Microbial and invertebrate activity at the litter-soil interface and its effect on decomposition in a Michigan agroecosystem

Background/Question/Methods The decomposition of organic material in soil is largely mediated by bacteria and fungi; invertebrates, however, can also play a significant role in this process.  While many soil invertebrates influence decomposition directly through litter-feeding, others, particularly the non-predatory soil mesofauna, may affect decomposition indirectly by feeding on microbes.  Few studies have examined the impact of soil mesofauna on litter decomposition in relation to seasonal patterns of microbial activity.  We conducted a litterbag experiment in conventional, no-till and old field ecosystems at the W.K. Kellogg Biological Station LTER near Kalamazoo, MI using corn and grass litter.  Our objectives were to a) assess the progression of microbial activity as well as the colonization and succession of soil mesofaunal communities over the course of the growing season and b) determine how microbes and mesofauna may interact to influence litter decomposition rates and the chemistry of litter.  Microbial activity was assessed by measuring potential extracellular enzyme activity levels of four enzymes (three hydrolase and one oxidative enzyme) from litterbags retrieved at weekly to monthly time intervals.  At each sampling date mesofauna were extracted from litterbags for community analysis using Berlese funnels.  Litter chemistry was determined using pyrolysis-gas chromatography/mass spectroscopy.   
Results/Conclusions The activity of hydrolase enzymes (N-acetyl-glucosaminidase, β-1,4-glucosidase and acid phosphatase) responded in the following sequence (old-field > no-till > conventional till), while phenol oxidase activity responded in the opposite sequence (CT > NT > OF).  Litter colonization by mesofauna was fastest in conventional tillage ecosystems but the populations fluctuated erratically among sample dates; by contrast, in no-till and old field ecosystems population sizes gradually increased until early September before dropping significantly in all ecosystems.  The most significant period of litter mass loss, in both corn and grass litterbags, coincided with a significant increase in microbial enzyme activity and arthropod abundance and total arthropod abundance was positively correlated with the activity of phenol oxidase.  We show that the influence of microbes and mesofauna on decomposition may occur in a successional pattern, and that the effects of mesofauna may require the preconditioning of litter by microbial activity.  Moreover, the succession of soil communities and related changes in decomposition rates are related to changes in the chemistry of litter over time.