Rajan Ghimire

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.

Greenhouse Gas Fluxes and Soil Carbon and Nitrogen Following Single Summer Tillage Event

No-till farming results in gradual buildup of soil organic matter (SOM) and re-introduction of tillage can often reverse it. However, tillage in low precipitation regions may be needed to manage weeds and disperse accumulation of immobile soil nutrients. The main objective of this study was to assess the effects of a single summer tillage on carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), soil water filled pore space (WFPS), dissolved organic carbon (DOC) and nitrate (NO3) in winter wheat summer fallow systems that were either tilled for the first time after nine years of no-till (NTT), not-tilled (no-till, NT) or were frequently tilled (conventional, CT; and organic, CF). The study was established in the US Central High Plains region where annual precipitation averaged 332±39 mm. Soil and gas samples were collected before the tillage event (time zero) and at 1hr, 5 hrs, 25 hrs and 50 hrs after. Immediate increases in CO2 and N2O fluxes were observed in all tilled treatments within the first 1 to 5 hours but 50-hr cumulative N2O and CO2 in NTT did not differ from Original Research Article Bista et al.; IJPSS, 6(4): 183-193, 2015; Article no.IJPSS.2015.109 184 the values observed in NT. Tillage however, resulted in a 22% greater 50-hr cumulative CH4 assimilation in NTT compared with NT and was comparable with CH4 in CT suggesting enhanced soil aeration. Soil NO3 did not change in NTT unlike in CT and CF and soil DOC did not increase in NTT until 25 hrs after when, it returned to levels comparable with time zero. In contrast, DOC in CT and CF continued to stay elevated after 50 hrs. In conclusion, single tillage event of a long-term notill performed on dry soil during summer did not negate benefits associated with SOM accrual and may be a viable alternative for farmers to address some of the management-related problems.

Water and nitrogen management effects on semiarid sorghum production and soil trace gas flux under future climate

External inputs to agricultural systems can overcome latent soil and climate constraints on production, while contributing to greenhouse gas emissions from fertilizer and water management inefficiencies. Proper crop selection for a given region can lessen the need for irrigation and timing of N fertilizer application with crop N demand can potentially reduce N2O emissions and increase N use efficiency while reducing residual soil N and N leaching. However, increased variability in precipitation is an expectation of climate change and makes predicting biomass and gas flux responses to management more challenging. We used the DayCent model to test hypotheses about input intensity controls on sorghum (Sorghum bicolor (L.) Moench) productivity and greenhouse gas emissions in the southwestern United States under future climate. Sorghum had been previously parameterized for DayCent, but an inverse-modeling via parameter estimation method significantly improved model validation to field data. Aboveground production and N2O flux were more responsive to N additions than irrigation, but simulations with future climate produced lower values for sorghum than current climate. We found positive interactions between irrigation at increased N application for N2O and CO2 fluxes. Extremes in sorghum production under future climate were a function of biomass accumulation trajectories related to daily soil water and mineral N. Root C inputs correlated with soil organic C pools, but overall soil C declined at the decadal scale under current weather while modest gains were simulated under future weather. Scaling biomass and N2O fluxes by unit N and water input revealed that sorghum can be productive without irrigation, and the effect of irrigating crops is difficult to forecast when precipitation is variable within the growing season. These simulation results demonstrate the importance of understanding sorghum production and greenhouse gas emissions at daily scales when assessing annual and decadal-scale management decisions’ effects on aspects of arid and semiarid agroecosystem biogeochemistry.