Non-symmetric responses of leaf onset date to natural warming and cooling in northern ecosystems.

The northern hemisphere has experienced regional cooling, especially during the global warming hiatus (1998-2012) due to ocean energy redistribution. However, the lack of studies about the natural cooling effects hampers our understanding of vegetation responses to climate change. Using 15,125 ground phenological time series at 3,620 sites since the 1950s and 31-year satellite greenness observations (1982-2012) covering the warming hiatus period, we show a stronger response of leaf onset date (LOD) to natural cooling than to warming, i.e. the delay of LOD caused by 1°C cooling is larger than the advance of LOD with 1°C warming. This might be because cooling leads to larger chilling accumulation and heating requirements for leaf onset, but this non-symmetric LOD response is partially offset by warming-related drying. Moreover, spring greening magnitude, in terms of satellite-based greenness and productivity, is more sensitive to LOD changes in the warming area than in the cooling. These results highlight the importance of considering non-symmetric responses of spring greening to warming and cooling when predicting vegetation-climate feedbacks.

Comprehensive quantification of global cropland ammonia emissions and potential abatement.

Ammonia (NH3) emissions mostly from agriculture result in air pollution and degrade human health. However, a full picture of soil NH3 emissions and associated abatement in cropping systems are not well understood. Here we present a thorough analysis of cropland NH3 emissions, discuss mitigation potential and assess associated abatement costs. Global cropland NH3 emissions account for 26% of total soil nitrogen losses, and are estimated as 22.8-31.2 Tg N yr-1 during 1996-2013 with the increase rate of 1.6% yr-1. Our results also show that, with no increase in nitrogen fertilizer, climate change can contribute to an additional 10% increase in cropland NH3 emissions in 2100 compared to the 2010 baseline. Instead, our scenario analysis show, cropland NH3 emissions will decline by 26% from 2010 to 2100 given a 0.5% yr-1 decrease in N fertilizer (with current technology and agricultural management level), considering the facts stronger control policies are expected to occur worldwide including Western Europe, the United States of America and China. The most ambitious management (with all known mitigation practices) can reduce cropland NH3 emissions by up (71%, 17.6 Tg N yr-1) at an abatement cost of US$524 billion. Our findings indicate that cropland NH3 emissions can be mitigated through adoption of appropriate human management practices with considerable economic costs, providing a critical reference for the future NH3 abatement strategies.

Spatial quantification method of grassland utilization intensity on the Qinghai-Tibetan Plateau: A case study on the Selinco basin.

Existing methods for spatial quantification of grassland utilization intensity cannot meet the demand for accurate detection of the spatial distribution of grassland utilization intensity in the Qinghai-Tibetan Plateau with high spatial resolution. In this paper, a method based on remote-sensing observations and simulations of grassland growth dynamics is proposed. The grassland enhanced vegetation index (EVI) time-series curve during the growing season characterizes the growth of grassland in the corresponding pixel; The deviation between the observed and potential EVI curves indicates the disturbance on grassland growth imposed by human activities, and it can characterize the grassland utilization intensity during the growing season. Based on the main idea described above, absolute and relative disturbances are calculated and used as quantitative indicators of grassland utilization intensity defined from different perspectives. Livestock amount at the pixel scale is obtained by pixel-by-pixel calculations based on the function relationship at the township scale between absolute disturbance and livestock density, which is specific quantitative indicator that considers the mode of grassland utilization. In simulating the potential EVI of grassland, the lag and accumulation effects of meteorological factors are investigated at the daily scale using a multi-objective genetic algorithm. Further, the nonlinear functions between multiple environmental factors (e.g., grassland type, topography, soil, meteorology) and the grassland EVI are established using an error back-propagation feedforward artificial neural network (ANN-BP) with parameter optimization. Finally, the potential EVIs of all grassland pixels are simulated on the basis of this model. The method is applied to the Selinco basin on the Qinghai-Tibetan Plateau and validated by examining the spatial consistency of the results with township-scale livestock density and grazing pressure. The final results indicate that the proposed method can accurately detect the spatial distribution of grassland utilization intensity which is appliable in the similar regions.

Soil Nitrous Oxide Emissions by Atmospheric Nitrogen Deposition over Global Agricultural Systems.

Agricultural soil is the main source of nitrous oxide (N2O) emissions which contribute to global warming and stratospheric ozone depletion. In recent decades, atmospheric nitrogen (N) deposition has increased dramatically as an important agricultural soil N input, while its effect on soil N2O emissions in the current and future climate change remains unknown. Here, we conducted a thorough analysis of the effect of N deposition and climate change on soil N2O emissions as well as their trends. Soil N2O emissions induced by N deposition accounted for 25% of global cropland soil N2O emissions. Global soil N2O emissions over croplands increased by 2% yr-1 during 1996-2013, of which N deposition could explain 15% of the increase. The emission factor of N deposition was ∼7 times that of N fertilizer plus manure (∼1%) through a more direct way, since N deposition including nitrate (NO3-) and ammonium (NH4+) could be directly used for nitrification and denitrification. By 2100, N deposition will increase by 80% and cropland soil N2O emissions will increase by 241% under the RCP8.5 scenario in comparison with the 2010 baseline. These results suggest that, under the background of increasing global N deposition, it is essential to consider its effects on soil N2O emissions in climatic change studies.

Random and systematic change analysis in land use change at the category level-A case study on Mu Us area of China.

Random and systematic change analysis is gradually becoming a common method for effectively detecting land use change signals from land transition matrix, but most researches focus only on the change characteristics at the transition level. This paper attempted to distinguish random and systematic changes at the category level, and to clarify the meanings of these two types of changes, as well as their indicative significances of change causes. This paper first calculated the random expected value of change area at the category level, and the deviation of the actual change area from the expected value. Then we proposed a method for setting a threshold of the deviation to clearly distinguish random and systematic changes. This method could eliminate the influence of land use classification errors on the distinction. Through analyzing the mathematical formulas of random expected area, this paper further clarified the meanings of random and systematic changes as well as their indicative significances to change causes. Land use change in Mu Us area of China was used as a case study. Practice showed that detecting and analyzing the random and systematic signals at the category level could accurately determine the change trend of land use, which could help to explore the relationship between land use change and external influences, especially human activities.

Lagged precipitation effect on plant productivity is influenced collectively by climate and edaphic factors in drylands.

Lagged precipitation effect explains a large proportion of annual aboveground net primary productivity in some dryland ecosystems. Using satellite-derived plant productivity and precipitation datasets in the Northern Hemisphere drylands during 2000-2018, we identify 1111 pixels mainly located in the Tibetan Plateau, the western US, and Kazakhstan where productivities are significantly correlated with previous-year precipitation (hereafter, the lagged type). Differences in climatic and edaphic factors between the lagged and unlagged (pixels where productivities are not correlated with previous-year precipitation) types are evaluated. Permutational multivariate analysis of variance shows that the two types differ significantly regarding six climatic and edaphic factors. Compared to unlagged type, water availability, soil organic carbon, total nitrogen, field capacity, silt content and radiation are more sensitive to changes in precipitation in lagged type. Water availability is the most important factor for distinguishing the two types, followed by soil organic carbon, total nitrogen, field capacity, soil texture, and radiation. Our study suggests that the altered sensitivities of several climatic and edaphic factors to precipitation collectively affect the lagged effect of precipitation on productivity in drylands.

Enhanced nitrous oxide emissions caused by atmospheric nitrogen deposition in agroecosystems over China.

Atmospheric nitrogen (N) deposition in China has been the largest worldwide. Yet the impacts of atmospheric N deposition on soil N2O emissions were often ignored by previous studies. Thus, we investigated how N deposition affected N2O emissions over China using the process-based model (DNDC, DeNitrification-DeComposition). Total soil N inputs were 194 kg N ha-1 in agricultural systems over China in 2010, including chemical N fertilizer (78%), atmospheric N deposition (12%), and crop residues N (10%). Annual N2O emissions induced by N deposition were estimated at 97 Gg N, occupying 43% of total soil N2O emissions (228 Gg N) in agricultural systems over China. In particular, the largest N2O emissions caused by atmospheric N deposition were found in South China, followed by North China Plain and Southwest China. The efficiency of N deposition generating N2O emissions (3.0%) over China was 4 times than that of N fertilizer (0.7%). N2O emissions induced by N deposition increased from 81 Gg in 2000 to 93 Gg in 2014 (by 1% yr-1), which was consistent with the long-term trend of N deposition. This suggests N deposition accelerated soil N2O emissions largely contributing to global warming. Our results also indicated that 62% and 10% of soil N2O emissions were reduced by applying a nitrification inhibitor and N fertilizer with 20% decrease. We highlight the significance of considering N deposition in determining total soil N2O emissions over China. The results provide an important scientific basis for the prediction of greenhouse effect caused by N deposition over China.

The constraints and driving forces of oasis development in arid region: a case study of the Hexi Corridor in northwest China.

The oasis, a special landscape with the integration of nature and humanity in the arid region, has undergone an enormous transformation during the past decades. To gain a better understanding of the tradeoff between economic growth and oases stability in the arid land, we took the oases in the Hexi Corridor as a case to explore the constraints of oases development and the driving factors of oases expansion. The dynamic changes and spatial distribution patterns underwent by the oases were examined using multispectral remote sensing imagery. The constraints of oasis development in arid land were investigated by the grid-transformed model, as well as the index system of driving forces was analyzed using the grey incidence model based on the data from statistics yearbooks. The oasis area in the Hexi Corridor had tremendous changes expanded 40% from 1986 to 2015, the stable oasis area was 9062 km2, while the maximum area reached 16,374 km2. The constraints for oases of topography, hydrology and heat condition are as follow: The elevation of oasis ranged from 1000 to 1800 m, peaked in 1500 m; the slope of oasis distribution was flatter than 3 degrees; the aspect of oases on slope land concentrated in northeast and north, accounting for more than 60%. The main driving forces of oasis spatial expansion in the arid region were population, water resource, economy, policies, and other factors. These results are expected to (1) improve the rationality of oasis development, and (2) promote the sustainable planning and management of oases in the arid land.

The critical role of local policy effects in arid watershed groundwater resources sustainability: A case study in the Minqin oasis, China.

Designed as a watershed groundwater restoration policy (WGRP), the Comprehensive Treatment Program of the Shiyang River Basin (CTSRB) was launched in 2006 to restore the groundwater resources in the Minqin oasis, northwestern China. This study sought to verify the recovery effects of CTSRB implementation from the perspective of groundwater depth. We reconstructed the spatio-temporal distribution of groundwater depth at interannual and pixel scales by using digital groundwater depth models (DGDMs), based on the ordinary kriging interpolation method. Using DGDMs data, various measures of the groundwater table (e.g., regional depths, surface areas, depletion cones, and conditions in irrigated regions including Ba, Quanshan, and Hu) were quantitatively analyzed and compared for the pre-CTSRB (2001-2006), CTSRB I (2006-2010), and CTSRB II (2010-2015) periods, for which spatial trends in the annual amplitudes of groundwater depth were compared. Finally, strategies that impacted the groundwater behavior before and during the CTSRB periods, possible indirect and adverse effects, and long-term strategies and prospects were discussed. The results showed that groundwater depth first declined sharply, before increasing slowly and stabilizing after implementation of the CTSRB. Areas of greater groundwater depth (<-20m) and four groundwater depletion cones expanded during the pre-CTSRB period, whereas variable shrinking trends were detected during the CTSRB period. Spatial analysis showed that groundwater recovery mainly occurred along the periphery of the three irrigated regions, among which recovery effects in Hu were more obvious than those in Quanshan and Ba, with pumping-well densities the main reason for the difference. Therefore, various strategies (increasing the surface water supply, reducing groundwater mining, and some other auxiliary measures) of CTSRB together supported groundwater recovery in the Minqin oasis. Overall, this research demonstrates an innovative perspective to verify the effects of WGRPs in arid and semi-arid areas.