Changes in Plant Biochemistry Under Varying Nitrogen Fertilization: Implications for the Soil Carbon Pools

Excess fertilization of an ecosystem can significantly increase nitrous oxide (N₂O) emissions without improvements to grain yield (McSwiney and Robertson, 2005). It is important to find the right combination of cover crop and fertilization rate to maximize crop yields and soil carbon sequestration, while minimizing greenhouse gas emissions. Fertilizer addition changes the nutrient status of an ecosystem, and can alter a plant's biochemical composition (i.e. percentage carbohydrates, lignin, lipids, and proteins) as well as biomass allocation (i.e. grain, reproductive parts, leaves & stems, or roots). When dealing with an agricultural ecosystem, shifts in either biochemistry or allocation can have many implications. One of most significant is the effect of plant biochemical changes on the plant litter that remains after harvest. Litter chemistry influences soil carbon pools and soil microbial communities. Here we present data from a corn agricultural ecosystem under a range of nitrogen fertilization rates (0 to 202 kg N/ha, both with a cover crop and without) at the Kellogg Biological Station-Living Field Laboratory (KBS-LFL) in Michigan, USA. We measured the plant biochemical composition of the corn using ¹³C nuclear magnetic resonance spectroscopy (¹³C NMR) in combination with a molecular mixing model (Baldock et al., 2004). We also measured the carbon oxidation state (C_ox) of plant samples via elemental analysis (Masiello et al. 2008). C_ox describes the bonding environment of carbon within the plant biomass, and can be used to calculate the oxidative ratio (i.e. photosynthetic quotient) of the ecosystem.

We will discuss observed changes in plant litter biochemistry (leaves & stems) and the implications of these changes for the soil carbon pools (lignin:N ratios), soil carbon sequestration, and what effect the use of a cover crop has on plant biochemistry. We will also explore the possible use of C_ox as a proxy for decomposition rate and biochemical recalcitrance.