• Energy Research

Land use changes from cropland to orchards in Eastern China have raised serious concerns about the regional nitrogen (N) cycle and greenhouse gas balance. We measured soil nitrous oxide (N2O) emissions and methane (CH4) uptake using manual static chambers in an apple orchard. The primary aims were to assess the effect of N fertilizer application on gas fluxes and quantify the site-specific N2O emission factor (EFd). Field experiments were arranged in a randomized block design with three N input rates (0, 800 and 2600/2000 kg N ha−1 year−1). We found that orchard soils were a negligible CH4 sink (−1.1 to −0.4 kg C ha−1 year−1). Annual N2O emissions responded positively to N input rates, ranging from 34.1 to 60.3 kg N ha−1 year−1. EFd ranged from 1.00% to 1.65% with a mean of 1.34%. The extremely large background emissions of N2O (34.1–34.3 kg N ha−1 year−1) most likely originated from nitrate accumulation in the soil profile because of historical overuse of N fertilizer. We conclude that (1) site-specific EFd is suitable for assessing regional direct N2O emissions from upland orchards; and (2) conventional fertilization regimes must be avoided, and reduced N input rates are recommended in the study region.

AbstractGlobally, about 50% of all arable soils are classified as acidic. As crop and plant growth are significantly hampered under acidic soil conditions, many farmers, but increasingly as well forest managers, apply lime to raise the soil pH. Besides its direct effect on soil pH, liming also affects soil C and nutrient cycles and associated greenhouse gas (GHG) fluxes. In this meta‐analysis, we reviewed 1570 observations reported in 121 field‐based studies worldwide, to assess liming effects on soil GHG fluxes and plant productivity. We found that liming significantly increases crop yield by 36.3%. Also, soil organic C (SOC) stocks were found to increase by 4.51% annually, though soil respiration is stimulated too (7.57%). Moreover, liming was found to reduce soil N2O emission by 21.3%, yield‐scaled N2O emission by 21.5%, and CH4 emission and yield‐scaled CH4 emission from rice paddies by 19.0% and 12.4%, respectively. Assuming that all acid agricultural soils are limed periodically, liming results in a total GHG balance benefit of 633−749 Tg CO2‐eq year−1 due to reductions in soil N2O emissions (0.60−0.67 Tg N2O‐N year−1) and paddy soil CH4 emissions (1.75−2.21 Tg CH4 year−1) and increases in SOC stocks (65.7–110 Tg C year−1). However, this comes at the cost of an additional CO2 release (c. 624–656 Tg CO2 year−1) deriving from lime mining, transport and application, and lime dissolution, so that the overall GHG balance is likely neutral. Nevertheless, liming of acid agricultural soils will increase yields by at least 6.64 × 108 Mg year−1, covering the food supply of 876 million people. Overall, our study shows for the first time that a general strategy of liming of acid agricultural soils is likely to result in an increasing sustainability of global agricultural production, indicating the potential benefit of liming acid soils for climate change mitigation and food security.

La connaissance de la biomasse des prairies et de sa dynamique est essentielle pour étudier les cycles régionaux du carbone et pour l'utilisation durable des ressources des prairies. Dans cette étude, nous avons étudié la variation spatio-temporelle de la biomasse dans les prairies de Xilingol du nord de la Chine. Des échantillons de biomasse sur le terrain et des ensembles de données de séries chronologiques MODIS ont été utilisés pour établir deux modèles empiriques basés sur la relation de l'indice de végétation à différence normalisée (NDVI) avec la biomasse aérienne (AGB) ainsi que celle de l'AGB avec la biomasse souterraine (BGB). Nous avons exploré plus en détail les contrôles climatiques de ces variations. Nos résultats ont montré que la biomasse était en moyenne de 99,01 Tg (1 Tg=10(12) g) sur une superficie totale de 19,6 × 10(4) km(2) et a fluctué sans tendance significative de 2001 à 2012. La densité moyenne de biomasse était de 505,4 g/m(2), avec 62,6 g/m(2) dans l'AGB et 442,8 g/m(2) dans le BGB, qui diminuait généralement du nord-est au sud-ouest et présentait une grande hétérogénéité spatiale. Le modèle d'AGB d'une année à l'autre était généralement cohérent avec la variation interannuelle des précipitations de la saison de croissance (SPG), montrant une corrélation positive robuste (R(2)=0,82, P<0,001), mais un modèle couplé opposé a été observé avec la température de la saison de croissance (GST) (R(2)=0,61, P=0,003). Les facteurs climatiques ont également affecté la distribution spatiale de l'AGB, qui a augmenté progressivement avec le gradient SPG (R(2)=0,76, P<0,0001) mais a diminué avec une augmentation de la TPS (R(2)=0,70, P<0,0001). Un indice d'humidité amélioré qui combinait les effets de la TPS et du SPG expliquait plus de variation de l'AGB que les précipitations seules (R(2)=0,81, P<0,0001). La relation entre AGB et GSP pourrait être ajustée par une fonction de puissance. Cette pente croissante des relations SPG-AGB le long du gradient SPG peut s'expliquer en partie par la configuration spatiale SPG-SGT dans Xilingol. Nos résultats suggèrent que les relations entre les facteurs climatiques et l'AGB peuvent dépendre de l'échelle et que des études multi-échelles et des données de terrain à long terme suffisantes sont nécessaires pour examiner les relations entre l'AGB et les facteurs climatiques. El conocimiento sobre la biomasa de los pastizales y su dinámica es fundamental para estudiar los ciclos regionales del carbono y para el uso sostenible de los recursos de los pastizales. En este estudio, investigamos la variación espacio-temporal de la biomasa en los pastizales de Xilingol del norte de China. Las muestras de biomasa basadas en campo y los conjuntos de datos de series de tiempo MODIS se utilizaron para establecer dos modelos empíricos basados en la relación del índice de vegetación de diferencia normalizada (NDVI) con la biomasa aérea (AGB), así como el de AGB con la biomasa subterránea (BGB). Exploramos más a fondo los controles climáticos de estas variaciones. Nuestros resultados mostraron que la biomasa promedió 99.01 Tg (1 Tg=10(12) g) en un área total de 19.6 × 10(4) km(2) y fluctuó sin una tendencia significativa de 2001 a 2012. La densidad media de biomasa fue de 505.4 g/m(2), con 62.6 g/m(2) en AGB y 442.8 g/m(2) en BGB, que generalmente disminuyó de noreste a suroeste y exhibió una gran heterogeneidad espacial. El patrón de AGB año a año fue generalmente consistente con la variación interanual en la precipitación de la temporada de crecimiento (GSP), mostrando una correlación positiva robusta (R(2)=0.82, P<0.001), pero se observó un patrón acoplado opuesto con la temperatura de la temporada de crecimiento (GST) (R(2)=0.61, P=0.003). Los factores climáticos también afectaron la distribución espacial de AGB, que aumentó progresivamente con el gradiente de GSP (R(2)=0.76, P<0.0001) pero disminuyó con un GST creciente (R(2)=0.70, P<0.0001). Un índice de humedad mejorado que combinó los efectos de GST y GSP explicó más variación en AGB que la precipitación sola (R(2)=0.81, P<0.0001). La relación entre AGB y GSP podría ajustarse mediante una función de potencia. Esta pendiente creciente de las relaciones GSP-AGB a lo largo del gradiente GSP puede explicarse en parte por el patrón espacial GST-GSP en Xilingol. Nuestros hallazgos sugieren que las relaciones entre los factores climáticos y la AGB pueden depender de la escala y que se necesitan estudios multiescala y suficientes datos de campo a largo plazo para examinar las relaciones entre la AGB y los factores climáticos. Knowledge about grassland biomass and its dynamics is critical for studying regional carbon cycles and for the sustainable use of grassland resources. In this study, we investigated the spatio-temporal variation of biomass in the Xilingol grasslands of northern China. Field-based biomass samples and MODIS time series data sets were used to establish two empirical models based on the relationship of the normalized difference vegetation index (NDVI) with above-ground biomass (AGB) as well as that of AGB with below-ground biomass (BGB). We further explored the climatic controls of these variations. Our results showed that the biomass averaged 99.01 Tg (1 Tg=10(12) g) over a total area of 19.6 × 10(4) km(2) and fluctuated with no significant trend from 2001 to 2012. The mean biomass density was 505.4 g/m(2), with 62.6 g/m(2) in AGB and 442.8 g/m(2) in BGB, which generally decreased from northeast to southwest and exhibited a large spatial heterogeneity. The year-to-year AGB pattern was generally consistent with the inter-annual variation in the growing season precipitation (GSP), showing a robust positive correlation (R(2)=0.82, P<0.001), but an opposite coupled pattern was observed with the growing season temperature (GST) (R(2)=0.61, P=0.003). Climatic factors also affected the spatial distribution of AGB, which increased progressively with the GSP gradient (R(2)=0.76, P<0.0001) but decreased with an increasing GST (R(2)=0.70, P<0.0001). An improved moisture index that combined the effects of GST and GSP explained more variation in AGB than did precipitation alone (R(2)=0.81, P<0.0001). The relationship between AGB and GSP could be fit by a power function. This increasing slope of the GSP-AGB relationships along the GSP gradient may be partly explained by the GST-GSP spatial pattern in Xilingol. Our findings suggest that the relationships between climatic factors and AGB may be scale-dependent and that multi-scale studies and sufficient long-term field data are needed to examine the relationships between AGB and climatic factors. تعد المعرفة بالكتلة الحيوية للأراضي العشبية وديناميكياتها أمرًا بالغ الأهمية لدراسة دورات الكربون الإقليمية والاستخدام المستدام لموارد الأراضي العشبية. في هذه الدراسة، بحثنا في التباين المكاني والزماني للكتلة الحيوية في مراعي زيلينغول في شمال الصين. تم استخدام عينات الكتلة الحيوية الميدانية ومجموعات بيانات السلاسل الزمنية لـ MODIS لإنشاء نموذجين تجريبيين بناءً على علاقة مؤشر الاختلاف الطبيعي للغطاء النباتي (NDVI) بالكتلة الحيوية فوق الأرض (AGB) بالإضافة إلى مؤشر الكتلة الحيوية تحت الأرض (BGB). كما استكشفنا الضوابط المناخية لهذه الاختلافات. أظهرت نتائجنا أن متوسط الكتلة الحيوية بلغ 99.01 تيراغرام (1 تيراغرام=10(12) غرام) على مساحة إجمالية قدرها 19.6 × 10(4) كم(2) وتذبذبت مع عدم وجود اتجاه كبير من عام 2001 إلى عام 2012. كان متوسط كثافة الكتلة الحيوية 505.4 جم/م(2)، مع 62.6 جم/م(2) في AGB و 442.8 جم/م(2) في BGB، والتي انخفضت بشكل عام من الشمال الشرقي إلى الجنوب الغربي وأظهرت عدم تجانس مكاني كبير. كان نمط AGB من سنة إلى أخرى متسقًا بشكل عام مع التباين بين السنوات في هطول الأمطار في موسم النمو (GSP)، مما يدل على وجود ارتباط إيجابي قوي (R(2)=0.82، P<0.001)، ولكن لوحظ وجود نمط مقترن معاكس مع درجة حرارة موسم النمو (GST) (R(2)=0.61، P=0.003). أثرت العوامل المناخية أيضًا على التوزيع المكاني لـ AGB، والذي زاد تدريجيًا مع تدرج GSP (R(2)=0.76، P<0.0001) ولكنه انخفض مع زيادة GST (R(2)=0.70، P<0.0001). أوضح مؤشر الرطوبة المحسن الذي جمع بين تأثيرات ضريبة السلع والخدمات ونظام الأفضليات المعمم تباينًا أكبر في AGB مقارنة بهطول الأمطار وحده (R(2)=0.81، P<0.0001). يمكن أن تتناسب العلاقة بين AGB و GSP من خلال وظيفة الطاقة. يمكن تفسير هذا الانحدار المتزايد لعلاقات GSP - AGB على طول تدرج نظام الأفضليات المعمم جزئيًا بالنمط المكاني لنظام الأفضليات المعمم في Xilingol. تشير النتائج التي توصلنا إليها إلى أن العلاقات بين العوامل المناخية و AGB قد تعتمد على المقياس وأن هناك حاجة إلى دراسات متعددة المقاييس وبيانات ميدانية كافية على المدى الطويل لفحص العلاقات بين AGB والعوامل المناخية.

A 3-year field experiment (October 2004-October 2007) was conducted to quantify N2O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

The direct measurement of denitrification dynamics and its product fractions is important for parameterizing process-oriented model(s) for nitrogen cycling in various soils. The aims of this study are to a) directly measure the denitrification potential and the fractions of nitrogenous gases as products of the process in laboratory, b) investigate the effects of the nitrate (NO 3 − ) concentration on emissions of denitrification gases, and c) test the hypothesis that denitrification can be a major pathway of nitrous oxide (N2O) and nitric oxide (NO) production in calcic cambisols under conditions of simultaneously sufficient supplies of carbon and nitrogen substrates and anaerobiosis as to be found to occur commonly in agricultural lands. Using the helium atmosphere (with or without oxygen) gas-flow-soil-core technique in laboratory, we directly measured the denitrification potential of a silt clay calcic cambisol and the production of nitrogen gas (N2), N2O and NO during denitrification under the conditions of seven levels of NO 3 − concentrations (ranging from 10 to 250 mg N kg−1 dry soil) and an almost constant initial dissolved organic carbon concentration (300 mg C kg−1 dry soil). Almost all the soil NO 3 − was consumed during anaerobic incubation, with 80–88 % of the consumed NO 3 − recovered by measuring nitrogenous gases. The results showed that the increases in initial NO 3 − concentrations significantly enhanced the denitrification potential and the emissions of N2 and N2O as products of this process. Despite the wide range of initial NO 3 − concentrations, the ratios of N2, N2O and NO products to denitrification potential showed much narrower ranges of 51–78 % for N2, 14–36 % for N2O and 5–22 % for NO. These results well support the above hypothesis and provide some parameters for simulating effects of variable soil NO 3 − concentrations on denitrification process as needed for biogeochemical models.

A large number of natural wetlands in northeast China have been reclaimed as farmland in the last few decades, and soybean is the main rain-fed crop here. For the depth understanding of nitrous oxide (N2O) emission from reclaimed soybean fields, using static opaque chamber method, we conducted a four-year N2O flux measurement at two adjacent soybean fields cultivated after wetland drainage in 1987 and 1993, respectively, in the Sanjiang Plain of northeast China Using static opaque chamber method,. Both sites had two treatments including soybean cropped and bare soils (i.e., SF87, BS87, SF93 and BS93). The results showed that soil N2O emission from all of the plots was severely inhibited by the low temperature in winter (November to March), while a N2O emission pulse occurred during the spring thaw (April and May). Temporal variation of the N2O fluxes during the growing season varied over all the four years but was mainly affected by soil water-filled pore space (WFPS). Intense rainfall events increased the intensity and duration of N2O pulses during the growing season, and most high fluxes were occurred at WFPS > 45%. The mean annual N2O emission from all treatments over four years was 4.8 ± 1.2 kg N ha-1 (ranges: 1.9-19.8), and one third of the emission originated from the spring-thaw. In addition, soybean growth did not increase N2O emissions during the growing season, which support the cancellation of N2O emission calculations from nitrogen fixed by legumes in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.

The alpine meadow ecosystem is one of the major vegetation biomes on the Qinghai-Tibetan Plateau, which hold substantial quantities of soil organic carbon. Pronounced grassland degradations (induced by overgrazing/climate change and further exacerbated by the subterranean rodent activities) that have widely occurred in this ecosystem may significantly alter the non-growing season carbon turnover processes such as carbon dioxide (CO2) efflux, but little is known about how the non-growing season CO2 emissions respond to the degradation (particularly the exacerbated degradations by plateau zokor), as most previous studies have focused primarily on the growing season. In this study, the effects of four degradation levels (i.e., the healthy meadow (HM), degraded patches (DP), 2-year-old zokor mounds (ZM2), and current-year zokor mounds (ZM1)) on CO2 emissions and corresponding environmental and agronomic variables were investigated over the two non-growing seasons under contrasting climatic conditions (a normal season in 2013-2014 and a "warm and humid" season in 2014-2015). The temporal variation in the non-growing season CO2 emissions was mainly regulated by soil temperature, while increasing degradation levels reduced the temperature sensitivity of CO2 emissions due to a reduction in soil water content. The cumulative CO2 emissions across the non-growing season were 587-1283 kg C ha-1 for all degradation levels, which varied significantly (p < 0.05) interannually. The degradation of alpine meadows significantly (p < 0.05) reduced the vegetation cover and aboveground net primary productivity as well as the belowground biomass, which are typically thought to decrease soil CO2 emissions. However, the non-growing season CO2 emissions for the degraded meadow, weighted by the areal extent of the DP, ZM2, and ZM1, were estimated to be 641-1280 kg C ha-1, which was significantly higher (p < 0.05) as compared with the HM in the warm and humid season of 2014-2015 but not in the normal season of 2013-2014. Additionally, grassland degradation substantially increased the productivity-scaled non-growing season CO2 emissions, which showed an exponential trend with increasing degradation levels. These results suggest that there is a strong connection between grassland degradation and soil carbon loss, e.g., in the form of CO2 release, pointing to the urgent need to manage degraded grassland restoration that contributes to climate change mitigation.

AbstractMaintaining or even increasing crop yields while reducing nitrous oxide (N2O) emissions is necessary to reconcile food security and climate change, while the metric of yield‐scaled N2O emission (i.e., N2O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta‐analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield‐scaled N2O emissions for maize, wheat and rice and associated potential mitigation options. Our results showed that the average yield‐scaled N2O emissions across all available data followed the order wheat (322 g N Mg−1, with the 95% confidence interval [CI]: 301–346) > maize (211 g N Mg−1, CI: 198–225) > rice (153 g N Mg−1, CI: 144–163). Yield‐scaled N2O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield‐scaled N2O emissions with increasing N use efficiency or in N2O emissions for production systems with cereal yields >10 Mg ha−1 (maize), 6.6 Mg ha−1 (wheat) or 6.8 Mg ha−1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade‐offs between crop production and N2O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced‐efficiency N fertilizers significantly reduced yield‐scaled N2O emissions at similar or even higher cereal yields. Our data‐driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production.