G. P. Robertson

Nutrient Imbalances in Agricultural Development

Nutrient additions to intensive agricultural systems range from inadequate to excessive—and both extremes have substantial human and environmental costs. Nutrient cycles link agricultural systems to their societies and surroundings; inputs of nitrogen and phosphorus in particular are essential for high crop yields, but downstream and downwind losses of these same nutrients diminish environmental quality and human well-being. Agricultural nutrient balances differ substantially with economic development, from inputs that are inadequate to maintain soil fertility in parts of many developing countries, particularly those of sub-Saharan Africa, to excessive and environmentally damaging surpluses in many developed and rapidly growing economies. National and/or regional policies contribute to patterns of nutrient use and their environmental consequences in all of these situations (1). Solutions to the nutrient challenges that face global agriculture can be informed by analyses of trajectories of change within, as well as across, agricultural systems.

Sustainable bioenergy production from marginal lands in the US Midwest

Legislation on biofuels production in the USA and Europe is directing food crops towards the production of grain-based ethanol, which can have detrimental consequences for soil carbon sequestration, nitrous oxide emissions, nitrate pollution, biodiversity and human health. An alternative is to grow lignocellulosic (cellulosic) crops on ‘marginal’ lands. Cellulosic feedstocks can have positive environmental outcomes and could make up a substantial proportion of future energy portfolios. However, the availability of marginal lands for cellulosic feedstock production, and the resulting greenhouse gas (GHG) emissions, remains uncertain. Here we evaluate the potential for marginal lands in ten Midwestern US states to produce sizeable amounts of biomass and concurrently mitigate GHG emissions. In a comparative assessment of six alternative cropping systems over 20 years, we found that successional herbaceous vegetation, once well established, has a direct GHG emissions mitigation capacity that rivals that of purpose-grown crops (−851 ± 46 grams of CO2 equivalent emissions per square metre per year (gCO2e m−2 yr−1)). If fertilized, these communities have the capacity to produce about 63 ± 5 gigajoules of ethanol energy per hectare per year. By contrast, an adjacent, no-till corn–soybean–wheat rotation produces on average 41 ± 1 gigajoules of biofuel energy per hectare per year and has a net direct mitigation capacity of −397 ± 32 gCO2e m−2 yr−1; a continuous corn rotation would probably produce about 62 ± 7 gigajoules of biofuel energy per hectare per year, with 13% less mitigation. We also perform quantitative modelling of successional vegetation on marginal lands in the region at a resolution of 0.4 hectares, constrained by the requirement that each modelled location be within 80 kilometres of a potential biorefinery. Our results suggest that such vegetation could produce about 21 gigalitres of ethanol per year from around 11 million hectares, or approximately 25 per cent of the 2022 target for cellulosic biofuel mandated by the US Energy Independence and Security Act of 2007, with no initial carbon debt nor the indirect land-use costs associated with food-based biofuels. Other regional-scale aspects of biofuel sustainability, such as water quality and biodiversity, await future study.

Role of denitrifier diversity in rates of nitrous oxide consumption in a terrestrial ecosystem

Abstract The ecosystem consequences of microbial diversity are largely unknown. We tested the hypothesis that soil microbial diversity influences ecosystem function by quantifying denitrification enzyme activity among denitrifying bacteria isolated from two geomorphically similar soils with significantly different in situ nitrous oxide (N2O) emission rates. We sampled soil from two southwest Michigan sites on the same soil series that differed in plant community composition and disturbance regime — a conventionally-tilled agricultural field and a never-tilled successional field. We isolated denitrifying bacteria from these soils, characterized them based on their fatty acid profiles, and compared denitrifier community composition for the two fields. For 31 representative isolates, we measured the sensitivity of nitrous oxide reductase (Nos) — which catalyzes the reduction of N2O to N2 — to low oxygen concentrations. Of the 93 denitrifying bacteria isolated from the agricultural field and 63 from the successional field, fatty acid profiles suggested the presence of 27 denitrifying taxa with only 12 common to both soils. In each field type the four numerically dominant taxa were either rare or absent in the other field. In addition, we found substantial diversity in the sensitivity of isolate Nos enzymes to oxygen, indicating that the taxonomic diversity present among denitrifiers in these two soils is functionally significant. These results demonstrate a clear physiological basis for differences in denitrifier community function previously described ( Cavigelli and Robertson, 2000. The functional significance of denitrifier community composition in a terrestrial ecosystem. Ecology 81, 229–241. ) and indicate that differences in denitrifier community composition alone can potentially influence in situ N2O production.

Evolution of CO2 and soil carbon dynamics in biologically managed, row-crop agroecosystems

Field CO2 production was related to soil carbon pools and fluxes determined by laboratory incubation of soils from agroecosystems designed to test the possibility of substituting biological for chemical inputs. Treatments included: conventional and organic-based row crops, woody and herbaceous perennial crops and historically tilled and never tilled successional fields. The CO2 efflux in corn and soybeans was affected by crop residues from previous years and growing season temperatures but not soil moisture. Overwinter cover crops and perennials such as alfalfa and poplar, resulted in fairly uniform fluxes of approximately 20 kg CO2‐C ha ˇ1 day ˇ1 throughout the non-frozen period. Highest fluxes occurred in alfalfa, historically tilled successional and never tilled, grassland successional treatments, although, highest aboveground productivity occurred in the corn and poplar. Summed, field CO2 fluxes were similar to residue-C inputs. Measurement of CO2 mineralized in extended incubations in the laboratory made it possible to use soil enzyme activity to determine the size and dynamics of soil C pools. The residue of acid hydrolysis defined the size of the resistant pool Cr. Carbon dating determined its mean residence time (MRT). Curve analyses of CO2 evolution plotted on a per unit time basis gave the active (Ca) and slow (Cs) pool sizes and decomposition rate constants ka and ks. Temperature correction factors provided field MRTs. The active pool of this coarse textured soil represents 2% of the soil C with a MRT of 30‐66 days. The slow pool represents 40‐45% of the SOC with field MRTs of 9‐13 years. The poplar soil has the greatest MRT for both the active and slow pools. The system approach to land use sustainability (SALUS) model, which predicts CO2 evolution from decomposition in the field as part of a plant growth ‐ soil process model, was tested using the decomposition parameters determined by incubation and 14 C dating. The model satisfactorily predicted the intra and inter year differences in field CO2 but over predicted fluxes from residues in the fall. It does not yet adequately consider a lag period during which the residues lose their hydrophobicity, are comminuted and colonized. # 1999 Elsevier Science B.V.

Denitrification in a clearcut Loblolly pine (Pinus taeda L.) plantation in the southeastern US

SummaryWe examined denitrification and nitrous oxide (N2O) production in intact soil cores removed from a clearcut southern pine site subjected to different harvest, site preparation, and herbicide treatments. Rates of N2O production in structurally intact soil cores incubated with acetylene showed that clearcutting stimulated denitrification but that rates varied by sample date and post-harvest site treatment. The site was harvested in December 1980. In September 1982 denitrification was greater in sheared, piled and disked (SPD) plots than in chopped or reference (uncut) plots; the following May, rates were higher in seven of the eight treatment plots than in the reference plot, and were highest in three of the four herbicide-treated plots. On both sample dates denitrification rates were correlated with nitrification potentials and nitrate pool sizes in the plots, and nitrate added to cores from all treatments significantly stimulated denitrification. Nitrate supply thus appeared to regulate denitrification at this site. Relative to harvest or site preparation losses of nitrogen, denitrification is not a major vector of N loss at this coniferous site; under post-harvest conditions, however, denitrification may be of the same magnitude as leaching losses.

Fluxes of CH4 and N2O in aspen stands grown under ambient and twice-ambient CO2

Elevated atmospheric CO2 has the potential to change below-ground nutrient cycling and thereby alter the soil-atmosphere exchange of biogenic trace gases. We measured fluxes of CH4 and N2O in trembling aspen (Populus tremuloides Michx.) stands grown in open-top chambers under ambient and twice-ambient CO2 concentrations crossed with ‘high’ and low soil-N conditions.Flux measurements with small static chambers indicated net CH4 oxidation in the open-top chambers. Across dates, CH4 oxidation activity was significantly (P < 0.05) greater with ambient CO2 (8.7 μg CH4-C m-2 h-1) than with elevated CO2 (6.5 μg CH4-C m-2 h-1) in the low N soil. Likewise, across dates and soil N treatments CH_4 was oxidized more rapidly (P < 0.05) in chambers with ambient CO2 (9.5 μg CH4-C m-2 h-1) than in chambers with elevated CO2 (8.8 μg CH4-C m-2 h-1). Methane oxidation in soils incubated in serum bottles did not show any response to the CO2 treatment. We suggest that the depressed CH4 oxidation under elevated CO2 in the field chambers is due to soil moisture which tended to be higher in the twice-ambient CO2 treatment than in the ambient CO2 treatment.Phase I denitrification (denitrification enzyme activity) was 12–26% greater under elevated CO2 than under ambient CO2 in the ‘high’ N soil; one sampling, however, showed a 39% lower enzyme activity with elevated CO2. In both soil N treatments, denitrification potentials measured after 24 or 48 h were between 11% and 21% greater (P < 0.05) with twice-ambient CO2 than with ambient CO2. Fluxes of N2O in the open-top chambers and in separate 44 cm2 cores ±N fertilization were not affected by CO2 treatment and soil N status.Our data show that elevated atmospheric CO2 may have a negative effect on terrestrial CH4 oxidation. The data also indicated temporary greater denitrification with elevated CO2 than with ambient CO2. In contrast, we found no evidence for altered fluxes of N2O in response to increases in atmospheric CO2

Nitrogen cycling in poplar stands defoliated by insects

Large-scale outbreaks of defoliating insects are common in temperate forests. These outbreaks are thought to be responsible for substantial cycling of nitrogen (N), and its loss from the system. Gypsy moth (Lymantria dispar) populations within poplar plots were manipulated over 2 years so that the ecosystem-wide consequences of catastrophic defoliation on N cycling could be examined. The quantities of N in leaf litter-fall, ammonia volatilization and soil N pools were estimated across the two seasons. Defoliated leaf biomass was estimated from experimentally derived approximate digestibility factors and added to the mass of senesced leaf to determine total annual leaf production. Throughout the growing season the defoliation treatment peaked at about 40% in year 1 and 100% in year 2. Rapid regrowth after defoliation meant that only 45% of the annual leaf biomass was consumed in the defoliation treatment in year 2, while control plots suffered about 20% consumption each year. In each year, defoliated plots produced 20% more leaf biomass and N than the controls, a phenomenon attributed to compensatory photosynthesis. No substantial losses of N via ammonia volatilization, nitrous oxide emission or nitrate leaching were observed. Neither was there any sustained or substantial gain in the soils microbial biomass or inorganic N pools. These observations suggest that the defoliated poplars were able to compete with soil microbes and N loss mechanisms for soil N as it became available, thereby ameliorating the effects of defoliation on soil nitrogen cycling. We conclude from this study that the N mineralized from defoliation residues was conserved in this plantation ecosystem.

Comparative water use by maize, perennial crops, restored prairie, and poplar trees in the US Midwest

Water use by plant communities across years of varying water availability indicates how terrestrial water balances will respond to climate change and variability as well as to land cover change. Perennial biofuel crops, likely grown mainly on marginal lands of limited water availability, provide an example of a potentially extensive future land cover conversion. We measured growing-season evapotranspiration (ET) based on daily changes in soil profile water contents in five perennial systems—switchgrass, miscanthus, native grasses, restored prairie, and hybrid poplar—and in annual maize (corn) in a temperate humid climate (Michigan, USA). Three study years (2010, 2011 and 2013) had normal growing-season rainfall (480–610 mm) whereas 2012 was a drought year (210 mm). Over all four years, mean (±SEM) growing-season ET for perennial systems did not greatly differ from corn (496 ± 21 mm), averaging 559 (±14), 458 (±31), 573 (±37), 519 (±30), and 492 (±58) mm for switchgrass, miscanthus, native grasses, prairie, and poplar, respectively. Differences in biomass production largely determined variation in water use efficiency (WUE). Miscanthus had the highest WUE in both normal and drought years (52–67 and 43 kg dry biomass ha−1 mm−1, respectively), followed by maize (40–59 and 29 kg ha−1 mm−1); the native grasses and prairie were lower and poplar was intermediate. That measured water use by perennial systems was similar to maize across normal and drought years contrasts with earlier modeling studies and suggests that rain-fed perennial biomass crops in this climate have little impact on landscape water balances, whether replacing rain-fed maize on arable lands or successional vegetation on marginal lands. Results also suggest that crop ET rates, and thus groundwater recharge, streamflow, and lake levels, may be less sensitive to climate change than has been assumed.

Regional nitrogen budgets: Approaches and problems

Regional nitrogen budgets are useful for assessing what is known about nitrogen cycling in important ecosystems of a region, for placing the various regional fluxes and pools into perspective, and for providing insight into the processes that regulate both regional and global nitrogen cycling. Existing regional budgest have been used both to study groundwater nitrate pollution and to help identify local ecosystems that are important on a land-use basis but that are poorly described biogeochemically. Avoidable problems common to many budgets include inappropriate compartment components, inadequate documentation, and unjustified certainty. Though imprecise, large-scale nutrient budgets at our present stage of understanding offer to researchers and system managers important advantages that would otherwise not be available.ResumenLos balances regionales de nitrógeno son útiles para estimar el estado del conocimiento sobre el ciclo de nitrógeno en los ecosistemas mas importantes de la región, para enfocar los flujos y reservas regionales en perspectiva y para adentrarse en los procesos que regulan tanto los ciclos regionales como los globales. Los balances regionales existentes se han utilizado para estudiar la contaminación de aguas freáticas con nitratos y para identificar aquellos ecosistemas localmente improtantes desde el punto de vista del uso de la tierra pero que son poco conocidos biogeoquímicamente. Algunos problemas que son comunes a muchos balances son: la selección de compartimientos inapropiados, documentación inadecuada y la certeza injustificada. Aun cuando sean imprecisos, los balances en gran escala, en el estado actual del conocimiento, aportan a investigadores y a quienes manjean los sistemas, algunas ventajas importantes que no estarían disponibles de otro modo.

Enhancing the soil and water assessment tool model for simulating N2O emissions of three agricultural systems

Abstract Nitrous oxide (N2O) is a potent greenhouse gas (GHG) contributing to global warming, with the agriculture sector as the major source of anthropogenic N2O emissions due to excessive fertilizer use. There is an urgent need to enhance regional‐/watershed‐scale models, such as Soil and Water Assessment Tool (SWAT), to credibly simulate N2O emissions to improve assessment of environmental impacts of cropping practices. Here, we integrated the DayCent models N2O emission algorithms with the existing widely tested crop growth, hydrology, and nitrogen cycling algorithms in SWAT and evaluated this new tool for simulating N2O emissions in three agricultural systems (i.e., a continuous corn site, a switchgrass site, and a smooth brome grass site which was used as a reference site) located at the Great Lakes Bioenergy Research Center (GLBRC) scale‐up fields in southwestern Michigan. These three systems represent different levels of management intensity, with corn, switchgrass, and smooth brome grass (reference site) receiving high, medium, and zero fertilizer application, respectively. Results indicate that the enhanced SWAT model with default parameterization reproduced well the relative magnitudes of N2O emissions across the three sites, indicating the usefulness of the new tool (SWAT‐N2O) to estimate long‐term N2O emissions of diverse cropping systems. Notably, parameter calibration can significantly improve model simulations of seasonality of N2O fluxes, and explained up to 22.5%–49.7% of the variability in field observations. Further sensitivity analysis indicates that climate change (e.g., changes in precipitation and temperature) influences N2O emissions, highlighting the importance of optimizing crop management under a changing climate in order to achieve agricultural sustainability goals.