The lower metal-humus complexes of Andisols in the present study as compared to Andisols from Réunion Island could be associated with the higher annual rainfall and temperature under tropical conditions that accelerated organic matter decomposition. In our study the negative correlation between soil pH and Fep and Alp was observed indicating favorable conditions for organo-metal complex formation under acidic conditions . Tonneijck et al. suggested SOM stabilization in volcanic ash soils in natural Andean ecosystems of Ecuador is through organo-metallic complex formation, low soil pH and toxic levels of Al, and physical protection of SOM in a very large micro-porosity.A place‐based approach for studying agricultural responses to climate change explores a broad set of biophysical and socioeconomic issues related to both greenhouse gas emissions and to adaptation to an uncertain climate. Few such studies exist. Instead, the scientific research on agriculture and climate change has focused on agricultural management practices to reduce the GHG emissions of carbon dioxide , nitrous oxide , and methane , or on the vulnerabilities of different crops to changes in seasonal weather, water supply, pests and diseases,pe grow bag and biophysical factors affecting agricultural production . These are only a few of the aspects necessary for planning for climate change in agricultural regions.
As many jurisdictions in the Western United States are now addressing regional impacts of climate change, there is a need for science‐based exploration tools for scientists, farmers, policymakers, and the general public to better understand the complexity of vulnerabilities and adaptation options for increasing agricultural sustainability in rural landscapes. California’s Climate Change Scenarios Project has focused on determining impacts from plausible climate change scenarios . Use of Global Circulation Models for future climate projections have used two scenarios from the International Panel on Climate Change that are based on story lines for high and low GHG emissions . For agriculture in California, climate change will have impacts on water availability, crop physiology, production , and pest and disease problems , especially for the A2 scenario by the end of this century. Addressing agricultural vulnerabilities and adaptive capacity is part of California’s new statewide climate adaptation strategy. A place‐based vulnerability approach deals with climate change as one of many other long‐range issues such as changes in commodity production, stewardship of natural resources, land use, population growth, and urbanization in a regional system. The capacity of a rural population to adapt with climate change and other uncertainties depends largely on its collective ability to assemble and process information and respond in site‐specific and context‐relevant ways .
Adaptive strategies will require input from many disciplines, including agronomy, ecology, economics, land use planning, and political science. And the involvement of multiple types of stakeholders must inform the assessment and planning process, so that adaptive management can proceed in response to a knowledge base that is continuously developing . The strong science‐policy interface for climate change in California has generated a great deal of agricultural interest in the implementation of the law to reduce statewide GHG emissions, California Assembly Bill 32 , known as the Global Warming Solutions Act of 2006.1 Under AB 32, the state’s GHG emissions are to be reduced to 1990 levels by 2020 through mandatory reporting, emission limits, and reduction measures, as implemented by the California Air Resource Board. It also establishes a goal of 80 percent reduction by 2050 and proposes a cap‐and‐trade policy for GHG emissions. Agricultural GHG emissions will not be included in the cap, but there may be potential for trading carbon offsets from agricultural practices. Senate Bill 375 connects land use planning with implementation of AB 32. It requires a Climate Action Plan for mitigation of GHG emissions in the unincorporated areas of each county in California. This process is engaging farmers and other agricultural stakeholders in detailed accounting of GHG emissions from production and processing practices, and thereby beginning to create greater awareness of vulnerabilities and adaptation options as well.
Yolo County is in the Sacramento Valley of Northern California. It extends westward from the Sacramento River to the Coast Range Mountains . The most important crops are tomatoes, alfalfa hay, wine grapes, and almonds. Upland summer‐dry grasslands and savannas are grazed by cattle. The few small towns and cities have experienced a changing mixture of urban, suburban, and farming‐based livelihoods through the past few decades. In Yolo County, there are approximately 500 farms with an average size of about 500 acres . Many farms produce sales ≥$100,000 per year. Yolo County is ranked 23 by value of sales of California’s 58 counties . Roughly 2 percent of the county’s production is consumed within the Sacramento region . The 653,452 acres of Yolo County are largely agricultural . Important farmland is 57 percent, and livestock grazing land is 24 percent, while urban and built‐up land is only 4.6 percent of the county’s acreage . During the past few decades, there has been a trajectory toward less crop diversification of county acreage, larger farm sizes, but fairly stable markets for commodities . Most commodities are managed with high intensification of agricultural inputs . The number of organic farms, however, is growing. A recent survey showed that many riparian corridors have low scores for soil quality and riparian health , and there is concern about transport of pesticides to the San Francisco Bay delta . Environmental quality is now receiving more attention, with active participation in programs from several agencies. Preservation of agricultural land has been a strong priority in Yolo County, and planning is focused on regional land use guidelines that maintain land in agricultural production and concentrate new development into urban areas . Regions within Yolo County are distinguished by their land forms , proximity to the Sacramento River and Delta , water availability , and the influence of small towns and cities. The regions differ in crop commodities. There is greater prevalence of wine grapes along the river, processing tomatoes in the alluvial plains, and organic fruits and vegetables in an isolated, narrow valley to the north. The regions also have different trends and targets for urban growth, rural housing, and wildlife habitat creation. Flooding along the Sacramento River poses the most significant regional hazard from climate change; water flows will increase by at least 25 percent by 2050 due to a decrease in snow pack in the Sierra Nevada .Climate simulations by Global Climate Models show that mean annual temperature will rise by 1°C to 3°C by 2050, the time frame of this case study . Heat wave days will increase two‐ to three‐fold by 2050. Precipitation is likely to decrease toward the end of the century, depending on the assumptions of each GCM. Hydrological changes suggest, however,growing bags that drought is already increasing and will become more severe and variable with time . Water supply has been considered the most uncertain aspect of climate change for farmers in Yolo County, who rely on groundwater for approximately 30 percent of their supply in a normal water year . It should be emphasized that GCM models are not “predictions,” but rather, are plausible scenarios of climate sequences over a long‐term period. The previous phase of this case study examined possible impacts of increased temperature and decreased precipitation on Yolo County crops . Horticultural crops will likely experience more problems from heat than field crops, due to greater temperature sensitivity of their reproductive biology, water content, visual appearance, and flavor quality . A warmer temperature regime is likely to shift more “hot‐season” horticultural crops, such as melon and sweet potato, into Yolo County’s horticultural “warm‐season” crop mix .
Warmer winter temperatures may allow “cool‐season” crops such as lettuce and broccoli, whose short growth seasons could permit two crops per year, unlike winter grains at present. Expansion of citrus production , and of heat and drought‐tolerant trees, such as olive , are likely options especially because reduction in winter chill hours will reduce flowering in stone fruits, nuts, and grapes . During the past 25 years, crop diversity has decreased in Yolo County . Diversity may increase if farmers find that resilience, especially to extreme events such as heat waves, is enhanced by a species mix that varies in stress tolerance . Forage production for livestock in upland grasslands and savannas may increase with warmer winter temperatures during the winter rainy season, but field experiments with elevated CO2 do not corroborate this expectation . More nitrogen limitation will likely occur under eCO2 . If N‐fixing legumes become more abundant in response to warmer winter temperatures, however, the N supply will increase. Thus, it is unclear if livestock production on these rangelands will actually increase due to climate change, especially in dry years, which require lower stocking rates, earlier animal removal dates, and transport to irrigated, permanent pasture.Pests and diseases are another major uncertainty: warmer temperatures can increase ranges and population sizes, and change the trophic interactions that currently provide biological control of invasive species . At present, no comprehensive compilations from California Department of Food and Agriculture or the National Plant Diagnostic Network exist to show new invasive species to target for a warmer climate . Some literature suggests that it is more efficient to focus on the spread of already naturalized species rather than from new potential invasive species at the importation stage . Yet, the Yolo County Agricultural Commissioner, John Young , notes that several recently arrived pests are becoming severe problems, such as the European grapevine moth in vineyards, spotted wing drosophila on cherries, and Japanese dodder on a wide range of cultivated and wild land plant species. Quarantines are especially difficult for Yolo County because so little of the crop production is consumed within its boundaries, and thus economic hardship occurs unexpectedly for all growers of a particular commodity. Discussions with the Yolo County University of California Cooperative Extension farm advisors indicated special concern for stripe rust on wheat , insect pests on nuts, medfly, corn ear worm on tomato, tomato spotted wilt virus, and earlier activity of perennial weeds such as bindweed . Very recently, alfalfa stem nematode has become a serious pest in the Sacramento Valley, possibly because winter minimum temperatures have reached the lower limit of reproduction for the species . On the other hand, some pests may become less serious; high summer temperatures are likely to reduce the fecundity and survival of the olive fly in this area, which will cause olive yields to increase . Decisions on strategies for adapting to these types of climate change vulnerabilities are not only made by growers. Public institutions, researchers, and non‐governmental organizations become involved in decision‐making by gathering information, stimulating awareness, and generating collective action. At present, California’s strong emphasis on reducing GHG emissions suggests that mitigation and adaptation should be dual components of climate change decision‐making. Some authors have made the case that most categories of adaptation measures have positive impacts on mitigation of GHG emissions . This may be too optimistic. First, agricultural soils may emit more potent GHG in a future CO2‐enriched atmosphere . Second, detailed analysis of crop management may show trade offs between mitigation and adaptation goals. An analysis of benefits of different management options for mitigation and adaptation benefits in Yolo County showed that synergies are often complex . Changes in crop diversity, irrigation methods, fertilizer management, and tillage practices often are more beneficial for either mitigation or adaptation. Rather than change a single practice, major changes in cropping systems will be needed to meet production and mitigation goals. For example, a conventional tomato system with furrow irrigation and knife injection of fertilizer emitted 3.4 times more N2O and had lower yields than an integrated tomato system with drip irrigation, reduced tillage and fertigation on the same soil type . But drip irrigation, unlike furrow irrigation, does not recharge groundwater,leaving farmers more vulnerable to long‐term drought. More comprehensive analysis of these complex relationships is needed.Analyzing changes in past crop acreages in relation to local climate history can provide a set of projections of potential climate‐induced changes in cropping patterns based on how farmers have responded to past climate change.