Mechanisms of soil carbon accrual and storage in bioenergy cropping systems
Author: Lisa Tiemann
Annual row cropping systems converted to perennial bioenergy crops tend to accrue soil C, likely a function of increased root production and decreased frequency of tillage; however very little is known about the mechanisms governing the accrual and stability of this additional soil C. To address this uncertainty, we assessed the formation and stability of aggregates and soil organic C (SOC) pools underswitchgrass, giant miscanthus, a native perennial grass mix and continuous corn treatments in Michigan and Wisconsin soils differing in both texture and mineralogy. We isolated different aggregate size fractions, > 2 mm, 0.5 ‚Äì 2 mm and < 0.5 mm, using a procedure intended to minimize alterations to aggregate biological and chemical properties. We determined SOC, permanganate oxidizable C (POXC), and microbial activities (i.e. enzyme activities and soil respiration rates) associated with these aggregates. Soil type strongly influenced the trajectory of aggregate formation and stabilization with differences between sites in mean aggregate size, stability, SOC and microbial activity under perennial versus corn cropping systems. At the Michigan site, soil microbial activities were highest in the > 2 mm aggregates, and higher under the perennial grasses compared to corn. Contrastingly, in Wisconsin soils, microbial activities were highest in the < 0.5 mm aggregates and evidence for soil C accrual under perennial grasses was observed only in a fast turnover pool in the < 0.5 mm aggregate class. Our results help explain cross site variability in soil C accrual under perennial bioenergy crops by demonstrating how interactions between belowground productivity, soil type, aggregation processes and microbial communities influence the rates and extent of SOC stabilization. Bioenergy cropping systems have the potential to be low-C energy sources but first we must understand the complex interactions controlling the formation and stabilization of SOC if we are to maximize soil C accrual.
Indirect effects of nitrogen amendments on organic substrate quality increase enzymatic activity driving decomposition in a mesic grassland
The fate of soil organic carbon (SOC) is determined, in part, by complex interactions between the quality of plant litter inputs, nutrient availability and the microbial communities that control decomposition rates. This study explores these interactions in a mesic grassland where C and nitrogen (N) availability and plant litter quality have been manipulated using both fertilization and haying for 7 years. We measured a suite of soil parameters including inorganic N, extractable organic C and N (EOC and EON), soil moisture, extracellular enzyme activity (EEA) and the isotopic composition of C and N in the microbial biomass and substrate sources. We use these data to determine how the activity of microbial decomposers was influenced by varying levels of substrate C and N quality and quantity and to explore potential mechanisms explaining the fate enhanced plant biomass inputs with fertilization. Oxidative EEA targeting relatively recalcitrant C pools was not affected by fertilization. EEA linked to the breakdown of relatively labile C rich substrates exhibited no relationship with inorganic N availability but was significantly greater with fertilization and associated increases in substrate quality. These increases in EEA were not related to increase in microbial biomass C. The ratio of hydrolytic C:N acquisition enzymes and d13C and d15N values of microbial biomass relative to bulk soil C and N, or EOC and EON suggest that microbial communities in fertilized plots were relatively C limited, a feature likely driving enhanced microbial efforts to acquire C from labile sources. These data suggest that in mesic grasslands, enhancements in biomass inputs and quality with fertilization can prompt increase in EEA within the mineral soil profile with no significant increases in microbial biomass. Our work helps elucidate the microbially mediated fate of enhanced biomass inputs that are greater in magnitude than the associated increases in mineral soil organic matter.