Symbiotic N nutrition, carbon accumulation, and water-use efficiency in species of the genus Aspalathus native to the Cape fynbos, South Africa

Abstract

Low soil moisture and N in the fynbos are a major constraint to plant growth and N2 fixation in legumes, including Aspalathus species. The 15N and 13C natural abundance techniques were used to study N nutrition and plant water relations in different Aspalathus species sampled from 13 study sites in the fynbos. The results showed that A. acuminata shoots recorded lower δ15N values at Kalbaskraal, Worcester Langerug and Roundebosch, resulting in higher %Ndfa at those sites relative to Elim, which registered greater δ15N values. However, A. hispida shoots showed the lowest δ15N at Elim and Dutoitskloof than Yzerfontein, and Silwerstroomstrand, resulting in high %Ndfa at Elim. Of A. cordata, A. astroites, A. hispida, and A. wildenowiana collected from Dutoitskloof differed in δ15N and %Ndfa. A. cordata and A. astroites shoots showed lower δ15N, leading to higher %Ndfa than A. wildenowiana, which exhibited greater shoot δ15N. Of four Aspalathus species (A. cordata, A. carnosa, A. capensis and A. juniperina) sampled from Silvermine, A. cordata revealed the lowest δ15N and hence the highest %Ndfa. Conversely, A. carnosa recorded the highest shoot δ15N and the lowest %Ndfa. At Kokrivier, shoot δ15N was much greater in A. zeyheri and lowest in A. incurvifolia, resulting in higher %Ndfa in the latter than the former. The Aspalathus species differed in water-use efficiency or shoot δ13C values, which ranged from −30.35‰ to −26.93‰. Rainfall, soil moisture deficit and altitude were the major determinants of shoot δ13C values, but how these factors influence shoot δ13C of plants remains to be determined.