Di Tian

Effects of nitrogen deposition on soil microbial communities in temperate and subtropical forests in China

Increasing nitrogen (N) deposition has aroused large concerns because of its potential negative effects on forest ecosystems. Although microorganisms play a vital role in ecosystem carbon (C) and nutrient cycling, the effect of N deposition on soil microbiota still remains unclear. In this study, we investigated the responses of microbial biomass C (MBC) and N (MBN) and microbial community composition to 4-5years of experimentally simulated N deposition in temperate needle-leaf forests and subtropical evergreen broadleaf forests in eastern China, using chloroform fumigation extraction and phospholipid fatty acid (PLFA) methods. We found idiosyncratic effects of N addition on microbial biomass in these two types of forest ecosystems. In the subtropical forests, N addition showed a significant negative effect on microbial biomass and community composition, while the effect of N addition was not significant in the temperate forests. The N addition decreased MBC, MBN, arbuscular mycorrhizal fungi, and the F/B ratio (ratio of fungi to bacteria biomass) in the subtropical forests, likely due to a decreased soil pH and changes in the plant community composition. These results showed that microbial biomass and community composition in subtropical forests, compared with the temperate forests, were sensitive to N deposition. Our findings suggest that N deposition may have negative influence on soil microorganisms and potentially alter carbon and nutrient cycling in subtropical forests, rather than in temperate forests.

Nitrogen deposition has minor effect on soil extracellular enzyme activities in six Chinese forests

Soil extracellular enzymes play a key role in mediating a range of forest ecosystem functions (i.e., carbon and nutrients cycling and biological productivity), particularly in the face of atmospheric N deposition that has been increasing at an unprecedented rate globally. However, most studies have focused only on surface soils in a single ecosystem. In this study, we aimed to determine whether the effect of simulated N deposition on the activities and ratios of soil enzymes changes with soil depth across six forest ecosystems in eastern China. We collected soil samples from three blocks×four soil depths (0-10cm, 10-20cm, 20-40cm and 40-60cm)×three N treatment levels (control, 50 and 100kgNha-1year-1) at each of the six forest ecosystems. We measured the activities of seven soil enzymes involved in C-, N- and P-cycling. We found that 4-5years of N addition had no significant effect on the activities and ratios of these enzymes in most cases. The interactions among N addition, site and soil depth on soil enzyme activities were not significant, except that acid phosphatase activity showed site-specific responses to N addition. Our findings suggest that the activities of soil enzymes involved in C- and N-cycling generally do not track simulated N deposition in the six forest ecosystems. Further work on plant, soil and microbial characteristics is needed to better understand the mechanisms of soil enzyme activities in response to N deposition in forest ecosystems.

An assessment on the uncertainty of the nitrogen to phosphorus ratio as a threshold for nutrient limitation in plants

Background and AimsnThe nitrogen (N) to phosphorus (P) ratio (N:P) has been widely used as a threshold for identifying nutrient limitations in terrestrial plants; however, the associated reliability has not been well assessed.nnnMethodsnThe uncertainty of nutrient limitations caused by the N:P threshold was evaluated using two approaches: fertilization experiments synthesized across multiple ecosystems; and random sampling simulation of the impacts of different nutrient sufficiencies and deficiencies.nnnKey ResultsnThe fertilization experiment data indicated that the types of nutrient limitation determined via N:P thresholds were partly inconsistent with the growth responses observed under N and P additions, i.e. under N:P thresholds of 14 and 16 (or 10 and 20), 32.5 % (or 16.2 %) of the data were inconsistent between these two. The random sampling simulation suggested that N:P thresholds may indicate N (or P) limitations when leaf N (or P) content is sufficient, whereas these thresholds may not indicate N (or P) limitations when leaf N (or P) content is deficient. The error risks calculated from the sampling simulation presented large fluctuations at small sample sizes and decreased as the thresholds of nutrient content sufficiency (or deficiency) increased (or decreased). The N:P thresholds of 10 and 20 showed lower error risks than the thresholds of 14 and 16.nnnConclusionsnThese findings highlight that canonical N:P thresholds have the potential to introduce a large uncertainty when used to detect plant nutrient limitations, suggesting that the error risks should be cautioned in future studies.

Drought effect on plant biomass allocation: A meta-analysis

Abstract Drought is one of the abiotic stresses controlling plant function and ecological stability. In the context of climate change, drought is predicted to occur more frequently in the future. Despite numerous attempts to clarify the overall effects of drought stress on the growth and physiological processes of plants, a comprehensive evaluation on the impacts of drought stress on biomass allocation, especially on reproductive tissues, remains elusive. We conducted a meta‐analysis by synthesizing 164 published studies to elucidate patterns of plant biomass allocation in relation to drought stress. Results showed that drought significantly increased the fraction of root mass but decreased that of stem, leaf, and reproductive mass. Roots of herbaceous plants were more sensitive to drought than woody plants that reduced reproductive allocation more sharply than the former. Relative to herbaceous plants, drought had a more negative impact on leaf mass fraction of woody plants. Among the herbaceous plants, roots of annuals responded to drought stress more strongly than perennial herbs, but their reproductive allocation was less sensitive to drought than the perennial herbs. In addition, cultivated and wild plants seemed to respond to drought stress in a similar way. Drought stress did not change the scaling exponents of the allometric relationship between different plant tissues. These findings suggest that the allometric partitioning theory, rather than the optimal partitioning theory, better explains the drought‐induced changes in biomass allocation strategies.

Nutrient resorption of Castanopsis eyrei varies at the defoliation peaks in spring and autumn in a subtropical forest, Anhui, China

Nutrient resorption from senescing leaves is a key nutrient conservation strategy in many ecosystems. In evergreen broad-leaved forests, nutrient resorption occurs with concomitant defoliation almost year-round. However, it is unclear whether nutrient resorption efficiency (NuRE) varies in different defoliation seasons for evergreen broad-leaved trees and how it may relate to soil nutrient availability. Castanopsis eyrei is a dominant tree species in evergreen broad-leaved forests in southern China, with distinct bimodal patterns of defoliation in spring and autumn. We investigated seasonal variation of the resorption efficiency of nitrogen (N) and phosphorus (P) of C. eyrei leaf and its relationship with soil mineral N by measuring the concentrations in leaf, leaf litter, and soil for two consecutive years. We hypothesized that leaf NuRE should be higher in seasons with low soil nutrient availability. We found that on average, soil mineral N content was 38.5xa0% lower during the autumn defoliation peak than in spring, and N and P concentrations in leaf litter at the spring defoliation peak were 10.0 and 26.7xa0% higher than at the autumn peak. Nitrogen NuRE at the autumnal defoliation peak was 5.1xa0% higher than that at the spring peak (49.3 vs 44.2xa0%), and the phosphorus NuRE in autumn was 6.8xa0% higher than that in spring (77.6 vs 69.8xa0%). Our findings suggest an inverse relationship between soil nutrient availability and NuRE and that the dynamics in NuRE should be incorporated into modeling of biogeochemical cycling in evergreen forest ecosystems with a bimodal defoliation pattern.

The response of tree growth to nitrogen and phosphorus additions in a tropical montane rainforest

Rapid increase of global nitrogen (N) deposition has greatly altered carbon cycles and functioning of forest ecosystems. Previous studies have focused on changes in carbon dynamics of temperate and subtropical forests through N enrichment experiments; however, the effects of N deposition on tree growth remain inconsistent, especially in tropical forests. Here, we conducted a five-year N addition experiment (0 and 50kgNha-1yr-1) in a tropical montane rain forest in Hainan Island, China, to explore the effects of enhanced N deposition on growth of trees. We also set phosphorus (P) treatment (50kgPha-1yr-1) and N+P treatment (50kgNha-1yr-1+50kgPha-1yr-1) to examine potential P limitation driven by N deposition. Our results showed that N addition has not significantly influenced tree growth, while P addition significantly increased the relative growth rate of small (diameter at breast height, DBH≤10cm) and medium (10<DBH≤20cm) trees. The combined N and P addition accelerated the growth of small trees, but did not affect the growth of medium and large (20cm<DBH) trees. These contrasting effects of N and P addition on tree growth indicate that the tropical montane forest is mainly limited by P, which suggests the importance of P in regulating growth of trees in tropical forests.