Cover crop enzyme activities and resultant soil ammonium concentrations under different tillage systems

Increased interest in managing agroecosystems for soil health has led to adoption of conservation management practices such as cover cropping and no-tillage. There is a dearth of knowledge surrounding cover crop residue N cycling and release, impacting subsequent N availability for the cash crop after cover crop termination. The microbial degradation of plant material and subsequent nutrient release is mediated by extracellular enzymes. This study used litter bag methodology to investigate the enzyme activities of three cover crop residues (cereal rye (CR) (Secale cereale L.), hairy vetch (HV) (Vicia villosa Roth), and a CR/HV mixture) in two tillage systems (no-tillage and reduced tillage) during the cover crop decomposition period in a corn (Zea mays L.) agroecosystem. The dynamics and amounts of β-glucosidase (EC 3.2.1.21) and urease (EC 3.5.1.5) enzyme activities on cover crop residue were quantified to determine if enzyme activities or stoichiometry are dependent on litter quality or can indicate N cycling potential. Results revealed that cover crop and tillage significantly impacted urease and β-glucosidase residue enzyme activity dynamics and amounts (p < 0.05). β-glucosidase activity of CR, HV, and a CR/HV mixture decreased by 76 %, 81 % and 77 % during the 2016 decomposition period, respectively. On average, urease activities of cover crop residues and soil ammonium concentrations increased during this period. In 2017, urease activities significantly decreased in all treatments (p < 0.05). β-glucosidase enzyme activity significantly decreased from 38 days after termination to 123 days after termination in all cover crop and tillage treatments (p < 0.05). Urease activities were significantly higher in the tillage treatments compared to no-tillage at the beginning of the 2017 decomposition period (p < 0.05). Soil ammonium concentrations significantly increased by 38 % across all treatments in 2017 during the decomposition period and concentrations were 12 times higher in 2017 at corn maturity compared to 2016. These results indicate potential for cover crops to stimulate residue β-glucosidase activity, suppress urease activity and ammonification, and increase soil N immobilization during corn N demanding growth stages. Furthermore, this study indicates that growers should consider optimum cover crop winter biomass accumulation levels to avoid potential N immobilization.