Effect of alfalfa-grass mixed culture and inoculation with Azotobacter and Rhizobium on soil biological properties and nutrient transformation activities

Abstract

Intercropping alfalfa (Medicago sativa) with grass offers yields equal to or greater than alfalfa monoculture, improves the quality of silaged fodder, and enhances resilience to drought and other stresses. Inoculating either alfalfa monoculture or mixed cultures (with a festucoid hybrid (Festulolium pabulare)) with plant growth-promoting rhizobacteria (PGPR) could potentially enhance plant growth, yield and soil quality. A monoculture of alfalfa and three different mixed cultures of alfalfa and festucoid hybrid at ratios 1:1, 2:1, and 3:1 were sown at a seeding rate of 30 kg·ha^-1 on small-scaled field plots (3 × 10 m). The soil type was Luvisol, either uninoculated or inoculated (⁓10.5 log10 CFUm^-2) with a commercial PGPR consortium containing (Azotobacter, Sinorhizobium meliloti, Bacillus megatherium). At the end of the trial, mixed soil samples (comprising 8 probes to a depth of 10 cm) were collected, and their biological properties were determined. Mixed cultures of alfalfa with the festucoid hybrid decreased nitrification; urease was lower by 8.5 % (alfalfa:festucoid 1:1), 36.5 % (2:1), and 49.7 % (3:1) compared to alfalfa control. d-glucose-induced respiration was higher by 55.4 % (2:1) and by 23.1 % (3:1), along with a negative trend in the nitrifying Archaea abundance. Nitrososphaeria relative abundance decreased from 4.5 % (1:1) to 9.4 % (3:1) compared to the control. Inoculation indirectly affected nitrogen (N) turnover in the mixed variants by increasing urease (2:1 inoculated 49.1 % over 2:1 uninoculated; 3:1 inoculated 36.5 % over 3:1 uninoculated value) and increased the relative abundance of Nitrososphaeria (alfalfa inoculated 7.3 % and 2:1 inoculated 4.2 % over uninoculated control). Inoculation enhanced phosphatase activity (1:1 inoculated 11.4 %; 2:1 inoculated 21.8 %, 3:1 inoculated 16.2 % over respective uninoculated values), specific soil respiration (alfalfa inoculated 146 %, 1:1 inoculated 192 %, 2:1 inoculated 3 % over uninoculated values), and brought a positive tendency in copiotrophic (Actinobacteria) relative abundance (alfalfa inoculated 10.2 %, 1:1 inoculated 6.1 %, 3:1 inoculated 3.4 % over respective uninoculated values), albeit it decreased fungal biomass. An increased rate of N₂ fixation and N assimilation in variants with high alfalfa: grass ratio decreased nitrification and increased mineralization of specific substrates. Inoculation neither directly enriched the soil with the introduced PGPR taxa nor shifted soil microbial diversity. However, it did prompt tendencies in community composition changes towards a higher proportion of nitrifiers and copiotrophs. Despite multiple changes in the tested experimental variants, no significant effect on the plant biomass of harvested crops was observed during the three years.