Soil Carbon and Nutrient Dynamics in Response to a Complete Woody Bioenergy Harvest Rotation

Short-rotation woody crops (SRWC) for energy production have been investigated in 1-15 yr rotations in the Southeast USA since the 1970s. Lessons from managed pine plantations in the region (~25 year rotations) demonstrate a need for N and P inputs, while K or Ca are only necessary with repeated harvest on specific sites. How such nutrient management knowledge from pine plantations translates to SRWC and to other species needs investigation. We report soil C and nutrient changes after 11-years in the upper 1-m of soil under two cottonwood clones (Populus deltoids; clones ST66 & S7C15), sweetgum (Liquidambar styraciflua), sycamore (Plantanus occidentalis), and loblolly pine (Pinus taeda) in a USA Sandhills location. Treatments are control (C), fertilization (F), irrigation (I), and their combination (IF). We observed growth gains from IF>F>I>C, although at age 9, depending on species, IF did not always exceed F (e.g., sycamore or sweetgum) and I did not always exceed C (i.e., pine). Total growth gains from C to IF ranged from ~50% in pine to ~300% in cottonwood clone ST66 and maximal above+belowground net primary productivity was observed in pine at 33 Mg ha^-1 yr^-1. At year 3, increased growth was associated with limited declines in soil C in response to fertilization or irrigation, with the exception of cottonwood clone ST66 showing a ~3 Mg-C ha^-1 increase in the upper 30 cm of soil. Results reported here are unique in having repeated soil measurements over a decade and sampling through 1 m of soil. At year 11, F or I had not significantly affected soil C concentrations, but declines observed from year 1 to 3 had stabilized in the upper two layers (0-15 and 15-30 cm). At depth (45-105 cm) soil C had increased, possibly due to increasing root growth or decay of roots from previous vegetation. There was some increase of N at depth, but no effect of F or I. N accumulations were relatively small compared to the decade of N inputs. Fertilization increased P and K concentrations over 11-yrs. In contrast, Ca declined with time as liming only occurred at the start of the rotation. CEC declined with Ca declines despite some increase in C. Management did not alter soil C concentration over the 1-m profile, but increased P and K. Nitrogen dynamics were consistent with pine plantations in that N demand is high, and despite high N inputs, soil N increases are limited.