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Lentic systems (lakes and reservoirs) are emission hotpots of nitrous oxide (N₂O), a potent greenhouse gas; however, this has not been well quantified yet. Here we examine how multiple environmental forcings have affected N₂O emissions from global lentic systems since the pre-industrial period. Our results show that global lentic systems emitted 64.6 ± 12.1 Gg N₂O-N yr⁻¹in the 2010s, increased by 126% since the 1850s. The significance of small lentic systems on mitigating N₂O emissions is highlighted due to their substantial emission rates and response to terrestrial environmental changes. Incorporated with riverine emissions, this study indicates that N₂O emissions from global inland waters in the 2010s was 319.6 ± 58.2 Gg N yr⁻¹. This suggests a global emission factor of 0.051% for inland water N₂O emissions relative to agricultural nitrogen applications and provides the country-level emission factors (ranging from 0 to 0.341%) for improving the methodology for national greenhouse gas emission inventories.

 

Abstract. Excessive anthropogenic nitrogen (N) inputs to the biosphere have disruptedthe global nitrogen cycle. To better quantify the spatial and temporalpatterns of anthropogenic N inputs, assess their impacts on thebiogeochemical cycles of the planet and the living organisms, and improvenitrogen use efficiency (NUE) for sustainable development, we have developeda comprehensive and synthetic dataset for reconstructing the History ofanthropogenic Nitrogen inputs (HaNi) to the terrestrial biosphere. The HaNi datasettakes advantage of different data sources in a spatiotemporally consistentway to generate a set of high-resolution gridded N input products from thepreindustrial period to the present (1860–2019). The HaNi dataset includes annual ratesof synthetic N fertilizer, manure application/deposition, and atmospheric Ndeposition on cropland, pasture, and rangeland at a spatial resolution of5 arcmin × 5 arcmin. Specifically, the N inputs are categorized, according to the Nforms and land uses, into 10 types: (1) NH4+-N fertilizer applied to cropland,(2) NO3--N fertilizer applied to cropland, (3) NH4+-N fertilizer applied to pasture,(4) NO3--N fertilizer applied to pasture, (5) manure N application oncropland, (6) manure N application on pasture, (7) manure N deposition onpasture, (8) manure N deposition on rangeland, (9) NHx-N deposition, and(10) NOy-N deposition. The total anthropogenic N (TN) inputs to globalterrestrial ecosystems increased from 29.05 Tg N yr−1 in the 1860s to267.23 Tg N yr−1 in the 2010s, with the dominant N source changing fromatmospheric N deposition (before the 1900s) to manure N (in the 1910s–2000s)and then to synthetic fertilizer in the 2010s. The proportion of syntheticNH4+-N in fertilizer input increased from 64 %in the 1960s to 90 % in the 2010s, while synthetic NO3--N fertilizerdecreased from 36 % in the 1960s to 10 % in the 2010s. Hotspots of TNinputs shifted from Europe and North America to East and South Asia duringthe 1960s–2010s. Such spatial and temporal dynamics captured by the HaNidataset are expected to facilitate a comprehensive assessment of the coupledhuman–Earth system and address a variety of social welfare issues, such as theclimate–biosphere feedback, air pollution, water quality, and biodiversity. Thedata are available at https://doi.org/10.1594/PANGAEA.942069(Tian et al., 2022). 

Abstract. Livestock manure nitrogen (N) and phosphorus (P) play an importantrole in biogeochemical cycling. Accurate estimation of manure nutrient isimportant for assessing regional nutrient balance, greenhouse gas emission,and water environmental risk. Currently, spatially explicit manure nutrientdatasets over a century-long period are scarce in the United States (US).Here, we developed four datasets of annual animal manure N and P productionand application in the contiguous US at a 30 arcsec resolution overthe period of 1860–2017. The dataset combined multiple data sourcesincluding county-level inventory data as well as high-resolution livestockand crop maps. The total production of manure N and P increased from 1.4 Tg N yr−1 and 0.3 Tg P yr−1 in 1860 to 7.4 Tg N yr−1 and 2.3 Tg P yr−1 in 2017, respectively. The increasing manure nutrient productionwas associated with increased livestock numbers before the 1980s andenhanced livestock weights after the 1980s. The manure application amountwas primarily dominated by production, and its spatial pattern was impactedby the nutrient demand of crops. The intense-application region mainlyenlarged from the Midwest toward the southern US and became moreconcentrated in numerous hot spots after the 1980s. The South Atlantic–Gulf and Mid-Atlantic basins were exposed to high environmental risks due to theenrichment of manure nutrient production and application from the 1970s tothe period of 2000–2017. Our long-term manure N and P datasets providedetailed information for national and regional assessments of nutrientbudgets. Additionally, the datasets can serve as the input data forecosystem and hydrological models to examine biogeochemical cycles interrestrial and aquatic ecosystems. Datasets are available at https://doi.org/10.1594/PANGAEA.919937 (Bian etal., 2020). 

Soil erosion and sedimentation problems remain a major water quality concern for making watershed management policies in the Mississippi River Basin (MRB). It is unclear whether the observed decreasing trend of stream suspended sediment loading to the mouth of the MRB over the last eight decades truly reflects a decline in upland soil erosion in this large basin. Here, we improved a distributed regional land surface model, the Dynamic Land Ecosystem Model, to evaluate how climate and land use changes have impacted soil erosion and sediment yield over the entire MRB during the past century. Model results indicate that total sediment yield significantly increased during 1980–2018, despite no significant increase in annual precipitation and runoff. The increased soil erosion and sediment yield are mainly driven by intensified extreme precipitation (EP). Spatially, we found notable intensified EP events in the cropland‐dominated Midwest region, resulting in a substantial increase in soil erosion and sediment yield. Land use change played a critical role in determining sediment yield from the 1910s to the 1930s, thereafter, climate variability increasingly became the dominant driver of soil erosion, which peaked in the 2010s. This study highlights the increasing influences of extreme climate in affecting soil erosion and sedimentation, thus, water quality. Therefore, existing forest and cropland Best Management Practices should be revisited to confront the impacts of climate change on water quality in the MRB.

 

Global carbon dioxide (CO₂) evasion from inland waters (rivers, lakes, and reservoirs) and carbon (C) export from land to oceans constitute critical terms in the global C budget. However, the magnitudes, spatiotemporal patterns, and underlying mechanisms of these fluxes are poorly constrained. Here, we used a coupled terrestrial–aquatic model to assess how multiple changes in climate, land use, atmospheric CO₂concentration, nitrogen (N) deposition, N fertilizer and manure applications have affected global CO₂evasion and riverine C export along the terrestrial‐aquatic continuum. We estimate that terrestrial C loadings, riverine C export, and CO₂evasion in the preindustrial period (1800s) were 1,820 ± 507 (mean ± standard deviation), 765 ± 132, and 841 ± 190 Tg C yr⁻¹, respectively. During 1800–2019, multifactorial global changes caused an increase of 25% (461 Tg C yr⁻¹) in terrestrial C loadings, reaching 2,281 Tg C yr⁻¹in the 2010s, with 23% (104 Tg C yr⁻¹) of this increase exported to the ocean and 59% (273 Tg C yr⁻¹) being emitted to the atmosphere. Our results showed that global inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans, highlighting the important role of inland waters in the global C balance, an amount that should be taken into account in future C budgets. Our analysis supports the view that a major feature of the global C cycle–the transfer from land to ocean–has undergone a dramatic change over the last two centuries as a result of human activities.

 

Human‐induced nitrogen–phosphorus (N, P) imbalance in terrestrial ecosystems can lead to disproportionate N and P loading to aquatic ecosystems, subsequently shifting the elemental ratio in estuaries and coastal oceans and impacting both the structure and functioning of aquatic ecosystems. The N:P ratio of nutrient loading to the Gulf of Mexico from the Mississippi River Basin increased before the late 1980s driven by the enhanced usage of N fertilizer over P fertilizer, whereafter the N:P loading ratio started to decrease although the N:P ratio of fertilizer application did not exhibit a similar trend. Here, we hypothesize that different release rates of soil legacy nutrients might contribute to the decreasing N:P loading ratio. Our study used a data‐model integration framework to evaluate N and P dynamics and the potential for long‐term accumulation or release of internal soil nutrient legacy stores to alter the ratio of N and P transported down the rivers. We show that the longer residence time of P in terrestrial ecosystems results in a much slower release of P to coastal oceans than N. If contemporary nutrient sources were reduced or suspended, P loading sustained by soil legacy P would decrease much slower than that of N, causing a decrease in the N and P loading ratio. The longer residence time of P in terrestrial ecosystems and the increasingly important role of soil legacy nutrients as a loading source may explain the decreasing N:P loading ratio in the Mississippi River Basin. Our study underscores a promising prospect for N loading control and the urgency to integrate soil P legacy into sustainable nutrient management strategies for aquatic ecosystem health and water security.

 

Phosphorus (P) control is critical to mitigating eutrophication in aquatic ecosystems, but the effectiveness of controlling P export from soils has been limited by our poor understanding of P dynamics along the land‐ocean aquatic continuum as well as the lack of well‐developed process models that effectively couple terrestrial and aquatic biogeochemical P processes. Here, we coupled riverine P biogeochemical processes and water transport with terrestrial processes within the framework of the Dynamic Land Ecosystem Model to assess how multiple environmental changes, including fertilizer and manure P uses, land use, climate, and atmospheric CO₂, have affected the long‐term dynamics of P loading and export from the Mississippi River Basin to the Gulf of Mexico during 1901–2018. Simulations show that riverine exports of dissolved inorganic phosphorus (DIP), dissolved organic phosphorus, particulate organic phosphorus (POP), and particulate inorganic phosphorus (PIP) increased by 42%, 53%, 60%, and 53%, respectively, since the 1960s. Riverine DIP and PIP exports were the dominant components of the total P flux. DIP export was mainly enhanced by the growing mineral P fertilizer use in croplands, while increased PIP and POP exports were a result of the intensified soil erosion due to increased precipitation. Climate variability resulted in substantial interannual and decadal variations in P loading and export. Soil legacy P continues to contribute to P loading. Our findings highlight the necessity to adopt effective P management strategies to control P losses through reductions in soil erosion, and additionally, to improve P use efficiency in crop production.

 

The large areal extent of hypoxia in the northern Gulf of Mexico has been partially attributed to substantial nitrogen (N) loading from the Mississippi River basin, which is driven by multiple natural and human factors. The available water quality monitoring data and most of the current models are insufficient to fully quantify N load magnitude and the underlying controls. Here we use a process‐based Dynamic Land Ecosystem Model to examine how multiple factors (synthetic N fertilizer, atmospheric N deposition, land use changes, climate variability, and increasing atmospheric CO₂) have affected the loading and delivery of total nitrogen (TN) consisting of ammonium and nitrate (dissolved inorganic N) and total organic nitrogen from the Mississippi River basin during 1901–2014. The model results indicate that TN export during 2000–2014 was twofold larger than that in the first decade of twentieth century: Dissolved inorganic N export increased by 140% dominated by nitrate; total organic nitrogen export increased by 53%. The substantial enrichment of TN export since the 1960s was strongly associated with increased anthropogenic N inputs (synthetic N fertilizer and atmospheric N deposition). The greatest export of TN was in the spring. Although the implementation of N reduction has been carried out over the past three decades, total N loads to the northern Gulf of Mexico have not decreased significantly. Due to the legacy effect from historical N accumulation in soils and riverbeds, a larger reduction in synthetic N fertilizer inputs as well as improved N management practices are needed to alleviate ocean hypoxia in the northern Gulf of Mexico.