Belowground impact of napier and guinea grasses grown for biofuel feedstock production

Abstract

Objectives of this study therefore were twofold. The first objective was to discern differences between species and accessions in the quantity and quality of C input after three cycles of ratooning to determine which accession of grass is best suited for potential soil C sequestration in Hawaiʻi. In order to quantify total amount of belowground C input, mass balance approach called total belowground C flux (TBCF) was used. Although a common assumption of the TBCF approach is that soil C pools are nearly at steady state (Giardina & Ryan, 2002), this may not be true in the system experiencing abrupt change from fallow grassland to intensively managed high yielding grass system like in this study. Therefore, changes in both soil and root C overtime were also quantified over the measurement period. The second objective was to develop a conceptual model of causal relationships among quantity and quality variables of C fluxes and pools in soil using structural equation modeling (SEM). Structural equation modeling is a multivariate statistical method which aims to disentangle the effect of the multiple explanatory variables into hypothesized causal pathways (Grace, 2006, Tabachnick & Fidell, 2007). Several recent ecological studies have used SEM to investigate the causal relationships among multicollinear predicting variables and their effect on soil CO2 efflux (FS) (Geng et al., 2012, Matias et al., 2012), soil C storage (Brahim et al., 2011, Jonsson & Wardle, 2010), and soil microbial community (Eisenhauer et al., 2012). The method can be either (1) a priori confirmation of statistical adequacy of proposed model, or (2) a posteriori modeling building from exploratory data analysis (Petraitis et al., 1996). This study utilized both approaches to model soil C dynamics. The variables in the models were selected after the data collection, while the initial model was formulated using theory from previous literature. Nevertheless, the purpose of SEM in this study was to delineate patterns of direct and indirect effects of explanatory variables by formulating causal model based on both results of this study and prior knowledge of soil C accumulation. Understanding of factors affecting soil C accumulation will guide selection and breeding of improved accessions for soil C sequestration.