G. Philip Robertson

Nitrous oxide emission from Australian agricultural lands and mitigation options: a review

Increases in the concentrations of greenhouse gases, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and halocarbons in the atmosphere due to human activities are associated with global climate change. The concentration of N2O has increased by 16% since 1750. Although the atmospheric concentration of N2O is much smaller (314 ppb in 1998) than of CO2 (365 ppm), its global warming potential (cumulative radiative forcing) is 296 times that of the latter in a 100-year time horizon. Currently, it contributes about 6% of the overall global warming effect but its contribution from the agricultural sector is about 16%. Of that, almost 80% of N2O is emitted from Australian agricultural lands, originating from N fertilisers (32%), soil disturbance (38%), and animal waste (30%). Nitrous oxide is primarily produced in soil by the activities of microorganisms during nitrification, and denitrification processes. The ratio of N2O to N2 production depends on oxygen supply or water-filled pore space, decomposable organic carbon, N substrate supply, temperature, and pH and salinity. N2O production from soil is sporadic both in time and space, and therefore, it is a challenge to scale up the measurements of N2O emission from a given location and time to regional and national levels. Estimates of N2O emissions from various agricultural systems vary widely. For example, in flooded rice in the Riverina Plains, N2O emissions ranged from 0.02% to 1.4% of fertiliser N applied, whereas in irrigated sugarcane crops, 15.4% of fertiliser was lost over a 4-day period. Nitrous oxide emissions from fertilised dairy pasture soils in Victoria range from 6 to 11 kg N2O-N/ha, whereas in arable cereal cropping, N2O emissions range from <0.01% to 9.9% of N fertiliser applications. Nitrous oxide emissions from soil nitrite and nitrates resulting from residual fertiliser and legumes are rarely studied but probably exceed those from fertilisers, due to frequent wetting and drying cycles over a longer period and larger area. In ley cropping systems, significant N2O losses could occur, from the accumulation of mainly nitrate-N, following mineralisation of organic N from legume-based pastures. Extensive grazed pastures and rangelands contribute annually about 0.2 kg N/ha as N2O (93 kg/ha per year CO2-equivalent). Tropical savannas probably contribute an order of magnitude more, including that from frequent fires. Unfertilised forestry systems may emit less but the fertilised plantations emit more N2O than the extensive grazed pastures. However, currently there are limited data to quantify N2O losses in systems under ley cropping, tropical savannas, and forestry in Australia. Overall, there is a need to examine the emission factors used in estimating national N2O emissions; for example, 1.25% of fertiliser or animal-excreted N appearing as N2O (IPCC 1996). The primary consideration for mitigating N2O emissions from agricultural lands is to match the supply of mineral N (from fertiliser applications, legume-fixed N, organic matter, or manures) to its spatial and temporal needs by crops/pastures/trees. Thus, when appropriate, mineral N supply should be regulated through slow-release (urease and/or nitrification inhibitors, physical coatings, or high C/N ratio materials) or split fertiliser application. Also, N use could be maximised by balancing other nutrient supplies to plants. Moreover, non-legume cover crops could be used to take up residual mineral N following N-fertilised main crops or mineral N accumulated following legume leys. For manure management, the most effective practice is the early application and immediate incorporation of manure into soil to reduce direct N2O emissions as well as secondary emissions from deposition of ammonia volatilised from manure and urine. Current models such as DNDC and DAYCENT can be used to simulate N2O production from soil after parameterisation with the local data, and appropriate modification and verification against the measured N2O emissions under different management practices.

THERMODYNAMIC CONSTRAINTS ON NITROGEN TRANSFORMATIONS AND OTHER BIOGEOCHEMICAL PROCESSES AT SOIL-STREAM INTERFACES

There is much interest in biogeochemical processes that occur at the interface between soils and streams since, at the scale of landscapes, these habitats may function as control points for fluxes of nitrogen (N) and other nutrients from terrestrial to aquatic ecosystems. Here we examine whether a thermodynamic perspective can enhance our mechanistic and predictive understanding of the biogeochemical function of soil-stream interfaces, by considering how microbial communities interact with variations in supplies of electron donors and acceptors. Over a two-year period we analyzed >1400 individual samples of subsurface waters from networks of sample wells in riparian wetlands along Smith Creek, a first-order stream draining a mixed forested-agricultural landscape in southwestern Michigan, USA. We focused on areas where soil water and ground water emerged into the stream, and where we could characterize subsurface flow paths by measures of hydraulic head and/or by in situ additions of hydrologic tracers. We found strong support for the idea that the biogeochemical function of soil-stream interfaces is a predictable outcome of the interaction between microbial communities and supplies of electron donors and acceptors. Variations in key electron donors and acceptors (NO 3 - , N 2 O, NH 4 + , SO 4 2- , CH 4 , and dissolved organic carbon [DOC]) closely followed predictions from thermodynamic theory. Transformations of N and other elements resulted from the response of microbial communities to two dominant hydrologic flow paths: (1) horizontal flow of shallow subsurface waters with high levels of electron donors (i.e., DOC, CH 4 , and NH 4 + ), and (2) near-stream vertical upwelling of deep subsurface waters with high levels of energetically favorable electron acceptors (i.e., NO 3 - , N 2 O, and SO 4 2- ). Our results support the popular notion that soil-stream interfaces can possess strong potential for removing dissolved N by denitrification. Yet in contrast to prevailing ideas, we found that denitrification did not consume all NO 3 - that reached the soil-stream interface via subsurface flow paths. Analyses of subsurface N chemistry and natural abundances of δ 15 N in NO 3 - and NH 4 + suggested a narrow near-stream region as functionally the most important location for NO 3 - consumption by denitrification. This region was characterized by high throughput of terrestrially derived water, by accumulation of dissolved NO 3 - and N 2 O, and by low levels of DOC. Field experiments supported our hypothesis that the sustained ability for removal of dissolved NO 3 - and N 2 O should be limited by supplies of oxidizable carbon via shallow flowpaths. In situ additions of acetate, succinate, and propionate induced rates of NO 3 - removal (∼1.8 g N.m -2 .d -1 ) that were orders of magnitude greater than typically reported from riparian habitats. We propose that the immediate near-stream region may be especially important for determining the landscape-level function of many riparian wetlands. Management efforts to optimize the removal of NO 3 - by denitrification ought to consider promoting natural inputs of oxidizable carbon to this near-stream region.

Sustainable Biofuels Redux

Science-based policy is essential for guiding an environmentally sustainable approach to cellulosic biofuels.

Fixing a critical climate accounting error

Rules for applying the Kyoto Protocol and national cap-and-trade laws contain a major, but fixable, carbon accounting flaw in assessing bioenergy. The accounting now used for assessing compliance with carbon limits in the Kyoto Protocol and in climate legislation contains a far-reaching but fixable flaw that will severely undermine greenhouse gas reduction goals (1). It does not count CO2 emitted from tailpipes and smokestacks when bioenergy is being used, but it also does not count changes in emissions from land use when biomass for energy is harvested or grown. This accounting erroneously treats all bioenergy as carbon neutral regardless of the source of the biomass, which may cause large differences in net emissions. For example, the clearing of long-established forests to burn wood or to grow energy crops is counted as a 100% reduction in energy emissions despite causing large releases of carbon.

Reconciling agricultural productivity and environmental integrity: a grand challenge for agriculture

Agriculture meets a major human need and both affects and depends on all other life support systems. Current trends point to continued human population growth and ever higher levels of consumption as the global economy expands. This will stress the capacity of agriculture to meet food needs without further sacrificing the environmental integrity of local landscapes and the global environment. Agricultures main challenge for the coming decades will be to produce sufficient food and fiber for a growing global population at an acceptable environmental cost. This challenge requires an ecological approach to agriculture that is largely missing from current management and research portfolios. Crop and livestock production systems must be managed as ecosystems, with management decisions fully informed of environmental costs and benefits. Currently, too little is known about important ecological interactions in major agricultural systems and landscapes and about the economic value of the ecosystem services assoc...

Spatial Variability in a Successional Plant Community: Patterns of Nitrogen Availability

We examined the spatial variability of N mineralization, nitrification, and denitrification at a resolution of 1 m over a 0.5-ha portion of an old field in southeast Michigan. Net mineralization and nitrification rates were estimated from changes in am- monium and nitrate during 45-d laboratory incubations of soil from >300 individual sample locations. Denitrification was estimated from nitrous oxide accumulation rates during 24-h incubations of intact cores (n = 252) under acetylene atmospheres at a pressure of 10 kPa. We used geostatistical procedures to characterize the spatial distributions of these and other soil variates. Semivariograms for all three N transformations showed a high degree of spatial de- pendence among points sampled within 1-40 m of one another. Nugget variances were 27-37% of structural variances, indicating that most of the variation within the sample populations for these rates could be attributed to spatial autocorrelation at a scale > 1 m. Isopleths calculated using punctual kriging algorithms show a nonuniform distribution of these transformations across the field. High rates of all processes occurred in swales on the northern edge of the sample area, but also occurred elsewhere in the field on drier, more level sites. These results indicate that spatial characteristics of the measured nitrogen transfor- mations in this old field are complex, and that only some of this complexity is associated with surface topography. Whether spatial complexity affects or mainly reflects plant com- munity structure is not known, but this small-scale heterogeneity may influence existing plant and microbial population dynamics and should be considered by those attempting to understand community dynamics or to quantify ecosystem-level nutrient fluxes.

THE FUNCTIONAL SIGNIFICANCE OF DENITRIFIER COMMUNITY COMPOSITION IN A TERRESTRIAL ECOSYSTEM

We tested the hypothesis that soil microbial diversity affects ecosystem func- tion by evaluating the effect of denitrifier community composition on nitrous oxide (N20) production. Denitrification is a major source of atmospheric N20, an important greenhouse gas and a natural catalyst of stratospheric ozone decay. The major environmental controls on denitrification rate and the mole ratio of N20 produced during denitrification have been incorporated into mechanistic models, but these models are, in general, poor predictors of in situ N20 flux rates. We sampled two geomorphically similar soils from fields in southwest Michigan that differed in plant community composition and disturbance regime: a con- ventionally tilled agricultural field and a never-tilled successional field. We tested whether denitrifier community composition influences denitrification rate and the relative rate of N20 production (AN20/A(N20 + N2)), or rN20, using a soil enzyme assay designed to evaluate the effect of oxygen concentration and pH on the activity of denitrification enzymes responsible for the production and consumption of N20. By controlling, or providing in nonlimiting amounts, all known environmental regulators of denitrifier N20 production and consumption, we created conditions in which the only variable contributing to differences in denitrification rate and rN2O in the two soils was denitrifier community composition. We found that both denitrification rate and rN2O differed for the two soils under controlled incubation conditions. Oxygen inhibited the activity of enzymes involved in N20 production (nitrate reductase, Nar; nitrite reductase, Nir; and nitric oxide reductase, Nor) to a greater extent in the denitrifying community from the agricultural field than in the community from the successional field. The Nar, Nir, and Nor enzymes of the denitrifying community from the successional field, on the other hand, were more sensitive to pH than were those in the denitrifying community from the agricultural field. Moreover, the denitrifying community in the soil from the successional field had relatively more active nitrous oxide reductase (Nos) enzymes, which reduce N20 to N2, than the denitrifying community in the agricultural field. Also, the shape of the rN20 curve with increasing oxygen was different for each denitrifying community. Each of these differences suggests that the denitrifying commu- nities in these two soils are different and that they do not respond to environmental regulators in the same manner. We thus conclude that native microbial community composition reg- ulates an important ecosystem function in these soils.

THERMODYNAMIC CONSTRAINTS ON NITROGENTRANSFORMATIONS AND OTHER BIOGEOCHEMICALPROCESSES AT SOIL–STREAM INTERFACES

There is much interest in biogeochemical processes that occur at the interface between soils and streams since, at the scale of landscapes, these habitats may function as control points for fluxes of nitrogen (N) and other nutrients from terrestrial to aquatic ecosystems. Here we examine whether a thermodynamic perspective can enhance our mechanistic and predictive understanding of the biogeochemical function of soil–stream interfaces, by considering how microbial communities interact with variations in supplies of electron donors and acceptors. Over a two-year period we analyzed >1400 individual samples of subsurface waters from networks of sample wells in riparian wetlands along Smith Creek, a first-order stream draining a mixed forested–agricultural landscape in southwestern Michigan, USA. We focused on areas where soil water and ground water emerged into the stream, and where we could characterize subsurface flow paths by measures of hydraulic head and/or by in situ additions of hydrologic tracers. We found strong support for the idea that the biogeochemical function of soil–stream interfaces is a predictable outcome of the interaction between microbial communities and supplies of electron donors and acceptors. Variations in key electron donors and acceptors (NO3−, N2O, NH4+, SO42−, CH4, and dissolved organic carbon [DOC]) closely followed predictions from thermodynamic theory. Transformations of N and other elements resulted from the response of microbial communities to two dominant hydrologic flow paths: (1) horizontal flow of shallow subsurface waters with high levels of electron donors (i.e., DOC, CH4, and NH4+), and (2) near-stream vertical upwelling of deep subsurface waters with high levels of energetically favorable electron acceptors (i.e., NO3−, N2O, and SO42−). Our results support the popular notion that soil–stream interfaces can possess strong potential for removing dissolved N by denitrification. Yet in contrast to prevailing ideas, we found that denitrification did not consume all NO3− that reached the soil–stream interface via subsurface flow paths. Analyses of subsurface N chemistry and natural abundances of δ15N in NO3− and NH4+ suggested a narrow near-stream region as functionally the most important location for NO3− consumption by denitrification. This region was characterized by high throughput of terrestrially derived water, by accumulation of dissolved NO3− and N2O, and by low levels of DOC. Field experiments supported our hypothesis that the sustained ability for removal of dissolved NO3− and N2O should be limited by supplies of oxidizable carbon via shallow flowpaths. In situ additions of acetate, succinate, and propionate induced rates of NO3− removal (∼1.8 g N·m−2·d−1) that were orders of magnitude greater than typically reported from riparian habitats. We propose that the immediate near-stream region may be especially important for determining the landscape-level function of many riparian wetlands. Management efforts to optimize the removal of NO3− by denitrification ought to consider promoting natural inputs of oxidizable carbon to this near-stream region.

An integrated conceptual framework for long-term social-ecological research

The global reach of human activities affects all natural ecosystems, so that the environment is best viewed as a social–ecological system. Consequently, a more integrative approach to environmental science, one that bridges the biophysical and social domains, is sorely needed. Although models and frameworks for social–ecological systems exist, few are explicitly designed to guide a long-term interdisciplinary research program. Here, we present an iterative framework, “Press–Pulse Dynamics” (PPD), that integrates the biophysical and social sciences through an understanding of how human behaviors affect “press” and “pulse” dynamics and ecosystem processes. Such dynamics and processes, in turn, influence ecosystem services –thereby altering human behaviors and initiating feedbacks that impact the original dynamics and processes. We believe that research guided by the PPD framework will lead to a more thorough understanding of social–ecological systems and generate the knowledge needed to address pervasive environmental problems.

Fatty acid methyl ester (FAME) profiles as measures of soil microbial community structure

Analysis of fatty acid methyl ester (FAME) profiles extracted from soils is a rapid and inexpensive procedure that holds great promise in describing soil microbial community structure without traditional reliance on selective culturing, which seems to severely underestimate community diversity. Interpretation of FAME profiles from environmental samples can be difficult because many fatty acids are common to different microorganisms and many fatty acids are extracted from each soil sample. We used principal components (PCA) and cluster analyses to identify similarities and differences among soil microbial communities described using FAME profiles. We also used PCA to identify particular FAMEs that characterized soil sample clusters. Fatty acids that are found only or primarily in particular microbial taxa-marker fatty acids-were used in conjunction with these analyses. We found that the majority of 162 soil samples taken from a conventionally-tilled corn field had similar FAME profiles but that about 20% of samples seemed to have relatively low, and that about 10% had relatively high, bacterial:fungal ratios. Using semivariance analysis we identified 21:0 iso as a new marker fatty acid. Concurrent use of geostatistical and FAME analyses may be a powerful means of revealing other potential marker FAMEs. When microbial communities from the same samples were cultured on R2A agar and their FAME profiles analyzed, there were many differences between FAME profiles of soil and plated communities, indicating that profiles of FAMEs extracted from soil reveal portions of the microbial community not culturable on R2A. When subjected to PCA, however, a small number of plated communities were found to be distinct due to some of the same profile characteristics (high in 12:0 iso, 15:0 and 17:1 ante A) that identified soil community FAME profiles as distinct. Semivariance analysis indicated that spatial distributions of soil microbial populations are maintained in a portion of the microbial community that is selected on laboratory media. These similarities between whole soil and plated community FAME profiles suggest that plated communities are not solely the result of selection by the growth medium, but reflect the distribution, in situ, of the dominant, culturable soil microbial populations.