Almond hull and shell organic matter amendments increase microbial biomass and multifunctionality in orchard soil and the undisturbed organic layer

Recycling almond hulls and shells as organic matter amendments in orchards can be used as a strategy to increase the soil microbial biomass of many functional community groups that metabolize a variety of different carbon substrates. In an almond orchard field trial, hull/shell amendments were applied to the soil surface with a compost spreader and remained undisturbed over time. The organic hull/shell layer supported high living microbial biomass and led to significant improvements in the soil microbial community beneath it. In the upper soil layer, soil bacteria were significantly higher under the amendments (mean of 47.6 ng bacterial biomass g−1 soil) compared to control soil (mean of 40.3 ng g−1) after one year. After a year and a half without soil disturbance, soil fungi were significantly higher under amendments (mean of 3014.9 ng g−1) compared to control soil (mean of 1483.3 ng g−1). The resulting soil microbial community included higher saprophytes, arbuscular mycorrhizal fungi, gram(−) and gram(+) bacteria, and actinomycetes than control soil, thus increasing microbial multifunctionality. Community-level physiological profiling demonstrated the amendment increased soil microbial activity in response to a range of carbon sources such as amino acids, carbohydrates, carboxylic acids, and polymers. The organic hull/shell layer itself supported high total microbial biomass across trophic levels, including protozoa. The amendment decomposed by 45 % of its initial dry weight in the first year and the carbon to nitrogen ratio (C:N) declined from 53:1 to 29:1. No effects of microbial nitrogen immobilization on tree nitrogen status were found and yields between treatments were similar. The organic layer created beneficial conditions for tree root growth, leading to higher almond root biomass density (means of 0.85 and 1.7 mg dry roots cm−3 in the control and amended soils, respectively) after a year and a half. Maintaining hull/shell amendments on the soil surface over time created a beneficial organic layer that promoted many functional microbial groups and nutrient cycling in the organic and upper soil layers. •Almond hull/shell amendments promoted microbial life in the soil and organic layer.•This amendment increased soil bacteria after 1 year and fungi after 1.5 years.•Soil microbial activity increased in response to a wide variety of C sources.•The amendment steadily decomposed, supporting high microbial biomass.•Almond root biomass increased under the