Use of foliar 15N and 13C abundance to evaluate effects of microbiotic crust on nitrogen and water utilization in Pinus massoniana in deteriorated pine stands of south China

We compared the foliar ¹⁵N and ¹³C values of Pinus massoniana growing on soils with and without microbiotic crust to examine the influence of the microbiotic crust on N and water use in plants in deteriorated watersheds in southern China. At our study site, litterfall and undergrowth had been intensively removed for fuel and soil N concentration was extremely low. Microbiotic crust covered the lower slope within the watersheds and pine trees were taller here than on the middle and upper slopes, although the crust reduced the amount of rainfall that could penetrate the soil. The foliar ¹⁵N values were greater (closer to zero) in pine trees growing on soil covered with microbiotic crust on the lower slope than on the middle and upper slopes, which lacked the microbiotic crust. These data suggest that P.massoniana may depend on N fixed by the microbiotic crust on the lower slope, and on N carried by precipitation on the middle and upper slopes. The microbiotic crust did not influence foliar ¹³C, an index for water use efficiency, in P.massoniana. The fact that P.massoniana biomass was greater on the lower slope, which is less permeable to rainfall, suggests that P.massoniana growth may be limited by the amount of available N rather than by water. The microbiotic crust may improve plant productivity by increasing N availability, despite its negative effect on water availability.