Surface-applied organic amendments modify soil biological activity down the soil profile of an irrigated vineyard

Organic amendments contain soluble organic carbon (C) that can be easily transported down the soil profile. This soluble organic C can modify the soil microbiome and consequently affect soil C dynamics. The effects of wood and bark chips mulch and a combined application of compost plus mulch on microbial function were assessed in soil samples collected from a Merlot (Vitis vinifera L.) vineyard in the semi-arid Okanagan Valley, British Columbia, Canada. Amendments were first applied in the spring of 2011 and re-applied every 2 years. The latest application was in August 2018; mulch and compost application rates in 2018 were 387 and 18.4 Mg ha^‑1 row^‑1 dry basis, respectively. Soil samples were collected from 15 cm intervals to 60 cm depth in November 2018 from replicate plots of each treatment and untreated control plots. Soil samples were incubated at 25°C and 55% water-filled pore space and CO₂ evolution was measured at 2, 4, 7, 10, 14, 22, 30, 38, 48, 58, 73, 88, and 102 days. For soil samples from the top 15 cm, CO₂ production was 95 and 110% greater in soil treated with mulch + compost and mulch only, respectively, than in untreated soil. For the 15 to 30 cm depth, CO₂ production in the same treatments was 66 and 94% greater, respectively, than in untreated soil. Microbial respiration in the top 30 cm was greater in the mulch only treatment than in the mulch + compost treatment. For soil from depths below 30 cm, CO₂ production in mulch + compost and mulch treatments was 33 and 27% greater than in untreated soil, respectively. Changes in C mineralization and CO₂ production were associated with altered microbial functional potential. The relative abundance of genes associated with the decomposition of complex organic matter, hemicellulose, cellulose and oligosaccharides doubled in surface soils of the amended treatments relative to the control treatment. Vertical distribution of soluble C in the soil profile can impact soil and plant health management and consequently affect vineyards' resilience to climate change.