Manganese and nitrogen coupled interaction on plant residue decomposition and soil carbon cycling

Avishesh Neupane, Biosystems Engineering & Soil Science, University of Tennessee, Knoxville, Knoxville, TN, Elizabeth Herndon, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, Thea Whitman, University of Wisconsin, Anthony Faiia, Earth and Planetary Sciences, University of Tennessee, Knoxville, Knoxville, TN and Sindhu Jagadamma, Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, Knoxville, TN

Background/Question/Methods
Recent studies have emphasized the potentially important but opposing effects of manganese (Mn) and nitrogen (N) enrichment on litter decomposition in forest systems. Increasing N deposition can lead to soil acidification that mobilizes Mn. This is particularly relevant in agroecosystems that depend heavily on N fertilization. However, no studies have examined the interactive effect of N and Mn fertilization on crop residue decomposition and soil carbon (C) cycling. Our current study aims to fill this gap. We conducted a laboratory microcosm experiment by adding 13C labeled plant residue to agricultural soils that received 225 kg N ha-1 yr-1 for 27 years and comparable soils that received no N fertilization. These soils also received one out of three levels of Mn: ambient (no additional Mn), low Mn (∼50 kg Mn ha-1), or high Mn (∼250 kg Mn ha-1). The soils were incubated at 65% water holding capacity at 21oC. Soil respiration was measured periodically to determine residue mineralization, and destructive sampling was done at day 30 to determine soil pH, dissolved organic C (DOC) and microbial biomass C (MBC).
Results/Conclusions
After 30 days of incubation, DOC, MBC, and cumulative CO2 production from residue was higher in long-term N fertilized soils than in soils without N fertilization, indicating that increased N availability promoted microbial biomass growth and decomposition of the residue Mn amendments at both levels significantly increased 30 days CO2 mineralization from residue in long-term N fertilized soil, but no such effect was found in soil without N fertilization. These results indicate that the effect of Mn on initial residue decomposition depends on soil N availability. Improving our understanding of the factors that alter residue decomposition and C stocks will ultimately help enhance soil health and predict ecosystem-climate feedback.