Rodney T. Venterea

Productivity limits and potentials of the principles of conservation agriculture

One of the primary challenges of our time is to feed a growing and more demanding world population with reduced external inputs and minimal environmental impacts, all under more variable and extreme climate conditions in the future. Conservation agriculture represents a set of three crop management principles that has received strong international support to help address this challenge, with recent conservation agriculture efforts focusing on smallholder farming systems in sub-Saharan Africa and South Asia. However, conservation agriculture is highly debated, with respect to both its effects on crop yields and its applicability in different farming contexts. Here we conduct a global meta-analysis using 5,463 paired yield observations from 610 studies to compare no-till, the original and central concept of conservation agriculture, with conventional tillage practices across 48 crops and 63 countries. Overall, our results show that no-till reduces yields, yet this response is variable and under certain conditions no-till can produce equivalent or greater yields than conventional tillage. Importantly, when no-till is combined with the other two conservation agriculture principles of residue retention and crop rotation, its negative impacts are minimized. Moreover, no-till in combination with the other two principles significantly increases rainfed crop productivity in dry climates, suggesting that it may become an important climate-change adaptation strategy for ever-drier regions of the world. However, any expansion of conservation agriculture should be done with caution in these areas, as implementation of the other two principles is often challenging in resource-poor and vulnerable smallholder farming systems, thereby increasing the likelihood of yield losses rather than gains. Although farming systems are multifunctional, and environmental and socio-economic factors need to be considered, our analysis indicates that the potential contribution of no-till to the sustainable intensification of agriculture is more limited than often assumed.

Climate, duration, and N placement determine N2O emissions in reduced tillage systems: a meta-analysis

No-tillage and reduced tillage (NT/RT) management practices are being promoted in agroecosystems to reduce erosion, sequester additional soil C and reduce production costs. The impact of NT/RT on N2 O emissions, however, has been variable with both increases and decreases in emissions reported. Herein, we quantitatively synthesize studies on the short- and long-term impact of NT/RT on N2 O emissions in humid and dry climatic zones with emissions expressed on both an area- and crop yield-scaled basis. A meta-analysis was conducted on 239 direct comparisons between conventional tillage (CT) and NT/RT. In contrast to earlier studies, averaged across all comparisons, NT/RT did not alter N2 O emissions compared with CT. However, NT/RT significantly reduced N2 O emissions in experiments >10xa0years, especially in dry climates. No significant correlation was found between soil texture and the effect of NT/RT on N2 O emissions. When fertilizer-N was placed at ≥5xa0cm depth, NT/RT significantly reduced area-scaled N2 O emissions, in particular under humid climatic conditions. Compared to CT under dry climatic conditions, yield-scaled N2 O increased significantly (57%) when NT/RT was implemented <10xa0years, but decreased significantly (27%) after ≥10xa0years of NT/RT. There was a significant decrease in yield-scaled N2 O emissions in humid climates when fertilizer-N was placed at ≥5xa0cm depth. Therefore, in humid climates, deep placement of fertilizer-N is recommended when implementing NT/RT. In addition, NT/RT practices need to be sustained for a prolonged time, particularly in dry climates, to become an effective mitigation strategy for reducing N2 O emissions.

Fertilizer source and tillage effects on yield-scaled nitrous oxide emissions in a corn cropping system.

Management practices such as fertilizer or tillage regime may affect nitrous oxide (N₂O) emissions and crop yields, each of which is commonly expressed with respect to area (e.g., kg N ha or Mg grain ha). Expressing N₂O emissions per unit of yield can account for both of these management impacts and might provide a useful metric for greenhouse gas inventories by relating N₂O emissions to grain production rates. The objective of this study was to examine the effects of long-term (>17 yr) tillage treatments and N fertilizer source on area- and yield-scaled N₂O emissions, soil N intensity, and nitrogen use efficiency for rainfed corn ( L.) in Minnesota over three growing seasons. Two different controlled-release fertilizers (CRFs) and conventional urea (CU) were surface-applied at 146 kg N ha(-1) several weeks after planting to conventional tillage (CT) and no-till (NT) treatments. Yield-scaled emissions across all treatments represented 0.4 to 1.1% of the N harvested in the grain. Both CRFs reduced soil nitrate intensity, but not N₂O emissions, compared with CU. One CRF, consisting of nitrification and urease inhibitors added to urea, decreased N₂O emissions compared with a polymer-coated urea (PCU). The PCU tended to have lower yields during the drier years of the study, which increased its yield-scaled N₂O emissions. The overall effectiveness of CRFs compared with CU in this study may have been reduced because they were applied several weeks after corn was planted. Across all N treatments, area-scaled N₂O emissions were not significantly affected by tillage. However, when expressed per unit yield of grain, grain N, or total aboveground N, N₂O emissions with NT were 52, 66, and 69% greater, respectively, compared with CT. Thus, in this cropping system and climate regime, production of an equivalent amount of grain using NT would generate substantially more N₂O compared with CT.

Biochar’s role as an alternative N-fertilizer: ammonia capture

BackgroundBiochar’s role as a carbon sequestration agent, while simultaneously providing soil fertility improvements when used as an amendment, has been receiving significant attention across all sectors of society, ranging from academia, industry, government, as well as the general public. This has lead to some exaggeration and possible confusion regarding biochar’s actual effectiveness as a soil amendment. One sparsely explored area where biochar appears to have real potential for significant impact is the soil nitrogen cycle.ScopeTaghizadeh-Toosi et al. (this issue) examined ammonia sorption on biochar as a means of providing a nitrogen-enriched soil amendment. The longevity of the trapped ammonia was particularly remarkable; it was sequestered in a stable form for at least 12xa0days under laboratory air flow. Furthermore, the authors observed increased 15N uptake by plants grown in soil amended with the 15N-enriched biochar, indicating that the 15N was not irreversibly bound, but, was plant-available.ConclusionsTheir observations add credence to utilizing biochar as a carrier for nitrogen fertilization, while potentially reducing the undesired environmental consequences through gas emissions, overland flow, and leaching.

Challenges and opportunities for mitigating nitrous oxide emissions from fertilized cropping systems

Nitrous oxide (N2O) is often the largest single component of the greenhouse-gas budget of individual cropping systems, as well as for the US agricultural sector as a whole. Here, we highlight the factors that make mitigating N2O emissions from fertilized agroecosystems such a difficult challenge, and discuss how these factors limit the effectiveness of existing practices and therefore require new technologies and fresh ideas. Modification of the rate, source, placement, and/or timing of nitrogen fertilizer application has in some cases been an effective way to reduce N2O emissions. However, the efficacy of existing approaches to reducing N2O emissions while maintaining crop yields across locations and growing seasons is uncertain because of the interaction of multiple factors that regulate several different N2O-producing processes in soil. Although these processes have been well studied, our understanding of key aspects and our ability to manage them to mitigate N2O emissions remain limited.

Effects of Elevated Carbon Dioxide and Increased Temperature on Methane and Nitrous Oxide Fluxes: Evidence from Field Experiments

Climate change could alter terrestrial ecosystems, which are important sources and sinks of the potent green-house gases (GHGs) nitrous oxide (N2O) and methane (CH4), in ways that either stimulate or decrease the magnitude and duration of global warming. Using manipulative field experiments, we assessed how N2O and CH4 soil fluxes responded to a rise in atmospheric carbon dioxide (CO2) concentration and to increased air temperature. Nitrous oxide and CH4 responses varied greatly among studied ecosystems. Elevated CO2 often stimulated N2O emissions in fertilized systems and CH4 emissions in wetlands, peatlands, and rice paddy fields; both effects were stronger in clayey soils than in sandy upland soils. Elevated temperature, however, impacted N2O and CH4 emissions inconsistently. Thus, the effects of elevated CO2 concentrations on N2O and CH4 emissions may further enhance global warming, but it remains unclear whether increased temperature generates positive or negative feedbacks on these GHGs in terrestri...

Calculating the detection limits of chamber-based soil greenhouse gas flux measurements.

Renewed interest in quantifying greenhouse gas emissions from soil has led to an increase in the application of chamber-based flux measurement techniques. Despite the apparent conceptual simplicity of chamber-based methods, nuances in chamber design, deployment, and data analyses can have marked effects on the quality of the flux data derived. In many cases, fluxes are calculated from chamber headspace vs. time series consisting of three or four data points. Several mathematical techniques have been used to calculate a soil gas flux from time course data. This paper explores the influences of sampling and analytical variability associated with trace gas concentration quantification on the flux estimated by linear and nonlinear models. We used Monte Carlo simulation to calculate the minimum detectable fluxes (α = 0.05) of linear regression (LR), the Hutchinson/Mosier (H/M) method, the quadratic method (Quad), the revised H/M (HMR) model, and restricted versions of the Quad and H/M methods over a range of analytical precisions and chamber deployment times (DT) for data sets consisting of three or four time points. We found that LR had the smallest detection limit thresholds and was the least sensitive to analytical precision and chamber deployment time. The HMR model had the highest detection limits and was most sensitive to analytical precision and chamber deployment time. Equations were developed that enable the calculation of flux detection limits of any gas species if analytical precision, chamber deployment time, and ambient concentration of the gas species are known.

A Mechanistic Treatment of the Dominant Soil Nitrogen Cycling Processes: Model Development, Testing, and Application

The broad impact of NO and N2O gas emissions on climate change are widely recognized [e.g., Mosier,1998; Vitousek et al., 1997], as are the effects of NO3x01 water contamination on human health [e.g., Kapoor and Viraraghavan, 1997] and eutrophication [e.g., Cloern, 2001]. Methods for evaluating the impacts of climate change, and fertilizer and water application techniques onN losses in agriculture are needed for both scientific investigations and management to limit N losses [e.g., Mosier et al., 1996; Matson et al., 1998; Subbarao et al., 2006]. In view of the needs for future increases in crop yield for food,fiber, and biofuel production, understanding the processes that regulate losses of solute and gaseous N species assumes even greater importance.

Nitric and nitrous oxide emissions following fertilizer application to agricultural soil: Biotic and abiotic mechanisms and kinetics

Emissions of nitric and nitrous oxide (NO and N2O) from agricultural soils may have several consequences, including impacts on local tropospheric and global stratospheric chemistry. Elevated NO and N2O emissions following application of anhydrous ammonia to an agricultural field in California were driven by the biological generation of nitrite (NO2−) and subsequent abiotic decomposition of nitrous acid (HNO2). Maximum fluxes of > 1000 ng NO-N cm−2 h−1 and > 400 ng N2O-N cm−2 h−1 were observed, and emissions of > 100 ng NO-N cm−2 h−1 and > 50 ng N2O-N cm−2 h−1 persisted for >4 weeks. Laboratory experiments were performed to determine rate coefficients and activation energies for HNO2-mediated NO and N2O production. Kinetic parameters describing the conversion of NO to N2O were measured and were found to vary with water-filled pore space (WFPS). Regression models incorporating HNO2, WFPS, and temperature accounted for 75–77% of the variability in field fluxes. A previously developed NO emissions model was modified to incorporate a kinetic expression for HNO2- and temperature-dependent production. The model tended to underestimate fluxes under low-flux conditions and overestimate fluxes under high-flux conditions. These data indicate that (1) control of acidity may be an effective means for minimizing gaseous N losses from fertilized soils and possibly for improving air quality in rural areas, (2) the transformation of HNO2-derived NO may be an important mechanism of N2O production even under relatively aerobic conditions, and (3) mechanistic models which account for spatial heterogeneity and transient conditions may be required to better predict field NO fluxes.

Simplified method for quantifying theoretical underestimation of chamber-based trace gas fluxes.

Closed chambers used to measure soil-atmosphere exchange of trace gases including nitrous oxide (N(2)O) and carbon dioxide (CO(2)) generate errors due to suppression of the gas concentration gradient at the soil-atmosphere interface. A method is described here for estimating the magnitude of flux underestimation arising from chamber deployment. The technique is based on previously established gas transport theory and has been simplified to facilitate application while preserving the fundamental physical relationships. The method avoids the use of nonlinear regression but requires knowledge of soil properties including texture, bulk density, water content, temperature, and pH. Two options are presented: a numerical technique which is easily adapted to spreadsheet application, and a graphical method requiring minimal calculation. In both cases, the magnitude of theoretical flux underestimation (TFU) is determined, taking into account effects of chamber geometry and deployment time, the flux-calculation scheme, and properties of the soil and gas under consideration. Application to actual data and recent studies confirmed that TFU can vary widely within and across sites. The analysis also revealed a highly linear correlation between soil water content and TFU, suggesting that previously observed relationships between water content and trace gas flux may in part reflect artifacts of chamber methodology. The method described here provides a practical means of improving the absolute accuracy of flux estimates and normalizing data obtained using different chamber designs in different soils.