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.

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.

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.

Current Situation and Key Parameters for Improving Wheat Quality in China.

Processing quality of winter-wheat is affected by genotype, environmental conditions, and crop husbandry practices. In the present study, a data set of 17 quality-related traits for 211 main winter-wheat varieties in China during 2006 to 2018 was extracted from China Wheat Quality Report. Analysis was carried out to evaluate the quality status and variations, to reveal correlation between quality-related traits, as well as to identify key influencing factors. Results indicated that the quality indicators of medium-gluten or medium-strong-gluten wheat varieties were acceptable, whereas those of weak- and strong-gluten wheat varieties were far below national standard, especially hardness index (HI), crude protein content (CPC), wet gluten content (WG), and water absorption for weak-gluten wheat and sedimentation value (SV), stability time (ST), and stretch area (SA) for strong-gluten wheat, respectively. Correlation analysis showed that WA, WG, development time, HI, CPC, falling number, ST, and tractility directly affected the overall quality of winter-wheat. CPC, SV, and WG in medium-gluten wheat had no significant correlation with the processing quality of noodles score, whereas gluten index significantly correlated with noodle score (P < 0.001). This implied that protein quality might play a more important role than protein quantity in determining medium-gluten wheat quality. Furthermore, analysis of variance showed that genetic characteristics (cultivars) had significant influences on the restriction indexes (SV, ST, and SA) of strong-gluten wheat, whereas genetic characteristics, environment conditions, and crop growing practices (cultivars, locations, and years) significantly affected the restriction indexes (HI, CPC, WG, and WA) of weak-gluten wheat. The results suggest that improvement of Chinese strong-gluten wheat should mainly focus on cultivating new varieties. As to weak-gluten wheat, cultivation and husbandry practices should be paid more attention to limit undesired high grain protein content.

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.

[Effects of dicyandiamide combined with nitrogen fertilizer on N2O emission and economic benefit in winter wheat and summer maize rotation system].

Aiming at the problems of excessive and unreasonable fertilizer application, lower nitrogen use efficiency, increasing N2O emission from soil and fertilizer in current intensified agricultural productions, a field experiment was conducted to study the effects of dicyandiamide (DCD) combined with nitrogen fertilizer application at different levels, i.e., 150, 225, 300 kg . hm-2, on N20 emission and relevant economic benefit in a typical winter wheat-summer maize rotation system in North China Plain. The results showed that DCD application decreased N2O emission fluxes and cumulative emissions by 25.6%-32.1% and 23.1%-31.1% in the year-round. There was a significant positive exponential correlation between N2O flux and soil surface temperature or soil moisture content. The effect of soil moisture on N2O emission was stronger in wheat season than in maize season, while the effect of temperature on N2O emission was on the contrary. The yields of winter wheat and summer maize with DCD addition were increased by 16.7%-24.6% and 29.8%-34.5%, respectively, and the average economic income of two seasons was increased by 7973.2 yuan . hm-2. Therefore, appropriate rate of N fertilizer combined with DCD could not only increase crop yield and economic income, but also reduce N2O emission. Considering environmental and economic benefit under this experimental condition, DCD combined with nitrogen of moderate level (total N amount 225 kg . hm-2) was a good nitrogen management mode in North China.