Martin Burger, Louise Jackson, Dennis Rolston, Kate Scow

Better understanding of biological and environmental factors that control soil microbial processes that affect N retention, and carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions from agricultural soil is needed to modify farming practices accordingly. The Long Term Research in Agricultural Systems (LTRAS) project at University of California at Davis has provided the context for studying differences between organic and conventional production of tomatoes in California’s Central Valley (Burger, 2002, Ph.D. Dissertation, University of California, Davis).

The organic system is a tomato-corn rotation and receives leguminous winter cover crops and composted manure in addition to harvest residue. No synthetic pesticides are applied. Average dry matter inputs are 23 Mg ha^-1 y^-1.

The conventional system is a tomato-wheat fallow rotation and receives harvest residue, inorganic fertilizer, and synthetic pesticides. Average dry matter inputs are 6.4 Mg ha^-1 y^-1.

Irrigation and rainfall stimulate soil microbial N transformations, including production and loss of the greenhouse gases, CO₂ and N₂O. Field and laboratory experiments on actual or simulated rewetting events gave the following results (Burger et al., 2005, Biology and Fetility of Soils):

• Microbial biomass C (MBC) and respiration, and CO₂ emissions were greater in the organic than conventional soil.
• The highest N₂O flux in the organic soil (0.94 mg N₂O-N m^-2 h^-1) occurred after manure and legume cover crop incorporation, and in the conventional soil (2.12 mg N₂O-N m^-2 h^-1) after inorganic N fertilizer inputs.
• Nitrification could have contributed to these N₂O fluxes since ammonium (NH₄⁺) concentrations were 19.1 and 32.2 ìg NH₄⁺-N g^-1 in the organic and conventional soil, respectively.
• Changes in soil moisture affected microbial activity and community structure, as determined by phospholipid ester-linked fatty acid (PLFA) analysis, as previously shown by Lundquist et al. (1999, Soil Biology and Biochemistry), which uses the cell membrane lipids within microorganisms as biomarkers for specific groups of organisms, more strongly than microbial biomass carbon.
• The higher activity of microbial communities at higher moisture levels and the ensuing effect on gaseous emissions indicate the importance of careful irrigation management, especially in the organic system with its propensity for higher CO₂, but not N₂O, emissions.

Soil N transformation rates and fates of NH₄⁺ and NO₃^- would be expected to differ between agricultural systems that receive high (organic) or low organic matter (conventional). To compare NH₄⁺ availability, competition between nitrifiers and heterotrophic microorganisms for NH₄⁺, and microbial NO₃^- assimilation in organic and conventional soils, chemical and biological soil assays and ¹⁵N isotope pool dilution and ¹⁵N tracer techniques were used (Burger et al., 2003, Soil Biology and Biochemistry). The results showed:

• Gross ammonification, potentially mineralizable N, and hot minus cold KCl-extracted NH₄⁺ were approx. twice as high in the organic compared to the conventional soil.

• Net estimated NO₃^- assimilation by microbes was between 32 and 45% of gross nitrification rates in both conventional and organic soil.

• In both soils, microbes assimilated more NO₃^- than NH₄⁺.
• Heterotrophic microbes assimilated less NH₄⁺ than NO₃^- probably because NH₄⁺ concentrations were low and because competition by nitrifiers was apparently strong.
• The organic soil released NH₄⁺ in a gradual manner and, compared to the conventional soil, and supported a more active microbial biomass with greater N demand that was met mainly by NO₃^- immobilization.

Plants were expected to alter soil nitrogen transformations, especially by increased immobilization of NH₄⁺. In microcosms containing organic production soil, gross nitrification and immobilization was measured in soil with tomato plants and in root exclosures (Burger et al., 2004, Plant and Soil). The results showed:

• Plant roots unexpectedly increased the amount of recently-produced NH₄⁺, based on appearance of ¹⁵NH₄⁺ after application of ¹⁵NO₃^-.
• Mechanisms for ¹⁵NH₄⁺ production were tested with models. Plant N efflux and/or and microbial biomass nitrogen turnover were the more likely possibilities, rather than dissimilatory NO₃^- reduction.
• Rapid recycling of NO₃^- to NH₄⁺ by plants and/or microbes may contribute to ecosystem nitrogen retention and would enable plants to capture more NH₄⁺ than is generally assumed.

The major findings of this work thus were:

• The higher microbial biomass and activity of the organic soils results in higher CO₂ emissions, but not consistently higher N₂O emissions, than the conventional soils, during periods after rewetting when losses are at a maximum.
• Microbial assimilation of NO₃^- was higher than NH₄⁺ in both soils, possibly due to high competition with nitrifiers.
• In both soils, NO₃^- dynamics are central to the retention and loss of N, even in the organic soil that only receives mineralizable N substrates.
• Conversion of NO₃^- to NH₄⁺ in the rhizosphere may contribute to greater nitrogen retention in the organic soil, especially if the mechanism is high turnover of microbial biomass.

References

Burger, M., L.E. Jackson, E.J. Lundquist, D.T. Louie, R.L. Miller, D.R. Rolston, and
     K.M. Scow. 2005. Microbial responses and nitrous oxide emissions during
     wetting and drying of organically and conventionally managed soil under
     tomatoes. Biology and Fertility of Soils 42: 109-118.
     ucce.ucdavis.edu/files/filelibrary/5472/23203.pdf
  
Lundquist, E.J., K.M. Scow, L.E. Jackson, S.L. Uesugi, and C.R. Johnson. 1999.   
     Rapid response of soil microbial communities from conventional, low input, and
     organic farming systems to a wet/dry cycle. Soil Biology and Biochemistry
     31:1661-1675.
     ucce.ucdavis.edu/files/filelibrary/5472/21420.pdf

Burger, M. and L.E. Jackson. 2003. Microbial immobilization of ammonium and
     nitrate in relation to ammonification and nitrification rates in organic and
     conventional cropping systems. Soil Biology and Biochemistry 35:29-36.
     ucce.ucdavis.edu/files/filelibrary/5472/21399.pdf

Burger, M. and L.E. Jackson. 2004. Plant and microbial nitrogen use and turnover:
     rapid conversion of nitrate to ammonium in soil with roots. Plant and Soil
     266:289-301.
     ucce.ucdavis.edu/files/filelibrary/5472/21400.pdf