长期氮沉降对杉木人工林叶、枝氮磷养分再吸收的影响

Responses of nitrogen and phosphorus resorption from leaves and branches to long-term nitrogen deposition in a Chinese fir plantation

为了解森林养分内循环对全球变化的响应,基于长期模拟氮沉降试验,研究了杉木(Cunninghamia lanceolata)人工林不同龄级(一年生、二年生和衰老)叶和枝的氮(N)、磷(P)养分分配及其再吸收特征,并分析了不同模拟N沉降处理时间(7年和14年)杉木叶N、P养分再吸收差异。在12年生杉木中开展模拟N沉降试验,以尿素(CO(NH2)2)为N源,设N0、N1、N2和N34个处理水平,施氮量分别为0、60、120和240kg·hm^-2·a^-1,每个处理重复3次。结果表明:(1)叶和枝在衰老过程中碳(C)、N和P含量逐渐降低,且叶的C、N和P含量比枝高;N含量大小依次为一年生叶>二年生叶>衰老叶>一年生枝>二年生枝>衰老枝,且N3>N2>N1>N0,而C:N则呈现相反的趋势;衰老器官的C:N、C:P、N:P比新鲜器官高;N沉降增加了不同龄级叶和枝(除二年生叶外)的N、N:P和C:P,但降低了P和C:N。(2)叶和枝的N、P养分再吸收率(REN、REP)随龄级的增加至衰老有规律地递减,且REP>REN;受长期N沉降的影响,REN叶(28.12%)<枝(30.00%),而REP则为叶(45.82%)>枝(30.42%);杉木叶和枝N:P与REN:REP之间存在极显著的线性相关关系。(3)随N沉降处理时间的增加,叶REN呈降低态势,各处理(N1、N2和N3)分别降低了9.85%、3.17%和11.71%;而REP则明显上升,分别增加了71.98%、42.25%和9.60%。研究结果表明:不同器官、不同龄级的养分再吸收率随氮沉降处理的水平、处理时间而所有不同;REN:REP与N:P之间存在紧密关系。

Aims Our objectives were to investigate differences in nutrient resorption between different plant organs (leaf and branch), among plants with different life spans (one-year old, two-year old and senesced), and under different duration of nitrogen (N) deposition treatments in a Chinese fir (Cunninghamia lanceolata) plantation. Methods The long-term N deposition experiment was conducted in a 12-year-old fir plantation of subtropical China. N deposition treatment was initiated in January 2004 until now, up-going 14 years. N deposition were designed at 4 levels of 0, 60, 120, and 240 kg·hm^-2·a^–1, indicated as N0, N1, N2, and N3, respectively, with 3 replicates for each treatment. The solution of CO(NH2)2 was sprayed on the forest floor each month. In the study, we measured N and phosphorus (P) concentrations and analyzed the pattern of nutrient resorption of mature and senescing leaves and branches. The different responses of needles N and P resorption after 7- and 14-year N deposition treatments were also compared. Important findings After 14 years of N deposition, (1) during the senescing process, leaf and branch C, N, and P content gradually decreased with increasing treatment duration, with higher content in leaf than in branch. N content decreased in the order of one-year old green leaf > two-year old green leaf > senescent leaf > one-year old living branch > two-year old living branch > senescent branch, and N3 > N2 > N1 > N0, with C:N showing the opposite trend. Senescent organs had higher C:N, N:P, and C:P than mature living organs. N deposition increased N, N:P, and C:P of mature living organs (except for the two-year old green leaf), while decreased P and C:N. (2) N resorption efficiency (REN) and P resorption efficiency (REP) of leaves and branches decreased gradually with increasing life span. REP was typically higher in leaf and branch than REN. Leaf had lower REN (28.12%) than branch (30.00%), but higher REP (45.82%) than branch (30.42%). A highly significant linear correlation existed between N:P and REN:REP in leaves and branches. (3) REN decreased but REP increased with the treatment duration of N deposition. The longer experimental duration (14 years) reduced REN by 9.85%, 3.17%, 11.71% under N1, N2, and N3 treatments, respectively, and increased REP by 71.98%, 42.25%, 9.60%, respectively, than the shorter treatment duration (7 years). In summary, the responses of essential nutrients resorption efficiency for different plant organs and life span varied with the levels and duration of N deposition treatment. REN:REP in leaf and branch were mostly driven by N:P of leaf and branch. The results highlight that nutrients resorption is significantly influenced by long-term N deposition.