Abstract:

Objective: This study is intended to explore whether and how slash pine (Pinus elliottii) nutrient acquisition and utilization strategies are effected by the initial planting (even-aged mixed) and replanting (uneven-aged mixed) of broad-leaved trees, in order to provide a scientific basis for management of the slash pine forests. Method: Our study was focused on slash pine, which are widely planted in the subtropical red soil hilly region of China. We conducted a long-term single-species and mixed experiment including three treatments (a 20 m×20 m plot per treatment) that pure slash pine plantation, mixed plantation of slash pine and Schima superba, mixed plantation of slash pine and S. superba (replanting S. superba for 14 years).At the age of 30 years and with similar site conditions, five 20 m × 20 m plots in each treatment were established.The rhizosphere soil, fine roots, twigs, fresh and senesced needles of slash pine standard trees were collected. The nitrogen (N) and phosphorus (P) in soil and plant organs were determined to evaluate the effects of different mixed methods on root nutrient capture, needles nutrient resorption and nutrient translocation, and to reveal the driving factors of nutrient acquisition strategies. Result: Initial and replanting broad-leaved trees increased the rhizosphere soil NH₄⁺-N and mineral N (MN) in slash pine (P<0.05). But there was no significant difference in nutrient contents between the initial planting and the replanting. Mixed broad-leaved trees increased the N and P concentrations of each organ in slash pine, and the mixed effect of the initial planting was better than that of the replanting. Slash pine had 14.04% and 46.16% higher root capture ability of N and P in the initial mixed plantation than the pure plantation, respectively. The needles N resorption efficiency in the initial mixed plantation significantly decreased by 6.07% compared with slash pine pure plantation, and the needles P resorption efficiency in the initial mixed plantation significantly increased by 15.49%. The lack of significant effect of broadleaf tree replanting on both root capture and needles resorption efficiency of slash pine because of the reduction in cost of roots nutrient capture compensating for the inhibition of root nutrient absorption. In addition, compared with pure plantation, the N translocation of transport roots to twigs and needles in the initial mixed plantation increased by 36.05% and 15.61%, respectively. And compared with pure plantation, the P translocation of transport roots to twigs and needles in the initial mixed plantation was decreased by 35.13% and 36.52%, respectively. There was no significant change in N translocation in the replanting of the mixed plantation, but the P translocation from transport roots to twigs and needles was 53.21% and 40.17% lower than that in the pure plantation. Conclusion: Mixed broad-leaved trees increased the nutrient content of rhizosphere soil and plant organs, and changed the nutrient acquisition strategy of slash pine in red soil hilly region.The initial planting promoted nutrient acquisition and utilization of slash pine with higher intensity than the replanting. In addition, the translocation process of nutrient in plants would shape the trade off between root capture and needles resorption.

Key words: mixed plantation of coniferous and broad-leaved trees, nutrient capture by root, nutrient resorption, nutrient acquisition strategy, Schima superba