Nutrient Management Research Database

Few studies address nutrient cycling during the transition period (e.g., 1–4 years following conversion) from standard to some form of conservation tillage. This study compares the influence of minimum versus standard tillage on changes in soil nitrogen (N) stabilization, nitrous oxide (N2O) emissions, short-term N cycling, and crop N use efficiency 1 year after tillage conversion in conventional (i.e., synthetic fertilizer-N only), low-input (i.e., alternating annual synthetic fertilizer- and cover crop-N), and organic (i.e., manure- and cover crop-N) irrigated, maize–tomato systems in California. To understand the mechanisms governing N cycling in these systems, we traced 15N-labeled fertilizer/cover crop into the maize grain, whole soil, and three soil fractions: macroaggregates (>250 mm), microaggregates (53–250 mm) and silt-and-clay (new (i.e., N derived from 15N-fertilizer or -15N-cover crop), with 173 kg N_new ha^-1 in the conventional system compared to 71.6 and 69.2 kg N_new ha^-1 in the low-input and organic systems, respectively. In the conventional system, more N_new was found in the microaggregate and silt-and-clay fractions, whereas, the N_new of the organic and low-input systems resided mainly in the macroaggregates. Even though no effect of tillage was found on soil aggregation, the minimum tillage systems showed greater soil fraction-N_new than the standard tillage systems, suggesting greater potential for N stabilization under minimum tillage. Grain-N_new was also higher in the minimum versus standard tillage systems. Nevertheless,minimum tillage led to the greatest N₂O emissions (39.5 g N₂O–N ha^-1 day[1]^-1) from the conventional cropping system, where N turnover was already the fastest among the cropping systems. In contrast, minimum tillage combined with the low-input system (which received the least N ha^-[1]1) produced intermediate N₂O emissions, soil N stabilization, and crop N use efficiency. Although total soil N did not change after 1 year of conversion from standard to minimum tillage, our use of stable isotopes permitted the early detection of interactive effects between tillage regimes and cropping systems that determine the trade-offs among N stabilization, N₂O emissions, and N availability.

This research took place at the UC Davis Russell Ranch experimental site, and consisted of three maize-tomato rotations:  conventional (synthetic N fertilizer applications), low-input (a combination of cover crop-N and synthetic N, applied in alternate years) and organic (yearly additions of composted manure and cover crop N).

• All plots had been under standard tillage practices since 1993.  In the spring of 2003 the research plots where split down the center into standard (12-15 tractor passes) and minimum tillage regimes (5-10 tractor passes).
• Legume cover crop residues (Vicia dasycarpa and Pisum sativum) where grown in a greenhouse and labeled with 99 at.% (¹⁵NH₄)₂SO₄. Composted manure and above and belowground biomass of the legume cover crop were then incorporated into the plots of low-input and organic systems.  Conventional systems received two applications of urea on bed-tops after maize was seeded.  
  
• Plots were furrow irrigated and manual cultivation was used to simulate tillage practices for the standard and minimum tillage regimes.  
• Soil samples were collected at harvest and assessed for bulk density, moisture content, soil aggregate fractions, mean weight diameter (MWD). Maize plants were harvested, weighted and sampled for elemental and isotopic N.   Whole soil and aggregate fractions, as well as grain and vegetative biomass were analyzed for elemental and isotopic N concentration, and the proportion of N from labeled N fertilizer, as well as Nitrogen Use Efficiency (NUE) and Mean residence times (MRT) of soil N was was calculated.  
• N2O chambers were also installed and N₂O fluxes were measured every 3 weeks in the early morning through mid-afternoon period.  

Notable results after one year of tillage treatments include:

• No differences in soil N levels between tillage systems or interactive effects between cropping system and tillage.
• Minimum tillage resulted in higher N_new than standard tillage in the conventional nutrient management system, and N_new was not affected by tillage in the low-input and organic systems.
  
• The amount of labeled nitrogen applied that was recovered in the grain, and the Nitrogen Use Efficiency (NUE) of aboveground biomass was higher in the minimum than in the standard tillage system.
• NUE in the conventional system with minimum tillage was the highest, while NUE in the conventional system with standard tillage was the lowest
• N_new was found mostly in microaggregate and silt-and-clay fractions in the conventional system, while in the low-input and organic systems, it was found mostly in macroaggregates.
• Minimum tillage and conventional crop management resulted in the highest N20 emissions of all treatments.