Accumulation of phosphorus and carbon and the dependency on biological N-2 fixation for nitrogen nutrition in Polhillia, Wiborgia and Wiborgiella species growing in natural stands in cape fynbos, South Africa

摘要

Species of the genera Polhillia, Wiborgia and Wiborgiella are endemic to the Cape fynbos, South Africa. These plants form root nodules with soil rhizobia that fix atmospheric nitrogen. Little is known about their N and P nutrition, as well as their water-use efficiency in this highly dry and nutrient-poor soil environment. Therefore, the aim of this study was to assess P nutrition and water-use efficiency in natural stands of Polhillia, Wiborgia and Wiborgiella species as well as their dependency on Biological N-2 fixation for nutrition. The delta N-15 natural abundance technique was used to measure the symbiotic performance. The Acid and alkaline phosphatase activities were assayed using p-nitrophenol method. 16S rRNA sequence was used to identify microsymbionts nodulating the test plants. The delta N-15 study showed a high dependency of the tested species on N-2 fixation. The N derived from fixation was 76-84% for Polhillia sps., 73-91% for Wiborgia sps., and 61-68% for Wiborgiella sessilifolia. The species differed in water-use efficiency, with delta C-13 values of -28.1 to -25.1 parts per thousand for Polhillia sps., -28.3 to -25.8 parts per thousand for Wiborgia sps., and - 27.7 to -27.1 parts per thousand for Wiborgiella sessilifolia. The rhizosphere acid and alkaline phosphatase activities were higher in P. brevicalyx and P. pollens, than in the other tested species and this resulted in greater available P in the rhizosphere soils and an increased P uptake and accumulation in the plant shoots. Based on 16S rRNA nucleotide sequence and phylogenetic analysis of root nodule isolates, a diverse and novel Mesorhizobium sps. nodulate the tested plant species in the fynbos ecosystem.