The Salinas is a losing stream with naturally transient flow and no surface water passing through the lower reaches for much of the summer. The aquifers in the alluvial valley are over drafted for agricultural production, causing saltwater intrusion. During the spring and summer, the Nacimiento and San Antonio reservoirs are operated by Monterey County Water Resources Agency to maintain required fish bypass flows at the Salinas River Diversion Facility, while maximizing recharge to the groundwater basin via the Salinas River bed . Today, the hydrological conditions of the Salinas River differ from that historically due to changes in basin land use and flow regulation. By 1901, groundwater pumping was underway, with wells drawing water from as deep as 75 m below the ground surface and lowering the water table below the ground by 3– 5 m . Valley bottoms were mostly converted to irrigated agriculture during the past century with a small proportion of urbanization. The Salinas Valley is primarily cropland, known as “America’s Salad Bowl” due to the prominence of vegetables and greens grown in the region while numerous vineyards are also present.The largest urban area in the Salinas River watershed is the City of Salinas, located in the lower portion of the watershed. The City population in 2010 was 150,441 while the six other smaller cities in the area have populations under 30,000 .

Anderson details the parallel histories in the Salinas Valley of land value and ownership,arandanos cultivo irrigation practices and areas, soil management, transportation, processing technology, and trade. Introduction of the turbine pump with a capacity to lift hundreds of feet in 1924 revolutionized groundwater extraction in the Valley. In 1889, only some 170 ha of the Valley was irrigated using historic ditch systems, less than 0.2% of the total cropped area. Crops included barley, wheat, beans, sugar beets and potatoes, and there were about 56,722 grazing cattle. By 1939, 30% of 139,400-ha agricultural production employed irrigation; about 56,600 ha consisted of vegetable crops introduced A total of ~1900 and 118,000 cattle grazed the Valley. Just 4 decades later, by 1980 dams were built on the Nacimiento and San Antonio rivers to supplement water supply and dam releases were used to recharge groundwater. By 1980, about half of the 181,300 ha in agricultural production was under irrigation, with grape production dramatically increasing from 60 ha in 1939 to 12,166 ha in 1980. Later, growers adopted water conservation technologies such as drip, sprinkler, and surge-flood irrigation presumably to reduce the farm water ‘footprint’ and water quality impacts associated with runoff from irrigation.Residuals between actual measured values and those determined from the best C–Q relationships were then used to describe changes in these relationships over time. We calculated residuals by subtracting the expected from observed sample values and then accumulated these over times. These residuals reveal systematic departures in sample C behavior from that of the rating curve model that is assumed to capture longer-term average behavior.

Residuals from the model represent a combination of measurement error, inadequacies of the model’s functional form, estimation error of the coefficients, and the influence of other variables that are not considered by the model. Thus, positive residuals indicate higher-than anticipated observed concentrations, while negative residuals indicate a lower-than anticipated observed concentration. The second phase involved the delineation of periods of persistent patterns in solute concentration behavior by sequentially summing, that is, accumulating C–Q WRTDS residuals over time. Periods of persistent positive or negative behavior were identified based on the local slope of the cumulative residual curve, with persistent positive or negative values identified by positive or negative slopes maintained over time. A third phase involved analysis of hydrogeochemical processing underlying solute behavior within the identified persistent periods of high or low. This was accomplished using compositional relations and ionic ratios among major dissolved species to gain insights into possible origins of water quality. Task 4 has two parts; the first part considers the load dynamics from discharge variability over the different periods to describe correlations between solute concentrations and river discharge events using best-fit log–log slopes, changes in flow-normalized concentrations, and changes in flow-normalized fluxes. The second part of phase 4 involved accounting for variability in solute concentrations not explained by instantaneous discharge. We tested the effects of hydrologic variables representing basin antecedent soil moisture “wetness”, seasonal stream flows, seasonal to extended dry conditions, and past stream flow events on C–Q relationships.

Seasonal flow effects were analyzed using relationships between solute WRTDS concentration–discharge relationships and flow rating curves computed from average daily flows for the water-year, fall and spring seasons. The effects of base flow, basin wetness , previous stream flow events, and extended periods of low/no flow conditions on WRTDS residuals were tested with the non-parametric Mann-Kendall trend analysis using the R package ‘Kendall’ . The Mann–Kendall Tau values indicate the strength and direction of monotonic trends, with −1 and 1 representing perfectly negative and positive monotonic trends, respectively. The p value was used to assess their significance. The Mann–Kendall test requires that the dependent variable response is monotonic in relation to the independent variable. The strength of the correlation between solute C–Q residuals and hydrologic variables was determined using Kendall’s Tau and the relationship was quantified using the Kendall-Theil robust line .Large amounts of manure are generated globally by livestock farming systems and include an estimated global N content of 81.5 to 128.3 Tg yr−1 . In China, annual manure production has rapidly increased from ~1.7 Pg in 1990 to 6.0 Pg in 2015, making it an important resource as an agricultural soil amendment. Manure application to agricultural lands has been demonstrated to improve soil fertility . It was also reported that manure application can increase soil N retention and decrease NO3 − leaching to reduce N loss when compared with synthetic fertilizer application . Compared to unfertilized or chemical fertilized soils, manure application also enhance soil C sequestration and thus to increase soil organic carbon . Therefore manure application is recommended as a beneficial practice to sustain soil productivity. However, high emissions of greenhouse gases, such as N2O, following manure application have been reported and should not be neglected, as the global warming potential of N2O is ~298 times greater than CO2 , 2006; Landman, 2007. Additionally, N2O can contribute to stratospheric ozone depletion . Thus, the benefits of manure application for decreasing soil carbon dioxide emission by soil C sequestration maybe offset by increased N2O emission. In general, N2O emission is regulated by both nitrification and denitrification processes . Nitrification by autotrophic nitrifiers occurs under aerobic conditions oxidizing NH4 + to NO3 −. In contrast, denitrification by heterotrophic denitrifiers transforms NO3 − to nitric oxide , N2O and nitrogen gas under anaerobic conditions using bio-available C as the electron donor. Several studies have reported the effects of manure properties on soil N2O emissions along with related mechanisms. Robertson and Tiedje showed that manures with high inorganic and organic N concentrations can potentially increase soil N2O emission as NH4 + and NO3 − + NO2 − are reaction substrates for nitrification and denitrification, respectively. In addition, manures with a high C content typically enhance N2O emissions by serving as a C substrates for denitrifiers , and increasing soil microbial activities to rapidly consume oxygen and form anaerobic microsites . In some cases,maceteros grandes reciclados manure can accelerate completion of the denitrification reaction by enhancing conversion of N2O to N2, especially in soils with intensive irrigation or high rainfall . Furthermore, manures with high C:N ratios may inhibit N2O emission by stimulating microbial growth and consuming inorganic N from indigenous soil sources . In addition, manure treatments, for example compost and digest, change manure physical, chemical and biological properties. These changes will impact manure C and N content, which directly and indirectly regulate nitrification and denitrification processes, resulting in influence of soil N2O emission after manure application.

Lastly, manure application can change soil physicochemical properties , which indirectly affect microbial activity and N cycling processes . Soil properties play important roles in regulating N2O emission. For instance, soil texture and structure strongly affect soil pore size and moisture retention, which determines soil gas diffusion and O2 availability . Low O2 availability in fine-textured soils would tend to favor growth of denitrifiers, leading to greater N2O emissions . Further, low soil C content would suppress denitrification due to the scarcity of C substrates resulting in lower microbial activity. Soils with neutral to higher pH values are generally more suitable for autotrophic nitrifiers and heterotrophic denitrifiers than strongly acidic soils . Additionally, agricultural practices and climate can influence soil N2O emission by changing soil structure, C content, pH and microbial activity . Although a number of previous laboratory studies have investigated how manure characteristics, soil properties and controlled environmental conditions affect N2O emission following manure application, the results and underlying mechanisms from field trials are still contradictory and complicated due to soil heterogeneity and variations in agricultural practices and climate conditions . Interactive processes affecting N2O emission in field trials are very complicated and likely produce different results than laboratory experiments. Thus, a comprehensive analysis is required to synthesize and better understand the factors regulating N2O emission resulting from manure application. A fundamental understanding is necessary to develop beneficial management practices to attenuate N2O emission associated with manure application. Therefore, we performed a meta-analysis to disentangle the links between N2O emission and key influencing factors that regulate soil N2O emission following manure application in field studies, as the results from field trials provide a more realistic response to real-world conditions. The objectives of this analysis were to investigate how manure application influences soil N2O emission fluxes and emission factors in field trials and to elucidate potential regulating mechanisms; and identify important factors related to soil properties, manure characteristics and agricultural practices that regulate N2O emission fluxes following manure application. Results from this study provide a scientific basis for developing strategies to mitigate soil N2O emission associated with manure application.Peer-reviewed articles that reported N2O emission following manure application in field trials were searched in the Web of Science . Literature prior to December 2017 with ‘manure’, ‘field’, and ‘N2O/nitrous oxide emission’ present in the title, keyword or abstract was collected. The following criteria were used to identify the studies for meta-analysis: studies were performed by field trial and with at least 3 replicates; studies reported soil N2O emissions in both manure applied treatments and non-manure controls; and at least one crop season was included at the same experimental site. If multiple growing seasons were available, each growing season was considered as a separate observation. If multiple crops were cultivated at different periods in the same experimental site, each crop type was considered as one observation, as crop type contributes greatly to changes in soil properties. If an experimental site included multiple measurements of N2O emission, only the final time point was chosen for this meta-analysis. When a treatment was applied as a mixed manure plus mineral fertilizer, the comparison was considered as one observation only if another treatment with the same mineral fertilizer application was set up as a control. In total, 262 observations from 44 publications met these criteria and were included in this analysis. Cumulative N2O emission , sample size and standard deviation in both manure application treatment and non-manure control were extracted. Data Thief software was used to extract data presented in figure format. If only the standard error was reported, MetaWin software was used to convert standard error to standard deviation. Climate regimes with contrasting annual temperatures and rainfalls have a strong control on soil microbial activity, and soil microbial activity is highly associated with N2O production. Our analysis showed that the warm temperate climate produced higher N2O emission compared to the cool temperate climate . This is consistent with Cantarel et al. findings of a strong correlation between increasing N2O emission with increasing temperature and rainfall. We attribute these results to higher microbial activity induced by higher temperature and rainfall in warmer climates . The higher microbial activity can increase soil C and N substrate availability by increasing microbial turnover rates , or contribute to more anaerobic microsites by increasing microbial respiration .