Soil fertility and fine root dynamics in response to 4 years of nutrient (N, P, K) fertilization in a lowland tropical moist forest, Panama

Abstract

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m -2 year -1 ), phosphorus (P as superphosphate, 5 g P m -2 year -1 ) and potassium (K as KCl, 5 g K m -2 year -1 ) within 38 ha of old-growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images.We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg -1 ), low concentrations of Bray-extractable phosphate (PO4 = 2.2 mg kg -1 ), and modest concentrations of KCl-extractable nitrate (NO3 = 5.0 mg kg -1 ) and KClextractable ammonium (NH4 = 15.5 mg kg -1 ). Added N increased concentrations of KCl-extractable NO3 and acidified the soil by one pH unit. Added P increased concentrations of Bray-extractable PO4 and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m -2 .The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species-rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.

Stable nitrogen isotope patterns of trees and soils altered by long-term nitrogen and phosphorus addition to a lowland tropical rainforest

“…The effects of altered P cycling on soil d 15 N values Phosphorus addition increased resin phosphate regardless of N co-addition (Table 1), which agrees with previous findings using different extraction methods Yavitt et al 2011). Interestingly, P addition also influenced soil nitrate d 15 N indirectly, as indicated by the N 9 P interaction for the fully replicated mid-rainy season resin nitrate (Table 2).…”

Nutrients limit photosynthesis in seedlings of a lowland tropical forest tree species

“…The first possibility is that these plants grow more in response to added nutrients by allocating less photosynthate to root growth which causes an increase in above-ground growth. Consistent with this hypothesis, at the site of this study, seedling root-to-shoot biomass ratio decreases with K addition (Santiago, unpublished data), and K addition decreases stand-level fine root biomass (Yavitt et al 2010;Wright et al 2011). The second possibility is that increased nutrient supply provides a resource that may be used to increase the uptake of another limiting resource such as light (Fahey et al 1998).…”

“…Foliar P also decreased with N addition. N addition in the same long-term fertilization experiment reduced soil pH by 0.5 pH units (Yavitt et al 2010), which may have affected the solubility of P or other elements. Foliar K increased by 8%, which was more modest than expected for an element that can be stored inorganically.…”

Effects of nitrogen and phosphorus supply on growth and physiological traits of two Larix species

“…Menge and Field (2007) found that N fertilization led to P limitation in six out of eleven previously N-limited ecosystems, mainly because N input could accelerate P cycling rates across a wide variety of terrestrial ecosystems (Marklein and Houlton, 2012). Nevertheless, most investigations on P fertilization have been conducted in N-rich Yavitt et al, 2011;Santiago et al, 2012;Mori et al, 2013) or P-limited conditions (De Groot et al, 2003;Wright et al, 2005), while only few studies have investigated how P fertilization influences productivity in N-limited ecosystems (Blevins et al, 2006;Carlson et al, 2014;Gotore et al, 2014).…”