Organic Matter and Nitrogen Cycles in a Pine Afforested Catchment with a Shrub Layer of Adenocarpus decorticans and Cistus laurifolius in South-eastern Spain

Organic matter and nitrogen cycles in two mediterranean woodyshrubs (Adenocarpus decorticans Boiss. andCistus laurifoliusL.) and two pine species were investigated during 1987–1989in a small, recently pine-afforested catchment in Sierra delos Filabres (Almería Province, south-eastern Spain).Pine cover was sparse because canopy closure was not yet completed,allowing strong shrub colonization in the clearings. The aimof this study was to compare the contribution of different annualfluxes and the distribution of organic matter and nitrogen inboth shrub and pine components for the entire catchment. Specialemphasis was placed on the N use strategy shown by the two shrubspecies, a symbiotic N₂fixing species (A. decorticans) and anon-fixing species (C. laurifolius). Our results showed that the characteristics of C and N cyclingin the Nacimiento catchment are mainly determined by the earlystage of pine layer development and the significant contributionof the two shrub species differing with respect to C acquisitionand N use. The pine layer was characterized by a low standing biomass,substantial percentages of needle biomass and high (net production/biomass)and (needle production/total net production) ratios. N uptakewas mainly diverted to biomass increment, as shown by the substantialallocation found in this study. Overall, these results are consistentwith the patterns described for even-aged stands during theearly-growth phase. Retranslocation supplied 21% of the annualN requirements for pine above-ground biomass production. C. laurifolius showed low litter-fall return, strong retranslocation,efficient utilization of N in bulk deposition and slow decomposition,suggesting a closer, self-regulated organic matter and N-conservingbehaviour thanA. decorticans . On the contrary,A. decorticansshowed high leaf turnover, strong litter-fall return, low retranslocationand fast mineralization of organic matter and nitrogen, indicatingthat internal organic matter and N cycling is less efficient,and thus, that this species is more dependent on the soil tomeet its N requirements. Adenocarpus decorticans Boiss.; biogeochemical cycles,Cistus laurifolius L.; mediterranean shrubs; nitrogen fixation; nitrogen use; pine afforestation; south-eastern Spain; woody legumes