Soil carbon dynamics and carbon sequestration potential of a wetness-prone perennial grass bioenergy field in northeastern United States

Background/Question/Methods

Many soils in the Northeastern United States are unsuitable for row crop agriculture due to seasonal water saturation or near-saturation (resulting from presence of shallow restrictive layers) and are being increasingly cited as a resource base for perennial bioenergy. Potential carbon (C) sequestration is a key ecosystem service and long term sustainability feature of perennial bioenergy systems. As soil organic carbon (SOC) changes might be difficult to demonstrate in the near-term, estimates of C fluxes entering and leaving soil over fixed time periods (seasonal or annual) done in conjunction to annual inventories of SOC will help better understand soil C dynamics in the shorter timeframe. The objective of the research is to assess the impact of soil moisture on SOC dynamics and Net Ecosystem Production (NEP) in perennial grass bioenergy cropping systems, exploring the links between soil depth, plant type and nitrogen (N) fertilization. C sequestration will be estimated as the difference between the change in plant biomass (aboveground and belowground) and the heterotrophic component of soil respiration. In situ soil respiration studies using automated infrared gas analyzers will help estimate seasonal/annual CO₂fluxes; subsequent partitioning of the autotrophic and heterotrophic components will be done by a regression approach. 

Results/Conclusions

This is a planned experimental approach at an existing 16 acre research site at Ithaca, NY, where initial hydrologic and soil characterization have been done; site had been fallow for more than fifty years before establishment in 2011. The randomized block design layout consists of sixteen strip plots for the following treatments: switchgrass (Panicum virgatum), switchgrass + fertilizer N, reed canarygrass (Phalaris arundinaceae) + N, and pre-existing grass as control. Five permanent sampling subplots (for annual estimations) have been established along the natural moisture gradient of each strip plot (~1 acre), based on initial water content measurements at the soil surface (0-12 cm) by time-domain reflectometry (TDR). Frequent TDR measurements have been normalized to determine relative wetness of all eighty subplots, which have been grouped under five wetness quintiles. The driest subplots have water contents approximately 0.8 times the field mean, whereas the more variable wettest subplots averaged 1.3 times the field mean. Though the % organic matter (OM) of surface soil (0-12cm) estimated by loss on ignition, for each cropping treatment shows highest values for the wettest quintiles, % OM values of subsoils (till 120 cm) show no clear trend. NEP values at selected representative subplots will be estimated.