Short-term effects of plant litter on the dynamics, amount, and stoichiometry of soil enzyme activity in agroecosystems

Abstract

Addition of plant litter can affect soil enzyme activity at three scales: dynamics, amount, and stoichiometry. In this study, we examined the dependence of soil enzyme activity at all three scales on litter quality. Soils of similar texture were collected from conventional and organic farming systems, the Center for Environmental Farming Systems, Goldsboro, North Carolina, USA. Soil samples were then amended with senescent pine needles, grass materials, and soybean residues of C:N ratio 139, 50, and 9, respectively, at 2 mg C g−1 soil, and the activities of soil β-glucosidase, exoglucanase, β-glucosaminidase, and phenol oxidase were measured over the course of 90-d incubation. Relationships between the dynamics of enzyme activity and litter quality appeared to be enzyme specific. Time patterns of soil β-glucosidase and β-glucosaminidase activity were independent of litter quality, with rapid increase in enzyme activity and reaching a peak several weeks after litter addition. In contrast, time patterns of polymer-degrading enzymes (exoglucanase and phenol oxidase) were dependent on litter quality. Exoglucanase activity showed a concave function with time following the addition of soybean residues or grass materials, but increased slightly following the addition of pine needles. Cumulative activities of soil enzymes were upregulated following litter addition and could be qualitatively assessed by litter C:N ratio. The activity of β-glucosaminidase was negatively related to litter C:N ratio, being greatest in soybean residues-amended soil. Litter of a low C:N ratio was generally better than litter of a high C:N ratio for increasing soil cellulase activity and vice versa for phenol oxidase. However, the stoichiometry of soil enzyme activity was decoupled with litter C:N ratio. Soybean residues and pine needles at opposite ends of the litter C:N range were more similar in the ratio of C- to N-acquiring enzyme activity. We also examined pH effects on the expression of added enzymes. Soil enzyme activities were enhanced as soil pH increased from 6 to 8. pH-associated changes in enzyme activity were generally smaller as compared to changes caused by other factors during the 42-d incubation. Our results suggest that litter effects on the dynamics, amount, and stoichiometry of soil enzyme activity were independent of soil pH. Litter C:N was a good indicator for the total amount, but not for the dynamics or stoichiometry of soil enzyme activity.