Louise E. Jackson

Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems

Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH4 ) and nitrate (NO3 ). To compare NH4 availability, competition between nitrifiers and heterotrophic microorganisms for NH4 , and microbial NO3 assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, 15 N isotope pool dilution and 15 N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH4 as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO3 and NH4 was estimated based on the uptake of 15 N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH4 were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO3 assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO3 than NH4 . Heterotrophic microbes assimilated less NH4 than NO3 probably because NH4 concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH4 in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO3 immobilization.

Responses of soil microbial processes and community structure to tillage events and implications for soil quality

The short-term responses of soil microbial processes and community structure to perturbation constitute one aspect of soil quality. Such responses are often associated with an increase in the emissions of greenhouse gases (i.e., CO2, NO, or N2O) and the accumulation and potential loss of nitrate by leaching. Here we describe our recent work on responses of soil carbon and nitrogen dynamics, microbial biomass, and microbial community structure to a tillage event in intensively managed vegetable crop systems in California. Our results indicate that CO2 emission is high for the first day after tillage, but respiration declines or remains constant, suggesting that physical processes are responsible for the high flux from the soil surface. Net mineralization and nitrate accumulation increase for several days after tillage, and this can be accompanied by higher denitrification rates. Tillage causes immediate changes in microbial community structure, based on phospholipid fatty acid (PLFA) analysis, but little concomitant change in total microbial biomass. Tillage events contribute to decreased soil quality by increasing emissions of greenhouse gases, and increasing the potential for nitrate leaching to groundwater, and these negative aspects must be weighed against the benefits of tillage for increasing the health and productivity of some crops. D 2003 Elsevier Science B.V. All rights reserved.

Roots, Nitrogen Transformations, and Ecosystem Services

This review considers some of the mechanistic processes that involve roots in the soil nitrogen (N) cycle, and their implications for the ecological functions that retain N within ecosystems: 1) root signaling pathways for N transport systems, and feedback inhibition, especially for NO(3)(-) uptake; 2) dependence on the mycorrhizal and Rhizobium/legume symbioses and their tradeoffs for N acquisition; 3) soil factors that influence the supply of NH(4)(+) and NO(3)(-) to roots and soil microbes; and 4) rhizosphere processes that increase N cycling and retention, such as priming effects and interactions with the soil food web. By integrating information on these plant-microbe-soil N processes across scales and disciplinary boundaries, we propose ideas for better manipulating ecological functions and processes by which the environment provides for human needs, i.e., ecosystem services. Emphasis is placed on agricultural systems, effects of N deposition in natural ecosystems, and ecosystem responses to elevated CO(2) concentrations. This shows the need for multiscale approaches to increase human dependence on a biologically based N supply.

Winter cover crops in a vegetable cropping system: Impacts on nitrate leaching, soil water, crop yield, pests and management costs

Plant-soil relationships in the surface soil layer affect other processes in agroecosystems, including crop productivity, nitrate leaching and plant-pest interactions. This study investigated the effect of altering surface soil dynamics, using a winter cover crop rotation, on biotic and abiotic characteristics of the soil profile. Two cover crop treatments, phacelia and Merced rye (Phacelia tanacetifolia cv. Phaci, and Secale cereale cv. Merced), with a fallow control, were planted in November after harvest of a broccoli crop on a commercial farm site, and were incorporated using reduced tillage techniques the following March. Changes in plant and soil N pools throughout the profile were described, emphasizing nitrate (NO3-N) leaching during winter, and N availability during the subsequent broccoli crop. Changes in other aspects of the ecosystem, such as plant-pest interactions and plant disease incidence, were monitored after cover crop incorporation. The on-fam~ economic costs of cover cropping were calculated. There was a 65-70% reduction in nitrate leaching from the cover-cropped plots compared with the fallow control during winter, because plant roots in the surface soil removed N and water that would have otherwise been lost from the profile. Incorporation caused sudden large surges in inorganic N pools, net mineralizable N, and microbial biomass N and C in the surface soil, which subsided within 6 weeks, by the time the broccoli crop was planted, but which did result in increased yield at harvest in the phacelia cover-cropped treatment. No insect or disease problems which threatened the cash crops were introduced or increased as a result of the cover crops. The economic analysis indicated that the costs of cover cropping were minor compared with conventional winter management of fallowed fields, and compared with the cost of producing broccoli. The cover crops therefore provided a clear advantage during winter by significantly reducing nitrate leaching, but the effects of one cover crop rotation on subsequent nutrient dynamics in the surface soil were mostly short-lived and possibly masked by large fertilizer applications.

Microbial responses and nitrous oxide emissions during wetting and drying of organically and conventionally managed soil under tomatoes

The types and amounts of carbon (C) and nitrogen (N) inputs, as well as irrigation management are likely to influence gaseous emissions and microbial ecology of agricultural soil. Carbon dioxide (CO2) and nitrous oxide (N2O) efflux, with and without acetylene inhibition, inorganic N, and microbial biomass C were measured after irrigation or simulated rainfall in two agricultural fields under tomatoes (Lycopersicon esculentum). The two fields, located in the California Central Valley, had either a history of high organic matter (OM) inputs (“organic” management) or one of low OM and inorganic fertilizer inputs (“conventional” management). In microcosms, where short-term microbial responses to wetting and drying were studied, the highest CO2 efflux took place at about 60% water-filled pore space (WFPS). At this moisture level, phospholipid fatty acids (PLFA) indicative of microbial nutrient availability were elevated and a PLFA stress indicator was depressed, suggesting peak microbial activity. The highest N2O efflux in the organically managed soil (0.94xa0mg N2O-N m−2 h−1) occurred after manure and legume cover crop incorporation, and in the conventionally managed soil (2.12xa0mg N2O-N m−2 h−1) after inorganic N fertilizer inputs. Elevated N2O emissions occurred at a WFPS >60% and lasted <2 days after wetting, probably because the top layer (0–150xa0mm) of this silt loam soil dried quickly. Therefore, in these cropping systems, irrigation management might control the duration of elevated N2O efflux, even when C and inorganic N availability are high, whereas inorganic N concentrations should be kept low during times when soil moisture cannot be controlled.

Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH4+) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of15NH4+ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH4+, we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,15N enriched nitrate (NO3−) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more15NH4+ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH4+-N present at this time in the soil with roots had been converted from NO3−. We estimated rates of conversion of NO3− to NH4+, and rates ofNH4+ immobilization by plants and microbes, by simulating N-flow of14+15N and15N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO3− reduction to NH4+ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO3− uptake, plant NH4+ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO3− to NH4+via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH4+ than generally assumed.

Organic Amendment and Tillage Effects on Vegetable Field Weed Emergence and Seedbanks

Evaluations of the effects of minimum tillage vs. conventional tillage and the effects of organic amendments (cover crops and compost) vs. no organic amendments were conducted in a California vegetable field. Weed densities were monitored, and soil samples were taken to measure the effects of the treatments on weed seedbanks and microbial biomass over a 24-mo period. Reduced tillage increased the density of shepherds-purse in the upper soil layer (0 to 15 cm) of the soil seedbank compared with conventional tillage. Evidence is presented that suggests relationships between organic amendments, weed population reductions, and increases in soil microbial biomass: (1) shepherds-purse emergence and seedbank densities were lower in the organic amendment plots, (2) microbial biomass was nearly always higher in the organic amendment plots, and (3) significant negative correlations between microbial biomass and burning nettle and shepherds-purse emergence densities were found. These results suggest that organic matter addition may lead to reduced weed emergence. Nomenclature: Burning nettle, Urtica urens L. #3 URTUR; shepherds-purse, Capsella bursa-pastoris L. Medik. # CAPBP. Additional index words: Alternative weed management, compost, cover crop, microbial biomass carbon, soil amendments, CAPBP, URTUR.

Assessment of best management practices for nutrient cycling: A case study on an organic farm in a Mediterranean-type climate

The effectiveness of best management practices (BMPs) designed primarily to protect surface water quality was assessed on a farm certified for organic tomato production to consider potential environmental quality and production tradeoffs. The BMPs included winter cover crops typically used in organic farming to cycle nutrients and reduce stormwater runoff; tailwater ponds designed to capture runoff; and tailwater return systems, which recycle runoff back to the field. The study took place at a 44 ha (108 ac) farm in Yolo County, California, over a two-year period. Monitoring throughout the winter rainy season showed cover crops successfully reduced runoff and loads of several constituents during the storm events, when compared to fallow. Total discharge was reduced by 44%, total suspended solids was reduced by 83%, ammonium was reduced by 33%, and dissolved organic carbon (DOC) was reduced by 58%. Estimates of leaching losses of DOC in the cover cropped fields, however, were 70% higher than the fallow fields in the winter rainy season and were 30% higher than the fallow fields in the summer irrigation season. During the summer irrigation season, the tailwater pond alone was highly effective in reducing losses of total suspended solids and volatile suspended solids to the neighboring riparian zone by 97% and 89%, respectively. The tailwater pond had no effect on dissolved reactive phosphorous and actually increased concentrations of nitrate-nitrogen (NO3−-N) in effluent by 40% and DOC by 20%. As was expected, the NO3−-N leaching measured by anion exchange resin bags and nitrous oxide emissions measured by static closed chambers was higher for the tailwater pond than the fallow field. Despite these differences, losses via NO3−-N leaching and nitrous oxide emissions accounted for only 24.7 and 0.48 kg N ha−1 y−1 (22.0 and 0.40 lb N ac−1), respectively, for the entire farm, even including ponds and ditches. When field and plot values were extrapolated to the entire tomato production area to understand the relative potential tradeoffs, results indicate that BMPs could be implemented without an impact on tomato marketable yields; the tailwater ponds higher nitrous oxide emissions would not significantly increase the overall emissions for tomato production given its relatively small size; and using tailwater ponds in combination with cover crops would decrease total suspended solids (TSS) losses compared to cover crops alone, with only minor increases in NO3−-N and DOC losses. Adding a tailwater return system to this combination of BMPs could help minimize these NO3−-N and DOC losses. Use of cover crops with a tailwater pond and tailwater return system are a combination of BMPS that can thus be recommended for organic production when considering multiple environmental outcomes.