Nutrient Limitation on Ecosystem Productivity and Processes of Mature and Old-Growth Subtropical Forests in China

Nitrogen (N) is considered the dominant limiting nutrient in temperate regions, while phosphorus (P) limitation frequently occurs in tropical regions, but in subtropical regions nutrient limitation is poorly understood. In this study, we investigated N and P contents and N:P ratios of foliage, forest floors, fine roots and mineral soils, and their relationships with community biomass, litterfall C, N and P productions, forest floor turnover rate, and microbial processes in eight mature and old-growth subtropical forests (stand age >80 yr) at Dinghushan Biosphere Reserve, China. Average N:P ratios (mass based) in foliage, litter (L) layer and mixture of fermentation and humus (F/H) layer, and fine roots were 28.3, 42.3, 32.0 and 32.7, respectively. These values are higher than the critical N:P ratios for P limitation proposed (16–20 for foliage, ca. 25 for forest floors). The markedly high N:P ratios were mainly attributed to the high N concentrations of these plant materials. Community biomass, litterfall C, N and P productions, forest floor turnover rate and microbial properties were more strongly related to measures of P than N and frequently negatively related to the N:P ratios, suggesting a significant role of P availability in determining ecosystem production and productivity and nutrient cycling at all the study sites except for one prescribed disturbed site where N availability may also be important. We propose that N enrichment is probably a significant driver of the potential P limitation in the study area. Low P parent material may also contribute to the potential P limitation. In general, our results provided strong evidence supporting a significant role for P availability, rather than N availability, in determining ecosystem primary productivity and ecosystem processes in subtropical forests of China.

氮（N）被认为是温带地区的主要限制性营养元素，而磷（P）限制则常见于热带地区，但亚热带地区的营养元素限制机制目前仍有待深入阐明。本研究针对中国鼎湖山生物圈保护区的8片成熟老龄亚热带森林（林龄>80年），调查了其叶片、森林枯落物层、细根与矿质土壤的氮、磷含量及氮磷比，并分析了这些指标与群落生物量、凋落物碳、氮、磷生产量、枯落物层周转速率以及微生物过程之间的关联。叶片、凋落物（L）层以及腐殖质发酵层与腐殖质层（F/H）混合物、细根的单位质量氮磷比平均值分别为28.3、42.3、32.0和32.7。这些数值高于已提出的磷限制临界氮磷比（叶片为16~20，枯落物层约为25）。显著偏高的氮磷比主要归因于这些植物材料较高的氮浓度。群落生物量、凋落物碳、氮、磷生产量、枯落物层周转速率以及微生物特性与磷相关指标的相关性强于氮相关指标，且通常与氮磷比呈负相关，这表明除一处受人为干扰的样地外，所有研究样地的磷有效性在调控生态系统生产能力与养分循环过程中均发挥着关键作用，而该受干扰样地的氮有效性可能同样重要。我们提出，氮富集可能是研究区域潜在磷限制的重要驱动因素，而低磷母质也可能加剧了这一潜在磷限制现象。总体而言，本研究结果提供了强有力的证据，证明在中国亚热带森林中，磷有效性而非氮有效性，是调控生态系统初级生产力与生态过程的关键因子。