Timothy A. Doane

Spectrophotometric Determination of Nitrate with a Single Reagent

Abstract A spectrophotometric procedure for determination of nitrate in water, soil extracts, and a variety of other sample types is described using one reagent solution which is easily prepared and stored. Sample and equipment requirements are minimal. Reduced chemical hazard, simplicity, and versatility represent improvements over existing methods. Limit of detection is 0.01 µg N mL−1 (0.72 μM ) or less, depending on the matrix.

Ammonia oxidation pathways and nitrifier denitrification are significant sources of N2O and NO under low oxygen availability

The continuous increase of nitrous oxide (N2O) abundance in the atmosphere is a global concern. Multiple pathways of N2O production occur in soil, but their significance and dependence on oxygen (O2) availability and nitrogen (N) fertilizer source are poorly understood. We examined N2O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O2 concentrations following urea or ammonium sulfate [(NH4)2SO4] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH3) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N2O production was determined in 36-h incubations in microcosms by 15N-18O isotope and NH3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH3 oxidation pathways increased as O2 concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O2 concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N2O production. Heterotrophic denitrification was responsible for all N2O production at 0% O2. Nitrifier denitrification was the main source of N2O production from ammonical fertilizer under low O2 concentrations with urea producing more N2O than (NH4)2SO4 additions. These findings challenge established thought attributing N2O emissions from soils with high water content to HD due to presumably low O2 availability. Our results imply that management practices that increase soil aeration, e.g., reducing compaction and enhancing soil structure, together with careful selection of fertilizer sources and/or nitrification inhibitors, could decrease N2O production in agricultural soils.

Short-term nitrogen-15 recovery vs. long-term total soil N gains in conventional and alternative cropping systems

Abstract Cropping systems receiving organic N additions may sequester more N than conventional systems receiving inorganic N inputs. A short-term study using 15N-labeled vetch (Vicia spp.) and 15N urea was conducted to determine whether 9 years of organic N inputs to a cropping system led to lower short-term 15N losses than a cropping system under conventional management. The three cropping systems were: a conventional, receiving 15N urea, a low-input receiving 15N urea or 15N-labeled vetch residue, and an organic receiving 15N-labeled vetch residue with unlabeled poultry manure. Nitrogen recovery was calculated as the amount of 15N recovered in maize (Zea mays L.) and in the soil. Total N recovery from 15N sources in the three cropping systems were similar after one growing season and unaffected by the form of 15N application, whether organic or inorganic. Similar results were reported from other studies conducted at Rodale (USA), Rothamsted (UK) and Cadriano (Italy) where short-term N recoveries from organic or inorganic 15N sources were similar in conventional and legume or manure based cropping systems. Therefore, short-term 15N recoveries appear to be unaffected by the form of 15N application and whether organic or inorganic N-based systems are evaluated. However, results from the short-term 15N tracer experiments did not reflect known long-term trends of increased total soil N in the organic N-based cropping systems. The apparent discrepancy between short-term 15N recoveries and long-term gains in total soil N may be due to differences in total N inputs between the various cropping systems or the inability of short-term 15N studies to accurately reflect the long-term N retention of various cropping systems.

Unprecedented carbon accumulation in mined soils: the synergistic effect of resource input and plant species invasion

Opencast mining causes severe impacts on natural environments, often resulting in permanent damage to soils and vegetation. In the present study we use a 14-year restoration chronosequence to investigate how resource input and spontaneous plant colonization promote the revegetation and reconstruction of mined soils in central Brazil. Using a multi-proxy approach, combining vegetation surveys with the analysis of plant and soil isotopic abundances (delta13C and delta15N) and chemical and physical fractionation of organic matter in soil profiles, we show that: (1) after several decades without vegetation cover, the input of nutrient-rich biosolids into exposed regoliths prompted the establishment of a diverse plant community (> 30 species); (2) the synergistic effect of resource input and plant colonization yielded unprecedented increases in soil carbon, accumulating as chemically stable compounds in occluded physical fractions and reaching much higher levels than observed in undisturbed ecosystems; and (3) invasive grasses progressively excluded native species, limiting nutrient availability, but contributing more than 65% of the total accumulated soil organic carbon. These results show that soil-plant feedbacks regulate the amount of available resources, determining successional trajectories and alternative stable equilibria in degraded areas undergoing restoration. External inputs promote plant colonization, soil formation, and carbon sequestration, at the cost of excluding native species. The introduction of native woody species would suppress invasive grasses and increase nutrient availability, bringing the system closer to its original state. However, it is difficult to predict whether soil carbon levels could be maintained without the exotic grass cover. We discuss theoretical and practical implications of these findings, describing how the combination of resource manipulation and management of invasive species could be used to optimize restoration strategies, counteracting soil degradation while maintaining species diversity.

Eliminating interference from iron(III) for ultraviolet absorbance measurements of dissolved organic matter

The presence of iron(III) has long been recognized as a difficulty when ultraviolet absorbance measurements of dissolved organic matter are desired. This interference was studied in water samples of diverse origins and properties, and a procedure is discussed which uses hydroxylamine to reduce up to 10 mg L(-1) (0.18 mM) of Fe(III) to non-interfering Fe(II). This procedure eliminated the effect of Fe(III) in all samples from about 220 to 400 nm, and removed interference down to 200 nm in most samples.

Short-term soil carbon dynamics of humic fractions in low-input and organic cropping systems

Observing changes in soil organic matter (SOM) is a fundamental part of defining the carbon cycle in natural and cultivated environments. However, relying on changes in the mass of soil C over short periods often produces conflicting results because of errors associated with sampling and analysis. In addition, C mass balance studies provide little interpretation of processes or turnover of specific C fractions. In the following study, we used C isotope and chemical separation of soil organic C to observe short-term soil C dynamics. With corn as the source of tracer C in two cover crop-based agricultural systems, natural abundance 13C measurements were used to identify changes in soil humic fractions (humic acid, fulvic acid, and humin) during two seasons under organic or low-input management treatments. All three fractions showed significant accumulation or turnover of C, with the fulvic acid fraction showing the most frequent but the smallest changes. The fulvic acid fraction showed a 5–9% turnover of C compared to 16% C turnover in the humic acid fraction. The stable soil C fraction defined as humin also exhibited an 8% turnover of C. The different humic fractions were affected at different times in the two treatments, supporting the idea that individual humic fractions may have different roles in C cycling depending on inputs and seasonal conditions.

Iron: the forgotten driver of nitrous oxide production in agricultural soil.

In response to rising interest over the years, many experiments and several models have been devised to understand emission of nitrous oxide (N2O) from agricultural soils. Notably absent from almost all of this discussion is iron, even though its role in both chemical and biochemical reactions that generate N2O was recognized well before research on N2O emission began to accelerate. We revisited iron by exploring its importance alongside other soil properties commonly believed to control N2O production in agricultural systems. A set of soils from Californias main agricultural regions was used to observe N2O emission under conditions representative of typical field scenarios. Results of multivariate analysis showed that in five of the twelve different conditions studied, iron ranked higher than any other intrinsic soil property in explaining observed emissions across soils. Upcoming studies stand to gain valuable information by considering iron among the drivers of N2O emission, expanding the current framework to include coupling between biotic and abiotic reactions.

Nitrogen supply from fertilizer and legume cover crop in the transition to no-tillage for irrigated row crops

In spite of potential benefits and positive assessments of reducing primary tillage operations, only a small part of irrigated row crops is currently managed using reduced tillage, for reasons that include concerns about its agronomic suitability for certain crop rotations. Three years of a tomato/corn rotation under standard and no-tillage management were used to understand the fate of a fertilizer and cover crop nitrogen (N) application. Uptake of both inputs was reduced under no-tillage during the year of application, in this case a tomato crop. As a result, more input N was retained in the soil in this system. The initial challenge of reduced tomato yields diminished as no-tillage management remained in place and the soil N reservoir developed. Corn production was not affected by tillage treatment. Inclusion of a legume cover crop increased the amount of fertilizer N retained in the soil over time, more so under no-tillage than under standard tillage, emphasizing the benefit of cover crops in reducing the amount of fertilizer required to maintain productivity. While acceptance of reduced tillage ultimately depends on economic performance, the results of this study support its agronomic viability for irrigated row crops.

Biochar Improves Soil Aggregate Stability and Water Availability in a Mollisol after Three Years of Field Application

A field experiment was carried out to evaluate the effect of organic amendments on soil organic carbon, total nitrogen, bulk density, aggregate stability, field capacity and plant available water in a representative Chinese Mollisol. Four treatments were as follows: no fertilization (CK), application of inorganic fertilizer (NPK), combined application of inorganic fertilizer with maize straw (NPK+S) and addition of biochar with inorganic fertilizer (NPK+B). Our results showed that after three consecutive years of application, the values of soil bulk density were significantly lower in both organic amendment-treated plots than in unamended (CK and NPK) plots. Compared with NPK, NPK+B more effectively increased the contents of soil organic carbon, improved the relative proportion of soil macro-aggregates and mean weight diameter, and enhanced field capacity as well as plant available water. Organic amendments had no obvious effect on soil C/N ratio or wilting coefficient. The results of linear regression indicated that the improvement in soil water retention could be attributed to the increases in soil organic carbon and aggregate stability.

Isotopic and nutritional evidence for species- and site-specific responses to N deposition and elevated CO2 in temperate forests

In this study we show that the effect of rising atmospheric CO2 levels on forest productivity is influenced by changes in nutrient availability caused by nitrogen (N) deposition. We used a dual-isotope approach (δ15N and δ13C), combined with dendrochronological and nutritional analyses, to evaluate the response of two dominant tree species in natural forest ecosystems near Mexico City (Pinus hartwegii—pine; Abies religiosa—fir). Our analysis focuses on changes that occurred over the past 50 years at two sites, one under high and one under low N deposition rates. Analyses of carbon isotope composition indicate increasing water-use efficiency in response to rising CO2 levels for both species and sites but this effect did not lead to improved tree growth. The magnitude and direction of shifts in 13C discrimination indicate a process of acclimation that varied with the rate of N deposition and species traits. Since the 1960s, strong negative responses to N deposition have been observed in fir trees, which showed altered foliar nutrition and growth decline, while the negative impacts of N deposition on pine trees remained undetectable until the 1990s. In recent years, both species have shown significant growth decline under high N deposition despite increasing atmospheric CO2. Multivariate analysis of leaf nutrients indicates that growth decline was prompted by depleted soil macronutrient (P, K, and Ca) and micronutrient (Cu, Fe, Zn, and Mn) availability. At both sites, fir trees were a better indicator of N deposition due to differences in canopy rainfall interception.