Jay B. Norton

Contrasting responses of Intermountain West grasses to soil nitrogen

The mechanisms responsible for soil-N-mediated species replacement of native perennial grasses by the invasive annual grasses cheatgrass (Bromus tectorum L.) and medusahead (Taeniatherum caput-medusae [L.] Nevski) on rangelands are not completely understood. In addition, the contributions of distinct forms of inorganic N (i.e., NH 4 + and NO 3 -) to these shifts in species composition are currently unclear. Consequently, we conducted a greenhouse experiment to test 2 hypotheses: 1) that low N availability reduces growth (root and shoot) and N allocation of invasive annual seedlings more than native perennial species, and 2) that seedling growth and N allocation of invasive annual grasses is more responsive than native perennial grasses when supplied with NO 3 - relative to NH 4 +. We grew seedlings of 2 annual grasses and the native perennial grasses bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love), and 4 populations of squirreltail (Elymus elymoides [Raf.] Swezey; E. multisetus [J.G. Smith] M.E. Jones) in separate pots and exposed them to treatments differing in N form and availability for 17 weeks. Unexpectedly, root and shoot growth of annual grasses were equal or greater than native perennial grasses under low N availability. Annual grasses took up more NO 3 - and allocated more growth and N to shoots than the perennial grasses (P < 0.05). Perennial grasses had significantly greater root:shoot dry mass ratios than the invasive annual grasses across treatments (P < 0.05). Invasive annual and native perennial grasses both had greater (P < 0.05) shoot and root mass and allocated more N to these structures when supplied with NO 3 - relative to NH 4 +. The ecological implications of these growth and N allocation patterns in response to N availability and form provide important clues regarding the specific traits responsible for differences in competitive ability between invasive annual and native perennial grasses on semiarid rangelands.

Soil morphology and organic matter dynamics under cheatgrass and sagebrush-steppe plant communities

Widespread cheatgrass (Bromus tectorum L.) invasion represents a major shift in species dominance that may alter ecosystem processes across much of the western US. To investigate differences following such conversion, soil morphology and organic matter under cheatgrass-dominated and native shrub-steppe vegetation were compared by standard soil analysis procedures at seven paired sites in Idaho and Utah. Results suggest that, following conversion to cheatgrass dominance, increased porosity and labile organic inputs enhance microbial decomposition in near-surface horizons beneath cheatgrass compared to adjacent soils under native vegetation. Enhanced decomposition could result in depletion of long-term SOM, leading to impoverished sites difficult to restore to native perennial vegetation.

Mediterranean annual grasses in western North America: kids in a candy store

In this issue of Plant and Soil, Blank and Sforza (2007) contribute to understanding of how exotic annual grasses invade ecosystems in the western United States. Their findings, that medusahead wildrye (Taeniatherum caput-medusae [L.] Nevski) was most productive on non-invaded US soil and that plants from US-derived seeds may have evolved to utilize higher nutrient concentrations, parallel other results that suggest soils of vulnerable western US ecosystems are functionally different (i.e., more available-nutrient rich) than soils where these weeds are native and noninvasive (Blumenthal 2005; Davis et al. 2000). While it has been well established that exotic annual grasses can perpetuate their own environment by fostering increased available nutrients (e.g., Ehrenfeld and Scott 2001; Norton et al. 2003), we must remember, for the sake of ecological restoration, that initial invasiveness is caused by chronic disturbances that disrupt native nutrient and organic matter cycles and increase nutrient availability. The principal difference between western US ecosystems and those where invasive annual grasses are native is the amount of time during which chronic disturbances have occurred. In the Mediterranean region, intensive management of grass and shrubland ecosystems depleted nutrients long ago, so that native vegetation is adapted to a “here today, gone tomorrow” strategy. In ecosystems of the arid and semiarid western USA, perennial-plantdominated native nutrient cycles were disrupted by intensive land use much more recently. Chronic disturbances in the form of extensive grazing and altered fire regimes began to unlock nutrients stored in soil organic matter (SOM) for millennia. When seeds of exotic annual grasses arrived they found that the candy store was open. In this commentary, we review evidence for major shifts in ecosystem processes across the semiarid western USA and we call for more research that links results of ecological restoration efforts to the growing body of knowledge about interactions between invasive annuals and invaded environments. In western US ecosystems vulnerable to weed invasion, the consequence of almost any type of ecosystem disruption is a shift from immobilizing, Plant Soil (2007) 298:1–5 DOI 10.1007/s11104-007-9364-8

Soil Microbial Substrate Properties and Microbial Community Responses under Irrigated Organic and Reduced-Tillage Crop and Forage Production Systems

Changes in soil microbiotic properties such as microbial biomass and community structure in response to alternative management systems are driven by microbial substrate quality and substrate utilization. We evaluated irrigated crop and forage production in two separate four-year experiments for differences in microbial substrate quality, microbial biomass and community structure, and microbial substrate utilization under conventional, organic, and reduced-tillage management systems. The six different management systems were imposed on fields previously under long-term, intensively tilled maize production. Soils under crop and forage production responded to conversion from monocropping to crop rotation, as well as to the three different management systems, but in different ways. Under crop production, four years of organic management resulted in the highest soil organic C (SOC) and microbial biomass concentrations, while under forage production, reduced-tillage management most effectively increased SOC and microbial biomass. There were significant increases in relative abundance of bacteria, fungi, and protozoa, with two- to 36-fold increases in biomarker phospholipid fatty acids (PLFAs). Under crop production, dissolved organic C (DOC) content was higher under organic management than under reduced-tillage and conventional management. Perennial legume crops and organic soil amendments in the organic crop rotation system apparently favored greater soil microbial substrate availability, as well as more microbial biomass compared with other management systems that had fewer legume crops in rotation and synthetic fertilizer applications. Among the forage production management systems with equivalent crop rotations, reduced-tillage management had higher microbial substrate availability and greater microbial biomass than other management systems. Combined crop rotation, tillage management, soil amendments, and legume crops in rotations considerably influenced soil microbiotic properties. More research will expand our understanding of combined effects of these alternatives on feedbacks between soil microbiotic properties and SOC accrual.

Long-term farming systems research in the central High Plains

In recent decades, there has been growing interest among farming and scientific communities toward integrated crop– range–livestock farming because of evidence of increased crop production, soil health, environmental services and resilience to increased climatic variability. This paper reviews studies on existing cropping systems and integrated crop– range–livestock systems across the USA which are relevant in the context of summarizing opportunities and challenges associated with implementing long-term crop–range–livestock systems research in the highly variable environment of the central High Plains. With precipitation ranging from 305 to 484mm and uncertain irrigation water supply, this region is especially vulnerable to changing moisture and temperature patterns. The results of our review indicate that diverse crop rotations, reduced soil disturbance and integrated crop–livestock systems could increase economic returns and agroecosystem resilience. Integrating agricultural system components to acquire unique benefits from small- to mediumsizedoperations,however,isachallengingtask.Thisisbecauseassessmentandidentificationofsuitablefarmingsystems, selection of the most efficient integration scheme, and pinpointing the best management practices are crucial for successful integration of components. Effective integration requires development of evaluation criteria that incorporate the efficiency of approaches under consideration and their interactions. Therefore, establishing the basis for more sustainable farming systems in the central High Plains relies on both long-term agricultural systems research and evaluation of short-term dynamics of individual components.

Alfalfa-grass biomass, soil organic carbon, and total nitrogen under different management approaches in an irrigated agroecosystem

Background and aimsManagement approach may influence forage production as well as soil organic carbon (SOC) and soil total nitrogen (STN) accrued beneath perennial grass-legume components of irrigated crop rotations. This study aimed to evaluate effects of conventional, certified organic, and reduced-tillage management approaches on above- and belowground biomass production and C and N content in alfalfa-grass mixture, and their relationships with SOC and STN.MethodsAn alfalfa-grass mixture was established in 2009 on four replications under a sprinkler irrigation system. Soil characteristics were analyzed at planting time in 2009. Aboveground biomass production, coarse and fine roots, SOC, STN, aboveground biomass C and N, and coarse- and fine-root C and N were quantified in samples collected during 2009–2011.ResultsConventional management produced more aboveground biomass than reduced-tillage and organic, but production under organic matched conventional and exceeded reduced-tillage in the last two harvests of the study. Root production was constant under the three approaches, but resulted in more SOC accrued under reduced-tillage than under the other two approaches.ConclusionsBiomass production was favored by conventional seedbed preparation and soil fertility management while SOC accrual was favored by minimum soil disturbance. In addition, aboveground biomass was influenced by seasonal air temperature, precipitation, and nutrient mineralization from the previous season, so above-/belowground allocation changed seasonally.

A social networks approach for strengthening participation in technology innovation: lessons learnt from the Mount Elgon region of Kenya and Uganda

Participatory approaches to agricultural technology development have not yet fully lived up to their promise to incorporate farmer knowledge. This paper introduces a social networks approach (SNA) to improve participatory research processes for co-innovation. Drawing upon findings from a collaborative project developing conservation agricultural production systems for smallholders in western Kenya and eastern Uganda, the paper explores farmer support networks to improve participation in technological innovation and development. Key research themes include: identifying farmers’ agricultural production networks; the local articulation of agricultural production networks and mindsets; using networks to facilitate meaningful participation in technology development; and disadvantages and advantages of using a social network approach. The introduction of SNA improved participatory research by building external and internal legitimacy for determining who participates, discouraging participatory attrition, and providing a meaningful forum for participation of all stakeholders. As a result, the introduction of SNA is demonstrated to show strong promise for improving processes of participatory technology development in agriculture.

Weed Dynamics during Transition to Conservation Agriculture in Western Kenya Maize Production

Weed competition is a significant problem in maize (Zea mays, L.) production in Sub-Saharan Africa. Better understanding of weed management and costs in maize intercropped with beans (Phaseolus vulgaris, L.) during transition to conservation agricultural systems is needed. Changes in weed population and maize growth were assessed for a period of three years at Bungoma where crops are grown twice per year and at Trans-Nzoia where crops are grown once per year. Treatments included three tillage practices: minimum (MT), no-till (NT) and conventional (CT) applied to three cropping systems: continuous maize/bean intercropping (TYPICAL), maize/bean intercropping with relayed mucuna after bean harvest (RELAY) and maize, bean and mucuna planted in a strip intercropping arrangement (STRIP). Herbicides were used in NT, shallow hand hoeing and herbicides were used in MT and deep hoeing with no herbicides were used in CT. Weed and maize performance in the maize phase of each cropping system were assessed at both locations and costs of weed control were estimated at Manor House only. Weed density of grass and forb species declined significantly under MT and NT at Manor House and of grass species only at Mabanga. The greatest declines of more than 50% were observed as early as within one year of the transition to MT and NT in STRIP and TYPICAL cropping systems at Manor House. Transitioning to conservation based systems resulted in a decline of four out of five most dominant weed species. At the same time, no negative impact of MT or NT on maize growth was observed. Corresponding costs of weed management were reduced by

Drastic Disturbance of Salt-Affected Soils in a Semi-Arid Cool Desert Shrubland

148.40 ha-1 in MT and

Multiple Knowledges for Agricultural Production: Implications for the Development of Conservation Agriculture in Kenya and Uganda

149.60 ha-1 in NT compared with CT. In conclusion, farmers can benefit from effective and less expensive weed management alternatives early in the process of transitioning to reduced tillage operations.