Effects of external and internal nutrient supplies on decomposition of wild rice, Zizania palustris.

Abstract

How external and internal nutrient supplies affects wild rice shoot and root litter decomposition and the nitrogen (N) and phosphorus (P) dynamics in decaying litter were examined in two experiments. In a site quality experiment, wild rice shoot and root litterbags were decayed in mesocosms in the field with added N or P or a control as ambient conditions, over 115 days. In the second litter quality experiment, wild rice plants were grown with N, P, both, or none, to produce litter of different qualities and then decayed for 168 days under controlled temperature in the laboratory. The impacts of both site and litter quality was greater on shoot litter than root litter. In the site quality experiment, P addition significantly increased wild rice shoot decay more than N addition and reduced P mineralization over time compared to the control. Neither adding N nor P influenced root decay, but adding P increased root P immobilization compared to the control, and the effect was much greater in root litter compared to shoot litter. In the litter quality experiment, P enrichment increased litter quality by increasing P concentrations and water-soluble compound concentrations in shoot litter and decreasing lignin concentrations in root litter, thereby increasing decay rates of both shoot and roots, although, the impact was much greater on shoot decay. N enrichment increased plant growth but reduced shoot litter quality and decay likely because these larger plants had greater lignin concentrations for structural support and lower phosphorus concentrations and lower N:P ratios. Increased lignin likely overrides a concomitant increase in nitrogen concentration in shoot litter and appears to control wild rice decomposition. Initial lignin:P,C:P, lignin+asid-solubles:P and the N:P ratios of shoot litter were correlated to the final mass remaining. Lignin and phosphorus appear to play a key role in driving wild rice decay through the opposite effects that each have on litter quality.