Nutrient resorption in Caragana microphylla along a chronosequence of plantations: Implications for desertified land restoration in North China

Resorption of nutrients before leaf death and abscission is an important energy conservation mechanism employed by plants to endure nutrient limitation. However, leguminous N-fixing plants are able to acquire N through biochemical fixation, so are less dependent of soil N availability, and thus probably resorb less N. In the semi-arid Horqin Sandy Land of North China, Caragana microphylla, a leguminous N-fixing shrub, is the dominant plant species and is widely used in vegetation re-establishment programs to stabilize shifting sand. In this study, green- and senesced-leaf nutrient concentrations of C. microphylla were examined to compare nutrient resorption patterns along an age sequence of plantations (11, 19, 27 and 36 years, respectively) and to detect if nutrient resorption was affected by soil nutrient availability. The results showed that there were significant effects (p < 0.01) of C. microphylla plantation age on nutrient resorption. N and P resorption efficiency decreased from 40% and 68%, respectively in the 11-yr old plantation to 32.0% and 55% in the 36-yr old plantation, while N and P concentrations of senesced leaves (indicators of resorption proficiency) increased from 17.5 and 0.56 mg g−1, respectively in the 11-yr old plantation to 24.8 and 0.91 mg g−1 in the 36-yr old plantation. Furthermore, N and P resorption proficiency of individual shrubs were negatively related to available N and P in the soil under the shrub canopy within and among plantations. This indicates that more foliar N and P was resorbed in lower nutrient sites and implies that C. microphylla in nutrient-poor environments is less dependent on day-to-day nutrient uptake and thereby adapted to severely desertified regions. Additionally, C. microphylla in the present study showed incomplete resorption of nitrogen, which suggests senesced-leaf fall returns highly decomposable litter to the soil, and thereby indirectly improves soil nutrient availability. The restoration of desertified land, therefore, may be accelerated with plantations of the N-fixing leguminous shrub C. microphylla.