William R. Horwath

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.

On-Farm Assessment of Soil Quality in California's Central Valley

Program, 1990; Mitchell et al., 1999). Mitchell et al. (1999) also reported a perceived decline in soil quality The high-value, large-scale crop production systems in the San among producers. As a result of these concerns, many Joaquin Valley (SJV) of California typically entail intensive tillage and large fertilizer and water inputs but few C additions to the soil. SJV producers have begun to question the long-term Such practices often contribute to a decline in soil quality. Our objecsustainability of their intensively managed agricultural tive for this participatory study was to examine the effects of supplesystems. mental C management practices (SCMPs) on various soil quality To help farmers in the SJV evaluate the soil quality indicators. To increase farmer participation, we conducted the study effects of alternative soil management practices, the West on farms using a variety of SCMPs, including cover crops, compost and Side On-Farm Demonstration Project (WSD) was conmanure amendments, and several different crop rotations common ducted from 1995 to 1998. This participatory research to the region. The SCMPs significantly changed a number of soil and extension program originally included 11 large-scale properties, including soil organic matter (SOM); total Kjeldahl N; SJV row-crop producers, University of California Coopmicrobial biomass C and N; exchangeable K; Olsen P; and extractable erative Extension researchers, USDA Natural Resources Fe, Mn, and Zn. A comparison including previously established, adjacent organic, conventional, and transitional fields in addition to the Conservation Service (NRCS) conservationists, USDAtreatment fields at one farm revealed significant differences in 16 of ARS scientists, and private-sector consultants. 18 soil quality indicators. A soil quality index computed for this farm Developing science-based guidelines to quantify imscored the established organic system significantly higher than the pacts of routinely used organic inputs in this region was conventional system. Our results suggest that significant changes in identified as an important priority among the project’s several soil quality indicators occur with a variety of SCMPs. This is farmer participants (Mitchell and Goodell, 1996). A especially noteworthy considering the intensive tillage, irrigation, and brief, written survey of 15 participants, conducted durhot, semiarid environment of the SJV, California, where increases in ing a routine project meeting, invited input about their SOM and related soil properties are generally not expected in a 3-yr interest in an indexing tool to evaluate soil quality (sensu study. Andrews and Carroll, 2001; Karlen et al., 1998). Fourteen of the respondents indicated that a soil quality assessment tool would be useful to compare manageW Fresno County in the San Joaquin Valley ment alternatives (one blank response) (S.S. Andrews, (SJV) of California is one of the world’s most J.P. Mitchell, and D.L. Karlen, unpublished data, 1999). productive agricultural regions. Farmers in this area Based on that level of participatory support, our project produce more than one-third of the county’s annual

Decomposition of rice straw and microbial carbon use efficiency under different soil temperatures and moistures.

3 objectives were to (i) facilitate information exchange billion agricultural output, making it the highest reveamong farmers, consultants, and researchers regarding nue-producing county in the USA (California Dep. of these soil management practices; (ii) monitor and evaluFood and Agric., 1997). Dominant crop rotations inate on-farm, side-by-side comparisons of various SCMPs; clude annual crops (Mitchell et al., 1999) such as proand (iii) demonstrate the use of a soil quality index cessing tomato (Lycopersicon esculentum L.), cotton (SQI) for the region. (Gossypium hirsutum L.), onion (Allium cepa L.), garlic (A. sativum L.), cantaloupe (Cucumis melo L. var. reticulatus Naud.), wheat (Triticum aestivum L.), sugarbeet MATERIALS AND METHODS (Beta vulgaris L.), and lettuce (Lactuca sativa L.). Site Descriptions The intense production practices used in this region Side-by-side comparisons of conventional and organicinclude frequent and intensive tillage, irrigation, and based production systems were established on 11 farms in extensive use of fertilizers and pesticides but few addiautumn 1995. The farms were located in the western SJV tions of organic amendments to the soil (Mitchell et al., between Mendota and Huron, CA. At the beginning of the 1999). These intensive practices have raised concerns project, we randomly designated adjacent fields at each farm about resource management and water consumption as to receive either conventional or alternative treatments. The well as environmental concerns such as fugitive dust, fields varied in size but generally ranged from 30 to 60 ha ground water quality, and food safety (SJV Drainage Abbreviations: BD, bulk density; CEC, cation exchange capacity; S.S. Andrews and D.L. Karlen, USDA-ARS, Natl. Soil Tilth Lab., EC, electrical conductivity; MBN, microbial biomass nitrogen; MDS, Ames, IA 50011; J.P. Mitchell and T.K. Hartz, Dep. of Vegetable minimum data set; NRCS, Natural Resources Conservation Service; Crops and Weed Sci., and W.R. Horwath, G.S. Pettygrove, and K.M. PC, principal component; PCA, principal component analysis; PMN, Scow, Dep. of Soils and Biogeochem., Univ. of California, Davis, CA potentially mineralizable nitrogen; SAFS, Sustainable Agriculture 95616; R. Mancinelli, Dep. of Crop Prod., Univ. of Tuscia, 01100, Farming Systems (Project); SAR, sodium adsorption ratio; SCMPs, Viterbo, Italy; and D.S. Munk, Univ. of California Coop. Ext., 1720 supplemental carbon management practices; SJV, San Joaquin Valley; S. Maple Ave., Fresno, CA 93702. Received 22 May 2000. *CorreSOM, soil organic matter; SQI, soil quality index; TKN, total Kjeldahl sponding author (andrews@nstl.gov). nitrogen; WSA, water-stable aggregates; WSD, West Side On-Farm Demonstration Project; x-K, exchangeable potassium. Published in Agron. J. 94:12–23 (2002).

Comparison of soil N availability and leaching potential, crop yields and weeds in organic, low-input and conventional farming systems in northern California

The management of crop residues has become an important aspect of sustaining long-term fertility in cropping systems. Incorporation of crop residues can change microbial processes, which aAect nutrient availability and hence crop yield. Carbon (C) use eAciency by soil microorganisms during rice straw decomposition was determined in a rice paddy soil, under aerobic and anaerobic (flooded) conditions at diAerent temperatures (5, 15, and 258C). Flooding had a tendency to reduce C mineralization and enhance methane (CH4) production; however, with decreasing temperature CH4 production became negligible. Our study showed that anaerobes recycled fermentation waste products during the long-term incubation resulting in a lower net residue-C mineralization in flooded systems compared to non-flooded conditions. As a result, we observed similar microbial production under flooded and non-flooded conditions even though anaerobes decomposed less straw-C than aerobes. These results indicate that a significant amount of decomposition occurred under flooded conditions, but because substrate use eAciency was higher, less straw-C was mineralized compared to aerobic conditions. Kinetic analyses of C mineralization curves confirmed that the C mineralized in the flooded treatment was mainly from labile pools with significant amounts coming from more recalcitrant pools, such as cellulose and lignin depending on temperature. The results are discussed in relation to nutrient availability in rice cropping systems. 7 2000 Elsevier Science Ltd. All rights reserved.

Nitrogen, weeds and water as yield-limiting factors in conventional, low-input, and organic tomato systems

Increasing dependence on off-farm inputs including, fertilizers, pesticides and energy for food and fiber production in the United States and elsewhere is of questionable sustainability resulting in environmental degradation and human health risks. The organic (no synthetic fertilizer or pesticide use), and low-input (reduced amount of synthetic fertilizer and pesticide use), farming systems are considered to be an alternative to conventional farming systems, to enhance agricultural sustainability and environmental quality. Soil N availability and leaching potential, crop yields and weeds are important factors related to agricultural sustainability and environmental quality, yet information on long-term farming system effects on these factors, especially in the organic and low-input farming systems is limited. Four farming systems: organic, low-input, conventional (synthetic fertilizer and pesticides applied at recommended rates) 4-year rotation (conv-4) and a conventional 2-year rotation (conv-2) were evaluated for soil mineral N, potentially mineralizable N (PMN), crop yields and weed biomass in irrigated processing tomatoes (Lycopersicon esculentumL.) and corn (Zea mays L.) from 1994 to 1998 in California’s Sacramento Valley. Soil mineral N levels during the cropping season varied by crop, farming system, and the amount and source of N fertilization. The organic and low-input systems showed 112 and 36% greater PMN pools than the conventional systems, respectively. However, N mineralization rates of the conventional systems were 100% greater than in the organic and 28% greater than in the low-input system. Average tomato fruit yield for the 5-year period (1994–1998) was 71.0 Mg ha −1 and average corn grain yield was 11.6 Mg ha −1 and both were not significantly different among farming systems. The organic system had a greater aboveground weed biomass at harvest compared to other systems. The lower potential risk of N leaching from lower N mineralization rates in the organic and low-input farming systems appear to improve agricultural sustainability and environmental quality while maintaining similar crop yields.

Methane pool and flux dynamics in a rice field following straw incorporation

The importance of nitrogen (N), weeds, and water as yield-limiting factors was evaluated over a 4-year period in tomato cropping systems under conventional, low-input, and organic management. The cropping systems studied were part of the Sustainable Agriculture Farming Systems (SAFS) Project at the University of California, Davis, a comparison of conventional and alternative farming systems in California’s Sacramento Valley. Water applied, soil N levels, plant N uptake, weed abundance, and tomato yield were measured and compared among treatments. Tomato yields ranged from just under 55 to over 90 t ha 1 and significant treatment differences were observed in 2 of the 4 years. Multivariate analyses, used to sort out the effects of N, weeds, and water, indicated all three factors influenced yields in this study but their relative importance was dependent upon the management system. Results indicated that N availability was most important in limiting yields in the organic system and water availability was more important under conventional management. Although weed abundance was relatively high in the organic system in 2 years of the study, weed competition for N was not evident. Instead, relative N input levels and N immobilization by soil microflora appeared to explain N uptake and tomato yield variation. The findings indicate that organic and low-input tomato systems in this region can produce yields similar to those of conventional systems but that the factors limiting yield may be more difficult to manage. ©1999 Elsevier Science B.V. All rights reserved.

Impacts of cropping systems on soil nitrogen storage and loss

Concerns for air quality have led to legislation restricting rice straw burning in some parts of the world. Consequently, growers must dispose of large amounts of residual rice straw by incorporation into the soil, which may have large effects on CH4 emissions from those fields. Our objective was to characterize how this recent change in management has affected overall CH4 emissions in a California rice field and establish relationships between organic matter availability, CH4 pool sizes and CH4 fluxes. Closed chamber measurements were used to monitor diurnal and post drain fluxes, to describe the seasonal pattern of CH4 emissions and estimate total CH4 fluxes on a large on-farm field trial during the 1997 growing season. Soil redox, temperature and plant growth and yield were also monitored. To establish relationships between CH4 pool sizes and fluxes, soil interstitial CH4 concentrations were monitored in the field and available organic matter in the spring was estimated with a laboratory incubation. Redox values in the soil were found to be 50 mV lower in plots in which straw had been incorporated (−275 mV) than those in which it had been burned (−225 mV). No significant treatment differences were seen in total soil organic matter contents in the spring. However, available organic matter was 1.5 times higher in straw incorporated than straw burned plots. Methane emissions peaked between 22.00 and 23.00 h on two different diurnal sampling dates. Methane emission after draining was about 10% of the flooded period total. A 5-fold increase in total CH4 emissions over the rice growing season was observed in plots in which rice straw had been incorporated each fall for 4 yr. Total cumulative CH4 flux, 1 May–1 October 1997, was 8.87 g C m−2 in incorporated, winter flooded plots; 9.52 g C m−2 in incorporated, non-winter flooded plots; 1.63 g C m−2 in burned, winter flooded plots; and 2.25 g C m−2 in burned, non-winter flooded plots. Soil CH4 concentrations at 10–15 cm depth was strongly associated with emissions to the atmosphere (r=0.89). A model developed by Nouchi et al. (1994) [Nouchi, I., Hosono, T., Aodi, K., Minami, K., 1994. Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water, rice biomass and temperature and its modeling. Plant and Soil 161, 195-208.] which could predict the CH4 flux based on soil CH4 concentrations and temperature was fit to our data. The model was very successful at predicting flux rates and cumulative fluxes because conductance (CH4 flux divided by CH4 concentration in soil water) was highly correlated with soil temperature (r=0.88) throughout the period of high CH4 emissions. Organic matter availability and CH4 pool and flux dynamics were altered by straw incorporation practices as evidenced by increased conductance at the same interstitial CH4 concentration and increased emissions per unit available organic matter in rice straw incorporated plots.

NIR and DRIFT-MIR spectrometry of soils for predicting soil and crop parameters in a flooded field

Abstract Organic and low-input cropping systems that use more C inputs are alternatives to conventional systems for sustaining long-term soil fertility. An understanding of the impacts of these cropping systems on N balance (N applied minus N removed in harvested plant material), storage and loss is necessary to improve long-term soil fertility and minimize the risk of environmental pollution. An evaluation of 4-year rotations of organic (N from legumes and composted manures), low-input (N from legumes and reduced amounts of synthetic fertilizers), and conventional (conv-4, N from synthetic fertilizers) and a conventional 2-year rotation (conv-2, N from synthetic fertilizers) on N balance, storage and loss was conducted from 1989 to 1998. Compared to the conv-2 system, the organic and conv-4 systems showed 119 and 8% greater cumulative N balances, respectively, over the duration of the study. However, N balance in the low-input system was 19% less than in conv-2 system. After 10 years of differential management, total N in the top 15 cm of soil was 1.46 g kg−1 in the organic, 1.26 g kg−1 in the low-input, 1.13 g kg−1 in the conv-4, and 1.1 g kg−1 in the conv-2 system. Compared to the conv-2 system, cumulative N losses for the organic, low-input and conv-4 systems were lower by 80, 92, and 10%, respectively. These findings suggest that organic and low-input cropping systems that add C to soil have the potential for storing N and making it available for future crop use, while minimizing the risk of environmental pollution.

Spatial Variability and Transport of Nitrate in a Deep Alluvial Vadose Zone

The increasing popularity of site-specific management (SSM) calls for fast, inexpensive, simultaneous analyses of large numbers of soil variables. The objective of this study was to assess the potential of near infrared (NIR) and diffuse reflectance Fourier transformed in the mid-infrared range (DRIFT-MIR) spectrometry for predicting crop and soil parameters in a flooded California rice field. Two transects of 400 m each were left unfertilized, and 100 sample locations were established. Soil samples were taken in spring, and crop and weed samples at harvest. IR spectra were linked to total soil C and N, mineralizable N, P Olsen, effective cation exchange capacity (eCEC) and exchangeable cations (Ca, Mg, Na and K), as well as yield, N uptake, biomass and weed biomass using partial least squares regression (PLSr). The PLSr models were calibrated using 50 random observations, and validated using the remaining 50 observations. For soil, predictions for eCEC, Ca and Mg were the most accurate, with r2 values of 0.83, 0.80 and 0.90 for NIR and 0.56, 0.60 and 0.61 for DRIFT-MIR. Correlations for P Olsen were 0.71 and 0.55, and for mineralizable N 0.46 and 0.21, respectively. No significant correlations were found for total soil C or N. For crop parameters, only weed pressure (r2 of 0.55 and 0.44) and straw biomass (0.30 and 0.34) yielded significant correlations. The correlation with weed pressure was an indirect effect due to better competition by weeds compared to rice under low soil fertility levels. For most parameters, standard errors of prediction were lower than reported in the literature. This indicates that the small range of variability within a field might be the limiting factor in predicting these parameters. It also illustrates the limited use of correlation coefficients in PLSr model validations. We concluded that NIR spectrometry shows promise for SSM, although its predictive power for parameters may vary from site to site. Moreover, predictive models remain unique for specific agroecosystems, and therefore have to be calibrated for every area. The fast and accurate predictions for Ca and Mg concentrations in the soil could be especially important in diagnosing and combating grass tetany, which strongly depends upon Ca and Mg concentrations in the soil.