Dashuan Tian

A global analysis of soil acidification caused by nitrogen addition

Nitrogen (N) deposition-induced soil acidification has become a global problem. However, the response patterns of soil acidification to N addition and the underlying mechanisms remain far from clear. Here, we conducted a meta-analysis of 106 studies to reveal global patterns of soil acidification in responses to N addition. We found that N addition significantly reduced soil pH by 0.26 on average globally. However, the responses of soil pH varied with ecosystem types, N addition rate, N fertilization forms, and experimental durations. Soil pH decreased most in grassland, whereas boreal forest was not observed a decrease to N addition in soil acidification. Soil pH decreased linearly with N addition rates. Addition of urea and NH4NO3 contributed more to soil acidification than NH4-form fertilizer. When experimental duration was longer than 20 years, N addition effects on soil acidification diminished. Environmental factors such as initial soil pH, soil carbon and nitrogen content, precipitation, and temperature all influenced the responses of soil pH. Base cations of Ca2+, Mg2+ and K+ were critical important in buffering against N-induced soil acidification at the early stage. However, N addition has shifted global soils into the Al3+ buffering phase. Overall, this study indicates that acidification in global soils is very sensitive to N deposition, which is greatly modified by biotic and abiotic factors. Global soils are now at a buffering transition from base cations (Ca2+, Mg2+ and K+) to non-base cations (Mn2+ and Al3+). This calls our attention to care about the limitation of base cations and the toxic impact of non-base cations for terrestrial ecosystems with N deposition.

Global patterns and substrate-based mechanisms of the terrestrial nitrogen cycle

Nitrogen (N) deposition is impacting the services that ecosystems provide to humanity. However, the mechanisms determining impacts on the N cycle are not fully understood. To explore the mechanistic underpinnings of N impacts on N cycle processes, we reviewed and synthesised recent progress in ecosystem N research through empirical studies, conceptual analysis and model simulations. Experimental and observational studies have revealed that the stimulation of plant N uptake and soil retention generally diminishes as N loading increases, while dissolved and gaseous losses of N occur at low N availability but increase exponentially and become the dominant fate of N at high loading rates. The original N saturation hypothesis emphasises sequential N saturation from plant uptake to soil retention before N losses occur. However, biogeochemical models that simulate simultaneous competition for soil N substrates by multiple processes match the observed patterns of N losses better than models based on sequential competition. To enable better prediction of terrestrial N cycle responses to N loading, we recommend that future research identifies the response functions of different N processes to substrate availability using manipulative experiments, and incorporates the measured N saturation response functions into conceptual, theoretical and quantitative analyses.

Nonlinear responses of ecosystem carbon fluxes and water-use efficiency to nitrogen addition in Inner Mongolia grassland

Summary Nitrogen (N) deposition is a continuous process and likely to affect ecosystem carbon (C) and water fluxes in a nonlinear way. However, experimental evidence is still lacking because most previous studies on these impacts usually used two discrete levels of N treatment. By a 12-year, 6-level N addition experiment in Inner Mongolia grassland, we found that the responses of C fluxes, including net ecosystem carbon exchange (NEE), gross ecosystem productivity (GEP), ecosystem respiration (ER), all exhibited nonlinear patterns with increasing N addition rate while that of evapotranspiration did not significantly change. As a result, the response of ecosystem water-use efficiency (EWUE) followed a similar pattern with NEE. These N-induced changes in C fluxes were greatly affected by the distribution of precipitation among different stages of growing seasons and mainly driven by the alterations in biomass production rather than soil temperature and soil moisture. This study highlights the importance of the nonlinearity of N addition impacts on ecosystem C fluxes, which should be incorporated into the global-C-cycling models for better predicting future C balance. Our results also have implications for the use of fertilization in restoring the degraded grasslands given that N addition can promote biomass production and ecosystem C uptake without additional water evapotranspiration but also change ecosystem composition.

Hierarchical Reproductive Allocation and Allometry within a Perennial Bunchgrass after 11 Years of Nutrient Addition

Bunchgrasses are one of the most important plant functional groups in grassland ecosystems. Reproductive allocation (RA) for a bunchgrass is a hierarchical process; however, how bunchgrasses adjust their RAs along hierarchical levels in response to nutrient addition has never been addressed. Here, utilizing an 11-year nutrient addition experiment, we examined the patterns and variations in RA of Agropyron cristatum at the individual, tiller and spike levels. We evaluated the reproductive allometric relationship at each level by type II regression analysis to determine size-dependent and size-independent effects on plant RA variations. Our results indicate that the proportion of reproductive individuals in A. cristatum increased significantly after 11 years of nutrient addition. Adjustments in RA in A. cristatum were mainly occurred at the individual and tiller levels but not at the spike level. A size-dependent effect was a dominant mechanism underlying the changes in plant RA at both individual and tiller levels. Likewise, the distribution of plant size was markedly changed with large individuals increasing after nutrient addition. Tiller-level RA may be a limiting factor for the adjustment of RA in A. cristatum. To the best of our knowledge, this study is the first to examine plant responses in terms of reproductive allocation and allometry to nutrient enrichment within a bunchgrass population from a hierarchical view. Our findings have important implications for understanding the mechanisms underlying bunchgrass responses in RA to future eutrophication due to human activities. In addition, we developed a hierarchical analysis method for disentangling the mechanisms that lead to variation in RA for perennial bunchgrasses.

Effects of functional diversity loss on ecosystem functions are influenced by compensation

Understanding the impacts of biodiversity loss on ecosystem functioning and services has been a central issue in ecology. Experiments in synthetic communities suggest that biodiversity loss may erode a set of ecosystem functions, but studies in natural communities indicate that the effects of biodiversity loss are usually weak and that multiple functions can be sustained by relatively few species. Yet, the mechanisms by which natural ecosystems are able to maintain multiple functions in the face of diversity loss remain poorly understood. With a long-term and large-scale removal experiment in the Inner Mongolian grassland, here we showed that losses of plant functional groups (PFGs) can reduce multiple ecosystem functions, including biomass production, soil NO3 -N use, net ecosystem carbon exchange, gross ecosystem productivity, and ecosystem respiration, but the magnitudes of these effects depended largely on which PFGs were removed. Removing the two dominant PFGs (perennial rhizomatous grasses and perennial bunchgrasses) simultaneously resulted in dramatic declines in all examined functions, but such declines were circumvented when either dominant PFG was present. We identify the major mechanism for this as a compensation effect by which each dominant PFG can mitigate the losses of others. This study provides evidence that compensation ensuing from PFG losses can mitigate their negative consequence, and thus natural communities may be more resilient to biodiversity loss than currently thought if the remaining PFGs have strong compensation capabilities. On the other hand, ecosystems without well-developed compensatory functional diversity may be much more vulnerable to biodiversity loss.

Size‐dependent nutrient limitation of tree growth from subtropical to cold temperate forests

The traditional paradigm is that plant growth at high latitudes is generally nitrogen (N) limited, whereas phosphorus (P) limitation occurs at low latitudes. However, this latitudinal pattern of nutrient limitation is not empirically tested and the underlying mechanisms are far from clear. Here we performed a coordinated experiment of N and/or P addition at three forest sites in China, a subtropical forest, a warm-temperate forest and a cold-temperate forest. By measuring relative growth rate (RGR) and leaf nutrient traits among different tree size groups, we assessed how they vary with nutrient addition and tree sizes and uncovered the likely mechanisms underlying these observed responses. Our results revealed that P addition enhanced the RGR of small trees (DBH 15cm) was not impacted by any nutrient addition treatment or soil nutrient variations at any site. Leaf P concentration and resorption efficiency in both small and large trees mostly showed a linear response to soil available P at subtropical and warm-temperate sites, while leaf N:P ratio in small trees elevated linearly with soil available N:P ratio at a cold-temperate site. Overall, this study presents robust experimental evidence that growth in small trees, not large trees, is primarily limited by P in subtropical and warm-temperate forests, but is co-limited by N and P in cold-temperate forests. This size-dependent nutrient limitation highlights the importance of considering tree size classes when assessing nutrient limitation in forest. A is available for this article.

Contrasting responses of phosphatase kinetic parameters to nitrogen and phosphorus additions in forest soils

Summary 1. Global changes include increasing nitrogen (N) and phosphorus (P) deposition, which affect microbial nutrient demand and biogeochemical cycles. The responses of P-mineralizing enzymes to these global change components are poorly defined and are not specified in forest soils differing in P content. 2. We chose one site in a P-rich and two sites in P-poor forests and established sixteen 20 × 20 m plots at each site. Control, either N only, P only, or combined N and P, were randomly distributed through each forest site with 4 replicates. We investigated the effects of N and P additions over four years on the phosphomonoesterase potential activity (Vmax), its half-saturation constant (Km), and its catalytic efficiency (Vmax/Km). 3. Without N and P additions, the enzyme kinetic parameters Vmax, Km, and Vmax/Km were higher in P-rich than in P-poor forest soils. These parameters increased with soil pH, SOC, TN, and TP contents increased. 4. Remarkably, P additions caused the Vmax and Km to increase in P-rich soils, but had no effect on Vmax/Km. P additions to P-poor soils resulted in a decrease in the Vmax/Km via the inhibitory effects of inorganic P on the Vmax. N additions had no effect on the Vmax/Km in P-rich and P-poor soils because of the similar increases in the Vmax and Km. The effects of combined N and P and P only additions to P-poor soils on the Vmax and Km were similar, but were stronger than the effects of N only or P only additions on the P-rich soils. 5. Phosphatase kinetic parameters were positively related to the availability of N and P in P-rich soils, but inorganic P inhibited phosphatase activity and caused a decrease in the catalytic efficiency in P-poor soils. More microbial community groups could contribute to the secretion of a broader spectrum of iso-enzymes under combined additions of N and P in P-rich soils. We conclude contrast responses of phosphatase kinetics to P and N inputs in P-rich and P-poor forest soils, while long-term N deposition might mitigate P limitation by increasing phosphatase secretion. This article is protected by copyright. All rights reserved.

Responses of soil enzymatic activities to transgenic Bacillus thuringiensis (Bt) crops - A global meta-analysis

Transgenic Bacillus thuringiensis (Bt) crops have been widely planted, and the resulting environmental risks have attracted extensive attention. To foresee the impacts of Bt crops on soil quality, it is essential to understand how Bt crops alter the soil enzymatic activities and what the important influencing factors are. We compiled data from 41 published papers that studied soil enzymatic activities with Bt crops and their non-Bt counterparts. The results showed that dehydrogenase and urease significantly increased, but neutral phosphatase significantly decreased under Bt crop cultivations without Bt residues incorporation. The activities of dehydrogenase, β-glucosidase, urease, nitrate reductase, alkaline phosphatase, and aryl sulfatase significantly increased under Bt crop cultivation with Bt residues incorporation. The response ratios of other enzymes were not significantly changed. Generally, the response ratios of soil enzymes were greater with Bt residues incorporation than those of Bt crop cultivations without Bt residues incorporation. Further, the response ratios of soil enzymes varied with Bt crop types and growth periods. It was the strongest under Bt cotton among Bt crops, and the significant responses usually appeared in the middle growth stages. The responses of soil enzymes ascribed more to the properties of Bt crops than to soil properties across sites. Given - significant responses of some soil enzymes to Bt crops, we recommended that soil environmental risks should be carefully evaluated over the transgenic crops.

Environmental variables better explain changes in potential nitrification and denitrification activities than microbial properties in fertilized forest soils

Because of increases in atmospheric nitrogen (N) deposition worldwide, nutrient imbalances and phosphorus (P) limitations in soil are aggravated, with the result that P fertilizer applications to terrestrial ecosystems worldwide may increase. Nitrification and denitrification in soil are major sources of nitrous oxide emissions, especially in soils treated with fertilizers. However, few researchers have studied how forest soils respond to nutrient additions, so we are not sure how the potential nitrification and denitrification activities (PNA and PDA, respectively) and microbial communities involved in these processes might respond when N and P are added to temperate and subtropical forest soils. We investigated how the PNA, PDA, the abundances and community compositions of nitrifiers and denitrifiers, and environmental properties, including soil pH, soil total and dissolved organic carbon, total and available N and phosphorus P, changed when N and/or P were added to subtropical and temperate forest soils. We quantified the abundance, and analyzed the composition, of functional marker genes of nitrifiers (ammonia-oxidizing bacteria and archaea amoA) and denitrifiers (nirK and nirS) using quantitative PCR and sequencing, respectively. We found that the PNA and PDA in the subtropical soil increased when P was added and PNA in the temperate forest soil increased when either N or P was added. The PNA and PDA were positively correlated with the abundance of ammonia-oxidizing bacteria and nirK-denitrifiers, respectively, in the subtropical forest soil but were not correlated with changes in corresponding community compositions in either of the forest soils. The soil total N to total P ratio explained most of the variabilities in the PNA and PDA in the subtropical forest soils, and the soil exchangeable ammonium concentrations and pH were the main controls on the PNA and PDA, respectively, in the temperate forest soils. Our results indicate that soil environmental conditions have more influence on variations in the PNA and PDA in forest soils fertilized with N and P than the corresponding microbial properties.