Alley Cropping Can Potentially Alter the Nitrogen and Carbon Soil Cycles and Increase the Abundance of Beneficial Bacteria in a Mediterranean Citrus Orchard

Abstract

Monocrop of mandarin leading to reduced soil biodiversity and functionality that must be changed to a sustainable agriculture practice such as alley cropping. In this study an attempt has been made to assess how two different alley cropping strategies promote soil bacterial diversity, microbial activities and the abundance of beneficial bacteria. Three treatments were established: (i) mandarin monoculture (MC); (ii) mandarin diversified with barley/vetch (summer) and with fava bean (winter) for 3 consecutive years (DIV1); and (iii) mandarin diversified with fava bean, purslane and cowpea (DIV2). Results reveal that alley cropping did not significantly affect alfa-diversity indices, but beta-diversity showed significant differences among the three treatments, indicating changes in the bacterial community. Specific genera such as Haliangium, Microbacterium, Pseudonocardia, Solirubrobacter and Sphingomonas, known as plant growth-promoting bacteria, showed higher relative abundances in DIV1 and DIV2 than MC. The genus Novosphingobium showed a higher relative abundance in DIV2, while MND1 showed a higher relative abundance in DIV1. Regarding potential gene abundances related to C and N cycling at the end of the experiment, only manB (hemicellulose degradation) showed a higher abundance in DIV2 than MC, while nifH (N fixation), amoA, and hao (nitrification) showed higher values in DIV1 and DIV2. Enzyme activities showed lower values in diversified treatments than in MC. Most significant changes were observed in the diversification of the alley with a sequence of different crops every year (DIV1), rather than repeating the same crops (DIV2). These alley cropping strategies (DIV1 and DIV2) seem an effective strategy to enhance the abundance of beneficial bacteria with increased potential activity related to N fixation and nitrification.