富士苹果萌芽至新梢旺长期肥料氮去向和土壤氮库盈亏

运用¹⁵N同位素示踪技术,以5年生‘烟富3’/SH6/平邑甜茶苹果为试材,研究了萌芽至新梢旺长期不同施氮水平(0、50、100、150、200、250 kg·hm^-2)下肥料氮的吸收利用、土壤残留和土壤氮库盈亏特点.结果表明:早春施氮后,15N均优先分配到根系中,然后转运用于地上部新生器官(果实、新生枝叶)的形态建造.新梢旺长期结束后(施氮2个月后),5.9%~9.9%的肥料氮被树体吸收,29.8%~33.4%的肥料氮残留在0~60 cm土体中,56.7%~64.4%的肥料氮通过其他途径损失.随施氮水平的提高,树体吸收的肥料氮量和土壤残留氮量逐渐增加,但肥料氮利用率和土壤残留率却不断降低,同时损失量和损失率不断增加.随施氮水平的提高,土壤氮素总平衡由亏缺转为盈余,且盈余量随施氮水平的提高而显著提高.表明施氮不足将会造成土壤氮肥力的下降;而过量施氮则会加剧土壤氮素累积,增加氮素污染风险.施氮水平与土壤氮素总平衡呈显著线性相关关系,拟合方程为:y=0.3511x-20.808(R^2=0.9927),当施氮量为59.27 kg·hm^-2时,由萌芽至新梢旺长期的土壤氮库达到平衡.     

Not only trees: Grasses determine African tropical biome distributions via water limitation and fire

Aim

Although much tropical ecology generally focuses on trees, grasses are fundamental for characterizing the extensive tropical grassy biomes (TGBs) and, together with the tree functional types, for determining the contrasting functional patterns of TGBs and tropical forests (TFs). To study the factors that determine African biome distribution and the transitions between them, we performed the first continental analysis to include grass and tree functional types.

Location

Sub‐Saharan Africa.

Time period

2000–2010.

Major taxa studied

Savanna and forest trees and C₄ grasses.

Methods

We combined remote‐sensing data with a land cover map, using tree functional types to identify TGBs and TFs. We analysed the relationships of grass and tree cover with fire interval, rainfall annual average and seasonality.

Results

In TGBs experiencing < 630 mm annual rainfall, grass growth was water limited. Grass cover and fire recurrence were strongly and directly related over the entire subcontinent. Some TGBs and TFs with annual rainfall > 1,200 mm had the same rainfall seasonality but displayed strongly different fire regimes.

Main conclusions

Water limitation to grass growth was fundamental in the driest TGBs, acting alongside the well‐known limitation to tree growth. Marked differences in fire regimes across all biomes indicated that fire was especially relevant for maintaining mesic and humid TGBs. At high rainfall, our results support the hypothesis of TGBs and TFs being alternative stable states maintained by a vegetation–fire feedback for similar climatic conditions.     

Influence of shaded systems on Xylosandrus compactus infestation in Robusta coffee along a rainfall gradient in Uganda

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