Effects of Excess Nitrogen (N) on Fine Root Growth in Tropical Forests of Contrasting N Status

• Published in: Forests Vol. 13; no. 8; p. 1328
• Main Authors: Zhu, Feifei, Gilliam, Frank S., Mulder, Jan, Yoh, Muneoki, Mo, Jiangming, Lu, Xiankai
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.08.2022
• Subjects: Acidic soils; Acidification; Aluminum; Biomass; Calcium ions; Calcium mobilization; Cell division; China; Decomposition; Depletion; Deposition; Environmental aspects; Fertilization; fine root chemistry; fine root vitality; Forest biomass; Forests; Forests and forestry; Growth; Iron; Land use; Magnesium; Nitrates; Nitrogen; nitrogen deposition; Nitrogen in the body; Nutrients; pH effects; Precipitation; Regression analysis; Roots (Botany); Soil acidification; Soil chemistry; Soil pH; Soils; tropical forest; Tropical forests; China
• ISSN: 1999-4907; 1999-4907
• DOI: 10.3390/f13081328

Elevated nitrogen (N) deposition may further acidify soils in tropical forests. Yet, we have limited evidence on this prediction and it remained unclear how this would affect fine root growth therein. Here, we report responses of fine root biomass, vitality, and chemistry, as well as related soil parameters to eight years of N additions in three tropical forests different in initial soil N status, with one primary forest being N-saturated, and another two younger forests (one secondary forest and one planted forest) less N-rich. Results showed that in the primary forest, fine root biomass decreased and fine root necromass increased following N addition, resulting in lower live fine root proportion (fine root vitality). Declining fine root vitality was associated with fine root Fe accumulation and soil acidification indicated by regression analysis. These alterations of fine root growth and chemistry co-occurred with soil pH decline, soil exchangeable Fe3+ mobilization, exchangeable Ca2+, and Mg2+ depletion after N treatments in the primary forest. In contrast, N addition only elevated fine root K, Al, and Fe content in the secondary forest. In the planted forest, moderate but significant decreases in soil pH, soil exchangeable K+, and Mg2+ were found after N treatment, with fine root biomass negatively correlated with soil exchangeable Al3+ and Al3+/(Ca2+ + Mg2+) ratio. Our results suggested that long-term N fertilization may negatively affect fine root growth, via severed soil acidification, Fe mobilization, and base cation depletion in highly acidified, N-saturated primary tropical forests. Initial forest N status, influenced by different land-use history, mediates N deposition effects on fine root growth.