Response of Nutrients and Sediment to Hydrologic Variables in Switchgrass Intercropped Pine Forest Ecosystems on Poorly Drained Soil

In the present study, we examined the relationships between (1) N, P, total organic carbon (TOC), and total suspended sediment (TSS) each and stream flow and water table elevation, individually (2) N, P, and TOC, each and TSS, and (3) stream water C/N ratios and stream flow in managed pine forests with various switchgrass treatments implemented on four watersheds in coastal North Carolina plain. The treatments included a young pine forest–natural understorey (27.5 ha), a young pine forest with switchgrass intercropped between pine rows replacing natural understorey (IC) (26.3 ha), a mature thinned pine forest (25.9 ha), and pure switchgrass (27.1 ha). Precipitation, flow, water table elevation, N, phosphate, TOC, and TSS concentrations were measured from November 2009 to June 2014 (switchgrass growth from May 2012 after site preparation (SP) that ended in April 2012). Relationships ( α = 0.05) among water quality and hydrologic variables were examined using a Spearman rank correlation coefficient and the principal component analysis (PCA). Nitrogen concentrations on IC were positively correlated with flow during SP. The export of nutrients and sediment from this drained pine plantation forest intercropped with switchgrass was affected by changes in hydrological and biochemical processes regulating the formation and transport of different water quality constituents during both site preparation and pine and switch growth periods. The PCA showed strong interaction between the hydrological and biochemical processes.