Abstract
Humid tropical forests are commonly characterized as N-rich but P-deficient. Increased N deposition may drive N saturation and aggravate P limitation in tropical forests. Thus, P addition is proposed to mitigate the negative effects of N deposition by stimulating N cycling. However, little is known regarding the effect of altered N and P supply on the nutrient status of understory plants in tropical forests, which is critical for predicting the consequences of disturbed nutrient cycles. We assessed the responses of N concentration, P concentration, and N:P ratios of seven understory species to N and P addition in an 8-year fertilization experiment in a primary forest in south China. The results showed that N addition had no effect on plant N concentration, P concentration, and N:P ratios for most species. In contrast, P addition significantly increased P concentration, and decreased N:P ratios but had no effect on plant N concentration. The magnitude of P concentration responses to P addition largely depended on the types of organs and species. The increased P was more concentrated in the fine roots and branches than in the leaves. The gymnospermous liana Gnetum montanum Markgr. had particularly lower foliar N: P (~9.8) and was much more responsive to P addition than the other species studied. These results indicate that most plants are saturated in N but have great potential to restore P in primary tropical forests. N deposition does not necessarily aggravate plant P deficiency, and P addition does not increase the retention of deposited N by increasing the N concentration. In the long term, P inputs may alter the community composition in tropical forests owing to species-specific responses.
Highlights
Humid tropical forests support the main organic carbon pool and harbor diverse species in terrestrial ecosystems [1]
We found that Cryptocarya chinensis Hemsl (CCH) and Cryptocarya concinna Hance (CCO) were quite similar in foliar nutrient concentrations and responses to P addition (Figure 4), as both belong to the same genus (Cryptocarya)
One plausible explanation is that the forest has already been N-saturated, and plant [N] has been at the optimal level. No factors, such as P addition, could further elevated plant [N], as a high load of N addition has no effect on plant [N], discussed above
Summary
Humid tropical forests support the main organic carbon pool and harbor diverse species in terrestrial ecosystems [1]. These tropical forests are commonly rich in N but are relatively poor in P availability because of the substantial biological N fixation and P leaching loss during pedogenesis [2]. Global N and P cycling has changed because of the increased N deposition and use of P fertilizers in the last several decades [3,4]. The effects of N input on plant N status have been widely studied. It is believed that N inputs increase plant N concentration ([N]) in N-limited ecosystems but have a minor effect on plant [N] in N-rich ecosystems [7].
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