Abstract
Conversion of lowland tropical rainforests to intensely fertilized agricultural land-use systems such as oil palm (Elaeis guineensis) plantations leads to changes in nitrogen (N) cycling. Although soil microbial-driven N dynamics has been largely studied, the role of the plant as a major component in N uptake has rarely been considered. We address this gap by comparing the root N contents and uptake in lowland rainforests with that in oil palm plantations on Sumatra, Indonesia. To this aim, we applied 15N-labeled ammonium to intact soil, measured the 15N recovery in soil and roots, and calculated the root relative N uptake efficiency for 10 days after label application. We found that root N contents were by one third higher in the rainforest than oil palm plantations. However, 15N uptake efficiency was similar in the two systems. This finding suggests that lower N contents in oil palm roots were likely caused by plant internal utilization of the absorbed N (e.g., N export to fruit bunches) than by lower ability to take up N from the soil. 15N recovery in roots was primarily driven by the amount of root biomass, which was higher in oil palm plantation than rainforest. The oil palms unveiled a high capacity to acquire N, offering the possibility of enhancing sustainable plantation management by reducing N fertilizer application.
Highlights
Conversion of tropical rainforests to cash crop monocultures is globally increasing (Curran et al, 2004; Corley, 2009; Hansen et al, 2009) triggering rural development and often improving the living standards of the smallholder farmers (Euler et al, 2017)
Soil microbial-driven N dynamics has been largely studied, the role of the plant as a major component in N uptake has rarely been considered. We address this gap by comparing the root N contents and uptake in lowland rainforests with that in oil palm plantations on Sumatra, Indonesia
The root vitality and ratio of the number of vital root tips to fine root biomass were significantly lower in the oil palm plantations in comparison with forest plots (P = 0.001; Table 1)
Summary
Conversion of tropical rainforests to cash crop monocultures is globally increasing (Curran et al, 2004; Corley, 2009; Hansen et al, 2009) triggering rural development and often improving the living standards of the smallholder farmers (Euler et al, 2017). Tropical soils generally have large pools of available N with high N-cycling rates (Hedin et al, 2009). Rainforest conversion to crop monocultures involves clearing, draining, and burning and thereby, decreases soil fertility (Dechert et al, 2004; Bringhurst and Jordan, 2015; Guillaume et al, 2016). In Sulawesi, conversion of the lower montane forests to cornfields resulted in the slowdown of N cycling by decreasing the gross N mineralization and turnover rates of ammonium (NH4+) and microbial N pools (Corre et al, 2006)
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