys to sustaining soil functions (Adhikari & Hartemink, 2016; Keestra et al., 2016; Mueller et al., 2013). In this regard, suitable management practices include tillage methods, residue management, amendments, soil fertility manage- ment and choice of crops (Bronick & Lal, 2005). Min et al. (2003) reported that alluvium soils under cover crops, such as alfalfa (Medicago sativa), have high soil aggregation induced by high crop root mass and easy litter decomposition and that this enhanced soil OC sequestration. Blanco-Canqui et al. (2013) stated that cover crops may be able to improve wet aggregate stability and reduce runoff loss. Arihara & Karasawa (2000) reported that previous crops might affect the growth of succeeding cover crops by altering the soil physical properties. The crop residues can improve soil quality through their impact on reducing risks of soil erosion, stabilising soil structure and providing energy for microbial processes (Indoria et al., 2017). Jia et al. (2006) suggested that the low C/N ratio in alfalfa residue might cause an acceleration of soil organic matter mineralisation leading to C depletion. Moreover, according to Kay & Munkholm (2011), the inclusion of cover crops can induce changes in soil physicochemical properties mainly through the alteration of physical habitat for soil biota and the modification in food sources and chemical environments. The main fluxes in soil are in fact of a bio- physicochemical character (e.g. emission of greenhouse gases; availability of oxygen, nutrient and water; organic C turnover) (Brevik et al., 2015) and are related to soil structure and aggregation (Adhikari & Hartemink, 2016). Consequently, the improvement of soil aggregation can result in a more efficient soil OC preservation. In mountain areas, soil aggregates are generally exposed to severe water erosion, negatively affecting soil functions and enhancing land degradation. Few investigations are available, however, on the induced effects of the use of cover crops on aggregation in mountain soils, and consequently, on soil OC preservation, nutrient supply and the properties of the biophysical habitat. Aggregate stability is often used as an indicator of soil structure (Six et al., 2000), because a good cohesion among aggregates can limit soil erosion and surface crusting, thus ensuring soil functionality. According to Six & Paustian (2014), it is important to understand how the different methods used to measure aggregate stability are related to, or represent, the different mechanisms of soil aggregate breakdown, in order to gain knowledge on the aggregate *Correspondence to: P. Guidi, Alma Mater Studiorum, Università di Bologna – DIPSA, Chimica Agraria e Pedologia Via Fanin 40, 40127 Bologna, Italy. E-mail: patrizia.guidi4@unibo.it Copyright © 2017 John Wiley & Sons, Ltd. land degradation & development Land Degrad. Develop. (2017) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.2771