Microbial community composition and activity in paired irrigated and non-irrigated pastures in New Zealand

Context Microorganisms are key for carbon (C) and nitrogen (N) cycling in soils supporting agricultural production. Aims We investigated the impacts of irrigation on microbial community structure and activity in New Zealand on 28 paired non-irrigated and irrigated grazed pasture sites where C and N had decreased under irrigation. Methods Microbial community structure and microbial biomass (phospholipid fatty acids) and activity (basal respiration, substrate-induced respiration (SIR), aerobically mineralisable N (AerMN)) were assessed. Key results Microbial biomass did not differ between irrigated and non-irrigated soils, but irrigated soils had increased gram-negative bacteria (P < 0.05), lower gram-positive:gram-negative ratio (P < 0.001) and lower fungal:bacterial ratio (P < 0.001) compared to non-irrigated soils. SIR and AerMN were greater in irrigated compared to non-irrigated soils. There were no differences in basal respiration between irrigation treatments. Greater prevalence of gram-negative bacteria (r-strategist) as well as decreases in actinomycetes and fungal to bacterial ratio, and increased SIR and AerMN suggest more rapid cycling of C and nutrients in irrigated systems where C had been lost. Conclusions We found clear evidence that irrigation alters microbial community structure and activity in New Zealand pasture systems. Implications Irrigation driven alteration of microbial populations may contribute to losses of soil SOM and soils’ ability to deliver ecosystem services.