Litter quantity and nitrogen immobilization cause oscillations in productivity of wild rice (Zizania palustris L.) in northern Minnesota

Microbial immobilization of nitrogen (N) in litter from one year's production may cause oscillations in biomass production if it delays N availability the following year. We tested whether shoot and root litter and plant density affect biomass and seed production of populations of wild rice (Zizannia palustris L.) grown in 378 l stock tank mesocosms over four consecutive years. Half the tanks were thinned to a uniform seedling density whereas density in the remaining tanks was allowed to fluctuate ad libitum. Litter treatments included both shoot litter removal, leaving only root litter, and retaining shoot litter intact with root litter. A separate greenhouse fertilizer experiment tested whether N and/or phosphorus (P) limited productivity. Responses to N additions were much stronger than to P additions. Annual production and N availability in the tanks were correlated with each other and followed a concurrent cycle from 2004 to 2008. Furthermore, production in tanks with shoot + root litter did not fluctuate more than tanks with only root litter. Root litter immobilized more nitrogen and for a longer period than shoot litter. Neither litter immobilized P. Density did not affect mean seed weight, total seed production, or mean plant weight, but total seed production declined in years following productive years and was high only following years of low litter production. Root litter may therefore be primarily responsible for the delays in N availability that cause cycles in biomass and seed production. Consequently, both wild rice litter quantity and quality play central roles in production and population dynamics of wild rice stands.