Combined legacy effects of drought and diverse plant residues: Fate and priming effects of glucose

Building soil organic carbon (SOC) is critical for agroecosystem functioning and resilience to climate change. SOC storage is driven by the accumulation of C in microbial biomass and residues (characterized by C stabilization efficiency, or CSE), formation of mineral-associated organic carbon (MAOC), and priming of existing SOC, all of which are disrupted by drought. Ecosystem recovery after drought may be improved by plant diversity/composition. We used a two-phase microcosm study with stable carbon (C) isotope (¹³C as glucose) tracing in an agricultural soil (silt loam). First, we amended soils with cover crop residues of varying diversity/composition and imposed a 30-day dry-down. Second, we allowed soils to recover from drought, then added and tracked ¹³C-glucose. All cover crop residues marginally increased glucose-CSE by 4.19 % at 24 h after glucose addition, decreased the accumulation of glucose-C in the MAOC fraction by 27.9 % at six months, and strongly influenced bacterial and fungal diversity and composition at both timepoints. Drought history did not influence microbial communities, but intensified glucose-driven priming of existing SOC, leading to positive priming in soils with no or monoculture residues and negative priming in soils with five-species mixture residues. Overall, our results indicate that cover crop residues (regardless of diversity/composition) have the potential to improve stabilization of newly added simple C as microbial biomass and residues, but prevent accumulation of that C as MAOC in the longer-term. At the same time, diverse plant residues can reduce simple C-induced priming of existing SOC after drought. Though limited to a single agricultural soil, these results shed light on how plant diversity/composition influences soil responses to global change.