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Aboveground litter production in forests is likely to increase as a consequence of elevated atmospheric carbon dioxide (CO(2)) concentrations, rising temperatures, and shifting rainfall patterns. As litterfall represents a major flux of carbon from vegetation to soil, changes in litter inputs are likely to have wide-reaching consequences for soil carbon dynamics. Such disturbances to the carbon balance may be particularly important in the tropics because tropical forests store almost 30% of the global soil carbon, making them a critical component of the global carbon cycle; nevertheless, the effects of increasing aboveground litter production on belowground carbon dynamics are poorly understood. We used long-term, large-scale monthly litter removal and addition treatments in a lowland tropical forest to assess the consequences of increased litterfall on belowground CO(2) production. Over the second to the fifth year of treatments, litter addition increased soil respiration more than litter removal decreased it; soil respiration was on average 20% lower in the litter removal and 43% higher in the litter addition treatment compared to the controls but litter addition did not change microbial biomass. We predicted a 9% increase in soil respiration in the litter addition plots, based on the 20% decrease in the litter removal plots and an 11% reduction due to lower fine root biomass in the litter addition plots. The 43% measured increase in soil respiration was therefore 34% higher than predicted and it is possible that this 'extra' CO(2) was a result of priming effects, i.e. stimulation of the decomposition of older soil organic matter by the addition of fresh organic matter. Our results show that increases in aboveground litter production as a result of global change have the potential to cause considerable losses of soil carbon to the atmosphere in tropical forests.

Nitrogen (N) and phosphorus (P) are essential nutrients for plant metabolism, and their availability often limits primary productivity. Whereas the effects of N availability on photosynthetic capacity are well established, we still know relatively little about the effects of P availability at a foliar level, especially in P‐limited tropical forests. We examined photosynthetic capacity, leaf mass per area (LMA) and foliar P fractions in five woody plant species after 6 years of N and P fertilization in a lowland tropical forest. Foliar N:P ratios indicated P limitation of the unfertilized plants; accordingly, photosynthetic P‐use efficiency (PPUE) and LMA decreased with P addition, and foliar N and P concentrations increased, whereas N addition had little effect on measured foliar traits. However, P addition enhanced photosynthetic capacity only in one species and not in other four species. We then assessed plant acclimation to low P availability by quantifying four fractions of foliar P representing different functional pools: structural P, metabolic P (including inorganic P), nucleic acid P, and residual P. We found that P addition enhanced the concentrations of metabolic, structural, and nucleic acid P fractions in all species, but the magnitude of the effect was species‐specific. Our findings indicate that tropical species acclimate to low P availability by altering allocation of foliar P to meet the demand of P for photosynthesis. Importantly, species typical of lowland tropical forests in East Asia maintained their photosynthetic rate under low P availability. We conclude that P limitation of leaf photosynthetic capacity may not be as common as previously assumed due to plant acclimation mechanisms in low‐P tropical forests. Species‐specific strategies to allocate P to different foliar fractions represent a potentially important adaptive mechanism for plants in P‐limited systems. 氮和磷是植物代谢过程中的重要养分也是限制生态系统初级生产力的主要因子。尽管我们对氮如何影响植物光合能力有了比较全面的认识，但在低磷的热带森林中，我们对土壤磷如何影响植物光合功能还缺乏系统的了解。本研究通过在热带森林的野外氮磷添加实验中测定不同植物的叶片光合能力和叶片功能性状以及叶片磷组分，系统的了解了这些植物在叶片尺度上的低磷环境的适应机制。叶片氮磷比的结果表明该森林是磷限制的生态系统，施磷降低了叶片磷利用效率和比叶重但增加了叶片磷和氮的浓度，而施氮对叶片性状无显著影响。施磷仅增加了一个广布种的叶片光合能力而对其他四个狭布种无显著影响。我们进一步了解了叶片磷组分的变化，发现施磷处理增加了所有物种的代谢磷、结构磷和核酸磷组分，但其增加的尺度因种而异。这些结果表明热带森林植物通过改变叶片中磷的分布来满足光合作用对磷的需求，并且东亚地区的热带森林的典型植物能够在低磷的土壤环境中维持相对稳定的光合速率。通过该项研究，我们认为由于热带植物进化的适应机制，低磷对热带森林植物的光合能力的限制比预想的要小的多。不同植物的叶片磷组分的分配策略是其重要的对低磷环境的适应机制。 A plain language summary is available for this article. Plain Language Summary

Soil respiration in forests contributes to significant carbon dioxide emissions from terrestrial ecosystems but it varies both spatially and seasonally. Both abiotic and biotic factors influence soil respiration but their relative contribution to spatial and seasonal variability remains poorly understood, which leads to uncertainty in models of global C cycling and predictions of future climate change. Here, we hypothesize that tree diversity, soil diversity, and soil properties contribute to local-scale variability of soil respiration but their relative importance changes in different seasons. To test our hypothesis, we conducted seasonal soil respiration measurements along a local-scale environmental gradient in a temperate forest in Northeast China, analyzed spatial variability of soil respiration and tested the relationships between soil respiration and a variety of abiotic and biotic factors including topography, soil chemical properties, and plant and soil diversity. We found that soil respiration varied substantially across the study site, with spatial coefficients of variation (CV) of 29.1%, 27.3% and 30.8% in spring, summer, and autumn, respectively. Soil respiration was consistently lower at high soil water content, but the influence of other factors was seasonal. In spring, soil respiration increased with tree diversity and biomass but decreased with soil fungal diversity. In summer, soil respiration increased with soil temperature, whereas in autumn, soil respiration increased with tree diversity but decreased with increasing soil nutrient content. However, soil nutrient content indirectly enhanced soil respiration via its effect on tree diversity across seasons, and forest stand structure indirectly enhanced soil respiration via tree diversity in spring. Our results highlight that substantial differences in soil respiration at local scales was jointly explained by soil properties (soil water content and soil nutrients), tree diversity, and soil fungal diversity but the relative importance of these drivers varied seasonally in our temperate forest.

We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine‐litter production, and fine‐root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 × 40 m each. The addition of K was associated with significant decreases in stand‐level fine‐root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1–10 cm in diameter at breast height) and a further marginally significant decrease in stand‐level fine‐root biomass. The addition of P was associated with a marginally significant (P = 0.058) increase in fine‐litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles.

Soil organic matter (SOM) in tropical forests is an important store of carbon (C) and nutrients. Although SOM storage could be affected by global changes via altered plant productivity, we know relatively little about SOM stabilisation and turnover in tropical forests compared to temperate systems. Here, we investigated changes in soil C and N within particle size fractions representing particulate organic matter (POM) and mineral-associated organic matter (MAOM) after 13 years of experimental litter removal (L–) and litter addition (L+) treatments in a lowland tropical forest. We hypothesized that reduced nitrogen (N) availability in L-plots would result in N-mining of MAOM, whereas long-term litter addition would increase POM, without altering the C:N ratio of SOM fractions. Overall, SOM-N declined more than SOM-C with litter removal, providing evidence of N-mining in the L– plots, which increased the soil C:N ratio. However, contrary to expectations, the C:N ratio increased most in the largest POM fraction, whereas the C:N ratio of MAOM remained unchanged. We did not observe the expected increases in POM with litter addition, which we attribute to rapid turnover of unprotected SOM. Measurements of ion exchange rates to assess changes in N availability and soil chemistry revealed that litter removal increased the mobility of ammonium-N and aluminium, whereas litter addition increased the mobility of nitrate-N and iron, which could indicate SOM priming in both treatments. Our study suggests that altered litter inputs affect multiple processes contributing to SOM storage and we propose potential mechanisms to inform future work.

Improved understanding of the nutritional ecology of arbuscular mycorrhizal (AM) fungi is important in understanding how tropical forests maintain high productivity on low-fertility soils. Relatively little is known about how AM fungi will respond to changes in nutrient inputs in tropical forests, which hampers our ability to assess how forest productivity will be influenced by anthropogenic change. Here we assessed the influence of long-term inorganic and organic nutrient additions and nutrient depletion on AM fungi, using two adjacent experiments in a lowland tropical forest in Panama. We characterised AM fungal communities in soil and roots using 454-pyrosequencing, and quantified AM fungal abundance using microscopy and a lipid biomarker. Phosphorus and nitrogen addition reduced the abundance of AM fungi to a similar extent, but affected community composition in different ways. Nutrient depletion (removal of leaf litter) had a pronounced effect on AM fungal community composition, affecting nearly as many OTUs as phosphorus addition. The addition of nutrients in organic form (leaf litter) had little effect on any AM fungal parameter. Soil AM fungal communities responded more strongly to changes in nutrient availability than communities in roots. This suggests that the ‘dual niches’ of AM fungi in soil versus roots are structured to different degrees by abiotic environmental filters, and biotic filters imposed by the plant host. Our findings indicate that AM fungal communities are fine-tuned to nutrient regimes, and support future studies aiming to link AM fungal community dynamics with ecosystem function.

Enhanced tropical forest productivity, facilitated by increasing carbon dioxide concentrations, could act as a substantial carbon sink. However, a long-term field experiment shows that increased leaf-litter inputs to the soil as productivity rises could stimulate the release of significant amounts of soil carbon, partially offsetting predicted gains in carbon storage. Tropical forests are a critical component of the global carbon cycle 1 and their response to environmental change will play a key role in determining future concentrations of atmospheric carbon dioxide (CO 2 ) 1 , 2 . Increasing primary productivity in tropical forests over recent decades has been attributed to CO 2 fertilization 3 , and greater biomass in tropical forests could represent a substantial sink for carbon in the future 3 , 4 . However, the carbon sequestration capacity of tropical forest soils is uncertain and feedbacks between increased plant productivity and soil carbon dynamics remain unexplored 5 , 6 . Here, we show that experimentally increasing litterfall in a lowland tropical forest enhanced carbon release from the soil. Using a large-scale litter manipulation experiment combined with carbon isotope measurements, we found that the efflux of CO 2 derived from soil organic carbon was significantly increased by litter addition. Furthermore, this effect was sustained over several years. We predict that a future increase in litterfall of 30% with an increase in atmospheric CO 2 concentrations of 150 ppm could release about 0.6 t C ha −1  yr −1 from the soil, partially offsetting predicted net gains in carbon storage. Thus, it is essential that plant–soil feedbacks are taken into account in predictions of the carbon sequestration potential of tropical forests.

1. The cycling of nutrients in litterfall is considered a key mechanism in the maintenance of tropical forest fertility but its importance has rarely been quantified experimentally. 2. We carried out a long-term (5 years), large-scale litter manipulation experiment in lowland semievergreen tropical forest to determine how changes in litterfall affect forest nutrient cycling. We hypothesized that: (i) long-term litter removal would decrease the forest's nutrient supply; (ii) litter addition would increase the forest's nutrient supply; (iii) soil and foliar nutrient concentrations would change in response to litter manipulation and would eventually affect above-ground productivity. 3. To test our hypotheses, we measured trunk growth, litterfall, and nutrient concentrations in live leaves, litter and soil in plots where litter was removed once a month (L-), litter was added once a month (L+) and controls (CT). 4. After 5 years, the concentration of nitrate in the soil and soil stocks of inorganic nitrogen were higher in the L+ plots and lower in the L -plots compared to the controls. Ammonium concentrations in the soil were also lower in the L -plots. Nitrogen in leaves and litter and the annual nitrogen return by litter were higher in the L+ plots, while potassium return was lower in the L -plots. Surprisingly, our treatments had little effect on phosphorus in soil, leaves or litter, even though lowland tropical forests are generally thought to be largely phosphorus limited. 5. Trunk growth of large trees was not affected by litter manipulation but rainy season litterfall from 2003 to 2008 was 13% higher in the L+ plots compared to the controls. 6. Synthesis. Litter removal affected forest nutrient cycling and productivity less than expected, probably because the soil at our site is moderately fertile. However, litter addition increased litterfall indicating that some limitation of forest production was removed by litter addition. We expected strong effects of litter manipulation on phosphorus cycling; instead, we found a stronger effect on nitrogen cycling. Our results suggest that litter is an important source of nutrients, in particular nitrogen, to trees in this lowland semi-evergreen tropical forest.

Tropical forest productivity is sustained by the cycling of nutrients through decomposing organic matter. Arbuscular mycorrhizal (AM) fungi play a key role in the nutrition of tropical trees, yet there has been little experimental investigation into the role of AM fungi in nutrient cycling via decomposing organic material in tropical forests. We evaluated the responses of AM fungi in a long-term leaf litter addition and removal experiment in a tropical forest in Panama. We described AM fungal communities using 454-pyrosequencing, quantified the proportion of root length colonised by AM fungi using microscopy, and estimated AM fungal biomass using a lipid biomarker. AM fungal community composition was altered by litter removal but not litter addition. Root colonisation was substantially greater in the superficial organic layer compared with the mineral soil. Overall colonisation was lower in the litter removal treatment, which lacked an organic layer. There was no effect of litter manipulation on the concentration of the AM fungal lipid biomarker in the mineral soil. We hypothesise that reductions in organic matter brought about by litter removal may lead to AM fungi obtaining nutrients from recalcitrant organic or mineral sources in the soil, besides increasing fungal competition for progressively limited resources.

To predict how species loss will affect ecosystems, it is important to consider how biodiversity influences processes such as decomposition.To predict how species loss will affect ecosystems, it is important to consider how biodiversity influences processes such as decomposition.