Widespread cooling of topsoil under nitrogen enrichment and implication for soil carbon flux.

Increasing reactive nitrogen (N) to terrestrial ecosystems is considered to enhance ecosystem carbon sink, which plays a critical role in ameliorating global warming. Besides this indirect buffering of temperature rise, the N-induced enhancement of vegetation growth may exert a biophysical cooling effect on soils. However, the magnitude and drivers of this cooling effect have rarely been evaluated. Here, using a global meta-analysis with 321 paired measurements, we demonstrated a widespread topsoil cooling (-0.30 °C in average) under anthropogenic N enrichment, which was primarily associated with the increase in aboveground biomass. This biophysical cooling could also buffer topsoil temperature rise by an average of 0.39 °C under experimental warming. Further, the reduced soil temperature was found to contribute to a reduction of soil respiration rate as temperature declines gradually. Overall, our results underpin a previously overlooked function of global N enrichment-the lowering of topsoil temperature, which suggests that the warming of topsoil may not be as fast as previously predicted under future global change scenarios. This biophysical cooling effect will also slow down soil carbon emissions and further mitigate climate warming.

Spatially explicit estimate of nitrogen effects on soil respiration across the globe.

Soil respiration (Rs), as the second largest flux of carbon dioxide (CO2 ) between terrestrial ecosystems and the atmosphere, is vulnerable to global nitrogen (N) enrichment. However, the global distribution of the N effects on Rs remains uncertain. Here, we compiled a new database containing 1282 observations of Rs and its heterotrophic component (Rh) in field N manipulative experiments from 317 published papers. Using this up-to-date database, we first performed a formal meta-analysis to explore the responses of Rs and Rh to N addition, and then presented a global spatially explicit quantification of the N effects using a Random Forest model. Our results showed that experimental N addition significantly increased Rs but had a minimal impact on Rh, not supporting the prevailing view that N enrichment inhibits soil microbial respiration. For the major biomes, the magnitude of N input was the main determinant of the spatial variation in Rs response, while the most important predictors for Rh response were biome specific. Based on the key predictors, global mapping visually demonstrated a positive N effect in the regions with higher anthropogenic N inputs (i.e., atmospheric N deposition and agricultural fertilization). Overall, our analysis not only provides novel insight into the N effects on soil CO2 fluxes, but also presents a spatially explicit assessment of the N effects at the global scale, which are pivotal for understanding ecosystem carbon dynamics in future scenarios with more frequent anthropogenic activities.

Nitrogen availability determines ecosystem productivity in response to climate warming.

One of the major uncertainties for carbon-climate feedback predictions is an inadequate understanding of the mechanisms governing variations in ecosystem productivity response to warming. Temperature and water availability are regarded as the primary controls over the direction and magnitude of warming effects, but some unexplained results signal that our understanding is incomplete. Using two complementary meta-analyses, we present evidence that soil nitrogen (N) availability drives the warming effects on ecosystem productivity more strongly than thermal and hydrological factors over a broad geographical scale. First, by synthesizing temperature manipulation experiments, a meta-regression model analysis showed that the warming effect on productivity is mainly driven by its effect on soil N availability. Sites with a higher warming-induced increase in N availability were characterized by stronger productivity enhancement and vice versa, suggesting that N is a limiting factor across sites. Second, a synthesis of full-factorial warming × N addition experiments demonstrated that N addition significantly weakened the positive warming effect, because the additional N induced by warming may not further benefit plant growth when N limitation is relieved, providing experimental evidence that N regulates the warming effect. Furthermore, we demonstrated that warming effects on soil N availability were modulated by changes in dissolved organic N and soil microbes. Overall, our findings enrich a new mechanistic understanding of the varying magnitudes of observed productivity response to warming, and the N scaling of warming effects may help to constrain climate projections.

[Response of root dynamics to nitrogen addition and the influencing factors in a Tibetan alpine steppe, China]. / é’è—é«˜åŽŸé«˜å¯’è‰åŽŸæ ¹ç³»åŠ¨æ€å¯¹æ°®æ·»åŠ çš„å“åº”åŠå ¶è°ƒæŽ§å› ç´ .

A field manipulative experiment was carried out during 2015 and 2016 to examine the changes and influencing factors of root production, turnover rate, and standing crop under different nitrogen (N) addition levels, i.e., 0, 1, 2, 4, 8 and 16 g N·m-2·a-1, in a Tibetan alpine steppe. The results showed that root production and standing crop decreased linearly or exponentially with increasing N addition rates. Compared with control, 16 g N·m-2·a-1 significantly reduced the two-year average root production and standing crop by 43.0% and 45.7%, respectively. Root turnover rate increased first and then decreased along the N addition gradient, with the maximum appearing under 2 and 4 g N·m-2·a-1 treatments for 2015 and 2016, respectively. Results from linear mixed-effects models showed that root starch content was the main factor modulating the N-induced changes in root production and turnover rate, explaining 21.7% and 25.4% of their variations. Root protein content mainly contributed to the variations in standing crop, with an explanation of 20.8% of its variance. Overall, N addition had negative effect on root production and standing crop, and low N promoted while high N inhibited root turnover rate. Root metabolic parameters were the main factors modulating the N-induced changes in root dynamics.

Response of soil N2O emission and nitrogen utilization to organic matter in the wheat and maize rotation system.

The appropriate nitrogen (N) fertilizer regulator could increase N utilization of crops and reduce N losses in the North China Plain. We investigated the effects of reduced inorganic-N rate combined with an organic fertilizer on nitrous oxide (N2O) emissions in winter wheat and summer maize rotation system. Simultaneously studied the effect of different treatments on N use efficiency (NUE), N balance and net income. After reducing the amount of nitrogen fertilizer in the wheat-corn rotation system, the results showed that the cumulative emission of soil N2O from the RN40% + HOM [40% of RN (recommended inorganic-N rate) with homemade organic matter] treatment was 41.0% lower than that of the RN treatment. In addition, the N production efficiency, agronomic efficiency, and apparent utilization were significantly increased by 50.2%, 72.4% and 19.5% than RN, respectively. The use of RN40% + HOM resulted in 22.0 and 30.1% lower soil N residual and N losses as compared with RN. After adding organic substances, soil N2O cumulative emission of RN40% + HOM treatment decreased by 20.9% than that of the HAN (zinc and humic acid urea at the same inorganic-N rate of RN) treatment. The N production efficiency, N agronomic efficiency and NUE of RN40% + HOM treatment were 36.6%, 40.9% and 15.3% higher than HAN's. Moreover, soil residual and apparent loss N were 23.3% and 18.0% less than HAN's. The RN40% + HOM treatment appears to be the most effective as a fertilizer control method where it reduced N fertilizer input and its loss to the environment and provided the highest grain yield.

Responses of Nitrogen Utilization and Apparent Nitrogen Loss to Different Control Measures in the Wheat and Maize Rotation System.

Nitrogen (N) is an essential macronutrient for plant growth and excessive application rates can decrease crop yield and increase N loss into the environment. Field experiments were carried out to understand the effects of N fertilizers on N utilization, crop yield and net income in wheat and maize rotation system of the North China Plain (NCP). Compared to farmers' N rate (FN), the yield of wheat and maize in reduction N rate by 21-24% based on FN (RN) was improved by 451 kg ha-1, N uptakes improved by 17 kg ha-1 and net income increased by 1671 CNY ha-1, while apparent N loss was reduced by 156 kg ha-1. The controlled-release fertilizer with a 20% reduction of RN (CRF80%), a 20% reduction of RN together with dicyandiamide (RN80%+DCD) and a 20% reduction of RN added with nano-carbon (RN80%+NC) all resulted in an improvement in crop yield and decreased the apparent N losses compared to RN. Contrasted with RN80%+NC, the total crop yield in RN80%+DCD improved by 1185 kg ha-1, N uptake enhanced by 9 kg ha-1 and net income increased by 3929 CNY ha-1, while apparent N loss was similar. Therefore, a 37-39% overall decrease in N rate compared to farmers plus the nitrification inhibitor, DCD, was effective N control measure that increased crop yields, enhanced N efficiencies, and improved economic benefits, while mitigating apparent N loss. There is considerable scope for improved N use effieincy in the intensive wheat -maize rotation of the NCP.

[Assessment and early warning of land ecological security in rapidly urbanizing coastal area: A case study of Caofeidian new district, Hebei, China].

Assessment and early warning of land ecological security (LES) in rapidly urbanizing coastal area is an important issue to ensure sustainable land use and effective maintenance of land ecological security. In this study, an index system for the land ecological security of Caofeidian new district was established based on the Pressure-State-Response (P-S-R) model. Initial assessment units of 1 km x 1 km created with the remote sensing data and GIS methods were spatially interpolated to a fine pixel size of 30 m x 30 m, which were combined with the early warning method (using classification tree method) to evaluate the land ecological security of Caofeidian in 2005 and 2013. The early warning level was classed into four categories: security with degradation potential, sub-security with slow degradation, sub-security with rapid degradation, and insecurity. Result indicated that, from 2005 to 2013, the average LES of Caofeidian dropped from 0.55 to 0.52, indicating a degradation of land ecological security from medium security level to medium-low security level. The areas at the levels of insecurity with rapid degradation were mainly located in the rapid urbanization areas, illustrating that rapid expansion of urban construction land was the key factor to the deterioration of the regional land ecological security. Industrial District, Shilihai town and Nanpu saltern, in which the lands at the levels of insecurity and sub-security with rapid degradation or slow degradation accounted for 58.3%, 98.9% and 81.2% of their respective districts, were at the stage of high early warning. Thus, land ecological security regulation for these districts should be strengthened in near future. The study could provide a reference for land use planning and ecological protection of Caofeidian new district.