Pasture in crop rotations influences microbial biodiversity and function reducing the potential for nitrogen loss from compost

Elsevier, Agriculture, Ecosystems & Environment, Volume 304, 1 December 2020, 107122
Jeth Walkup, Zachary Freedman, James Kotcon, Ember M. Morrissey

Agroecosystems make up a significant portion of terrestrial ecosystems and receive a disproportionally high amount of terrestrial nitrogen inputs from fertilizer, leading to nitrogen loss and associated environmental problems. Integrated crop livestock systems, such as pasture-integrated crop rotations, may be more environmentally sustainable however the long-term effects of this management practice on soil microorganisms and nitrogen transformations are not well understood. To address this knowledge gap, samples were collected from a long-term experiment designed to assess the impacts of, and interaction between, compost application and pasture-integrated crop rotations. The compost-input and pasture-integrated rotation treatments increased soil organic matter by 41 % and 14 % respectively, carbon mineralization by 127 % and 146 %, and extracellular enzyme activity by 74 % and 35 %, suggesting enhanced heterotrophic microbial activity. Compost addition led to rapid ammonification, increased potential nitrification rates ∼3-fold, and enhanced nitrate concentration by ∼75%. Further, compost addition more than doubled the abundance of prokaryotic ammonia oxidizing organisms (AOO). These effects of compost on nitrogen cycling were greatly mitigated by the pasture-integration likely due to enhanced microbial decomposition activity associated with carbon inputs from perennial grasses. Specifically, pasture-integration mitigated increases in potential nitrification, AOO abundance, and soil nitrate concentrations. The rates of potential nitrification were correlated with the abundance of AOO, highlighting the connection between nitrogen cycling processes and microbial community composition. These patterns in nitrifier abundances, potential nitrification rates, and nitrate levels indicate that the long-term integration of pasture in crop rotations has the potential to reduce early-season nitrogen loss without reducing crop productivity. This work demonstrates how management practices that facilitate favorable interactions between microbial carbon and nitrogen cycling may enhance agroecosystem sustainability and reduce nitrogen pollution.