The impact of annual harvesting of switchgrass intercropped in loblolly pine plantations on soil compaction and long-term soil productivity

Background/Question/Methods

Using forest biomass for biofuel production has emerged as a promising approach. However, very little research has focused on how to simultaneously grow biomass for biofuel production while still managing for high quality wood products. Furthermore, the potential effects this could have on long-term soil productivity is critical for the viability of such an intensive management system. As a result of this gap in scientific knowledge, a long term study has been established by Catchlight Energy LLC (a Chevron|Weyerhaeuser joint venture) on land owned and managed by Weyerhaeuser Company within the lower coastal plain of North Carolina to investigate effects of intercropping and biomass management on site productivity and sustainability. The study consists of seven treatments implemented within 0.81 hectare plots, replicated four times, with and without logging residuals: 1) loblolly pine (Pinus taeda L.) plantation, 2) loblolly pine with switchgrass (Panicum virgatum L.) interplanted between beds, and 3) loblolly pine establishment with flat-planted pine trees between crop tree beds. Stands of pure switchgrass were also established. Site preparation for each treatment varied in intensity and number of entries with heavy equipment. Furthermore, treatments with switchgrass require annual entries to cut, rake, and bale this feedstock. Impacts of these non-traditional forest management approaches on soil sustainability and productivity were evaluated. Soil compaction measurements were obtained using a soil penetrometer to a depth of 30 cm at 2.5 cm intervals at five different time periods: 1) before and 2) after treatment installation, 3) following a cut only treatment at the end of year one, 4) at end of year two and 5) year three which both included cutting, raking and baling of switchgrass.

Results/Conclusions

Clearing and site preparation that were completed for treatment installation did not adversely affect soil strength. Repeated activities associated with switchgrass harvesting caused a significant increase in soil strength (switchgrass > pine with switchgrass > pine = pine with extra row) in the top 15cm only. However, it is possible that these conditions could improve over time as switchgrass’ prolific rooting system continues to enhance soil structure and increase soil organic matter. Additionally, this increase in soil strength did not adversely affect pine or switchgrass growth.