Effects of residue removal and tillage on greenhouse gas emissions in continuous corn systems as simulated with RZWQM2.

Agricultural production is a major source of carbon dioxide (CO2) and nitrous oxide (N2O) globally. The effects of conservation practices on soil CO2 and N2O emissions remain a high degree of uncertainty. In this study, soil CO2 and N2O emissions under different residue and tillage practices in an irrigated, continuous corn system, were investigated using the Root Zone Water Quality Model (RZWQM2). Combinations of no/high stover removal (NR and HR, respectively) and no-till/conventional tillage (NT and CT, respectively) field experiments were tested over the four crop-years (Apr. 2011-Apr. 2015). The model was calibrated using the NRCT, and validated with other treatments. The simulation results showed that soil volumetric water content (VWC) in the NR treatments (i.e., NRCT and NRNT) was 1.3%-1.9% higher than that in the HR treatments (i.e., HRCT and HRNT) averaged across the four years. A higher amount of CO2 and N2O emissions were simulated in the NRCT across the four years (annual average: 7034 kg C/ha/yr for CO2 and 3.8 kg N/ha/yr for N2O), and lower emissions were in the HRNT (annual average: 6329 kg C/ha/yr and 3.7 kg N/ha/yr for N2O). A long-term simulation (2001-2015) suggested that the CO2 and N2O emissions were closely correlated with the stover removal degree (SRD), tillage, VWC, soil temperature (ST), years in management (Y), and fertilizer application. Stover and tillage practices had cumulative effects on CO2 emissions. The simulated annual CO2 emissions in 1st year from NRCT, NRNT, and HRCT were 7.8%, 0.0%, and 7.7% higher than that from HRNT, respectively; then the emissions in 15th year were 63.6%, 47.7%, and 29.1% higher, respectively. Meanwhile, there were no cumulative effects on N2O emissions. The results also demonstrated that the RZWQM2 is a promising tool for evaluating the long-term effects of CO2 and N2O emissions on different conservation practices.

Management controls the net greenhouse gas outcomes of growing bioenergy feedstocks on marginally productive croplands.

Bio-based energy is key to developing a globally sustainable low-carbon economy. Lignocellulosic feedstock production on marginally productive croplands is expected to provide substantial climate mitigation benefits, but long-term field research comparing greenhouse gas (GHG) outcomes during the production of annual versus perennial crop-based feedstocks is lacking. Here, we show that long-term (16 years) switchgrass (Panicum virgatum L.) systems mitigate GHG emissions during the feedstock production phase compared to GHG-neutral continuous corn (Zea mays L.) under conservation management on marginally productive cropland. Increased soil organic carbon was the major GHG sink in all feedstock systems, but net agronomic GHG outcomes hinged on soil nitrous oxide emissions controlled by nitrogen (N) fertilizer rate. This long-term field study is the first to demonstrate that annual crop and perennial grass systems respectively maintain or mitigate atmospheric GHG contributions during the agronomic phase of bioenergy production, providing flexibility for land-use decisions on marginally productive croplands.

Soil Carbon Response to Projected Climate Change in the US Western Corn Belt.

The western US Corn Belt is projected to experience major changes in growing conditions due to climate change over the next 50 to 100 yr. Projected changes include increases in growing season length, number of high temperature stress days and warm nights, and precipitation, with more heavy rainfall events. The impact these changes will have on soil organic carbon (SOC) needs to be estimated and adaptive changes in management developed to sustain soil health and system services. The process-based model CQESTR was used to model changes in SOC stocks (0-30 cm) of continuous corn ( L.) and a corn-soybean [ (L.) Merr.] rotation under disk, chisel, ridge, and no-tillage using projected growing season conditions for the next 50 yr. Input for the model was based on management and harvest records from a long-term tillage study (1986-2015) in eastern Nebraska, and model output was validated using measured changes in SOC from 1999 to 2011 in the study. The validated model was used to estimate changes in SOC over 17 yr under climatic conditions projected for 2065 under two scenarios: (i) crop yields increasing at the observed rate from 1971 to 2016 or (ii) crop yields reduced due to negative effects of increasing temperature. CQESTR estimates of SOC agreed well with measured SOC ( = 0.70, < 0.0001). Validated model simulated changes in SOC under projected climate change differed among the three soil depths (0-7.5, 7.5-15, and 15-30 cm). Summed over the 0- to 30-cm depth, there were significant three-way interactions of year × rotation × yield ( = 0.014) and year × tillage × yield ( < 0.001). As yield increased, SOC increased under no-tillage continuous corn but was unchanged under no-tillage corn-soybean and ridge tillage regardless of cropping system. Under chisel and disk tillage, SOC declined regardless of cropping system. With declining yields SOC decreased regardless of tillage or cropping system. These results highlight the interaction between genetics and management in maintaining yield trends and soil C.

Long-term no-till and stover retention each decrease the global warming potential of irrigated continuous corn.

Over the last 50 years, the most increase in cultivated land area globally has been due to a doubling of irrigated land. Long-term agronomic management impacts on soil organic carbon (SOC) stocks, soil greenhouse gas (GHG) emissions, and global warming potential (GWP) in irrigated systems, however, remain relatively unknown. Here, residue and tillage management effects were quantified by measuring soil nitrous oxide (N2 O) and methane (CH4 ) fluxes and SOC changes (ΔSOC) at a long-term, irrigated continuous corn (Zea mays L.) system in eastern Nebraska, United States. Management treatments began in 2002, and measured treatments included no or high stover removal (0 or 6.8 Mg DM ha-1  yr-1 , respectively) under no-till (NT) or conventional disk tillage (CT) with full irrigation (n = 4). Soil N2 O and CH4 fluxes were measured for five crop-years (2011-2015), and ΔSOC was determined on an equivalent mass basis to ~30 cm soil depth. Both area- and yield-scaled soil N2 O emissions were greater with stover retention compared to removal and for CT compared to NT, with no interaction between stover and tillage practices. Methane comprised <1% of total emissions, with NT being CH4 neutral and CT a CH4 source. Surface SOC decreased with stover removal and with CT after 14 years of management. When ΔSOC, soil GHG emissions, and agronomic energy usage were used to calculate system GWP, all management systems were net GHG sources. Conservation practices (NT, stover retention) each decreased system GWP compared to conventional practices (CT, stover removal), but pairing conservation practices conferred no additional mitigation benefit. Although cropping system, management equipment/timing/history, soil type, location, weather, and the depth to which ΔSOC is measured affect the GWP outcomes of irrigated systems at large, this long-term irrigated study provides valuable empirical evidence of how management decisions can impact soil GHG emissions and surface SOC stocks.

Arbuscular mycorrhizal fungi differ in their ability to regulate the expression of phosphate transporters in maize (Zea mays L.).

Previous studies have found that some phosphate (Pi) starvation inducible transporter genes are downregulated and arbuscular mycorrhizal (AM) inducible Pi transporter genes are upregulated in maize roots associated with the fungus Glomus intraradices. However, little is known about the functional diversity of different AM fungal species in influencing the expression of Pi transporters in maize roots. Here, we studied the expression of two Pi transporter genes ZEAma:Pht1;3 (Pi starvation inducible) and ZEAma:Pht1;6 (AM inducible) in maize root colonized by different AM fungal inoculants. Non-mycorrhizal maize, maize colonized by Glomus deserticola (CA113), Glomus intraradices (IA506), Glomus mosseae (CA201), Gigaspora gigantea (MN922A) and the co-inoculation of all four species were established. The expression patterns of the two genes were quantified using real-time, reverse transcription polymerase chain reaction. The expression level of ZEAma:Pht1;6 was 26-135 times higher in AM plants than in non-mycorrhizal maize roots, whereas the expression level of ZEAma:Pht1;3 was five to 44 times lower in AM plants than in non-mycorrhizal plants. Expression of the two genes differed with inoculation treatment, and increasing the diversity of AM fungi in maize roots led to greater expression of ZEAma:Pht1;6 as well as Pi uptake in shoots. The expression of ZEAma:Pht1;6 was significantly positively correlated with AM colonization rate, concentration of AM biomarkers in maize roots, Pi uptake and dry weight of shoot, but negatively correlated with the expression of ZEAma:Pht1;3. Addition of Pi fertilizer at a low concentration significantly increased the expression of ZEAma:Pht1;6 but had no effect on the expression of ZEAma:Pht1;3.

Distribution and quantification of antibiotic resistant genes and bacteria across agricultural and non-agricultural metagenomes.

There is concern that antibiotic resistance can potentially be transferred from animals to humans through the food chain. The relationship between specific antibiotic resistant bacteria and the genes they carry remains to be described. Few details are known about the ecology of antibiotic resistant genes and bacteria in food production systems, or how antibiotic resistance genes in food animals compare to antibiotic resistance genes in other ecosystems. Here we report the distribution of antibiotic resistant genes in publicly available agricultural and non-agricultural metagenomic samples and identify which bacteria are likely to be carrying those genes. Antibiotic resistance, as coded for in the genes used in this study, is a process that was associated with all natural, agricultural, and human-impacted ecosystems examined, with between 0.7 to 4.4% of all classified genes in each habitat coding for resistance to antibiotic and toxic compounds (RATC). Agricultural, human, and coastal-marine metagenomes have characteristic distributions of antibiotic resistance genes, and different bacteria that carry the genes. There is a larger percentage of the total genome associated with antibiotic resistance in gastrointestinal-associated and agricultural metagenomes compared to marine and Antarctic samples. Since antibiotic resistance genes are a natural part of both human-impacted and pristine habitats, presence of these resistance genes in any specific habitat is therefore not sufficient to indicate or determine impact of anthropogenic antibiotic use. We recommend that baseline studies and control samples be taken in order to determine natural background levels of antibiotic resistant bacteria and/or antibiotic resistance genes when investigating the impacts of veterinary use of antibiotics on human health. We raise questions regarding whether the underlying biology of each type of bacteria contributes to the likelihood of transfer via the food chain.

Links among nitrification, nitrifier communities, and edaphic properties in contrasting soils receiving dairy slurry.

Soil biotic and abiotic factors strongly influence nitrogen (N) availability and increases in nitrification rates associated with the application of manure. In this study, we examine the effects of edaphic properties and a dairy (Bos taurus) slurry amendment on N availability, nitrification rates and nitrifier communities. Soils of variable texture and clay mineralogy were collected from six USDA-ARS research sites and incubated for 28 d with and without dairy slurry applied at a rate of ~300 kg N ha(-1). Periodically, subsamples were removed for analyses of 2 M KCl extractable N and nitrification potential, as well as gene copy numbers of ammonia-oxidizing bacteria (AOB) and archaea (AOA). Spearman coefficients for nitrification potentials and AOB copy number were positively correlated with total soil C, total soil N, cation exchange capacity, and clay mineralogy in treatments with and without slurry application. Our data show that the quantity and type of clay minerals present in a soil affect nitrifier populations, nitrification rates, and the release of inorganic N. Nitrogen mineralization, nitrification potentials, and edaphic properties were positively correlated with AOB gene copy numbers. On average, AOA gene copy numbers were an order of magnitude lower than those of AOB across the six soils and did not increase with slurry application. Our research suggests that the two nitrifier communities overlap but have different optimum environmental conditions for growth and activity that are partly determined by the interaction of manure-derived ammonium with soil properties.

Phosphorus fractionation in manure from swine fed traditional and low-phytate corn diets.

Traditional corn (Zea mays L.) (TC), the primary grain used in swine (Sus scrofa) diets, stores a majority of its P as phytate, which is largely unavailable for digestion by nonruminant animals. Low-phytate corn (LPC) contains similar amounts of total P but a smaller percentage of P as phytate. When fed to swine, LPC increases P utilization and reduces P content of manure. While differences in P content between manure from animals fed TC and LPC diets have been documented, solubility and lability of manure P have not been compared. Manure P was characterized in manure from swine fed either LPC or TC diets in 2000 and 2001. Total P was lower (20 vs. 34 g kg(-1)) and N to P ratio was higher (4.5 vs. 3.3) in LPC manure than in TC manure. Manures were sequentially extracted with deionized water, 0.5 M NaHCO3, 0.1 M NaOH, and 1.0 M HCl. Extracts were analyzed for inorganic and total P. Most P (approximately 80%) in the extracts was in the inorganic form. Concentration of P in the water-extractable fraction was lower for LPC manure (10.2 g kg(-1) in 2000 and 9.7 g kg(-1) in 2001) than for TC manure (13.6 g kg(-1) in 2000 and 17.0 g kg(-1) in 2001). Percentage of total P in each extract was in the order of: H2O (60%), HCl (22%), NaHCO3 (12%), NaOH (8%), and residue (<1%). Total P and distribution of P in extracts indicates swine are able to utilize more P contained in LPC feed but the composition of P excreted in LPC manure is similar to TC manure. Solubility, crop availability, and lability of P in LPC manure should be similar to that of TC manure.