Nitrogen Stocks Research Articles

Forest thinning increases soil carbon stocks in China

Thinning is a common forest management practice. However, its impact on soil organic carbon (SOC) stocks is still unknown. Therefore, we conducted a meta-analysis of 270 measurements from 77 articles to evaluate the effect of forest thinning on SOC stocks in mineral soil (0–30 cm) across planted forests in China. The results showed that, compared to reference (non-thinned) plantations, thinning significantly increased SOC stocks in planted forests by 7.2%. Among different thinning intensities, moderate thinning (35–55% of thinning intensity) increased SOC stocks in planted forests (+16.1%) more than other levels of thinning intensity. However, the positive effect of thinning on SOC stock was significant > 5 years after harvesting. In the humidity-restricted areas (humidity index (HI) < 30), the increase in SOC stocks after thinning was significantly higher than that of other areas. In addition, thinning increased the sensitivity (i.e. slope value) of soil total nitrogen (TN) stocks changes to SOC stocks. Therefore, we conclude that forest thinning strategy is potentially a viable silvicultural measure to increase SOC fixation in planted forests. Our results provide a reference for the formulation and implementation of future forest management strategies.

Afforestation of a pasture in Norway did not result in higher soil carbon, 50 years after planting

Afforestation of marginal cultivated land is an internationally approved climate mitigation strategy, however, with uncertain implications for soil organic carbon (SOC) storage. We examined the effect of forest planting by measuring SOC at two adjacent sites: one with a Norway spruce forest planted in 1968 and one actively grazed pasture. Both sites had similar land-use history before forest planting, and they were as similar as possible in all other edaphic factors. There were no significant differences in SOC stocks down to 30 cm mineral soil, 7.15 and 8.51 kg C m−2 in the forest plantation and pasture respectively. Only a minimal build-up of an O horizon, less than 2 cm, was observed in the plantation. The SOC stocks of the plantation and pasture were not significantly different from that of a nearby old forest, 7.17 kg C m−2. When comparing these three land-uses we found that there were significant differences in the upper 10 cm of the soil with regard to other soil properties. Nitrogen (N) stock and pH were significantly lower in the old forest compared to the plantation, which again was significantly lower than that of the pasture. The opposite was the case for the C/N ratio. We conclude that there were no significant differences in SOC stocks in the upper 30 cm 50 years after afforestation with Norway spruce, but that there is still a legacy from the former cultivation that may influence both productivity and organic matter dynamics.

CARBON, NITROGEN, AND ORGANIC MATTER OF SOIL IN AN INTEGRATED CROP-LIVESTOCK SYSTEM

This study aimed to evaluate the effect of the integrated crop-livestock system on the contents and stocks of total organic carbon, total nitrogen, and soil organic matter. The experiment was carried out between April 2016 and February 2017, in a randomized block design in a strip-block scheme, with additional control treatment (fallow) and four repetitions. The treatments consisted of two densities of oat sowing (40 and 60 kg ha-1) in strips A and different grazing frequencies (without grazing - WG, one grazing - 1G, and two grazing - 2G) in strips B, besides fallow as a control treatment. The determinations of the contents of total organic carbon (TOC), total nitrogen (TN), and organic matter (SOM) were carried out in the soil layers of 0-5, 5-10, and 10-20 cm. Samplings were carried out after oat management, in the winter, and after soybean harvest, in the summer. The sowing density of black oats and the grazing frequencies were not able to promote changes in the total organic carbon and in the TOC stock, regardless of the time of evaluation. The use of 40 kg ha-1 of oat seeds would be recommended, as it does not promote changes in the increase in MOS, thus generating savings in the acquisition of seeds. The use of the lowest density of oat without grazing and one grazing promoted changes on the total nitrogen in the 5-10 cm layer after the removal of oats, when compared to the fallow, and it presented content similar to the management with two grazings.

Spatial variability of carbon and nitrogen stocks in integrated management systems and pasture in a cerrado region

The conversion of the native Cerrado into agricultural systems promotes a reduction in the input of organic residues on the soil, which can compromise the contents of soil organic carbon. The spatial distribution of carbon and nitrogen stocks in the soil is normally influenced by environmental and anthropogenic factors. This research aimed to map and evaluate the spatial variability of carbon and nitrogen stocks in the soil, in different integrated systems and pasture areas in the edaphoclimatic conditions of the Cerrado biome, in the state of Maranhão. The work was set up in a Red-Yellow Latosol with different management methods: crop-livestock integration with no-till farming, crop-livestock integration with patch scarification and harrowing, crop-livestock-forest integration, and pasture. The samples were removed at a depth of 0.0-0.20 m, in a grid with a regular interval of 50 m, totaling 193 points. The data were subjected to descriptive statistics, geostatistics, and kriging interpolation. The mean and median values are similar for the carbon and nitrogen stocks, in their respective management systems, indicating symmetric data distribution, confirmed by the asymmetry and kurtosis values close to zero. The spatial distribution of the carbon stocks is more homogeneous in the crop-livestock integration with no-till farming, whereas the crop-livestock integration with patch scarification and harrowing presents greater homogeneity in nitrogen distribution.

Fluxes, stocks and availability of nitrogen in evergreen broadleaf and fir forests: similarities and differences

In this study, nitrogen fluxes or flows in litterfall, nitrogen stocks and available nitrogen in soils of two plots representing evergreen broadleaf and Bulgarian fir forests were assessed. Both plots are in good quality sites and for this reason, the litterfall quantities and nitrogen fluxes were relatively high. The woody litterfall flux of nitrogen was significantly higher in the fir forest than in the evergreen broadleaf one. The total nitrogen stock was higher in the soil under the fir forest. However, the percentage of the available nitrogen (ammonium + nitrates) was significantly higher in the upper 20 cm soil layer of the evergreen broadleaf forest in spite of the higher average C/N ratios in the foliar litterfall of the broadleaf forest and insignificant difference of the C/N ratios in all soil layers of the two ecosystems. The microclimatic conditions (higher soil temperatures in the evergreen broadleaf forest) is probable possible cause for this difference. The available nitrogen in the soils and its retranslocation from senescing leaves cover the nitrogen requirements of trees. It is hypothesized that trees may also take up nitrogen from deeper soil layers.

Linking Cover Crop Residue Quality and Tillage System to CO2-C Emission, Soil C and N Stocks and Crop Yield Based on a Long-Term Experiment

Cover crops (CC), particularly legumes, are key to promote soil carbon (C) sequestration in no-tillage. Nevertheless, the mechanisms regulating this process need further elucidation within a broad comprehensive framework. Therefore, we investigated effects of CC quality: black oat (Avena strigosa Schreb) (oat), common vetch (Vicia sativa L.) (vetch), and oat + vetch on carbon dioxide-C (CO2-C) emission (124 days) under conventional- (CT), minimum- (MT) and no-tillage (NT) plots from a long-term experiment in Southern Brazil. Half-life time (t1/2) of CC residues and the apparent C balance (ACB) were obtained for CT and NT. We linked our data to long-term (22 years) soil C and nitrogen (N) stocks and crop yield data of our experimental field. Compared to CT, NT increased t1/2 of oat, oat + vetch and vetch by 3.9-, 3.1- and 3-fold, respectively; reduced CO2-C emissions in oat, oat + vetch and vetch by 500, 600 and 642 kg ha−1, respectively; and increased the ACB (influx) in oat + vetch (195%) and vetch (207%). For vetch, CO2-C emission in MT was 77% greater than NT. Legume CC should be preferentially combined with NT to reduce CO2-C emissions and avoid a flush of N into the soil. The legume based-NT system showed the greatest soil C and N sequestration rates, which were significantly and positively related to soybean (Glycine max (L.) Merrill) and maize (Zea mays L.) yield. Soil C (0–90 cm depth) and N (0–100 cm depth) sequestration increments of 1 kg ha−1 corresponded to soybean yield increments of 1.2 and 7.4 kg ha−1, respectively.

Long-term evaluation of the initiative 4‰ under different soil managements in Mediterranean olive groves

The 4‰ initiative implementation has increased the emphasis and interest in soil carbon and nitrogen storage in the last few years. This study evaluated the dynamics of soil organic carbon and total nitrogen under rain-fed olive groves over a long-term period (2004–2019). The management practices associated with achieving the 4‰ initiative objectives and the depth of analysis to measure the effectiveness of the initiative have generated uncertainties and wide debate in the scientific community. To contribute to this debate from a farm level, the objective of this study was to analyse the effects of conventional tillage and no-tillage with bare soil by using herbicides (after land management change from conventional tillage) on carbon and nitrogen stocks in complete soil profiles (depth > 100 cm) over 15 years in a Mediterranean olive grove. Soil samples were collected from each farm and analysed for carbon content and physical-chemical characteristics. This study indicates that management practices evaluated resulted in soil organic carbon and total nitrogen contents decreasing in soil, with a reduction >30% in all horizons. Results highlight a significant depletion of soil organic carbon stock with a significant decarbonisation process (−1.8 Mg C ha−1 yr−1) and total nitrogen stock (−0.57 and − 0.41 Mg N ha−1 yr−1) on average under both managements (no-tillage no tillage with herbicide and conventional tillage respectively) as compared to the initial situation. Furthermore, it was demonstrated that deep horizons are significant reservoirs of carbon (>50% in all cases) and in woody crops, its analysis within the dynamics of soil organic carbon stocks proposed by the 4‰ initiative was relevant. With these results, no-tillage with bare soil by using herbicides was demonstrated as an unsustainable agricultural practice and it is proposed to change the current soil management to sustainable management that increases the C inputs to achieve the 4‰ targets.

Carbon and Nitrogen Stocks and Soil Quality in an Area Cultivated with Guinea Grass under the Residual Effect of Nitrogen Doses

This study examines the residual effect of nitrogen (N) doses on the carbon (C) and N stocks and on soil quality in an area cultivated with guinea grass. The pastures received three annual doses of N (100, 200 and 300 kg ha−1) from 2015 to 2017. In 2018, N fertilization was not applied so the residual effect of the nutrient could be characterized. Soil chemical attributes, C and N stocks, density and penetration resistance and root system characteristics were evaluated at different depths. No difference was observed between the N doses for soil density, which averaged 1.22 and 1.25 g cm−3 under and between the tussocks of guinea grass, respectively. Penetration resistance was affected by the N doses post-grazing, with the highest value (1.9 MPa) observed in pastures that received 300 kg ha−1 of N for three consecutive years. Root dry mass was not affected by the N doses. There was no effect of N doses on the average (19.7 mg ha−1) or total (134.3 mg ha−1) C stock in the soil. The total N stock did not change (11.3 mg ha−1) in response to the N doses; however, the average N stock was higher in the soil cultivated with guinea grass under the N dose of 300 kg ha−1 (1.7 mg ha−1). The N doses had little interference with the soil chemical and physical aspects. Regardless of the dose, high C and N stocks were observed in the soil cultivated with guinea grass. Therefore, when properly managed, intensive pasture-based animal production systems become important allies of the environment.

Impacts of land use and cropland management on soil organic matter and greenhouse gas emissions in the Brazilian Cerrado

AbstractThe Brazilian Cerrado is a large and expanding agricultural frontier, representing a hotspot of land‐use change (LUC) from natural vegetation to farmland. It is known that this type of LUC impacts soil organic matter (SOM) dynamics, particularly labile carbon (C) pools (living and non‐living), decreasing soil health and agricultural sustainability, as well as increasing soil greenhouse gas (GHG) emissions, and accelerating global climate change. In this study, we quantified the changes in the quantity and quality of SOM and GHG fluxes due to changes in land use and cropland management in the Brazilian Cerrado. The land uses studied were native vegetation (NV), pasture (PA) and four croplands, including the following management types: conventional tillage with a single soybean crop (CT), and three no‐tillage systems with two crops cultivated in the same year (i.e., soybean/sorghum (NTSSo), soybean/millet (NTSMi) and maize/sorghum (NTMSo)). Soil and gases were sampled in the rainy season (November, December and January) and dry season (May, July and September). The highest soil C and nitrogen (N) stocks (6.7 kg C m−2 and 0.5 kg N m−2, 0–0.3‐m layer) were found under NV. LUC reduced C stocks by 25% in the CT and by 10% in the PA and NT. Soil N stocks were 30% lower in the PA and NTMSo and 15% lower in the croplands with soybean compared to NV. δ13C values clearly distinguished between the C‐origin from NV (−25‰) and that from other land uses (−16‰). Soil (0–0.1 m) under NV also presented higher labile‐C (625 g C m−2), microbial‐C (70 g C m−2) and microbial‐N (5.5 g N m−2), whereas other land uses presented values three times lower. GHG emissions (expressed as C‐equivalent) were highest in the NV (1.2 kg m−2 year−1), PA (1.3 kg m−2 year−1) and NTMSo (0.9 kg m−2 year−1) and were positively related to the higher SOM turnover in these systems. Our results suggest that in order to maintain SOM, it is necessary to adopt “best” management practices, that provide large plant residue inputs (above‐ and belowground). This can be seen as a pathway to achieving high food production with low GHG emissions.

Nitrogen stocks and flows in an acid sulfate soil

Besides causing acidification, acid sulfate (AS) soils contain large nitrogen (N) stocks and are a potential source of N loading to waters and nitrous oxide (N2O) emissions. We quantified the stocks and flows of N, including crop yields, N leaching, and N2O emissions, in a cultivated AS soil in western Finland. We also investigated whether controlled drainage (CD) and sub-irrigation (CDI) to keep the sulfidic horizons inundated can alleviate N losses. Total N stock at 0–100 cm (19.5 Mg ha−1) was smaller than at 100–200 cm (26.6 Mg ha−1), and the mineral N stock was largest below 170 cm. Annual N leaching (31–91 kg N ha−1) plus N in harvested grain (74–122 kg N ha−1) was 148% (range 118–189%) of N applied in fertilizers (90–125 kg N ha−1) in 2011–2017, suggesting substantial N supply from soil reserves. Annual emissions of N2O measured during 2 years were 8–28 kg N ha−1. The most probable reasons for high N2O emission rates in AS soils are concomitant large mineral N pools with fluctuating redox conditions and low pH in the oxidized subsoil, all favoring formation of N2O in nitrification and denitrification. Although the groundwater level was higher in CD and CDI than in conventional drainage, N load and crop offtake did not differ between the drainage methods, but there were differences in emissions. Nitrogen flows to the atmosphere and drainage water were clearly larger than those in non-AS mineral soils indicating that AS soils are potential hotspots of environmental impacts.

Changes in soil organic carbon and total nitrogen in apple orchards in different climate regions on the Loess Plateau

The Loess Plateau is the largest high-quality apple-producing area in China. However, the responses of soil organic carbon (SOC) and total nitrogen (TN) concentrations and stocks to long-term cultivation in apple orchards under different climate regions remain unclear. We conducted soil sampling in apple orchards distributed throughout the main apple-production area on the Loess Plateau. A total of 28 soil samples (max depth 500 cm) from stands of different ages at 7 soil sampling sites were collected to evaluate the changes in SOC and TN concentrations and stocks; furthermore, C:N ratios across apple orchards in different climate regions were evaluated. Compared with those in the semihumid region, SOC concentration and stock in the 0–500 cm soil profile were 6.6% and 32.1% lower, respectively, in the semiarid region. Furthermore, over the entire soil profile, TN concentration and stock were 12.1% and 38.4% lower, respectively, in the semiarid region than in the semihumid region. However, the C:N ratios in the semiarid region were higher than those in the semihumid region. SOC and TN concentrations generally decreased with increasing soil depth, and those in the 0–120 cm soil layer were significantly higher than those at the other soil depths in both climate regions. The most important factors, which control the variability of SOC and TN, were precipitation, plantation age, topographical properties (altitude and landform of apple orchards) and soil type. These results not only provide information on the effects of changes in SOC and TN concentrations and stocks and C:N ratios but also provide regional-level information that can aid the sustainable development of apple orchards on the Loess Plateau.

Changes in soil carbon and nitrogen stocks following degradation of alpine grasslands on the Qinghai‐Tibetan Plateau: A meta‐analysis

AbstractUntil now, nearly 90% of alpine grasslands on the Qinghai‐Tibetan Plateau (QTP) have been degraded. However, there is currently no consensus on how soil organic carbon (SOC) and soil total nitrogen (STN) stocks vary with the degradation succession of alpine grasslands in this region. Here, a meta‐analysis was conducted to quantify the dynamics of SOC and STN stocks in topsoil (0–30 cm) at different degradation stages of alpine grasslands on the QTP. The results showed that grassland degradation led to average losses of 48 and 39% for SOC and STN stocks, respectively. The changes in SOC and STN stocks following grassland degradation did not differ significantly between grassland types, but were significantly affected by grassland degradation stage. The reductions in both SOC and STN stocks increased with the degradation stage, and the highest reductions were all found at extreme degradation stage. The results indicated that the depletion of SOC and STN pools were aggravated with the degradation succession of grassland. The tightly coupled SOC and STN implied that the depletion of SOC stock was closely related to that of STN stock during the process of grassland degradation. Positive relationships were detected between the dynamics of vegetation coverage and that of SOC or STN stock, indicating that vegetation coverage could not only reflect grassland degradation, but might also be a potential indicator of SOC and STN status. The findings suggest that preventing the degradation succession of alpine grasslands is vital to maintain or promote SOC and STN levels on the QTP.

Carbon and Nitrogen Balance in an Oxisol Influenced by Sewage Sludge and Mineral Fertilization

ABSTRACT Most conventional agricultural production systems have difficulty maintaining carbon (C) and nitrogen (N) stocks in soils. However, the use of organic sources in fertilization, such as sewage sludge, can reverse these processes of degradation of soil organic matter. The objective of this study was to evaluate C and N stocks and balance in organic matter fractions of a Oxisol under pineapple after fertilization with sewage sludge and mineral fertilizers. A randomized block experimental design was used with four replications in a 2 × 2 factorial arrangement, corresponding to two pineapple cultivars (‘Vitória’ and ‘Smooth Cayenne’) and two fertilization tips (sewage sludge and mineral fertilizers). Soil samples were collected at the depths of 0–20, 20–40, and 40–60 cm to characterize the stocks of total organic carbon (TOC), total nitrogen (TN), labile carbon (LC), carbon in the fulvic acid fraction (FAF), in the humic acid fraction (HAF), and in the humin fraction (HUM), and the humification index (HI). Fertilization with sewage sludge increased the carbon and nitrogen stocks in the labile and stable fractions of the soil compared to the soil that received only mineral fertilization and compared to the condition at the beginning of cultivation. The increase in the TOC stock was 8 t ha−1, with accumulated storage of 148 t ha−1 in the 0–60 cm depth. The TN stock increased 2 t ha−1, with accumulated storage of 15 t ha−1 in the 0–60 cm depth. Therefore, fertilization with sewage sludge contributed to the agricultural sustainability of the soil.

Incorporating soil aggregate-associated indicators into evaluating ecological responses of degraded estuarine wetlands to freshwater replenishment at different intensity: A case study from the Yellow River Delta, China

Management intensity was considered an important factor in evaluating and regulating the anthropogenic restoration measures for a degraded ecosystem. By comparing selected conventional soil physicochemical, microbial and incorporating aggregate-associated indices between wetland plot with high-intensity freshwater replenishment (HI wetlands) and wetland plot with low-intensity freshwater replenishment (LI wetlands), edaphic responses of degraded estuarine wetlands to freshwater replenishment at different intensity were assessed in the present study. The results showed that high-intensity freshwater replenishment exerted better soil salinization and alkalization alleviation effects on degraded estuarine wetlands, which caused lower EC and pH values in HI wetlands than those in LI wetlands. And soil microbial biomass was promoted, with freshwater replenishment intensity increased, as indicated by MBC and total PLFAs. Minor alternation of soil microbial community composition was observed with an elevated abundance of arbuscular mycorrhizal fungi (AMF) in HI wetlands. On the other hand, freshwater replenishment at low intensity was found to contribute to organic matter accumulation, as indicated by both higher SOC concentrations and carbon stocks in LI wetlands than those in HI wetlands. And soil aggregate-associated indicators provided more scientific clues that freshwater replenishment at low intensity favored the formation of large macroaggregates, and thus supported higher soil aggregate stability. Moreover, a transfer of allocations of aggregate-associated carbon and nitrogen stocks from microaggregates to large macroaggregates was induced as the freshwater replenishment intensity decreased, and the principle governing factors were the proportion of large macroaggregates in soils, soil salinity and microbial decomposition rates based on multivariate statistical analysis. The findings of this study could provide basic data regarding soil physicochemical properties, microbial characteristics and soil structure stability in restored wetlands of the Yellow River Delta with freshwater replenishment at different intensity and guiding adaptive management strategies of freshwater replenishment to enhance the ecological functions of estuarine wetlands.

Importance of sugarcane straw maintenance to prevent soil organic matter depletion in a Nitisol in the central-southern region of Brazil

Sugarcane straw is a potential feedstock to increase bioenergy production. However, leaving straw on the soil surface is important to sustaining soil organic matter (SOM). We conducted a field experiment to evaluate the impacts of two years of straw removal on SOM in a sugarcane area – on an Eutric Nitisol in south-eastern Brazil – with low straw production (7 Mg ha–1). The experiment was a randomised complete block design (four blocks) with four rates of straw removal: (i) no removal, (ii) medium removal, (iii) high removal and (iv) total removal. We evaluated straw decomposition, soil carbon (C) and nitrogen (N) stock (0–30 cm), labile C, and microbial biomass C and N (0–10 cm). Increases in the straw removal rates reduced decomposition, with 30–42% of the initial straw amount remaining on the soil after one year (first year) and 33–38% remaining at the end of the second year. During the first year, no effect was found on soil C (8 kg m–2) and N (0.7 kg m–2) stocks; however, during the second year, total straw removal reduced soil C stock by 10% (0–30 cm) compared with no straw removal. Straw removal rates negatively affected the labile C (6–10% and 20–25%, first and second year respectively) and microbial C (correspondingly 10–15% and 21–27%). Microbial N was not altered in the first year, but decreased 26% due to straw maintenance (no removal) during the second year. Total straw removal decreased the C management index (CMI) by 15% (first year) compared with no straw removal. However, during the second year, all rates of straw removal decreased the CMI: by 15% for medium, 22% for high and 26% for total removal. Our findings indicate that any straw removal in areas with low straw production negatively affects SOM, initially through reduction in labile C pools. Although slight impacts on soil C and N stock were detected, reduction in straw-derived inputs stimulates microbial utilisation of soil C pools, and consequently reduces SOM over time.

Effects of groundwater tables and salinity levels on soil organic carbon and total nitrogen accumulation in coastal wetlands with different plant cover types in a Chinese estuary

Groundwater tables and soil salinity can greatly affect the biogeochemical cycles of carbon and nitrogen by altering soil physicochemical properties. Coastal wetlands with different plant cover types (including bare land, Phragmites australis, and Suaeda salsa wetlands) and groundwater table wetlands (including low groundwater table, medium groundwater table, and high groundwater table wetlands) were investigated in 2012–2013 to characterize the spatial and temporal changes and the effects of groundwater tables on soil organic carbon (SOC) and total nitrogen (TN) in the Yellow River Estuary, China. The results showed that SOCs in 2012 was higher than that in 2013 regardless of the groundwater tables, while TNs was higher in 2013 than in 2012 except in the BL soil with low groundwater tables and the PA wetlands with middle groundwater tables. The SOCs and TNs were mainly concentrated in the top 20 cm soils. SOCc and TNc were fitted very well with soil depth by the general linear fit (p < 0.01). The Lorentz models were ideal for the relationships among SOC, TN and electrical conductivity (EC), indicating that certain EC values could improve carbon and nitrogen accumulation. The theoretical threshold of EC for the maximum SOC and SOCs was 2.2 ms/cm, and the maximum TN and TNs highly depended on groundwater tables (0.9–1.0 ms/cm for LW, 1.6–1.8 ms/cm for MW and 3.0 ms/cm for HW). The general linear model results showed that SOC was positively correlated with TN at the same groundwater table (p < 0.01). Additionally, a positive linear relationship was also observed among SOC, TN and silt + clay contents for all groundwater tables (p < 0.05). These findings indicated that carbon and nitrogen stocks might be regulated by groundwater tables and salinity to improve carbon sinks in coastal wetlands.

Changes in Ecosystem Nitrogen and Carbon Allocation with Black Mangrove (Avicennia germinans) Encroachment into Spartina alterniflora Salt Marsh

Increases in temperature are expected to facilitate encroachment of tropical mangrove forests into temperate salt marshes, yet the effects on ecosystem services are understudied. Our work was conducted along a mangrove expansion front in Louisiana (USA), an area where coastal wetlands are in rapid decline due to compounding factors, including reduced sediment supply, rising sea level, and subsidence. Marsh and mangrove ecosystems are each known for their ability to adjust to sea-level rise and support numerous ecosystem services, but there are some differences in the societal benefits they provide. Here, we compare carbon and nitrogen stocks and relate these findings to the expected effects of mangrove encroachment on nitrogen filtration and carbon sequestration in coastal wetlands. We specifically evaluate the implications of black mangrove (Avicennia germinans) encroachment into Spartina alterniflora-dominated salt marsh. Our results indicate that black mangrove encroachment will lead to increased aboveground carbon and nitrogen stocks. However, we found no differences in belowground (that is, root and sediment) nitrogen or carbon stocks between marshes and mangroves. Thus, the shift from marsh to mangrove may provide decadal-scale increases in aboveground nitrogen and carbon sequestration, but belowground nitrogen and carbon sequestration (that is, carbon burial) may not be affected. We measured lower pore water nitrogen content beneath growing mangroves, which we postulate may be due to greater nitrogen uptake and storage in mangrove aboveground compartments compared to marshes. However, further studies are needed to better characterize the implications of mangrove encroachment on nitrogen cycling, storage, and export to the coastal ocean.

A new dataset of soil carbon and nitrogen stocks and profiles from an instrumented Greenlandic fen designed to evaluate land-surface models

Abstract. Arctic and boreal peatlands play a major role in the global carbon (C) cycle. They are particularly efficient at sequestering carbon because their high water content limits decomposition rates to levels below their net primary productivity. Their future in a climate-change context is quite uncertain in terms of carbon emissions and carbon sequestration. Nuuk fen is a well-instrumented Greenlandic fen with monitoring of soil physical variables and greenhouse gas fluxes (CH4 and CO2) and is of particular interest for testing and validating land-surface models. But knowledge of soil carbon stocks and profiles is missing. This is a crucial shortcoming for a complete evaluation of models, as soil carbon is one of the primary drivers of CH4 and CO2 soil emissions. To address this issue, we measured, for the first time, soil carbon and nitrogen density, profiles and stocks in the Nuuk peatland (64∘07′51′′ N, 51∘23′10′′ W), colocated with the greenhouse gas measurements. Measurements were made along two transects, 60 and 90 m long and with a horizontal resolution of 5 m and a vertical resolution of 5 to 10 cm, using a 4 cm diameter gouge auger. A total of 135 soil samples were analyzed. Soil carbon density varied between 6.2 and 160.2 kg C m−3 with a mean value of 50.2 kg C m−3. Mean soil nitrogen density was 2.37 kg N m−3. Mean soil carbon and nitrogen stocks are 36.3 kg C m−2 and 1.7 kg N m−2. These new data are in the range of those encountered in other arctic peatlands. This new dataset, one of very few in Greenland, can contribute to further development of joint modeling of greenhouse gas emissions and soil carbon and nitrogen in land-surface models. The dataset is open-access and available at https://doi.org/10.1594/PANGAEA.909899 (Morel et al., 2019b).

Do different land use changes in a deciduous forest ecosystem result in alterations in soil organic C and total N stocks?

This study sought to evaluate how soil organic carbon (SOC) and total nitrogen (STN) stocks vary following the conversion of a deciduous natural forest (DNF) to a paddy rice farm (PRF), a seventy-year-old Pinus koraiensis monoculture (CP70), a mixed forest (MF) plantation, and a P. koraiensis monoculture resulting from the re-plantation of a portion of the CP70 attacked by bugs. Soil samples (0–30 cm) were collected from DNF, PRF, CP7, CP70 and MF in the Gwangneung experimental site located in Gyeonggi-do (South Korea), and analyzed for bulk density, pH, SOC, STN content and stocks (0–30 cm). The conversion of DNF into PRF showed 23.9% SOC stocks and 54.5% STN stocks increases. However, there were no significant changes in STN or SOC stocks after DNF was reforested for seven or 70 years or when PRF was reforested to MF. The conversion of a DNF to a PRF increased the SOC and STN stock, which might be due to over 63 years’ fertilizations with urea and diammonium phosphate, but reforestation with a P. koraiensis monoculture for 70 years and/or mixed forest was enough time to restore SOC and STN stocks to natural levels.

A reactive nitrogen budget for forest land and wetlands in Latvia and Estonia

ABSTRACT Human intervention in global nitrogen cycling has led to excess reactive nitrogen (Nr) flows to the environment, impacting terrestrial ecosystems as well as the quality of the atmosphere and waterbodies. This paper presents the results of a study that quantified and compared the main flows of Nr through forest land and wetlands in Latvia and Estonia using substance flow analysis approach based on data from statistics, research results, reports, monitoring programmes, data bases and expert estimations. Input and output flows to and from the forest land and wetlands both in Latvia and Estonia are well balanced, with a trend to be a Nr sink (+1.2 kT N yr−1) for Latvia and a Nr source (−6.5 kT N yr−1) for Estonia. Major input flow to the studied pools in Estonia is a soil nitrogen stock used for forest growth, while in Latvia, it is atmospheric deposition and biological fixation. Major output flow of Nr in both countries occurs via wood and peat for industrial uses. Emissions of Nr to the atmosphere and hydrosphere from forest land and wetlands are significantly lower compared to human-related emissions such as industries, energy production, agriculture, human settlements, and transport.