Effect of soil moisture on carbon mineralization in wetness-prone perennial grass bioenergy cropping soils of Northeastern United States: A laboratory incubation study

Background/Question/Methods

Many soils in the Northeastern United States unsuitable for row crop agriculture due to seasonal water saturation or near-saturation are being increasingly cited as a resource base for perennial bioenergy. Soil organic carbon (SOC) accumulation is reported as a primary benefit of perennial grass bioenergy cropping systems. Carbon (C) mineralization studies done in conjunction with annual SOC inventories will help better understand soil C dynamics. The sensitivity of soil heterotrophic respiration to moisture is being evaluated in wetness-prone soils of a ~16 acre bioenergy field at Ithaca, NY. The field trial consists of quadruplicate cropping treatments (with 5 subplots each) spread across varying natural moisture gradient conditions. Wet, mid and relatively dry representative soils of switchgrass, switchgrass +fertilizer N, reed canarygrass +fertilizer N and pre-existing control (50 year fallow) plots were chosen for the incubation experiment. A full factorial combination of soils from four crop types, three field wetness classes, three replicated field locations and three lab moisture-adjusted levels were used. Soils were incubated for 30 weeks at 30°C and the electrical conductivity was measured at periodic intervals (potassium hydroxide trap method).

Results/Conclusions

Volumetric water content measurements in the field have been normalized to determine relative wetness of eighty subplots grouped under five wetness quintiles; the driest subplots having water content approximately 0.8 times and the wettest subplots averaging 1.3 times the field mean. Soils from 36 (out of the 80) subplots chosen for the incubation experiment were sieved (< 4 mm) and air dried for 48 hours prior to incubation. All air dried soil samples were then adjusted to long term equivalent levels of high, mid and low (0.52 gm/gm, 0.38 gm/gm and 0.28 gm/gm respectively) field moisture content, before starting incubation. With each measurement of electrical conductivity, soil was weighed and water content was readjusted. The % C for the soil samples at the beginning varied between 2.3% to 5.0% (mean 3.2%, std dev 0.71). The preliminary results from the first 6 weeks indicate that the high and medium lab moisture-adjusted soils (for high and mid field moisture equivalents), across all crop treatments showed higher C mineralization (measured in cumulative CO₂ mg/gm soil) than low lab moisture-adjusted soils. The low field moisture equivalent soils across all crop treatments showed less variation in C mineralization among all lab adjusted water contents, than the high and mid field moisture equivalents. The final results and conclusions are being prepared.