Kerri L. Steenwerth

Soil microbial community composition and land use history in cultivated and grassland ecosystems of coastal California

Phospholipid ester-linked fatty acid (PLFA) profiles were used to evaluate soil microbial community composition for 9 land use types in two coastal valleys in California. These included irrigated and non-irrigated agricultural sites, non-native annual grasslands and relict, never-tilled or old field perennial grasslands. All 42 sites were on loams or sandy loams of similar soil taxa derived from granitic and alluvial material. We hypothesized that land use history and its associated management inputs and practices may produce a unique soil environment, for which microbes with specific environmental requirements may be selected and supported. We investigated the relationship between soil physical and chemical characteristics, management factors, and vegetation type with microbial community composition. Higher values of total soil C, N, and microbial biomass (total PLFA) and lower values of soil pH occurred in the grassland than cultivated soils. The correspondence analysis (CA) of the PLFA profiles and the canonical correspondence analysis (CCA) of PLFA profiles, soil characteristics, and site and management factors showed distinct groupings for land use types. A given land use type could thus be identified by soil microbial community composition as well as similar soil characteristics and management factors. Differences in soil microbial community composition were highly associated with total PLFA, a measure of soil microbial biomass, suggesting that labile soil organic matter affects microbial composition. Management inputs, such as fertilizer, herbicide, and irrigation, also were associated with the distinctive microbial community composition of the different cultivated land use types.

Responses of soil microbial processes and community structure to tillage events and implications for soil quality

The short-term responses of soil microbial processes and community structure to perturbation constitute one aspect of soil quality. Such responses are often associated with an increase in the emissions of greenhouse gases (i.e., CO2, NO, or N2O) and the accumulation and potential loss of nitrate by leaching. Here we describe our recent work on responses of soil carbon and nitrogen dynamics, microbial biomass, and microbial community structure to a tillage event in intensively managed vegetable crop systems in California. Our results indicate that CO2 emission is high for the first day after tillage, but respiration declines or remains constant, suggesting that physical processes are responsible for the high flux from the soil surface. Net mineralization and nitrate accumulation increase for several days after tillage, and this can be accompanied by higher denitrification rates. Tillage causes immediate changes in microbial community structure, based on phospholipid fatty acid (PLFA) analysis, but little concomitant change in total microbial biomass. Tillage events contribute to decreased soil quality by increasing emissions of greenhouse gases, and increasing the potential for nitrate leaching to groundwater, and these negative aspects must be weighed against the benefits of tillage for increasing the health and productivity of some crops. D 2003 Elsevier Science B.V. All rights reserved.

Influence of Floor Management Technique on Grapevine Growth, Disease Pressure, and Juice and Wine Composition: A Review

Vineyard floor management has multiple goals that encompass improving weed management and soil conservation, reducing soil resource availability to control vine vigor, and influencing desirable aspects in wine quality. This review addresses the effects of cultivation, weed control, cover crops, and mulch on vine growth and balance, disease pressure, yield, and juice and wine quality in many growing regions (Australia, New Zealand, South Africa, Europe, and the western United States); offers recommendations for practical use; and highlights research needs. In the last decade, more literature has been published on mulching and cover cropping than on cultivation and herbicide use, suggesting stronger interest in cover cropping and mulching practices for vineyards. Cover crops have the potential to improve soil and vine health, can be adapted to many climates and soils, and may influence vine vigor by adjusting parameters such as the length of their growth period, coverage of the vineyard floor, and aggressiveness. Cover crops increased juice soluble solids, anthocyanins, and other phenolic components and decreased titratable acidity and pH. They were associated with red wines judged superior to those issued from non-cover-cropped vines. Use of organic mulches resulted in improved vine balance, soil water content, and friability, increased yields, and reduced pathogen and pest pressure. Plastic and fabric mulches remain impractical due to high installation cost. Application of newer techniques such as flame weeding or soil steaming is limited due to difficulty in targeting the appropriate stage of weed growth and limited susceptibility of some weed species to these techniques. Research needs include development of multiyear, multidisciplinary studies that use a mechanistic approach to link management practices to soil processes, grapevine responses, grape and wine composition, and sensory characteristics.

Climate-smart agriculture global research agenda: scientific basis for action

BackgroundClimate-smart agriculture (CSA) addresses the challenge of meeting the growing demand for food, fibre and fuel, despite the changing climate and fewer opportunities for agricultural expansion on additional lands. CSA focuses on contributing to economic development, poverty reduction and food security; maintaining and enhancing the productivity and resilience of natural and agricultural ecosystem functions, thus building natural capital; and reducing trade-offs involved in meeting these goals. Current gaps in knowledge, work within CSA, and agendas for interdisciplinary research and science-based actions identified at the 2013 Global Science Conference on Climate-Smart Agriculture (Davis, CA, USA) are described here within three themes: (1) farm and food systems, (2) landscape and regional issues and (3) institutional and policy aspects. The first two themes comprise crop physiology and genetics, mitigation and adaptation for livestock and agriculture, barriers to adoption of CSA practices, climate risk management and energy and biofuels (theme 1); and modelling adaptation and uncertainty, achieving multifunctionality, food and fishery systems, forest biodiversity and ecosystem services, rural migration from climate change and metrics (theme 2). Theme 3 comprises designing research that bridges disciplines, integrating stakeholder input to directly link science, action and governance.OutcomesIn addition to interdisciplinary research among these themes, imperatives include developing (1) models that include adaptation and transformation at either the farm or landscape level; (2) capacity approaches to examine multifunctional solutions for agronomic, ecological and socioeconomic challenges; (3) scenarios that are validated by direct evidence and metrics to support behaviours that foster resilience and natural capital; (4) reductions in the risk that can present formidable barriers for farmers during adoption of new technology and practices; and (5) an understanding of how climate affects the rural labour force, land tenure and cultural integrity, and thus the stability of food production. Effective work in CSA will involve stakeholders, address governance issues, examine uncertainties, incorporate social benefits with technological change, and establish climate finance within a green development framework. Here, the socioecological approach is intended to reduce development controversies associated with CSA and to identify technologies, policies and approaches leading to sustainable food production and consumption patterns in a changing climate.

Assessment of carbon in woody plants and soil across a vineyard-woodland landscape

BackgroundQuantification of ecosystem services, such as carbon (C) storage, can demonstrate the benefits of managing for both production and habitat conservation in agricultural landscapes. In this study, we evaluated C stocks and woody plant diversity across vineyard blocks and adjoining woodland ecosystems (wildlands) for an organic vineyard in northern California. Carbon was measured in soil from 44 one m deep pits, and in aboveground woody biomass from 93 vegetation plots. These data were combined with physical landscape variables to model C stocks using a geographic information system and multivariate linear regression.ResultsField data showed wildlands to be heterogeneous in both C stocks and woody tree diversity, reflecting the mosaic of several different vegetation types, and storing on average 36.8 Mg C/ha in aboveground woody biomass and 89.3 Mg C/ha in soil. Not surprisingly, vineyard blocks showed less variation in above- and belowground C, with an average of 3.0 and 84.1 Mg C/ha, respectively.ConclusionsThis research demonstrates that vineyards managed with practices that conserve some fraction of adjoining wildlands yield benefits for increasing overall C stocks and species and habitat diversity in integrated agricultural landscapes. For such complex landscapes, high resolution spatial modeling is challenging and requires accurate characterization of the landscape by vegetation type, physical structure, sufficient sampling, and allometric equations that relate tree species to each landscape. Geographic information systems and remote sensing techniques are useful for integrating the above variables into an analysis platform to estimate C stocks in these working landscapes, thereby helping land managers qualify for greenhouse gas mitigation credits. Carbon policy in California, however, shows a lack of focus on C stocks compared to emissions, and on agriculture compared to other sectors. Correcting these policy shortcomings could create incentives for ecosystem service provision, including C storage, as well as encourage better farm stewardship and habitat conservation.

Cover Crops and Tillage in a Mature Merlot Vineyard Show Few Effects on Grapevines

Permanent cover crops are commonly used in vineyard floor management because of their beneficial effects to soil and vine health, but studies evaluating their competitive effects on vines have been conducted primarily in nonirrigated vineyards. Future air quality regulations could mandate the use of no-till floor management practices in California’s Central Valley. We evaluated the combined effects of cover crop type (oats alone or oats grown with legumes) and tillage on soil nutrient availability, vine nutrition, growth, and yield characteristics of Vitis vinifera cv. Merlot grown under regulated deficit irrigation in a commercial vineyard from 2008 to 2010. Five treatments were used: Resident Vegetation (RV) + Till, Oats + Till, Oats/Legumes +Till, Oats + NoTill, and Oats/Legumes + NoTill. No differences in soil nutrient availability were found among the treatments. Of the numerous nutritional constituents analyzed on leaf petioles and blades, only NO3-Npetiole was affected by floor management. At nearly all growth stages among all years, NO3-Npetiole of tilled treatments was twice the no-till treatments. At harvest, yield, mean cluster weight, cluster number per vine, and aboveground cover crop biomass differed among treatments in 2009 and/or 2010 but not in the first year (2008); however, responses were not consistent among treatments within each respective year. Importantly, yields were similar from all four cover crop treatments compared to the typical management (RV + Till), suggesting that use of cover crops and/or no-till practices may be implemented in an irrigated vineyard with little immediate effect on grape productivity in mature vineyards.

Tightly-Coupled Plant-Soil Nitrogen Cycling: Comparison of Organic Farms across an Agricultural Landscape.

How farming systems supply sufficient nitrogen (N) for high yields but with reduced N losses is a central challenge for reducing the tradeoffs often associated with N cycling in agriculture. Variability in soil organic matter and management of organic farms across an agricultural landscape may yield insights for improving N cycling and for evaluating novel indicators of N availability. We assessed yields, plant-soil N cycling, and root expression of N metabolism genes across a representative set of organic fields growing Roma-type tomatoes (Solanum lycopersicum L.) in an intensively-managed agricultural landscape in California, USA. The fields spanned a three-fold range of soil carbon (C) and N but had similar soil types, texture, and pH. Organic tomato yields ranged from 22.9 to 120.1 Mg ha-1 with a mean similar to the county average (86.1 Mg ha-1), which included mostly conventionally-grown tomatoes. Substantial variability in soil inorganic N concentrations, tomato N, and root gene expression indicated a range of possible tradeoffs between yields and potential for N losses across the fields. Fields showing evidence of tightly-coupled plant-soil N cycling, a desirable scenario in which high crop yields are supported by adequate N availability but low potential for N loss, had the highest total and labile soil C and N and received organic matter inputs with a range of N availability. In these fields, elevated expression of a key gene involved in root N assimilation, cytosolic glutamine synthetase GS1, confirmed that plant N assimilation was high even when inorganic N pools were low. Thus tightly-coupled N cycling occurred on several working organic farms. Novel combinations of N cycling indicators (i.e. inorganic N along with soil microbial activity and root gene expression for N assimilation) would support adaptive management for improved N cycling on organic as well as conventional farms, especially when plant-soil N cycling is rapid.

Management to Reduce Greenhouse Gas Emissions in Western U.S. Croplands

Agriculture is a major activity in the western U.S. with approximately 57 million ha of harvested cropland of which 27% is irrigated; however, irrigated crops account for a high proportion of the economic returns because of their high economic value. We sought to summarize greenhouse gas (GHG) flux research from crop production systems in the western U.S. published from 2005 to 2011. Limited GHG emissions data were found from irrigated cropping systems in California (grain, rice, vegetable, orchards), Texas (cotton), Colorado (corn), and Washington (corn and potato), and from dryland wheat systems in Montana and North Dakota. Converting from conventional tillage (CT) to minimum-till (MT) or no-till (NT) production generally sequestered soil organic carbon (SOC) and reduced carbon dioxide (CO 2 ) emissions in many cropping systems, but not all. Methane (CH 4 ) flux was not greatly influenced by crop management practices, except in rice and manure production systems. Nitrous oxide (N 2 O) emissions were affected by N availability, climatic factors, irrigation, and crop management practices, and tended to be lower under dryland than irrigated cropping conditions. Reducing N fertilization rate and selecting the right N source can reduce N 2 O emissions as much as 50%. Use of microjet sprinkler or subsurface drip irrigation reduced N 2 O emissions in vineyards and orchards as much as 50% compared to surface drip systems. Available GHG data could be used to verify models and develop local mitigation practices, but due to the large diversity of cropping systems and ecoregions, and a lack of representative cropping system GHG databases, generalized mitigation recommendations for the western U.S. are not possible at this time.

Vulnerability of California specialty crops to projected mid-century temperature changes

Increasing global temperatures are likely to have major impacts on agriculture, but the effects will vary by crop and location. This paper describes the temperature sensitivity and exposure of selected specialty crops in California. We used literature synthesis to create several sensitivity indices (from 1 to 4) to changes in winter minimum and summer maximum temperature for the top 14 specialty crops. To estimate exposure, we used seasonal period change analysis of mid-century minimum and maximum temperature changes downscaled to county level from CMIP5 models. We described crop vulnerability on a county basis as (crop sensitivity index × county climate exposure × area of crop in county); individual crop vulnerabilities were combined to create an aggregate index of specialty crop vulnerability by county. We also conducted analyses scaled by crop value rather than area, and normalized to total specialty crop area in each county. Our analyses yielded a spatial assessment highlighting seasons and counties of highest vulnerability. Winter and summer vulnerability are correlated, but not highly so. High-producing counties (e.g., Fresno County in the San Joaquin Valley) are the most vulnerable in absolute terms, while northern Sacramento Valley counties are the most vulnerable in relative terms, due to their reliance on heat-sensitive perennial crops. Our results illustrate the importance of examining crop vulnerability from different angles. More physiological and economic research is needed to build a comprehensive picture of specialty crop vulnerability to climate change.

Effects of Various Vineyard Floor Management Techniques on Weed Community Shifts and Grapevine Water Relations

Adoption of permanent cover crops and no-till systems is considered integral to achieving the California state air quality standards regulating airborne particulate matter, as indicated in Senate Bill 656. Imposition of such management techniques in a vineyard could create competition with vines for limited water resources. We evaluated the effects of cover crops that were either tilled or just mowed on vineyard floor composition, weed populations, vine water relations and growth, and fruit composition over three years within a mature commercial Merlot vineyard subjected to deficit irrigation in Lodi, San Joaquin Valley, California. The vineyard floor in this experiment supported resident vegetation that was tilled (standard grower practice), an oat cover crop, or a legume/oat cover crop. The two planted cover crops were either tilled or mowed (i.e., no-till). Biomass of cover crops, weeds, and legumes varied by year and treatment, but consistent effects among treatments were not observed. Weed species composition and cover segregated with the presence or absence of tillage rather than cover crop type and the weed species composition of the resident vegetation was distinct from those in the cover crop treatments. Some treatments, like ‘Oats/Legumes + NoTill’, ‘Oats + NoTill’, ‘Oats/Legumes + Till’, and ‘Resident Vegetation + Till’ reduced soil water content (Өv) in at least one of the three shallow soil layers spanning 0 to 30 cm, 30 to 60 cm, and 60 to 100 cm in 2008, and ‘Oats/Legumes + NoTill’ also dried the two upper layers in 2009, but these differences had no consistent influence on plant water status. Distinctions in Өv between years were attributed partly to the cessation of rainfall two months earlier in 2008 than in 2009, despite similar total annual quantities. Significant reductions in Өv imposed by the ‘Oats/Legumes + NoTill’ treatments reduced vine vegetative growth in two of three years, but these effects did not manifest in yield and fruit composition. Values for the Ravaz index for two of the three years indicate that the vineyard was overcropped for all treatments, but maximizing production in this region is a common practice. Weak competitive effects of the cover crops for water were likely associated with the use of a well-established mature vineyard and demonstrated that these management strategies could be employed to improve air quality to meet California air quality regulations with limited effects on vine water status and production.