David Harris

The 18O signature of biogenic nitrous oxide is determined by O exchange with water

To effectively mitigate emissions of the greenhouse gas nitrous oxide (N(2)O) it is essential to understand the biochemical pathways by which it is produced. The (18)O signature of N(2)O is increasingly used to characterize these processes. However, assumptions on the origin of the O atom and resultant isotopic composition of N(2)O that are based on reaction stoichiometry may be questioned. In particular, our deficient knowledge on O exchange between H(2)O and nitrogen oxides during N(2)O production complicates the interpretation of the (18)O signature of N(2)O.Here we studied O exchange during N(2)O formation in soil, using a novel combination of (18)O and (15)N tracing. Twelve soils were studied, covering soil and land-use variability across Europe. All soils demonstrated the significant presence of O exchange, as incorporation of O from (18)O-enriched H(2)O into N(2)O exceeded their maxima achievable through reaction stoichiometry. Based on the retention of the enrichment ratio of (18)O and (15)N of NO(3)(-) into N(2)O, we quantified O exchange during denitrification. Up to 97% (median 85%) of the N(2)O-O originated from H(2)O instead of from the denitrification substrate NO(3)(-).We conclude that in soil, the main source of atmospheric N(2)O, the (18)O signature of N(2)O is mainly determined by H(2)O due to O exchange between nitrogen oxides and H(2)O. This also challenges the assumption that the O of N(2)O originates from O(2) and NO(3)(-), in ratios reflecting reaction stoichiometry.

A Large-Scale Screen for Mutagen-Sensitive Loci in Drosophila

In a screen for new DNA repair mutants, we tested 6275 Drosophila strains bearing homozygous mutagenized autosomes (obtained from C. Zuker) for hypersensitivity to methyl methanesulfonate (MMS) and nitrogen mustard (HN2). Testing of 2585 second-chromosome lines resulted in the recovery of 18 mutants, 8 of which were alleles of known genes. The remaining 10 second-chromosome mutants were solely sensitive to MMS and define 8 new mutagen-sensitive genes (mus212–mus219). Testing of 3690 third chromosomes led to the identification of 60 third-chromosome mutants, 44 of which were alleles of known genes. The remaining 16 mutants define 14 new mutagen-sensitive genes (mus314–mus327). We have initiated efforts to identify these genes at the molecular level and report here the first two identified. The HN2-sensitive mus322 mutant defines the Drosophila ortholog of the yeast snm1 gene, and the MMS- and HN2-sensitive mus301 mutant defines the Drosophila ortholog of the human HEL308 gene. We have also identified a second-chromosome mutant, mus215ZIII-2059, that uniformly reduces the frequency of meiotic recombination to <3% of that observed in wild type and thus defines a function required for both DNA repair and meiotic recombination. At least one allele of each new gene identified in this study is available at the Bloomington Stock Center.

The Impact of Elevated Atmospheric [CO2] on Soil C and N Dynamics: A Meta-Analysis

The current rise in atmospheric [CO2], a consequence of human activities such as fossil fuel burning and deforestation, is thought to stimulate plant growth in many ecosystems (Bazzaz and Fajer 1990). Gifford (1994) suggested that the resulting increase in C assimilation by plants and its subsequent sequestration in the soil could counterbalance CO2 emissions. However, higher plant growth rates in a CO2-rich world can only be sustained if the soil supplies plants with additional nutrients (Zak et al. 2000; Luo et al. 2004). Therefore, the effect of elevated (e)[CO2] on soil N availability is of key importance when predicting the potential for C storage in terrestrial ecosystems. In short-term experiments, soil N availability can decrease (Diaz et al. 1993) or increase (Zak et al. 1993) under e[CO2], depending on the response of the soil microbial community. Moreover, plants under e[CO2] can increase N uptake at the expense of microbial N consumption (Hu et al. 2001). Clearly, the impact of higher [CO2] levels on C and N dynamics in terrestrial ecosystems depends on a set of complex interactions between soil and plants. Also, the establishment of equilibrium between soil organic matter (SOM) input and decomposition can take up to decades or longer. Therefore, we need longterm experiments under realistic field situations to predict changes in ecosystems under future [CO2]. The use of open-top chambers (OTC) and free-air carbon dioxide enrichment (FACE) techniques allowed for CO2 fumigation studies under far more realistic conditions than before (Rogers et al. 1983; Hendrey 1993; Chapter 2). Over the past two decades, many OTC and FACE experiments have been conducted, covering a wide range of terrestrial ecosystems. Soil characteristics

Estimation of Field N Mineralization from Laboratory Incubation for Sugar Beet Production in Michigan

Abstract Optimum and economic sugar‐beet (Beta vulgaris L.) production requires an accurate prediction of the fertilizer nitrogen (N) required, and this in turn requires an accurate estimation of the quantity of N mineralized from soil organic matter. The objectives of this study were to 1) estimate cumulative net N mineralization (Nm) in a long‐term aerobic incubation study and 2) develop a model that predicts field cumulative net N mineralization (Nt) in Misteguay silty clay soil coupled with predictions of N lost in the 0- to 45‐cm depth. Laboratory data from soil incubations were fit to linear and one‐pool exponential models to predict field N mineralization. Rates of mineralization in linear and exponential models were adjusted for field air temperatures (T), and predicted cumulative net N mineralization (Nt) values were corrected for soil moisture content (W). Calculated field cumulative net N mineralization amounts were 93.5 N kg ha−1 and 84.1 N kg ha−1 in 1993 and 1994, respectively. Predicted amounts of N leached were 6.92 kg N ha−1 and 35.7 kg N ha−1 for 1993 and 1994 sugar‐beet growing seasons, respectively. The exponential model predicted Nt better than the linear model, and Nt values were 93 kg N ha−1 and 120 kg N ha−1 in 1993 and 1994, respectively. The results of this study provided information on the potential amounts of N, which mineralizes during the sugar‐beet growing season in Misteguay silty clay soil as well as the probable amounts of N leached from the 0- to 45‐cm soil layer. Thus, this model can be a valuable tool for use in the process of developing reliably good recommendations of fertilizer rates of N during wet or dry years needed to achieve economically optimum sugar‐beet production.

SPATIAL AND TEMPORAL DISTRIBUTION OF 15N TRACER AND TEMPORAL PATTERN OF N UPTAKE FROM VARIOUS DEPTHS BY SUGARBEET

Assessment of nitrogen (N) fertilizer needs for sugarbeet requires knowledge of the amount of N available from the soil including the profile below the plow layer. Field studies were conducted to examine 1) spatial and temporal distribution of 15N tracer applied at different soil depths and 2) temporal pattern of N uptake by sugarbeet from various depths. Tracer 15N was applied in confined spots at depths of 30, 75 and 120 cm. Soil samples were collected at 5, 8 and 16 weeks after planting and analyzed for atom excess and mineral N. For the second objective, tracer N was applied on the surface and at depths of 30, 60, 90, 120, and 150 cm. Sugarbeet plants were collected 3, 6, 9, 12, 15, and 18 weeks after planting. Soil mineral N declined with time at all depths and the decline was most pronounced at 30-cm deep. Lateral movement of tracer 15N 10 cm from the point of injection was detected at the 75 and 120 cm depths 8-weeks after planting. Tracer N was recovered from all depths 9 weeks after planting but greatest amounts were recovered from the top 30-cm depth of soil throughout the sugarbeet growing season. Nitrogen uptake by sugarbeet was mostly from the surface and 30 cm deep. The findings of this study suggests that while sugarbeet recovers N from far below the plow layer the N available in or near the top 30 cm is preferentially utilized in sugarbeet nutrition.