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

Abstract

Organic matter and nitrogen cycles in two mediterranean woody shrubs ( Adenocarpus decorticansBoiss. and Cistus laurifoliusL.) and two pine species were investigated during 1987–1989 in a small, recently pine-afforested catchment in Sierra de los Filabres (Almerı́a Province, south-eastern Spain). Pine cover was sparse because canopy closure was not yet completed, allowing strong shrub colonization in the clearings. The aim of this study was to compare the contribution of different annual fluxes and the distribution of organic matter and nitrogen in both shrub and pine components for the entire catchment. Special emphasis was placed on the N use strategy shown by the two shrub species, a symbiotic N 2fixing species ( A. decorticans) and a non-fixing species ( C. laurifolius). Our results showed that the characteristics of C and N cycling in the Nacimiento catchment are mainly determined by the early stage of pine layer development and the significant contribution of the two shrub species differing with respect to C acquisition and 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 uptake was mainly diverted to biomass increment, as shown by the substantial allocation found in this study. Overall, these results are consistent with the patterns described for even-aged stands during the early-growth phase. Retranslocation supplied 21% of the annual N requirements for pine above-ground biomass production. C. laurifoliusshowed 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-conserving behaviour than A. decorticans. On the contrary, A. decorticansshowed high leaf turnover, strong litter-fall return, low retranslocation and fast mineralization of organic matter and nitrogen, indicating that internal organic matter and N cycling is less efficient, and thus, that this species is more dependent on the soil to meet its N requirements.