Measurements and APSIM modelling of soil C and N dynamics

Process-based models capture our understanding of key processes that interact to determine productivity and environmental outcomes. Combining measurements and modelling together help assess the consequences of these interactions, identify knowledge gaps and improve understanding of these processes. Here, we present a dataset (collected in a two-month fallow period) and list potential issues related to use of the APSIM model in predicting fluxes of soil water, heat, nitrogen (N) and carbon (C). Within the APSIM framework, two soil water modules (SoilWat and SWIM3) were used to predict soil evaporation and soil moisture content. SWIM3 tended to overestimate soil evaporation immediately after rainfall events, and SoilWat provided better predictions of evaporation. Our results highlight the need for testing the modules using data that includes wetting and drying cycles. Two soil temperature modules were also evaluated. Predictions of soil temperature were better for SoilTemp than the default module. APSIM configured with different combinations of soil water and temperature modules predicted nitrate dynamics well, but poorly predicted ammonium-N dynamics. The predicted ammonium-N pool empties several weeks after fertilisation, which was not observed, indicating that the processes of mineralisation and nitrification in APSIM require improvements. The fluxes of soil respiration and nitrous oxide, measured by chamber and micrometeorological methods, were roughly captured by APSIM. Discrepancies between the fluxes measured with chamber and micrometeorological techniques highlight difficulties in obtaining accurate measurements for evaluating performance of APSIM to predict gaseous fluxes. There was uncertainty associated with soil depth, which contributed to surface emissions. Our results showed that APSIM performance in simulating N2O fluxes should be considered in relation to data precision and uncertainty, especially the soil depths included in simulations. Finally, there was a major disconnection between the predicted N loss from denitrification (N2 + N2O) and that measured using the 15N balance technique.

New methods to quantify NH3 volatilization from fertilized surface soil with urea

Gaseous N losses from soil are considerable, resulting mostly from ammonia volatilization linked to agricultural activities such as pasture fertilization. The use of simple and accessible measurement methods of such losses is fundamental in the evaluation of the N cycle in agricultural systems. The purpose of this study was to evaluate quantification methods of NH3 volatilization from fertilized surface soil with urea, with minimal influence on the volatilization processes. The greenhouse experiment was arranged in a completely randomized design with 13 treatments and five replications, with the following treatments: (1) Polyurethane foam (density 20 kg m-3) with phosphoric acid solution absorber (foam absorber), installed 1, 5, 10 and 20 cm above the soil surface; (2) Paper filter with sulfuric acid solution absorber (paper absorber, 1, 5, 10 and 20 cm above the soil surface); (3) Sulfuric acid solution absorber (1, 5 and 10 cm above the soil surface); (4) Semi-open static collector; (5) 15N balance (control). The foam absorber placed 1 cm above the soil surface estimated the real daily rate of loss and accumulated loss of NH3N and proved efficient in capturing NH3 volatized from urea-treated soil. The estimates based on acid absorbers 1, 5 and 10 cm above the soil surface and paper absorbers 1 and 5 cm above the soil surface were only realistic for accumulated N-NH3 losses. Foam absorbers can be indicated to quantify accumulated and daily rates of NH3 volatilization losses similarly to an open static chamber, making calibration equations or correction factors unnecessary.

Fertilizer 15N balance in a coffee cropping system: a case study in Brazil

Knowledge about the fate of fertilizer nitrogen in agricultural systems is essential for the improvement of management practices in order to maximize nitrogen (N) recovery by the crop and reduce N losses from the system to a minimum. This study involves fertilizer management practices using the 15N isotope label applied in a single rate to determine the fertilizer-N balance in a particular soil-coffee-atmosphere system and to deepen the understanding of N plant dynamics. Five replicates consisting of plots of about 120 plants each were randomly defined within a 0.2 ha coffee plantation planted in 2001, in Piracicaba, SP, Brazil. Nine plants of each plot were separated in sub-plots for the 15N balance studies and treated with N rates of 280 and 350 kg ha-1 during 2003/2004 and 2004/2005, respectively, both of them as ammonium sulfate enriched to a 15N abundance of 2.072 atom %. Plant shoots were considered as separate parts: the orthotropic central branch, productive branches, leaves of productive branches, vegetative branches, leaves of vegetative branches and fruit. Litter, consisting of dead leaves accumulated below the plant canopy, was measured by the difference between leaves at harvest and at the beginning of the following flowering. Roots and soil were sampled down to a depth of 1.0 at intervals of 0.2 m. Samples from the isotopic sub-plots were used to evaluate total N and 15N, and plants outside sub-plots were used to evaluate dry matter. Volatilization losses of NH3 were estimated using special collectors. Leaching of fertilizer-N was estimated from deep drainage water fluxes and 15N concentrations of the soil solution at 1 m soil depth. At the end of the 2-year evaluation, the recovery of 15N applied as ammonium sulfate was 19.1 % in aerial plant parts, 9.4 % in the roots, 23.8 % in the litter, 26.3 % in the fruit and 12.6 % remaining in the 0_1.0 m soil profile. Annual leaching and volatilization losses were very small (2.0 % and 0.9 %, respectively). After two years, only 6.2 % N were missing in the balance (100 %) which can be attributed to other non-estimated compartments and experimental errors. Results show that an enrichment of only 2 % atom 15N allows the study of the partition of fertilizer-N in a perennial crop such as coffee during a period of two years.

Estimating nitrous oxide emissions from flood-irrigated alkaline grey clays

Concern has mounted over recent decades regarding the emission of nitrous oxide (N2O) to the atmosphere through human activities. Modern agriculture has contributed to this with elevated use of nitrogenous fertilizers and irrigation. Irrigated cotton grown on alkaline heavy clay soils often uses nitrogen fertiliser inefficiently, due largely to N loss (commonly 50–100 kg N/ha) through denitrification. However, the amount of denitrified N emitted as N2O has rarely been measured. This paper derives estimates of the quantities of N2O emitted from N fertiliser applied to alkaline grey clays.A negative exponential function between the N2O/N2 mole fraction and soil pH was derived from a search of laboratory and field studies published by numerous authors using a wide range of soil types. A greater proportion of N2O relative to N2 is emitted from acid soils; approximately equivalent amounts of each gas are emitted from soil of pH 6.0. For the alkaline grey clays (pH 8.3–8.5), the N2O/N2 mole fraction was about 0.024.The quantities of N2O emitted from alkaline grey clays during the growth of a cotton crop were estimated by applying this relationship to 15N balance studies where N fertiliser losses had been measured. Using this approach, about 2 kg N/ha (~1.1% of the N applied) was calculated to be lost as N2O during the cotton-growing season. This is similar to the value of 1.25% commonly used to estimate N2O emissions from N fertiliser, but this estimation should only be applied to alkaline soils; a larger percentage of the fertiliser N denitrified from acid soils should be emitted as N2O-N. These estimates of N2O emissions require validation with field experimentation.The low (negligible) values for N2O emission from flooded fields compared with laboratory observations are discussed. It is possible that high N2O emissions observed under laboratory conditions result from the shallow depth of soil, reducing the opportunity for N2O to be further reduced as it diffuses through the soil profile. Management strategies that have the potential to reduce N2O emissions are discussed.

Gaseous nitrogen losses from urea applied to maize on a calcareous fluvo-aquic soil in the North China Plain

Gaseous nitrogen losses, by NH3 volatilisation and denitrification, are mainly responsible for the low recovery of N fertiliser applied to irrigated maize on the North China Plain. Two field experiments were conducted to measure NH3 volatilisation and nitrification-denitrification losses from urea applied to maize (Zea mays L.) grown on a calcareous fluvo-aquic soil (Aquic Inceptisol) in Fengqiu County, Henan Province. The first was carried out in June 1998 (urea applied at 75 kg N/ha 3 weeks after sowing), and the second in July 1998 (urea applied at 200 kg N/ha 6 weeks after sowing). Each experiment included 3 treatments-control, surface-broadcast (SB), and deep point placement (DP) or broadcast followed by irrigation (BI). NH3 loss was measured by a micrometeorological method (NH3 sampler). Denitrification (N2 + N2O) was measured by the acetylene inhibition-intact soil core technique, and N2O emission was also measured in the absence of acetylene. The recovery of applied N was measured by a 15N balance technique. When urea was surface broadcast (SB) 3weeks (75 kg N/ha) and 6weeks (200 kg N/ha) after sowing, 44 and 48% of the applied N was lost by NH3 volatilisation, respectively. The corresponding losses from the BI and DP treatments were only 18% and 11%, respectively. Denitrification was a significant process in this well-drained sandy soil, with average loss rates of 0.26-0.43 kg N/ha.day in the controls (from resident soil N), compared with 0.52-0.63 kg N/ha.day in the surface fertiliser treatments. Deep placement of urea reduced the denitrification rate to an average of 0.3 kg N/ha.day. The net denitrification loss from the fertiliser was <2% of the applied N, except for the SB urea treatment in the second experiment. The application of N fertiliser as urea increased N2O emissions from c. 0.3 to c. 2.3 kg N/ha over 57 days in the second experiment, with average N2O emission rates in the control and SB treatment of 0.006 and 0.042 kg N/ha.day, respectively. The significantly lower ratio of N2 /N2O in the urea treatments compared with the control suggested that nitrification of applied N may have contributed to N2O production. Alternatively, the ratio of N2 /N2O during denitrification may have changed with the greater supply of NO3 -. denitrification, maize, NH3 volatilisation, N2O emission.

Resolution of the 15N balance enigma?

The enigma of soil nitrogen balance sheets has been discussed for over 40 years. Many reasons have been considered for the incomplete recovery of 15N applied to soils, including sampling uncertainty, gaseous N losses from plants, and entrapment of soil gases. The entrapment of soil gases has been well documented for rice paddy and marshy soils but little or no work appears to have been done to determine entrapment in drained pasture soils. In this study 15N-labelled nitrate was applied to a soil core in a gas-tight glovebox. Water was applied, inducing drainage, which was immediately collected. Dinitrogen and N2O were determined in the flux through the soil surface, and in the gases released into the glovebox as a result of irrigation or physical destruction of the core. Other components of the N balance were also measured, including soil inorganic-N and organic-N. Quantitative recovery of the applied 15N was achieved when the experiment was terminated 484 h after the 15N-labelled material was applied. Nearly 23% of the 15N was recovered in the glovebox atmosphere as N2 and N2O due to diffusion from the base of the soil core, convective flow after irrigation, and destructive soil sampling. This 15N would normally be unaccounted for using the sampling methodology typically employed in 15N recovery experiments.

Measurement of gaseous emissions from denitrification of applied N-15 .I. Effect of cover duration

Measurement of gas emissions from denitrification of applied N has been restricted because of the lack of a convenient method. Recently a method using an electric are to measure 15N contents of dinitrogen (N2) and nitrous oxide (N2;O) in air has been developed. Gas emissions from denitrification of applied 15N were determined using this method for gas analysis of the 15N2 and 15N2O captured beneath an air-tight soil cover. Loss of 15N was calculated from gas emission measurements by two methods, accumulation of daily emissions and from the peak 15N emission value by assuming linear increase and decrease over the period of emissions. Losses estimated at low emissions with incomplete soil saturation were similar (1.9 - 5.6% 15N applied) for the two methods. Losses estimated at higher emissions with complete soil saturation were higher when calculated using peak emission values (14.8 - 28.5%) rather than accumulated daily emissions (9.5 - 18.7%). Losses estimated by emissions were compared with 15N loss estimated by mass balance at the completion of two successive soil saturations. As daily cover duration was shortened, gas emission estimates of loss more closely approximated total gaseous 15N as estimated from unaccounted for 15N in the15N balance. With shortest cover duration (15 min day-1) there was close agreement (94% estimated from peak emissions) with 15N loss estimated by 15N balance. A strategy for quantitatively estimating 15N loss by emission measurements is suggested.