Nutrient Limitation in Three Lowland Tropical Forests in Southern China Receiving High Nitrogen Deposition: Insights from Fine Root Responses to Nutrient Additions

Abstract

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.

Root distribution of trees and crops: competition and/or complementarity.

“…Vitousek and Sanford (1986) found that shoot:root ratios in tropical forest decrease with decreasing soil fertility. Leuschner et al, (2007) found that the ratio of root biomass to above-ground biomass increased 10 fold with altitude in tropical forest at 1050 and 3060 m elevation, and Zhu et al (2013) found that nitrogen addition to an N-rich old growth forest decreased fine root biomass by 31%. A review of above-and below-ground production in forest ecosystems found that the highest fine root biomasses were found at locations where soils were high in Al and Fe and nutrient limited (Vogt et al, 1996).…”

Phosphorus amendment mitigates nitrogen addition-induced phosphorus limitation in two plant species in a desert steppe, China

“…This ecosystem, unlike ecosystems such as bogs and tropical forests that are typically P limited (i.e., high soil N:P ratio), has been found to be experiencing high P but low N in soils (Delgado-Baquerizo et al 2013). This raises the question: will N deposition be beneficial to this desert steppe, or will it lead to a similar shift from N to P limitation as has been observed in bogs and tropical forests (i. e., Phuyal et al 2008;Zhu et al 2013)?…”

“…Since N deposition affects soil N content more positively than soil C and P contents, researchers have hypothesized that excessive long term N deposition may cause an increase in the ecosystem N:P ratio, leading to N saturation and eventual P limitation (Bobbink et al 1998;van Heerwaarden et al 2003;Menge and Field 2007;Vitousek et al 2010;Li et al 2012). To date, this prediction has been tested in many ecosystem types, such as three tropical forests in southern China (Zhu et al 2013), an ombrotrophic bog in Whim Moss, Edinburgh, UK (Phuyal et al 2008), a calcareous grassland in Derbyshire Dales, UK (Phoenix et al 2004), a fire-derived and N-rich Hawaiian grassland (D'Antonio and Mack 2006), and several temperate forests (Gress et al 2007;Braun et al 2010;Naples and Fisk 2010). Despite the well-documented evidence in other ecosystems, our knowledge of the effect of N deposition remains incomplete for the desert steppe.…”

Responses of soil acid phosphatase and beta-glucosidase to nitrogen and phosphorus addition in two subtropical forests in southern China

“…P addition has been suggested as an effective management practice because P addition may mitigate the negative effects caused by adding N alone, such as mitigating the decrease in litter decomposition rates [31], microbial biomass [32] and live fine root biomass [33]. According to previous studies, adding P alone seems to only affect soil phosphatase activity but not b-glucosidase activity.…”