Zhu Feifei, Yoh Muneoki, Gilliam Frank S, Lu Xiankai, Mo Jiangming
Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China ; United Graduate School of Agriculture Science, Tokyo University of Agriculture and Technology, Tokyo, Japan ; University of Chinese Academy of Sciences, Beijing, China.
Institute of Agriculture, Tokyo University of Agriculture and Technology, Tokyo, Japan.
PLoS One. 2013 Dec 20;8(12):e82661. doi: 10.1371/journal.pone.0082661. eCollection 2013.
Elevated nitrogen (N) deposition to tropical forests may accelerate ecosystem phosphorus (P) limitation. This study examined responses of fine root biomass, nutrient concentrations, and acid phosphatase activity (APA) of bulk soil to five years of N and P additions in one old-growth and two younger lowland tropical forests in southern China. The old-growth forest had higher N capital than the two younger forests from long-term N accumulation. From February 2007 to July 2012, four experimental treatments were established at the following levels: Control, N-addition (150 kg N ha(-1) yr(-1)), P-addition (150 kg P ha(-1) yr(-1)) and N+P-addition (150 kg N ha(-1) yr(-1) plus 150 kg P ha(-1) yr(-1)). We hypothesized that fine root growth in the N-rich old-growth forest would be limited by P availability, and in the two younger forests would primarily respond to N additions due to large plant N demand. Results showed that five years of N addition significantly decreased live fine root biomass only in the old-growth forest (by 31%), but significantly elevated dead fine root biomass in all the three forests (by 64% to 101%), causing decreased live fine root proportion in the old-growth and the pine forests. P addition significantly increased live fine root biomass in all three forests (by 20% to 76%). The combined N and P treatment significantly increased live fine root biomass in the two younger forests but not in the old-growth forest. These results suggest that fine root growth in all three study forests appeared to be P-limited. This was further confirmed by current status of fine root N:P ratios, APA in bulk soil, and their responses to N and P treatments. Moreover, N addition significantly increased APA only in the old-growth forest, consistent with the conclusion that the old-growth forest was more P-limited than the younger forests.
热带森林中氮（N）沉降增加可能会加速生态系统的磷（P）限制。本研究在中国南方的一片老龄林和两片较年轻的低地热带森林中，研究了五年的氮和磷添加对细根生物量、养分浓度以及土壤酸性磷酸酶活性（APA）的影响。由于长期的氮积累，老龄林的氮储量高于两片较年轻的森林。从2007年2月到2012年7月，设置了以下四个实验处理水平：对照、施氮（150 kg N ha(-1) yr(-1)）、施磷（150 kg P ha(-1) yr(-1)）和氮磷共施（150 kg N ha(-1) yr(-1)加150 kg P ha(-1) yr(-1)）。我们假设，在氮含量丰富的老龄林中，细根生长会受到磷有效性的限制，而在两片较年轻的森林中，由于植物对氮的大量需求，细根生长主要会对施氮做出反应。结果表明，五年的施氮仅显著降低了老龄林中活细根生物量（降低了31%），但显著提高了所有三片森林中死细根生物量（提高了64%至101%），导致老龄林和松林中活细根比例下降。施磷显著增加了所有三片森林中的活细根生物量（增加了20%至76%）。氮磷共施显著增加了两片较年轻森林中的活细根生物量，但老龄林中未增加。这些结果表明，所有三片研究森林中的细根生长似乎都受到磷的限制。细根氮磷比现状、土壤中酸性磷酸酶活性及其对氮磷处理的反应进一步证实了这一点。此外，施氮仅显著增加了老龄林中的酸性磷酸酶活性，这与老龄林比年轻森林更受磷限制的结论一致。
