Modeling global oceanic nitrogen deposition from food systems and its mitigation potential by reducing overuse of fertilizers.

Growing population and consumption pose unprecedented demands on food production. However, ammonia emissions mainly from food systems increase oceanic nitrogen deposition contributing to eutrophication. Here, we developed a long-term oceanic nitrogen deposition dataset (1970 to 2018) with updated ammonia emissions from food systems, evaluated the impact of ammonia emissions on oceanic nitrogen deposition patterns, and discussed the potential impact of nitrogen fertilizer overuse. Based on the chemical transport modeling approach, oceanic ammonia-related nitrogen deposition increased by 89% globally between 1970 and 2018, and now, it exceeds oxidized nitrogen deposition by over 20% in coastal regions including China Sea, India Coastal, and Northeastern Atlantic Shelves. Approximately 38% of agricultural nitrogen fertilizer was excessive, which corresponds to 15% of global oceanic ammonia-related nitrogen deposition. Policymakers and water quality managers need to pay increasingly more attention to ammonia associated with food production if the goal of reducing coastal nitrogen pollution is to be achieved for Sustainable Development Goals.

Nitrogen deposition accelerates soil carbon sequestration in tropical forests.

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO2 and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.

Manganese Oxidation States in Volcanic Soils across Annual Rainfall Gradients.

Manganese (Mn) exists as Mn(II), Mn(III), or Mn(IV) in soils, and the Mn oxidation state controls the roles of Mn in numerous environmental processes. However, the variations of Mn oxidation states with climate remain unknown. We determined the Mn oxidation states in highly weathered bulk volcanic soils (primary minerals free) across two rainfall gradients covering mean annual precipitation (MAP) of 0.25-5 m in the Hawaiian Islands. With increasing MAP, the soil redox conditions generally shifted from oxic to suboxic and to anoxic despite fluctuating at each site; concurrently, the proportions of Mn(IV) and Mn(II) decreased and increased, respectively. Mn(III) was low at both low and high MAP, but accumulated substantially, up to 80% of total Mn, in soils with prevalent suboxic conditions at intermediate MAP. Mn(III) was likely hosted in Mn(III,IV) and iron(III) oxides or complexed with organic matter, and its distribution among these hosts varied with soil redox potentials and soil pH. Soil redox conditions and rainfall-driven leaching jointly controlled exchangeable Mn(II) in soils, with its concentration peaking at intermediate MAP. The Mn redox chemistry was at disequilibrium, with the oxidation states correlating with long-term average soil redox potentials better than with soil pH. The soil redox conditions likely fluctuated between oxic and anoxic conditions more frequently at intermediate than at low and high MAP, creating biogeochemical hot spots where Mn, Fe, and other redox-sensitive elements may be actively cycled.

Water balance affects foliar and soil nutrients differently.

Water balance influences soil development, and consequently plant communities, by driving weathering of soil minerals and leaching of plant nutrients from the soil. Along gradients in water balance, soils exhibit process domains where chemical properties are relatively stable punctuated by pedogenic thresholds where soil chemical properties change rapidly with little additional change in water balance. We ask if plant macronutrient concentrations in leaves also exhibit non-linear trends along water balance gradients, and if so, how these non-linearities relate to those in soils. We analyze foliar nutrient concentrations and foliar N:P ratios from eight species that span a range of growth forms along three water balance gradients (three of the species are found on multiple gradients). The gradients are located on basaltic substrate of different ages and have previously been characterized by studies on soil development. We find that maximum concentrations of foliar macronutrients occur at an intermediate water balance. As with soil nutrients, time mediates the effect of water balance on foliar nutrients, such that plants on older soils attain maximum nutrient concentrations at a lower water balance. On both a young, 20 ky and an old, 4100 ky water balance gradient, foliar nutrients reach peak concentrations at a water balance greater than the threshold for depletion of rock-derived nutrients in surface soils. Our findings suggest that plant acquisition of essential nutrients is imperfectly predicted by overall soil nutrient availability because the regulation of internal nutrient pools by plants makes nutrient pools within leaves partially independent of soil nutrient availability.

Foliar ẟ15N patterns in legumes and non-N fixers across a climate gradient, Hawai'i Island, USA.

Recent studies from the Hawaiian Islands showed that pedogenic thresholds demarcate domains in which rock-derived nutrient dynamics remain similar across wide variations in rainfall. These thresholds appear related to certain aspects of N cycling, but the degree to which they correspond to patterns of biological N fixation (BNF)-the dominant input of N into less-managed ecosystems-remains unclear. We measured aboveground plant biomass, foliar nutrient concentrations, and foliar δ15N along a climate gradient on ~ 150,000-year-old basaltic substrate to characterize foliar N sources and spatially relate them to soil nutrients. Patterns in legume δ15N correspond to known pedogenic thresholds along the rainfall gradient, with low δ15N values (~ 0 to - 2‰) occurring in the dry, biologically inactive domain and the wet, highly weathered domain. Elevated δ15N in the middle, fertile domain suggests a greater reliance of legumes on soil N where it has accumulated over time. Non-legume face N deficiencies throughout most of the gradient while legumes maintain low C:N ratios via symbiotic BNF. However, legume abundance declines outside the fertile domain, limiting ecosystem N inputs. Breakpoints in legume δ15N data suggest that P (and potentially other nutrients) limits BNF and, by extension, legume abundance in wet region. Nutrients may also constrain legume abundance in the dry domain, but pedogenic effects could not be isolated from climatic constraints at the dry sites. We conclude that pedogenic thresholds defined by climate can be informative of foliar δ15N patterns in cases where legumes are not directly constrained by climate, land use, or other external factors.

Changes in belowground biodiversity during ecosystem development.

Belowground organisms play critical roles in maintaining multiple ecosystem processes, including plant productivity, decomposition, and nutrient cycling. Despite their importance, however, we have a limited understanding of how and why belowground biodiversity (bacteria, fungi, protists, and invertebrates) may change as soils develop over centuries to millennia (pedogenesis). Moreover, it is unclear whether belowground biodiversity changes during pedogenesis are similar to the patterns observed for aboveground plant diversity. Here we evaluated the roles of resource availability, nutrient stoichiometry, and soil abiotic factors in driving belowground biodiversity across 16 soil chronosequences (from centuries to millennia) spanning a wide range of globally distributed ecosystem types. Changes in belowground biodiversity during pedogenesis followed two main patterns. In lower-productivity ecosystems (i.e., drier and colder), increases in belowground biodiversity tracked increases in plant cover. In more productive ecosystems (i.e., wetter and warmer), increased acidification during pedogenesis was associated with declines in belowground biodiversity. Changes in the diversity of bacteria, fungi, protists, and invertebrates with pedogenesis were strongly and positively correlated worldwide, highlighting that belowground biodiversity shares similar ecological drivers as soils and ecosystems develop. In general, temporal changes in aboveground plant diversity and belowground biodiversity were not correlated, challenging the common perception that belowground biodiversity should follow similar patterns to those of plant diversity during ecosystem development. Taken together, our findings provide evidence that ecological patterns in belowground biodiversity are predictable across major globally distributed ecosystem types and suggest that shifts in plant cover and soil acidification during ecosystem development are associated with changes in belowground biodiversity over centuries to millennia.

Policy distortions, farm size, and the overuse of agricultural chemicals in China.

Understanding the reasons for overuse of agricultural chemicals is critical to the sustainable development of Chinese agriculture. Using a nationally representative rural household survey from China, we found that farm size is a strong factor that affects the use intensity of agricultural chemicals across farms in China. Statistically, a 1% increase in farm size is associated with a 0.3% and 0.5% decrease in fertilizer and pesticide use per hectare (P < 0.001), respectively, and an almost 1% increase in agricultural labor productivity, while it only leads to a statistically insignificant 0.02% decrease in crop yields. The same pattern was also found using other independently collected data sources from China and an international panel analysis of 74 countries from the 1960s to the 2000s. While economic growth has been associated with increasing farm size in many other countries, in China this relationship has been distorted by land and migration policies, leading to the persistence of small farm size in China. Removing these distortions would decrease agricultural chemical use by 30-50% and the environmental impact of those chemicals by 50% while doubling the total income of all farmers including those who move to urban areas. Removing policy distortions is also likely to complement other remedies to the overuse problem, such as easing farmer's access to modern technologies and knowledge, and improving environmental regulation and enforcement.

Plant acclimation to long-term high nitrogen deposition in an N-rich tropical forest.

Anthropogenic nitrogen (N) deposition has accelerated terrestrial N cycling at regional and global scales, causing nutrient imbalance in many natural and seminatural ecosystems. How added N affects ecosystems where N is already abundant, and how plants acclimate to chronic N deposition in such circumstances, remains poorly understood. Here, we conducted an experiment employing a decade of N additions to examine ecosystem responses and plant acclimation to added N in an N-rich tropical forest. We found that N additions accelerated soil acidification and reduced biologically available cations (especially Ca and Mg) in soils, but plants maintained foliar nutrient supply at least in part by increasing transpiration while decreasing soil water leaching below the rooting zone. We suggest a hypothesis that cation-deficient plants can adjust to elevated N deposition by increasing transpiration and thereby maintaining nutrient balance. This result suggests that long-term elevated N deposition can alter hydrological cycling in N-rich forest ecosystems.

Nitrogen dynamics along a climate gradient on geologically old substrate, Kaua'i, Hawai'i.

We evaluated N dynamics on a climate gradient on old (> 4 million year) basaltic substrate on the Island of Kaua'i, Hawai'i, to evaluate the utility of pedogenic thresholds and soil process domains for understanding N cycling in terrestrial ecosystems. Studies of nitrogen dynamics on the climate gradient on a younger basaltic substrate (~ 150,000 year) had found a good match between soil process domains and N cycling processes. Here we measured net N mineralization and nitrification by incubation, and δ15N of total soil N, to determine whether the soil process domains on the older gradient were equally useful for interpreting N cycling and thereby to explore the general utility of the approach. Net N mineralization varied from 0 to 1.7 mg kg-1 d-1 across the old Kaua'i gradient, and δ15N varied from + 3 to + 11 ο/οο, both ranges similar to those on the younger substrate. However, while the pattern of variation with climate was similar for δ15N, the highest rates of mineralization on the old gradient occurred where forests were dominated by the native N fixer Acacia koa. This occurred in sites wetter than the process domain associated with high net N mineralization on the gradient on younger substrate. We conclude that soil process domains based on rock-derived nutrients are not always useful for evaluating N dynamics, especially where the distribution of biological N fixers is controlled by factors other than rock-derived nutrients.

Integrated reactive nitrogen budgets and future trends in China.

Reactive nitrogen (Nr) plays a central role in food production, and at the same time it can be an important pollutant with substantial effects on air and water quality, biological diversity, and human health. China now creates far more Nr than any other country. We developed a budget for Nr in China in 1980 and 2010, in which we evaluated the natural and anthropogenic creation of Nr, losses of Nr, and transfers among 14 subsystems within China. Our analyses demonstrated that a tripling of anthropogenic Nr creation was associated with an even more rapid increase in Nr fluxes to the atmosphere and hydrosphere, contributing to intense and increasing threats to human health, the sustainability of croplands, and the environment of China and its environs. Under a business as usual scenario, anthropogenic Nr creation in 2050 would more than double compared with 2010 levels, whereas a scenario that combined reasonable changes in diet, N use efficiency, and N recycling could reduce N losses and anthropogenic Nr creation in 2050 to 52% and 64% of 2010 levels, respectively. Achieving reductions in Nr creation (while simultaneously increasing food production and offsetting imports of animal feed) will require much more in addition to good science, but it is useful to know that there are pathways by which both food security and health/environmental protection could be enhanced simultaneously.

Variation in Rapa Nui (Easter Island) land use indicates production and population peaks prior to European contact.

Many researchers believe that prehistoric Rapa Nui society collapsed because of centuries of unchecked population growth within a fragile environment. Recently, the notion of societal collapse has been questioned with the suggestion that extreme societal and demographic change occurred only after European contact in AD 1722. Establishing the veracity of demographic dynamics has been hindered by the lack of empirical evidence and the inability to establish a precise chronological framework. We use chronometric dates from hydrated obsidian artifacts recovered from habitation sites in regional study areas to evaluate regional land-use within Rapa Nui. The analysis suggests region-specific dynamics including precontact land use decline in some near-coastal and upland areas and postcontact increases and subsequent declines in other coastal locations. These temporal land-use patterns correlate with rainfall variation and soil quality, with poorer environmental locations declining earlier. This analysis confirms that the intensity of land use decreased substantially in some areas of the island before European contact.

Controls of nitrogen cycling evaluated along a well-characterized climate gradient.

The supply of nitrogen (N) constrains primary productivity in many ecosystems, raising the question "what controls the availability and cycling of N"? As a step toward answering this question, we evaluated N cycling processes and aspects of their regulation on a climate gradient on Kohala Volcano, Hawaii, USA. The gradient extends from sites receiving <300 mm/yr of rain to those receiving >3,000 mm/yr, and the pedology and dynamics of rock-derived nutrients in soils on the gradient are well understood. In particular, there is a soil process domain at intermediate rainfall within which ongoing weathering and biological uplift have enriched total and available pools of rock-derived nutrients substantially; sites at higher rainfall than this domain are acid and infertile as a consequence of depletion of rock-derived nutrients, while sites at lower rainfall are unproductive and subject to wind erosion. We found elevated rates of potential net N mineralization in the domain where rock-derived nutrients are enriched. Higher-rainfall sites have low rates of potential net N mineralization and high rates of microbial N immobilization, despite relatively high rates of gross N mineralization. Lower-rainfall sites have moderately low potential net N mineralization, relatively low rates of gross N mineralization, and rates of microbial N immobilization sufficient to sequester almost all the mineral N produced. Bulk soil δ15 N also varied along the gradient, from +4‰ at high rainfall sites to +14‰ at low rainfall sites, indicating differences in the sources and dynamics of soil N. Our analysis shows that there is a strong association between N cycling and soil process domains that are defined using soil characteristics independent of N along this gradient, and that short-term controls of N cycling can be understood in terms of the supply of and demand for N.

Fungal endophyte communities reflect environmental structuring across a Hawaiian landscape.

We surveyed endophytic fungal communities in leaves of a single tree species (Metrosideros polymorpha) across wide environmental gradients (500-5,500 mm of rain/y; 10-22 °C mean annual temperature) spanning short geographic distances on Mauna Loa Volcano, Hawai'i. Using barcoded amplicon pyrosequencing at 13 sites (10 trees/site; 10 leaves/tree), we found very high levels of diversity within sites (a mean of 551 ± 134 taxonomic units per site). However, among-site diversity contributed even more than did within-site diversity to the overall richness of more than 4,200 taxonomic units observed in M. polymorpha, and this among-site variation in endophyte community composition correlated strongly with temperature and rainfall. These results are consistent with suggestions that foliar endophytic fungi are hyperdiverse. They further suggest that microbial diversity may be even greater than has been assumed and that broad-scale environmental controls such as temperature and rainfall can structure eukaryotic microbial diversity. Appropriately constrained study systems across strong environmental gradients present a useful means to understand the environmental factors that structure the diversity of microbial communities.

Litter quality versus soil microbial community controls over decomposition: a quantitative analysis.

The possible effects of soil microbial community structure on organic matter decomposition rates have been widely acknowledged, but are poorly understood. Understanding these relationships is complicated by the fact that microbial community structure and function are likely to both affect and be affected by organic matter quality and chemistry, thus it is difficult to draw mechanistic conclusions from field studies. We conducted a reciprocal soil inoculum × litter transplant laboratory incubation experiment using samples collected from a set of sites that have similar climate and plant species composition but vary significantly in bacterial community structure and litter quality. The results showed that litter quality explained the majority of variation in decomposition rates under controlled laboratory conditions: over the course of the 162-day incubation, litter quality explained nearly two-thirds (64%) of variation in decomposition rates, and a smaller proportion (25%) was explained by variation in the inoculum type. In addition, the relative importance of inoculum type on soil respiration increased over the course of the experiment, and was significantly higher in microcosms with lower litter quality relative to those with higher quality litter. We also used molecular phylogenetics to examine the relationships between bacterial community composition and soil respiration in samples through time. Pyrosequencing revealed that bacterial community composition explained 32 % of the variation in respiration rates. However, equal portions (i.e., 16%) of the variation in bacterial community composition were explained by inoculum type and litter quality, reflecting the importance of both the meta-community and the environment in bacterial assembly. Taken together, these results indicate that the effects of changing microbial community composition on decomposition are likely to be smaller than the potential effects of climate change and/or litter quality changes in response to increasing atmospheric CO2 concentrations or atmospheric nutrient deposition.

A unifying framework for dinitrogen fixation in the terrestrial biosphere.

Dinitrogen (N(2)) fixation is widely recognized as an important process in controlling ecosystem responses to global environmental change, both today and in the past; however, significant discrepancies exist between theory and observations of patterns of N(2) fixation across major sectors of the land biosphere. A question remains as to why symbiotic N(2)-fixing plants are more abundant in vast areas of the tropics than in many of the mature forests that seem to be nitrogen-limited in the temperate and boreal zones. Here we present a unifying framework for terrestrial N(2) fixation that can explain the geographic occurrence of N(2) fixers across diverse biomes and at the global scale. By examining trade-offs inherent in plant carbon, nitrogen and phosphorus capture, we find a clear advantage to symbiotic N(2) fixers in phosphorus-limited tropical savannas and lowland tropical forests. The ability of N(2) fixers to invest nitrogen into phosphorus acquisition seems vital to sustained N(2) fixation in phosphorus-limited tropical ecosystems. In contrast, modern-day temperatures seem to constrain N(2) fixation rates and N(2)-fixing species from mature forests in the high latitudes. We propose that an analysis that couples biogeochemical cycling and biophysical mechanisms is sufficient to explain the principal geographical patterns of symbiotic N(2) fixation on land, thus providing a basis for predicting the response of nutrient-limited ecosystems to climate change and increasing atmospheric CO(2).

Cellulose (delta)18O is an index of leaf-to-air vapor pressure difference (VPD) in tropical plants.

Cellulose in plants contains oxygen that derives in most cases from precipitation. Because the stable oxygen isotope composition, δ(18)O, of precipitation is associated with environmental conditions, cellulose δ(18)O should be as well. However, plant physiological models using δ(18)O suggest that cellulose δ(18)O is influenced by a complex mix of both climatic and physiological drivers. This influence complicates the interpretation of cellulose δ(18)O values in a paleo-context. Here, we combined empirical data analyses with mechanistic model simulations to i) quantify the impacts that the primary climatic drivers humidity (e(a)) and air temperature (T(air)) have on cellulose δ(18)O values in different tropical ecosystems and ii) determine which environmental signal is dominating cellulose δ(18)O values. Our results revealed that e(a) and T(air) equally influence cellulose δ(18)O values and that distinguishing which of these factors dominates the δ(18)O values of cellulose cannot be accomplished in the absence of additional environmental information. However, the individual impacts of e(a) and T(air) on the δ(18)O values of cellulose can be integrated into a single index of plant-experienced atmospheric vapor demand: the leaf-to-air vapor pressure difference (VPD). We found a robust relationship between VPD and cellulose δ(18)O values in both empirical and modeled data in all ecosystems that we investigated. Our analysis revealed therefore that δ(18)O values in plant cellulose can be used as a proxy for VPD in tropical ecosystems. As VPD is an essential variable that determines the biogeochemical dynamics of ecosystems, our study has applications in ecological-, climate-, or forensic-sciences.

Integrated soil-crop system management for food security.

China and other rapidly developing economies face the dual challenge of substantially increasing yields of cereal grains while at the same time reducing the very substantial environmental impacts of intensive agriculture. We used a model-driven integrated soil-crop system management approach to develop a maize production system that achieved mean maize yields of 13.0 t ha(-1) on 66 on-farm experimental plots--nearly twice the yield of current farmers' practices--with no increase in N fertilizer use. Such integrated soil-crop system management systems represent a priority for agricultural research and implementation, especially in rapidly growing economies.

Resource-use efficiency and plant invasion in low-resource systems.

No species can maximize growth, reproduction and competitive ability across all environments, so the success of invasive species is habitat-dependent. Nutrient-rich habitats often experience more invasion than resource-poor habitats, a pattern consistent with traits generally associated with successful invaders (high growth rates, early reproduction and many offspring). However, invaders do colonize resource-poor environments, and the mechanisms that allow their success in these systems are poorly understood. Traits associated with resource conservation are widespread among species adapted to resource-poor environments, and invasive species may succeed in low-resource environments by employing resource conservation traits such as high resource-use efficiency (RUE; carbon assimilation per unit of resource). We investigated RUE in invasive and native species from three habitats in Hawaii where light, water or nutrient availability was limiting to plant growth. Here we show that across multiple growth forms and broad taxonomic diversity invasive species were generally more efficient than native species at using limiting resources on short timescales and were similarly efficient when RUE measures were integrated over leaf lifespans. Our data challenge the idea that native species generally outperform invasive species under conditions of low resource availability, and suggest that managing resource levels is not always an effective strategy for invasive species control.

Amplified temperature dependence in ecosystems developing on the lava flows of Mauna Loa, Hawai'i.

Through its effect on individual metabolism, temperature drives biologically controlled fluxes and transformations of energy and materials in ecological systems. Because primary succession involves feedbacks among multiple biological and abiotic processes, we expected it to exhibit complex dynamics and unusual temperature dependence. We present a model based on first principles of chemical kinetics to explain how biologically mediated temperature dependence of "reactant" concentrations can inflate the effective temperature dependence of such processes. We then apply this model to test the hypothesis that the temperature dependence of early primary succession is amplified due to more rapid accumulation of reactants at higher temperatures. Using previously published data from the lava flows of Mauna Loa, HI, we show that rates of vegetation and soil accumulation as well as rates of community compositional change all display amplified temperature dependence (Q(10) values of approximately 7-50, compared with typical Q(10) values of 1.5-3 for the constituent biological processes). Additionally, in young ecosystems, resource concentrations increase with temperature, resulting in inflated temperature responses of biogeochemical fluxes. Mauna Loa's developing ecosystems exemplify how temperature-driven, biologically mediated gradients in resource availability can alter the effective temperature dependence of ecological processes. This mechanistic theory should contribute to understanding the complex effects of temperature on the structure and dynamics of ecological systems in a world where regional and global temperatures are changing rapidly.