Increased C and N substrates can supply more essential substrates for N cycling microorganisms. For instance, Song et al. demonstrated the importance of substrate availability to fast growth of temperature-sensitive N2O producing microorganisms. The microbiome shift was closely associated with fast N mineralization at warm temperatures, resulting in increased N2O emissions. The increased microbial mineralization can produce more CO2, leading to O2 depletion , and eventually accelerated denitrification . The tropical zone with the highest annual temperature/rainfall and microbial activities had lower N2O emission compared to the warm temperate zone . We attribute this to accelerated completion of denitrification , C substrate loss and less accumulation of inorganic N. High rainfall may create wet and O2 limited conditions , which can accelerate completion of the denitrification process by converting N2O to N2 . Heavy rains may also transport C/N substrates and N2O formation deeper into the soil profile, where relatively more N2O can be consumed before it escapes to the atmosphere. Further, N cycling in tropical systems is generally very efficient between the soil and vegetation, which limits the accumulation of NH4 + and NO3 − in the soil thereby attenuating nitrification and denitrification processes. Hence, lower N2O emission was observed in tropical compared to warm temperate climates . With respect to crop type, our analysis showed that manure application increased N2O emission in soils of all upland crops, except for beans . A lack of enhanced N2O emissions from paddy rice cultivation following manure application was also noted and attributed to: the dominantly anaerobic conditions associated with paddy rice cultivation that limits nitrification and promotes conversion of N2O to N2 ; and low sample size in rice systems may affect the statistical robustness. In general,macetas cuadradas grandes cultivation of leguminous beans uptakes large amounts of base cations from soils and release H+ , leading to lower soil pH and based-cation fertility.

This may inhibit N2O production, as nitrifiers and denitrifiers prefer relatively neutral or mildly alkaline environments . Additionally, leguminous beans are N2-fixers and tend to receive lower manure applications resulting in lower production of N2O compared to other crops. The WFPS had a significant effect on N2O emission, with soils having a moderate WFPS experiencing the highest N2O emission . Soils with WFPS at 50–90% appear to provide the optimum conditions for denitrifier activity and N2O production. At these intermediate WFPS conditions, there is likely some O2 available to allow nitrification to proceed and the generation of NO3 − provides substrate for denitrification to occur in adjacent anaerobic microsites. In contrast, the major processin soils with WFPS b50% is nitrification with denitrification inhibited by the presence of O2 . When the WFPS is N90%, soil porosity is water-saturated, leading to greater conversion of N2O to N2 under strongly anaerobic conditions . It was notable that short-term application of manure produced higher N2O emission than long-term application . While the exact mechanisms remain unknown, one possible reason is that manure application enhances microbial growth and proliferation and stimulates soil N cycling by providing more available substrates and generating more anaerobic microsites . Once the N cycling microorganisms adapt to regular manure application, they may become less responsive to further manure applications over time. In addition, regular application of manure may lead to higher microbial biomass and therefore a higher capacity of soil microbial community to retain N, resulting in more uptake of N by the microbial community and less N2O emission. Another possibility is improved soil abiotic properties resulting from long-term manure application. As manures are applied annually, several soil properties would be progressively altered to a new steady-state compared to initial soil conditions.Zhou et al. showed no differences in N2O emissions from different manure sources , consistent with the findings of our meta-analysis. Raw manure resulted in higher N2O emission than pre-treated manure, consistent with the results of Nkoa . In general, raw manure has higher inorganic N and a lower C: N ratio than pre-treated manure . Higher inorganic N contents induce higher N2O emission, as NO3 − and NH4 + are essential substrates for denitrification and nitrification, respectively. Manures with a high C:N ratio would enhance microbial N assimilation , resulting in uptake of inorganic N from indigenous soil sources.

The lack of available N substrates would thereby decrease soil N2O emission. However, Zhu et al. demonstrated that manure pre-treatment did not reduce N2O emissions and Chantigny et al. showed no difference in N2O emissions between pre-treated and raw manures. We attribute these contradictory results to factors such as the high heterogeneity of manure, contrasting manure sources and pretreatment methods. In this meta-analysis, we did not specify manure forms or pretreatments for manure . Instead, we focused on the in-situ response of N2O emission to manure application from the perspective of agricultural soil rather than manure source management. As showed in Fig. 4, pre-treated manure showed lower effect size compared to raw manure. Manure treatment, for example, compost and digest, will change the physical, chemical and biological properties of the manure radically, resulting in the difference for N and C content in raw/pre-treated manure and soil N2O emission after manure application. Thus, a detailed quantitative index of manure characteristics may be more suitable for explaining the mechanisms mediating N2O emission from soil than qualitative categorical descriptions such as manure preparation and manure type. The overall increase in soil N2O emission resulting from manure application was consistently greater than zero , and the responses of N2O emission differed with manure characteristics. Different microbial activity and growth induced by different manure characteristics likely account for differences in N2O emission. In this analysis, manures with the highest N content had the highest soil N2O emissions compared with manures with medium and low N contents . This is in accordance with the consensus that higher inorganic N availability directly enhances nitrification-denitrification processes, resulting in higher N2O emission. Our analysis also found that manures with medium C content or C:N ratio had significantly lower N2O emission compared to those with lower or higher C contents and C:N ratios. Normally, when manures have a C:N ratio b 5 or low C content, they provide ample N for microbial growth and proliferation, resulting in net N mineralization .

Excessive inorganic N produced from mineralization can stimulate soil nitrification and denitrification processes, contributing to increased soil N2O emission. When the C:N ratio increases, the N content in manure cannot meet the N requirement for microbial growth and proliferation, and the microorganisms will utilize indigenous N from the soil resulting in microbial N immobilization . This process competes with heterotrophic denitrification and autotrophic nitrification to utilize the NO3 − and NH4 + substrates, respectively. Further, high manure C:N ratios or C content may initially enhance microbial activity, leading to consumption of O2 and development of anaerobic conditions . As a result, denitrification may persist for longer time periods, leading to increasing N2O emission .Soil texture did not significantly affect N2O emission following manure application . This is contradictory with several previous laboratory studies that found higher N2O emissions from fine-texture soils than coarse-texture soils . In general, soil texture strongly affects soil pore distribution, and thereby regulates water and O2 availability . Soils with coarse textures would favor nitrification as the dominant process . In contrast, denitrification preferentially occur in soils with fine textures ,frambuesas cultivo where O2 availability is often low . However, our analysis showed no difference in N2O emission between soils with coarse and fine textures from field trials . This was probably due to the long-term effects of manure application to fields, as continuous and intensive application can greatly change initial soil properties .In general, nitrifiers prefer neutral to moderately alkaline conditions , and heterotrophic denitrifiers are more active in neutral rather than acidic environments . Thus, N2O emission may be expected to be higher in neutral or alkaline soils compared to acidic soils. In contrast, our analysis revealed that the initial soil pH had no significant effect on N2O emission, contradictory with some previous laboratory studies . A potential reason for this discrepancy may result from manure being an effective acidic soil amelioration amendment that can increase soil pH . After manure application, the final soil pH may be increased to a neutral or alkaline value attenuating possible effects from the initially acidic soil conditions. Given this potential pH buffering and/or soil acidity amelioration effect, the activity of nitrifier and denitrifier communities between initially different pH soils may not be as pronounced as expected based on the initial soil pH values. Our analysis further found that initial soil organic C content significantly affected N2O emission and soils with moderate SOC content had the largest N2O emission. We attributed this to differential C-use efficiency among microorganisms. Soils with low SOC often have low microbial activity , which may lead to low N2O emission. Soils with high SOC may have their C persevered by chemical/physical protection mechanisms or SOC may have a high C/N ratio resulting in N-limitation for microbes. Additional research is warranted to better understand the role of soil carbon dynamics in N2O emission.

Overall, initial soil properties were not highly predictive of N2O emission response to manure application in field trials. As our analysis utilized a global dataset, several interacting factors that regulate N2O emission within a given site are obscured by combing with data from other regions. Additionally, intensive manure application may substantially alter the initial soil properties, making them non-representative of post-manure application conditions. In addition, the lack of significant effects of soil properties may be related to many confounding factors in the field trials, which may obscured the individual effect that can be observed in laboratory experiments on N2O emission with manure application. Compared to field trials, laboratory experiments are typically short-term incubations and receive less cumulative manure application . Therefore, WFPS, which can be controlled and measured during field experiments, is often a better predictor of N2O emission than initial soil properties, such as soil texture, pH and organic matter. Using real world data generated from field trials for our meta-analysis was an important distinction of our analysis since laboratory experiments are not able to capture all the complexities and interaction associated with field trials.California’s electricity system is undergoing unprecedented change. California’s current goals call for meeting 50% of the state’s retail electricity sales with renewable energy by 2030 and reducing greenhouse gas emissions to 40% below 1990 levels by 2030 . A 50% renewable electricity system in California will have a high penetration of variable solar and wind generation. Fluctuations and uncertainty of variable generation will make the operation of an already complex electricity system even more complicated. One way to offset the unpredictability of renewable resources is through DR programs, by which end users are induced to change their electric demand to match the supply. Historically, DR resources have been used to reduce the system level peaks . As California moves closer to its target of 50% renewables, traditional DR can provide local reliability, but more importantly faster time scale DR services will be more important for facilitating the intermittency of renewable generation. With higher penetration of intermittent renewable sources, the grid needs to deal with generation variability. Intra-hour variability and short-duration ramps are one of the immediate challenges faced by a 50% renewable grid. However, other challenges arise as the California grid decarbonizes over time. Historically peak hours were defined as the hours between 12pm-6pm . Proliferation of solar generation in California is forcing those peak hours to shift to later hours in the day 1 . This is most commonly referred to as the “Duck Curve” , where the increased solar generation is significantly dropping the net electricity demand during the day, which in turn results in significant ramps in the later hours . Agricultural irrigation pumping is a significant component of California’s electric demand and a resource that can provide DR services to the grid and contribute to its stability. In addition, distribution feeders that serve agricultural customers often have low diversity in their types of customer loads, and exercise of a large number of irrigation pumps on a single feeder can cause over-voltage issues .