Linkages between phosphorus transformations and carbon decomposition in a forest soil

Abstract

Phosphorus mineralization is chemically coupled with organic matter (OM) decomposition in surface horizons of a mixed-conifer forest soil from the Sierra Nevada, California, and is also affected by the disturbance caused by forest harvesting. Solution13C nuclear magnetic resonance (NMR) spectroscopy of NaOH extracts revealed a decrease of O-alkyl and alkyl-C fractions with increasing degree of decomposition and depth in the soil profile, while carbonyl and aromatic C increased. Solid-state13C-NMR analysis of whole soil samples showed similar trends, except that alkyl C increased with depth. Solution31P-NMR indicated that inorganic P (P1) increased with increasing depth, while organic-P (Po) fractions decreased. Close relationships between P mineralization and litter decomposition were suggested by correlations between P1 and C fractions (r = 0.82, 0.81, −0.87, and −0.76 for carbonyl, aromatic, alkyl and O-alkyl fractions, respectively). Correlations for diester-P and pyrophosphate with O-alkyl (r = 0.63 and 0.84) and inverse correlations with aromatics (r = −0.74 and −0.72) suggest that mineralization of these P fractions coincides with availability of C substrate. A correlation between monoester P and alkyl C (r = 0.63) suggests mineralization is linked to breakdown of structural components of the plant litter. NMR analyses, combined with Hedley-P fractionation, suggest that post-harvest buildup of labile P in decomposed litter increases the potential for leaching of P during the first post-harvest season, but also indicates reduced biological activity that transports P from litter to the mineral soil. Thus, P is temporarily stored in decomposed litter, preventing its fixation by mineral oxides. In the mineral horizons,31P-NMR provides evidence of decline in biologically-available P during the first post-harvest season.